# Exploring the Dust Content of Galactic Halos with Herschel III. NGC 891

J. H. Yoon,1 C. L. Martin,1 S. Veilleux,2,3 M. Meléndez,2,4 T. Mueller,5 K. D.
Gordon,4 G. Cecil,6 J. Bland-Hawthorn,7 C. Engelbracht8
1Department of Physics, University of California, Santa Barbara, CA 93106, USA
2Department of Astronomy, University of Maryland, College Park, MD 20742, USA
3Joint Space-Science Institute, University of Maryland, College Park, MD
20742, USA
4Space Telescope Science Institute, Baltimore, MD 21218, USA
5Max-Planck-Institute for Extraterrestrial Physics (MPE), 85748 Garching,
Germany
6Department of Physics and Astronomy, University of North Carolina, Chapel
Hill, NC 27599
7Department of Physics, University of Sydney, Sydney, NSW 2006, Australia
8Department of Astronomy, University of Arizona, Tucson, AZ 85721, USA E-mail:
<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present deep far-infrared observations of the nearby edge-on galaxy NGC 891
obtained with the Herschel Space Observatory and the Spitzer Space Telescope.
The maps confirm the detection of thermal emission from the inner
circumgalactic medium (halo) and spatially resolve a dusty superbubble and a
dust spur (filament). The dust temperature of the halo component is lower than
that of the disk but increases across a region of diameter $\approx 8.0$ kpc
extending at least 7.7 kpc vertically from one side of the disk, a region we
call a superbubble because of its association with thermal X-ray emission and
a minimum in the synchrotron scaleheight. This outflow is breaking through the
thick disk and developing into a galactic wind, which is of particular
interest because NGC 891 is not considered a starburst galaxy; the star
formation rate surface density, 0.03$~{}{\rm M_{\odot}}$ yr-1 kpc-2, and gas
fraction, just $10\%$ in the inner disk, indicate the threshold for wind
formation is lower than previous work has suggested. We conclude that the star
formation surface density is sufficient for superbubble blowout into the halo,
but the cosmic ray electrons may play a critical role in determining whether
this outflow develops into a fountain or escapes from the gravitational
potential. The high dust-to-gas ratio in the dust spur suggests the material
was pulled out of NGC 891 through the collision of a minihalo with the disk of
NGC 891. We conclude that NGC 891 offers an example of both feedback and
satellite interactions transporting dust into the halo of a typical galaxy.

###### keywords:

galaxies: halos – galaxies: ISM – galaxies: photometry – galaxies: starburst –
galaxies: star formation – galaxies: infrared: galaxies

††pubyear: 2020††pagerange: Exploring the Dust Content of Galactic Halos with
Herschel III. NGC 891–Exploring the Dust Content of Galactic Halos with
Herschel III. NGC 891

##  1 Introduction

Studying giant spiral galaxies beyond the Local Group determines whether the
assembly of the Milky Way was typical. Having a vantage point outside the
system also makes observations of the stellar halo and circumgalactic medium
(CGM) easier. The disk component can be completely separated from the halo,
for example, when such galaxies are viewed edge-on. The edge-on orientation of
the nearby galaxy, NGC 891, is ideal for studying the interaction between the
disk, CGM, and satellite galaxies.

NGC 891 resides in a spiral-rich group representative of typical environments
and has a luminosity similar to the group’s brightest member, NGC 1023. NGC
891 is similar to the Milky Way in spiral type and rotation speed (Swaters et
al., 1997). In both galaxies, a large fraction of the halo stars have been
accreted from satellites (Mouhcine et al., 2010; Bell et al., 2008) and recent
star formation is elevated (compared to the disk as a whole) in a molecular
ring and central circumnuclear disk (Scoville et al., 1993).

The warm ionized gas in the interstellar medium of NGC 891 is more extended
than the Reynold’s layer (Reynolds, 1989) in the Milky Way, and the central
surface density of the warm ionized medium is roughly twice as high in NGC 891
as in the Milky Way (Rand et al., 1990; Dettmar, 1990). The molecular gas mass
of NGC 891 is 2.5 times larger than that of the Milky Way, and the current
star formation rate (SFR) is proportionally higher. Star formation in NGC 891
is more active than in the Milky Way disk yet not high enough to be considered
a starburst. The specific SFR and stellar mass of NGC 891 are similar to many
COS Halos galaxies at redshift $z\approx 0.2$ (Tumlinson et al., 2011; Werk et
al., 2014). The proximity of NGC 891, however, has made it possible to
directly detect emission from the circumgalactic medium (CGM). The recent
21-cm detection of cold gas out to 90 - 120 kpc along the minor axis (Das et
al., 2020), combined with the X-ray detection of the virialized halo gas
(Hodges-Kluck et al., 2018) and the well-studied synchroton halo (Schmidt et
al., 2019), together provide excellent observational contraints on the
properties of the inner CGM.

The vertical distribution of hot gas (Bregman & Pildis, 1994), neutral gas
(Swaters et al., 1997), molecular gas (Garcia-Burillo et al., 1992), warm dust
(McCormick et al., 2013), and stars (van der Kruit & Searle, 1981) have been
measured in NGC 891 and found to contain extraplanar components. A full 30% of
the HI resides in a halo (Oosterloo et al., 2007). The thermal emission from
the halo dust indicates a larger dust mass than was originally estimated from
absorption features (Howk & Savage, 1997, 2000; Popescu et al., 2000; Bianchi,
2008; Popescu et al., 2011). The first maps of the dust mass and temperature
showed a strong correlation with the distribution of stellar mass and star
formation, respectively (Hughes et al., 2014). The X-ray halo of NGC 891 was
the first detected in a non-starburst galaxy, but it has remained
controversial as to whether the hot gas was heated by galactic accretion
(Hodges-Kluck & Bregman, 2012) or feedback processes (Strickland et al.,
2004a, b; Tüllmann et al., 2006).

The presence of cold dust in NGC 891 has been known for several decades, but
little was known about its spatial distribution due to the low resolution of
the far-infrared and submm observations. Observations of NGC 891 with
Spitzer/MIPS resolved disk and halo components at 24 ${\mu}$m but lacked the
spatial resolution at longer wavelengths required to map the spectral energy
distribution, hereafter the SED (GO-20528, PI C. Martin). As part of the Very
Nearby Galaxies Survey, a Herschel Guaranteed Time Key Project, the large
vertical extent of the dust in NGC 891 was resolved with Herschel PACS/SPIRE
(Hughes et al., 2014)

In this paper, we combine deeper Herschel observations (OT1_sveilleu_2, PI S.
Veilleux) with the Spitzer/MIPS data to better describe the amount of halo
dust and its origin. This work builds on our examination of NGC 4631 (Meléndez
et al., 2015, hereafter Paper I), a lower mass galaxy with a higher SFR
surface density (Tüllmann et al., 2006), and six nearby dwarf galaxies
including NGC 1569 (McCormick et al., 2018, hereafter Paper II). The
interstellar dust in NGC 891 plays an important role in its appearance. Young,
star-forming regions, for example, are much more apparent along the northeast
(approaching) side of the disk compared to the receding (southwest) half due
to the accumulation of dust along the inner edges of the trailing spiral arms.
Our deep observations directly show the relative importance of stellar
feedback and satellite interactions for building a dusty halo. This new
perspective should prove generally valuable for understanding the dust-to-gas
ratio in galaxy halos (Ménard & Fukugita, 2012), and the processes taking
place in the outskirts of galaxies in general (Veilleux et al., 2020).

Our presentation is structured as follows. In Section 2, we present the far-
IR/submm observations. The morphological components of the dust distribution
are defined in Section 3 before describing our fits to the multi-band
photometry and the resulting physical properties in Section 4. In Section 5,
we argue that the superbubble will become a galactic wind and discuss the
origin of a puzzling, extraplanar dust spur. Our conclusions are briefly
summarized in Section 6.

We adopt a distance of 9.77 Mpc (Ferrarese et al., 2000), which is the
weighted mean of distances derived from HST observations of planetary nebula
and surface brightness fluctuations. This distance gives an angular scale of
47.37 pc/″.

##  2 Observation and Data Reduction

###  2.1 Spitzer/IRAC and Spitzer/MIPS data

Spitzer images at 3.6 ${\mu}$m, 4.5 ${\mu}$m, and 24 ${\mu}$m are used in this
study. Images taken with the Infrared Array Camera (IRAC) at 3.6 ${\mu}$m and
4.5 ${\mu}$m cover the entire galaxy (PID: 3, PI: G. Fazio). The observations
consist of a series of dithered images taken with exposure times from 0.4 to
96.80 seconds. The basic calibrated data were obtained from the Spitzer
archive and combined into a mosaic using the montage package
(http://montage.ipac.caltech.edu/) correcting for variations in the overall
level and rejecting outliers. The point source flux accuracy is $\sim$2%
(Reach et al., 2005) and the extended source surface brightness accuracy is
estimated at $\sim$4%.

We observed NGC 891 with the Multiband-Imaging Photometer for Spitzer (MIPS,
Rieke et al., 2004) at 24 ${\mu}$m (PID: 20528, PI: C. Martin). The images
were obtained using the medium rate, scan-map mode and the scan legs of
$0.5^{\circ}$ with 148″ cross-scan offsets. This observation consists of 5
scans (Obs. id 14815488, 14815744, 14816000, 14816256, 14816512) with total
exposure times of 1600 seconds. The data were reduced with the MIPS Data
Analysis Tool v3.04 (DAT Gordon et al., 2005). Extra steps were carried out to
improve the images including readout offset correction, array averaged
background subtraction (using a low order polynomial fit to each leg, with the
region including NGC 891 excluded from this fit), and exclusion of the first
five images in each scan leg due to boost frame transients. The point source
flux accuracy is $\sim$2% (Engelbracht et al., 2007) and the extended source
surface brightness accuracy is estimated at $\sim$4%.

The 24 ${\mu}$m image resolves stars and background galaxies, which we mask
out for accurate photometry of the galaxy. We identified background galaxies
using SExtractor. Briefly, we set the detection threshold to 1.5$\sigma$ in
surface brightness. The sources identified by the segmentation map were then
replaced by the values in the background map at the same locations.

###  2.2 Herschel/PACS and Herschel/SPIRE Data

The Photodetector Array Camera and Spectrometer (PACS, Poglitsch et al., 2010)
observation for NGC 891 was conducted as our cycle 1 open-time program
(OT1_sveilleu_2, PI: S. Veilleux). This program utilizes PACS blue 70 ${\mu}$m
and red 160 ${\mu}$m channels with 6 scan position angles at 55∘, 70∘, 85∘,
95∘, 110∘, and 125∘ (Obs IDs: 1342237999, 1342238000, 1342238001, 1342238002,
1342238003, and 1342238004). The exposure time per each scan was 5596 seconds
(total 9.3 hours) including overhead. The scan map leg length and separation
were 4′ and 4″ and the number of scan map legs was 60. Each scan map was
repeated three times for redundancy reasons.

We also retrieved, reduced, and analyzed the PACS data from The Herschel EDGE-
on galaxy Survey (HEDGES, Murphy, 2011, OT2_emurph01_3) for the analysis of
extended structure as these data have a larger field-of-view than do our
observations. For the blue channel (70 ${\mu}$m), the scan angle was 45∘ and
135∘ with an exposure time of 6550 seconds. The scan map leg length and
separation were 14$\aas@@fstack{\prime}$6 and 14$\aas@@fstack{\prime\prime}$0
and the number of scan map legs was 70. For the green channel (100 ${\mu}$m),
the exposure time was 8977 seconds at scan angles of 45∘ and 135∘. The scan
map leg length and separation were 14$\aas@@fstack{\prime}$6 and
20$\aas@@fstack{\prime\prime}$0 and the number of scan map legs was 51.

The observation in 250 ${\mu}$m, 350 ${\mu}$m, and 500 ${\mu}$m bands were
performed using the Spectral and Photometric Imaging Receiver (SPIRE, Griffin
et al., 2010) onboard Herschel Space Observatory. Our SPIRE images are the
part of the KINGFISH open-time key program, KPGT_cwilso01_1, from Herschel
guaranteed time key project,Physical Processes in the Interstellar Medium of
Very Nearby Galaxies111http://hedam.lam.fr/VNGS/index.php (ID: 1342189430,
Griffin et al., 2010). The beam sizes are 465.39, 822.58, and 1768.66 arcsec2
at 250, 350, and 500 ${\mu}$m, respectively (SPIRE Data Reduction Guide, Table
6.7) and the exposure times were 1678 seconds for each channel.

###  2.3 Data Reduction

For the PACS data, we used the Herschel Interactive Processing Environment
(HIPE, Ott, 2010) version 10.0 for data reduction. This reduction procedure
contains the extraction of the calibration tree for the data processing,
electronic crosstalk correction, flat-fielding, and unit conversion from volts
to Janskys per pixel. The final map was created with the algorithm implemented
in Scanamorphos (v24.0, Roussel, 2013). The error and weight maps were also
produced with Scanamorphos and the error map is defined as the error on the
mean brightness in each pixel.

The SPIRE data were processed from level 0 up to level 1 with HIPE scripts in
the Scanamorphos distribution. The same steps as PACS processing were
conducted. The thermal drifts are subtracted by using the smoothed series of
thermistors located on the detector array as the input of the drift model (see
Ott (2010) for details). A more detailed description of the data reduction can
be found in Paper I.

Both sets of PACS observations contain a low-amplitude artifact parallel to
the major axis on the eastern side of the disk midplane. It is 70″ away to the
south-east and the flux in the ghosts is less than 0.5% of the corresponding
object flux. This region immediately east of the disk midplane was not used in
our analysis of the vertical structure of the disk. The amplitude of the
feature is small enough, that it has a negligible effect on our integrated
flux measurements.

Figure 1: Spitzer MIPS 24 ${\mu}$m, Herschel PACS 70 and 160 ${\mu}$m, and
Herschel SPIRE 250, 350, and 500 ${\mu}$m images of NGC 891 The color bars
show surface brightness on a logarithmic scale; the maximum central surface
brightness is white, and the images are grayed out below a S/N ratio of 1.5.
The black arrow indicates the dust spur to the south-east. The red circles
indicate the size of the point spread function. The color bar ranges (in
Janskys per pixel on a logarithmic scale) are as follows: 0.0032 to 63 (24
micron), $10^{-5}$ to 0.079 (70 micron), $10^{-5}$ to 0.40 (160 micron),
$10^{-4}$ to 0.50 (250 micron), $10^{-4}$ to 0.25 (350 micron), and $10^{-4}$
to 0.16 (500 micron).

###  2.4 Integrated Photometry of NGC 891 & Global Properties

Figure 1 compares the new (24, 70, 160 ${\mu}$m) images to the SPIRE images at
250, 350, and 500 ${\mu}$m. We define the center of NGC 891 by the location of
the maximum 4.5 ${\mu}$m IRAC intensity, a proxy for the center of the stellar
bulge. In Figure 1 and subsequent images, the coordinates (0,0) reference this
position. Table 1 provides the corresponding sky coordinates.

We measured fluxes from the 24, 70, 160, 250, 350, and 500 ${\mu}$m images. In
order to match the point spread function (PSF), we convolved the images with
the kernels provided by Aniano et al. (2011). The image pixel scale and
coordinates were then matched using the IDL code hrebin.pro and hastrom.pro in
the Goddard package. These processes conserve total image flux.

The aperture adopted for the integrated photometry is based on a growth curve
measured from photometry of the SPIRE 500 ${\mu}$m image in increasingly
larger apertures. It is an elliptical aperture of semi-major axis of
7$\aas@@fstack{\prime}$35 and semi-minor axis of 3$\aas@@fstack{\prime}$68
that includes most of the dust emission seen in 24 ${\mu}$m image after
convolution to the PSF of the 500 ${\mu}$m image. Figure 2 shows this aperture
on the 24 ${\mu}$m image.

Within an annulus at larger radii, we masked point sources and background
galaxies based on the morphology in the 24 ${\mu}$m data. The sky background
was taken as the mean data value following 2$\sigma$ clipping. The variance in
this region was used to compute the standard deviation of the mean. This
uncertainty on the mean background dominates the total background error for
large apertures. We also include a term for the pixel-to-pixel statistical
variance in the background.

The error budget is dominated by the calibration uncertainties, which exceed
the uncertainties from both background subtraction and photon noise. Following
Paper I, we conservatively estimate the background error at 10% in all bands.

We computed color corrections for the PACS and SPIRE fluxes assuming a
blackbody spectrum modified by the dust emissivity, $\kappa_{\nu}=\kappa_{\rm
0}(\nu/\nu_{\rm 0})^{\beta}$ with $\beta=2$, and a blackbody temperature of
$T=30$ K.222 As the PACS and SPIRE pipelines compute fluxes assuming a
monochromatic spectrum, we applied a color correction. See section 4.3 in
“PACS Photometer Passbands and Color Correction Factors for Various Source
SEDs”, PICC-ME-TN-038, version 1.0, and Table 5.7 in SPIRE Handbook, HERSCHEL-
DOC-0798, version 2.5. We divided the original flux measurements by the
following correction factors: 0.977, 1.037, 0.9796, 0.9697, and 0.9796 for,
respectively, 70 ${\mu}$m, 160 ${\mu}$m, 250 ${\mu}$m, 350 ${\mu}$m, and 500
${\mu}$m. The estimated aperture corrections were small compared to the
photometric errors and were therefore not applied.

The integrated fluxes listed in Table 1 for the 24, 70, 160, 250, 350, and 500
${\mu}$m bands. Our measurements agree with both Bendo et al. (2012) and
Hughes et al. (2014) to within the $1\sigma$ errors.

Table 1: Properties of NGC 891

Property | Value
---|---
RA (J2000)a | 2:22:33.0
DEC (J2000)a | 42:20:53
d (Mpc)b | 9.77
$V_{\rm circ}$ ( km s-1)c | 225
SFR ($~{}{\rm M_{\odot}}$ yr-1) | 5.0
$F_{\nu}^{24\mu m}({\rm Jy})$ | $6.2\pm 0.6$
$F_{\nu}^{70\mu m}({\rm Jy})$ | $100\pm 10$
$F_{\nu}^{160\mu m}({\rm Jy})$ | $334\pm 33$
$F_{\nu}^{250\mu m}({\rm Jy})$ | $167\pm 17$
$F_{\nu}^{350\mu m}({\rm Jy})$ | $72\pm 7$
$F_{\nu}^{500\mu m}({\rm Jy})$ | $26\pm 3$

aCoordinates of the maximum intensity in the 4.5 ${\mu}$m IRAC image.
bDistance from Ferrarese et al. (2000). cRotation speed from Swaters et al.
(1997).

Figure 2: Spitzer/MIPS 24 ${\mu}$m image for NGC 891 adaptively smoothed after
subtraction of background sources. The color scale for the surface brightness
is logarithmic. The scale height and scale length of the stars are indicated
by the rectangle, $\pm 0\aas@@fstack{\prime}5$ (1.4 kpc) by $\pm
7\aas@@fstack{\prime}35$ (20.9 kpc) (Ibata et al., 2009). The extended
emission forms a halo which we define by an ellipse (semi-major axis of
$7\aas@@fstack{\prime}35$ and semi-minor axis of 3$\aas@@fstack{\prime}$68, or
10.5 kpc). An arrow indicates the dust spur. Figure 3: Adaptively smoothed
maps. We applied ADAPTSMOOTH (Zibetti, 2009) with a minimum S/N = 5 pixel-1.
Left: The Herschel 250 ${\mu}$m map overlaid with the H I emission contours
from Oosterloo et al. (2007). The contour levels for H I emission are 0.01,
0.03, 0.08, 0.16, 0.47, 0.78, 1.56, 3.11, 6.23, 9.34$\times\rm
10^{21}~{}cm^{-2}$. Middle: The Herschel 250 ${\mu}$m map overlaid with the
radio continuum contours from Oosterloo et al. (2007). The contour levels for
the radio continuum are 0.1, 0.4, 2, and 10 mJy beam-1. Right: The Spitzer 24
${\mu}$m map with the H I contours. The discrete sources in the background
were masked prior to the smoothing.  Figure 4: The PACS 70 ${\mu}$m/160
${\mu}$m color map. The arrow points at the western superbubble. Left: The
X-ray contours for the adaptively smoothed Chandra image (0.3-2.0 keV)
presented by Hodges-Kluck & Bregman (2012); the contour levels are 9, 10, 15,
20, 30, 40, and 50 counts s-1 deg-2. Top-right: The same color maps overlaid
with radio continuum contours. The spatial resolution is 17″ $\times$ 12″ and
the rms noise is 23 $\mu$Jy beam-1 (Oosterloo et al., 2007). The contour
levels for the radio continuum are 0.1, 0.4, 2, and 10 mJy beam-1. Bottom-
right: The same color maps overlaid with H$\alpha$ surface brightness contours
from an H$\alpha$ image taken at Steward Observatory by C. Engelbracht and
calibrated to the total luminosity reported in Hoopes et al. (1999). Contours
are shown at $\Sigma(H\alpha)=9.9\times 10^{-15},1.0\times 10^{-14},1.23\times
10^{-14},2.61\times 10^{-15},6.09\times 10^{-15},1.02\times 10^{-13}$ ergs s-1
cm-2 arcsec-2.

##  3 Morphological Features of the Warm Dust in NGC 891

The 24 ${\mu}$m image in Figure 2 resolves several structures contributing to
the thermal emission. The brightest of these are thin and thick dust
components aligned with the center of the galaxy, components we will refer to
as the dusty disk and dusty halo, respectively. Modeling these components in
the longer wavelength Herschel images is sensitive to how the PSF is modeled
(Bocchio et al., 2016).

We adopt the scale heights that Bocchio et al. (2016) fitted to the vertical
surface brightness profiles. Figure 2 of their paper shows that the shape of
the vertical surface brightness profile requires a two-component fit. The
thin, unresolved dust component significantly reduces the scale height of the
fitted thick dust component. A consequence of this interplay is that the thick
dust component has the largest 24 ${\mu}$m scale height above the region of
the stellar disk where the thin dust component is faintest. Based on the
vertical profile perpendicular to a region of the disk with low star formation
activity, Bocchio et al. (2016) conclude that the scale height of the dusty
halo is $h_{z}=1.36\pm 0.01$ kpc. The scale height of the dusty halo is
therefore comparable to that of the thick stellar disk, whose size we have
indicated in Figure 2.

Our analysis describes two non-axisymmetric components of the far-infrared
morphology: (1) the large filament marked by the arrow in Figure 2, and (2) a
dusty superbubble erupting from the center of the galaxy. The superbubble was
detected previously at X-ray energies (Hodges-Kluck & Bregman, 2012; Hodges-
Kluck et al., 2018), and we present the first description of its dust content.
The filament, or dust spur, is a new discovery enabled by the spatial
resolution and sensitivty of the new observations.

###  3.1 The Dust Spur

The arrow in Fig. 2 indicates a filament of 24 ${\mu}$m emission extending
6$\aas@@fstack{\prime\prime}$90 (19.6 kpc) southeast of the dust halo. We
discovered this filament in the Spitzer MIPS data and confirmed it using
Herschel. Figure 1 shows the SPIRE detections. The structure is outside the
field-of-view of our new PACS images, so we examined the PACS images obtained
previously by The Herschel EDGE-on galaxy Survey (HEDGES, Murphy, 2011) and
found a clear detection at at 160 ${\mu}$m, a few clumps of emission at 100
${\mu}$m, and weaker emission at 70 ${\mu}$m. The far-infrared filament is
therefore not an instrumental artifact.

We looked for a background cluster of galaxies at this location but found none
in either X-ray images or optical images.333The $z=0.0184$ cluster Abell 347
lies to the southeast at 02:25:50.9,+41:52:30 (NED). Our photometry indicates
a broad spectral energy distribution, which we show is consistent with thermal
dust emission in Section 4. The far-infrared filament may therefore be a
component of the dusty halo of NGC 891, and we will refer to this
morphological feature as the dust spur. Spectroscopy of an emission line would
confirm its association with NGC 891; alternatively, higher-resolution mapping
would be useful to definitively rule out an unresolved galaxy cluster.

The morphology of the dust spur shows no connection to the center of the
galaxy, making an association with a galactic outflow unlikely. When projected
on the sky, the long-axis of the dust spur is roughly perpendicular to the
disk. The filament emerges at a galactocentric radius $R$ =
4$\aas@@fstack{\prime}$0 (11 kpc) and extends 6$\aas@@fstack{\prime}$9 (19.6
kpc) to the southeast, and has a width of $\sim$ 2$\aas@@fstack{\prime}$0 (6
kpc).

The dust spur is not coincident with the large filament of neutral hydrogen
described by Oosterloo et al. (2007) (see also Pingel et al., 2018). In Figure
3, we show their H I 21-cm emission contours on the Herschel and Spitzer
images. The lowest contour at $N(HI)=1\times 10^{19}\rm~{}cm^{-2}$ extends 22
kpc northwest of NGC 891. The dust spur extends to the southeast, in contrast,
and is not visible in the H I contours. We will show that the H I filament and
the dust spur have different compositions in Section 4.

The middle panel of Fig. 3 compares the dust emission to the radio continuum.
The overall shape of the radio continuum contours is similar to the
distribution of far-infrared emission, whereas the H I contours show a lower
ratio of vertical height to galactocentric radius.

A close inspection of the dust spur suggests a spatial coincidence between the
brightest emission regions at 250 ${\mu}$m and the local maxima in the the
faintest radio continuum contours. This correlation should be interpreted
cautiously. Unresolved sources in the SPIRE map could be background sources
unrelated to NGC 891, and the radio detections have low signal-to-noise. If
the radio sources are at the redshift of the dust spur, then the radio flux
levels of the clumps, 0.2 mJy beam-1, combined with the fluxes listed in Table
3, are consistent with far-IR – radio relation defined by star-forming regions
(Yun et al., 2001; Bell, 2003). These data raise the possibility that the dust
spur is a region of active star formation.

###  3.2 The Superbubble

We smoothed the 70 ${\mu}$m image to match the resolution of the 160 ${\mu}$m
image, and then we divided the smoothed 70 ${\mu}$m image by the 160 ${\mu}$m
image to produce a color map from the PACS data. This color map resolves a
broad plume of thermal dust emission, indicated by the arrow in Figure 4. This
structure emerges from a region of the disk within roughly
1$\aas@@fstack{\prime}$8 (5.1 kpc) of the nucleus, slightly inside the
3$\aas@@fstack{\prime}$1 radius of the molecular ring. It extends northwest,
roughly perpendicular to the disk, reaching a height of
2$\aas@@fstack{\prime}$7 (7.7 kpc). We refer to this structure as the
superbubble but will argue in Section 5 that a galactic wind is actually
developing here. We will show that this region has a slightly higher dust
temperature than the surrounding halo in Section 4.

Contours of the soft X-ray emission (Hodges-Kluck & Bregman, 2012) outline the
perimeter of the dusty superbubble. In deeper X-ray observations, this hot
($kT=0.71\pm 0.01$ keV) gas is concentrated near the most active region of
star formation and coexists with gas near the virial temperature, $kT=0.20\pm
0.01$ keV (Hodges-Kluck et al., 2018). Harder diffuse X-ray emission is
detected within 5 kpc of the galactic center; its physical origin remains
unclear.

The soft X-ray emission from galactic winds is produced mostly at the
interface between the hot wind and a cooler component of the multi-phase
outflow (Strickland et al., 2004a, b). The smaller panels in Figure 4 overlay
the radio continuum contours and the H$\alpha$ contours on the PACS color map.
The H$\alpha$ image shows one prominent filament extending well into the dusty
bubble, but the H$\alpha$ image is not sensitive enough to determine the
amount of correlation with the superbubble morphology at other wavelengths. We
have shown in Figure 3 that the radio contours closely follow the morphology
of the dusty halo. As would therefore be expected, the radio contours do not
describe the PACS color map well.

The fate of the superbubble will be determined in large part by the CGM. The
inner CGM of NGC 891 contains much more cold gas than hot, virialized gas
(Hodges-Kluck et al., 2018; Das et al., 2020)

###  3.3 The Dusty Halo

On the eastern side of the disk, the 24 ${\mu}$m emission extends
2$\aas@@fstack{\prime}$03 (5.76 kpc) over most of the disk, reaching a larger
distance along the dust spur (Fig. 3). On the western side of the NGC 891
disk, the dusty halo is detected out to 3$\aas@@fstack{\prime}$68 (10.5 kpc)
in the 24 ${\mu}$m map.

The extent of the PAH emission is similar to that of the cold dust. McCormick
et al. (2013) estimate a symmetric extent of $\pm 5\aas@@fstack{\prime}0$ with
a bulk height, $H_{e,PAH}=7.1$ kpc.

##  4 Dust Temperature and Mass

We can derive the dust mass, $M_{\rm d}$, from the opacity per unit dust mass
and the optical depth at far-infrared wavelengths. We fit a simple, single-
temperature modified blackbody (MBB) function to the photometry in the
wavelength range $100\leq\ \lambda(\mu{\rm m})\leq\ 500$. The dust spectrum
consists of a blackbody spectrum at the dust temperature, $B_{\nu}(T_{d})$,
modified by the dust opacity, $\kappa_{\nu}$, such that

$\displaystyle F_{\nu}={{M_{\rm d}\kappa_{\nu}B_{\nu}(T_{\rm
d})}\over{D^{2}}},$ (1)

where $D$ is the distance to the galaxy. This simple approach yields accurate
dust masses and temperatures provided the normalization and the spectral index
of the opacity are consistent with full dust models, which include the
distribution of dust grain properties and a range of interstellar radiation
fields (Bianchi, 2013). We adopt a dust opacity model,
$\kappa_{\nu}=\kappa_{\rm 0}(\nu/\nu_{\rm 0})^{\beta}$. The normalization,
$\kappa_{\rm 0}=1.92~{}\rm cm^{2}~{}g^{-1}$ at 350 ${\mu}$m, and spectral
index, $\beta=2.0$, are based on the Milky Way model presented in Draine et
al. (2007).

In the next section, we validate our approach using the spectral energy
distribution for the entire galaxy. In Section 4.2, we then describe the dust
properties of the morphological components identified in the previous section
and defined in Figure 5. We begin with the dust mass and temperature of each
region as a whole, and then we explore these properties on a pixel-by-pixel
basis. We estimate dust-to-gas ratios for the various components in Section
4.4.

###  4.1 Validation of the Spectral Energy Distribution Fitting

The top panel of Figure 6 shows the SED for the entire galaxy. We fit only the
70 $-$ 500 ${\mu}$m data because the 24 ${\mu}$m flux comes from a warmer dust
component. The single-temperature, MBB model with $\beta\equiv 2$ gives a dust
mass of $M_{d}=1.1\times 10^{8}$${\rm~{}M_{\odot}}$ and dust temperature
$T=21.7\pm 0.2$ K. We also plot the fit with $\beta$ taken as a free
parameter. A comparison shows that our fitted dust parameters are robust to
minor changes in the dust emissivity; see Table 2 for a quantitative
comparison.

The dominant error term is the 10% uncertainty in the flux calibration. To
determine the uncertainty on the fitted mass and temperture, we scale the
photometry by 10% and refit the models. We find that systematically increasing
(or decreasing) all the fluxes can increase (decrease) the fitted dust masses
by $\sim$10-20%, but dust temperature varies by less than a degree. Our
results confirm the dust mass and temperature obtained previously by Hughes et
al. (2014); Bocchio et al. (2016).

The total dust mass fit to the FIR observations is nearly twice that inferred
from radiative transfer modeling of the distribution of optical and near-IR
emission. For example, when scaled to our adopted distance, Xilouris et al.
(1998) found $5.3\times 10^{7}$${\rm~{}M_{\odot}}$ of dust. A significant
fraction of the dust presumably goes undetected because little optical/near-IR
radiation emerges from some regions of the galaxy. Previous work has suggested
that this hidden dust could reside in a geometrically thinner dust disk
(Popescu et al., 2000, 2011) or in clumpy clouds associated with molecular gas
(Bianchi, 2008). Seon et al. (2014) considered NGC 891 specifically and
introduced a thin dust disk of scale height $\approx 5\arcsec$ to
simultaneously describe the distribution of GALEX UV emission (Morrissey et
al., 2007) and Herschel SPIRE photometry (Bianchi & Xilouris, 2011).

For completeness, we point out that the impact of smaller dust grains (grain
radii $<50$ Å) on our fits is constrained by the 24 ${\mu}$m flux. We fit a
two temperature model to the full SED including the 24 ${\mu}$m flux. The
temperature of a warmer component is not well constrained by this single data
point, so we assumed a dust temperature of $T=220$ K to facilitate comparison
to NGC 4631. In Paper I, we found an additional dust component at 220 K in NGC
4631. In their single-temperature, MBB fits, they treated the 70 ${\mu}$m flux
as an upper limit because the contribution from the warmer component was non-
negligible. We find, however, that the contribution of this warmer component
to the 70 ${\mu}$m flux is only 0.3% for this two temperature model fitted to
NGC 891. Assuming that the peak wavelength of the MBB curve for the warmer
component is shorter than 24 ${\mu}$m, we vary the temperature of the warmer
component, and the contribution to 70 ${\mu}$m flux is 1.3% at most.
Therefore, we consider 70 ${\mu}$m data point in the same way as other
wavelength data in the fitting process. For an assumed warm dust temperature
of 220 K, we fit a two-component model and find dust mass in the 220 K
component is negligible ($\sim 0.00003$%) compared with dust mass in the $\sim
22$ K component. The mass in the cool component in this two-component fitting
does not deviate from the mass in the one-component, cool component only,
fitting as shown in Table 2.

Figure 5: Apertures used for far-infrared photometry shown relative to H I
21-cm contours from Oosterloo et al. (2007). The semi-major and semi-minor
axises are 7$\aas@@fstack{\prime}$4 (21 kpc) and 3$\aas@@fstack{\prime}$7
(10.5 kpc), respectively. We define the thickness of the disk by the thick
stellar disk with the width of $\pm 0\aas@@fstack{\prime}5$ (1.4 kpc). The
inner disk has a width of 4$\aas@@fstack{\prime}$1 (11.7 kpc) which includes
the three star-forming clumps. The other rectangle marks the aperture we use
for the superbubble photometry. The height and width of the superbubble are
2$\aas@@fstack{\prime}$7 (7.7 kpc) and 2$\aas@@fstack{\prime}$8 (8.0 kpc),
respectively. The ellipse encompasses the extended halo emission in the lowest
resolution band. We define the aperture for the dust spur southeast of the
galaxy by the 250 ${\mu}$m contour at 1.7 mJy/pix, which is 3$\sigma$ of the
background noise, and outside the extended dust halo. We define an aperture
northwest of the galaxy in order to place upper limits on the far-infrared
emission from the H I spur. Each dust clump and hot spot (star forming clump)
listed in Table 3 is labeled with a number.

###  4.2 SED Fitting of Dust Components

Table 2 summarizes the dust properties for the regions labeled in Figure 5.
The photometry shown in Figure 6 was obtained after degrading the resolution
of the 70, 160, 250, and 350 ${\mu}$m maps to match that of the 500 ${\mu}$m
map; see details in Section 2.

Our analysis constrains the contribution from dust well beyond the galactic
plane where the starlight is too faint to model its attenuation. Inspection of
Table 2 indicates 87% of the dust mass is associated with the galactic disk
(log ($M_{\rm dust}/M_{\odot}$)$=7.970\pm 0.006$) with 65% of the disk dust
concentrated in the inner disk (log ($M_{\rm dust}/M_{\odot}$)$=7.781\pm
0.012$). We associate roughly 13% of the total dust mass with the halo
component at $|z|>0\aas@@fstack{\prime}5$, or $\pm\ 1.42$ kpc; the same halo
aperture includes 20% of the total H I mass. This difference can be attributed
to the lower dust-to-gas ratio of the halo. Roughly 25% of the halo dust mass
(log ($M_{\rm dust}/M_{\odot}$)$=7.163\pm 0.053$) comes from the superbubble
region, while the dust spur, log ($M_{\rm dust}/M_{\odot}$)$=5.88\pm 0.02$,
makes a smaller contribution.

The halo dust temperature ($T=19.24\pm 0.62$ K) is significantly cooler than
the disk dust ($T=21.89\pm 0.06$ K). The superbubble dust ($T=20.61\pm 0.03$
K) is warmer than the surrounding halo but cooler than the disk. The emission
from the dust spur is weak at 70 ${\mu}$m compared to the SPIRE bands, and the
fitted dust temperature is considerably lower than that of the disk or halo.

Figure 6: The best fit MBB model for integrated flux of each part of NGC 891
in 24 ${\mu}$m, 70 ${\mu}$m, 160 ${\mu}$m, 250 ${\mu}$m, 350 ${\mu}$m, and 500
${\mu}$m bands. The regions are defined in Figure 5. The flux for 24 ${\mu}$m
is not used for the fitting as it traces different dust components. Gray-
dashed line is the MBB model fits when $\beta$ is fixed at 2. Table 2: Dust
properties derived from the MBB model fitting.

Componentd | Total | Inner Disk | Disk | Halo | Superbubble | Dust Spur |  H I Spur
---|---|---|---|---|---|---|---
log ${\rm M}_{\rm dust}$ | 8.025$\pm$0.005 | 7.781$\pm$0.012 | 7.970$\pm$0.006 | 7.163$\pm$0.053 | 6.468$\pm$0.003 | 5.883$\pm$0.019 |  $<3.443$ a
$\prime\prime$ | 8.018$\pm$0.007 | 7.800$\pm$0.018 | 7.970$\pm$0.013 | 7.061$\pm$0.074 | 6.474$\pm$0.004 | 5.868$\pm$0.061 | …
$\beta$ | 2 | 2 | 2 | 2 | 2 | 2 | 2
$\prime\prime$ | 1.97$\pm$0.03 | 2.09$\pm$0.07 | 2.00$\pm$0.05 | 1.66$\pm$0.22 | 2.03$\pm$0.02 | 1.96$\pm$0.16 | …
T (K) | 21.72$\pm$0.05 | 22.49$\pm$0.12 | 21.89$\pm$0.06 | 19.25$\pm$0.62 | 20.61$\pm$0.03 | 16.57$\pm$0.17 | 19.25a
$\prime\prime$ | 21.89$\pm$0.15 | 22.02$\pm$0.37 | 21.89$\pm$0.26 | 21.30$\pm$1.45 | 20.50$\pm$0.08 | 16.79$\pm$0.81 | …
$\chi_{\nu}^{2}$ | 0.02 | 0.11 | 0.03 | 1.05 | 0.01 | 0.08 | …
$\prime\prime$ | 0.02 | 0.09 | 0.05 | 0.75 | 0.01 | 0.11 | …
log $({\rm M}_{\rm HI}/{\rm M}_{\odot}$) b | 9.64 | 9.09 | 9.48 | 9.10 | 8.07 | $<7.22$ | 7.20
${\rm M}_{\rm dust}$/${\rm M}_{\rm gas}$ c | 0.0088 | 0.0082 | 0.0095 | 0.0084 | 0.025 | $>0.0046$ | $<0.00017$
$\prime\prime$ | 0.0087 | 0.0086 | 0.0095 | 0.0067 | 0.025 | $>0.0044$ | …

a The dust mass limit is computed by rescaling the MBB model to satisfy the
FIR flux limits of the H I spur assuming $\beta=2$ and the halo dust
temperature. b The H I masses were measured from H I map of Oosterloo et al.
(2007). c The gas mass was computed as $M_{\rm
gas}=1.36(M(\text{H\,{I}})+M({\rm H_{2}}))$, where the coefficient accounts
for the gas mass in helium. d Alternate rows compare fixed and free spectral
indices for the dust opacity.

###  4.3 Pixel by Pixel Analysis

Figure 7: Pixel-to-pixel dust mass, dust temperature, and $\chi_{\nu}^{2}$ of
the model fit with $\beta\equiv 2$ are shown. Each pixel is
$9\aas@@fstack{\prime\prime}0\times 9\aas@@fstack{\prime\prime}0$. The disk
boundary defined in Figure 5 is indicated by the white-dotted line in each
panel.

We also examined the variation in dust properties at the resolution limit of
the 500 ${\mu}$m image. We fit the SED of all pixels that had $S/N>3\sigma$ in
all five bands, essentially the entire disk component and the disk – halo
interface. Hughes et al. (2014) previously found the variation in dust
temperature to be more uneven than that of the dust mass. Figure 7 shows our
resulting maps of the temperature and dust surface density, i.e., the column
density of dust in mass units.

The dust surface density decreases with distance from the nucleus along the
major axis. In the vertical direction, the dust surface density drops very
quickly to values less than 0.50${\rm~{}M_{\odot}}$ pc-2.

The temperature range in Fig. 8 is similar to the $T_{d}=17-24$ K range found
previously by Hughes et al. (2014). The hotspots identified in both the 24
${\mu}$m map and the 70 ${\mu}$m/160 ${\mu}$m color map stand out as regions
with $T_{d}\geq\ 22.5$ K in Figure 7. This region of the disk is surrounded by
a molecular ring (Israel et al., 1999; Scoville et al., 1993). The dust cools
off with increasing radius in the disk beyond the molecular ring.

We compare the dust temperature and surface density directly to the 3.6
${\mu}$m and 24 ${\mu}$m emission in Figure 8. The 24 ${\mu}$m emission is a
good proxy for the star formation rate surface density (Calzetti et al.,
2007), while the 3.6 ${\mu}$m emission traces the stellar mass density. We
have color coded the points in Figure 8 by their location in NGC 891.

The dust in the midplane of the inner disk has temperature, $T_{D}\approx
22-23$ K and a high mass column, as indicated by its location in the upper
right of Figure 8. With increasing vertical height at fixed radius, the
temperaure decreases by roughly one degree as the surface brightness drops by
an order of magnitude. This locus (blue points) defines a narrow band that
continues smoothly into the emission from the superbubble region where the
temperature drops a few more degrees (orange and yellow points). Hughes et al.
(2014) produced color maps that identify this same spectral transition with
height above the disk. Adding the halo emission immediately above the inner
disk extends this correlation to dust temperatures $T_{D}\lower
2.0pt\hbox{$\buildrel{\scriptstyle<}\over{\scriptstyle\sim}$}20$ K. Based on
this correlation between dust temperature and 24 ${\mu}$m emission, the dust
in and above the inner disk is heated primarily by the starlight from the
star-forming regions.

The width of the SB(24 ${\mu}$m) – $T_{D}$ locus in the inner disk correlates
with radial distance from the nucleus. For example, following the midplane
points in Figure 8 from the inner disk to the outer disk (green points)
follows the upper edge of the triangular locus. At fixed 24 ${\mu}$m surface
brightness, dust in the midplane of the outer disk is cooler than dust above
the plane at small radii. Equivalently, at fixed dust temperature, the
superbubble and halo emission are not as bright at 24 ${\mu}$m as the outer
disk. We expect a transition in the outer disk toward heating by an older
stellar population, and it seems plausible this transition manifests as the
broad triangle of green points in Figure 8. Consistent with this
interpretation, the 3.6 ${\mu}$m emission from the midplane points in the
outer disk (green points with cyan dot) are strongly correlated with the dust
temperature. Dust above the midplane of the outer disk fills the interior of
the triangular locus, as might be expected for dust heated by a mixture of the
two stellar populations.

We acknowledge that the temperature correlations look nearly identical against
the near-infrared (3.6 ${\mu}$m) and mid-infrared (24 ${\mu}$m) emission, so
the temperatures have the same correlations with the tracers of stellar mass
and star formation, respectively. It seems likely that this happens because
the mass and SFR both decrease with separation from the center of the galaxy.

The panels on the right side of Figure 8 show power law correlations between
infrared emission and dust surface density. For the inner disk, the
correlation is tighter in the mid-infrared (24 ${\mu}$m), more closely related
to SFR surface density. In the outer disk, the 3.6 ${\mu}$m emission shows the
stronger correlation, consistent with a closer relation to stellar mass
surface density.

Figure 8:  Pixel-to-pixel correlations between dust properties and shorter
wavelength emission not used in the SED fitting. The regions shown correspond
to the map in Figure 7. In this plot, midplane is defined as the region within
$|z|\leq 0\aas@@fstack{\prime}7$ (1.99 kpc) of the disk plane.

###  4.4 Dust-to-gas Ratio

Table 2 compares the dust mass to the gas mass, where all measurements taken
from the literature have been scaled to an NGC 891 distance of 9.77 Mpc. The
total H I mass of NGC 891 is then $4.4\times
10^{9}$${\rm~{}M_{\odot}}$(Oosterloo et al., 2007). The total CO(1-0)
intensity (Scoville et al., 1993; Garcia-Burillo et al., 1992) correspondes to
a molecular gas mass of $M({\rm H_{2}})\approx 4.2\times
10^{9}\mbox{${\rm~{}M_{\odot}}$}$ for the Bolatto et al. (2013) calibration of
the CO-to-$\rm H_{2}$ conversion factor, $X_{\rm CO}=2\times 10^{20}{\rm
cm}^{-2}{\rm[K~{}km~{}s^{-1}]}^{-1}$. The ratio of molecular-to-atomic gas is
close to unity, $M({\rm H_{2}})/M(\text{H\,{I}})\approx 0.95$, which is higher
than the median of 0.6 found for Sab galaxies (Obreschkow & Rawlings, 2009).
Including the mass contribution from helium, the total mass of cold gas is
$1.2\times 10^{10}$${\rm~{}M_{\odot}}$.

Comparing our measured dust mass to the atomic and molecular gas mass, we find
a global gas-to-dust ratio $M_{\rm gas}/M_{\rm dust}\approx 110$, where the
uncertainty introduced by the $X_{\rm CO}$-factor is 30% (Bolatto et al.,
2013). The gas-to-dust ratios for the SINGS galaxies range from 100 to 340
after scaling the values in Table 4 of Draine et al. (2007) to the same
$X_{\rm CO}$ and including the helium mass. On this scale, the gas-to-dust
ratio of the Milky Way is 200 based on the $M_{\rm dust}/M_{\rm H}\approx
0.007$ ratio given by Draine et al. (2007). Based on its atomic and molecular
gas content, NGC 891 is dustier than the median Sab galaxy.

The molecular gas is concentrated in the inner disk of NGC 891, so it is not
surprising that the inner disk is the dustiest region. The H I disk is larger
than the molecular disk. Oosterloo et al. (2007) attribute slightly over 70%
of the total H I to the disk, and our measurements of their H I map indicate
the inner disk H I mass is $1.22\times 10^{9}$${\rm~{}M_{\odot}}$, slightly
less than half the mass of the entire H I disk. The total gas mass of the
inner (and full) disk is then $7.4\times 10^{9}$${\rm~{}M_{\odot}}$
($9.8\times 10^{9}$${\rm~{}M_{\odot}}$). These disk masses do not include the
small contribution from warm ionized gas (Rand et al., 1990; Dettmar, 1990),
but they do include the mass contribution from helium, as described in the
notes to Table 2.

The filament northwest of NGC 891 contains at least $1.6\times
10^{7}$${\rm~{}M_{\odot}}$ of H I(Oosterloo et al., 2007). No thermal dust
emission is detected from the H I filament. The upper limit on the dust mass
excludes gas-to-dust ratios similar to galactic disks. If the H I filament is
accreting gas as Oosterloo et al. (2007) suggest, then the lack of dust
emission favors low metallicity gas. Alternatively, if the H I filament is
fountain gas, the timescale to recycle the disk gas must be long enough to
destroy even large grains, which seems unlikely based on the large amount of
dust that persists in the CGM (Ménard & Fukugita, 2012).

The properties of the dust spur southeast of NGC 891 contrast sharply with
those of the H I filament. At the distance of NGC 891, the far-infrared
filament contains nearly $8\times 10^{5}$${\rm~{}M_{\odot}}$ of dust. Based on
the dust mass, this circumgalactic gas was once in a galaxy, either NGC 891 or
a satellite. Using the H I map (Oosterloo et al., 2007), the H I mass in the
dust spur is less than $2\times 10^{7}$${\rm~{}M_{\odot}}$. The exceptionally
low ratio of neutral hydrogen gas to dust is a puzzle. Perhaps a background
source will be found for the infrared emission. An alternative, however, is
that most of the gas mass is either molecular or ionized.

The mass ratio of H I to dust in the inner CGM of NGC 891 is indistinguishable
from the galactic disk. We find this surprising because the neutral fraction
of halo gas is around 1% (Popping et al., 2009; Bland-Hawthorn et al., 2017).
The dust was presumably made inside galaxies and transported into the halo via
outflows or stripping. Gas clouds are normally destroyed by these processes
unless special conditions are met, so we would not expect the galactic gas to
remain neutral. The inner CGM of NGC 891 contains more neutral gas than the
median COS (Cosmic Origins Spectrograph) Halos galaxy (Prochaska et al., 2017;
Das et al., 2020). The disk-like ratio of H I to dust certainly suggests that
the excess of neutral gas in the inner CGM of NGC 891 was removed from
galaxies. We will discuss the extrapolated dust mass of the CGM in Section
5.3.

Table 3: FIR and Radio Fluxes within NGC 891

Part | log $L_{\rm FIR}$ | log $L_{\rm 60\mu m}$ | log $L_{\rm Cont}$ | $q_{\rm IR}$ | $q_{\rm 60\mu m}$
---|---|---|---|---|---
| (${~{}\rm L_{\odot}}$) | (${~{}\rm L_{\odot}}$) | ($\rm W~{}Hz^{-1}$) |  |
(1) | (2) | (3) | (4) | (5) | (6)
Total | 10.44 | 9.64 | 22.00 | 2.45 | 2.19
Inner Disk | 10.30 | 9.55 | 21.75 | 2.56 | 2.32
Disk | 10.41 | 9.62 | 21.87 | 2.55 | 2.29
Halo | 9.28 | 8.32 | 21.39 | 1.90 | 1.56
Bubble | 8.75 | 7.82 | 20.81 | 1.96 | 1.64
Dust Spur | 7.60 | 6.04 | 19.13 | 2.48 | 1.93
Dust Spur1 | 7.23 | 6.08 | 18.68 | 2.56 | 2.13
Dust Spur2 | 6.91 | 5.27 | 17.68 | 3.24 | 2.71
Dust Spur3 | 7.12 | 5.50 | 18.78 | 2.35 | 1.76
Dust Spur4 | 6.56 | 5.18 | 18.30 | 2.27 | 1.76
Hotspot1 | 9.76 | 9.02 | 21.20 | 2.57 | 2.33
Hotspot2 | 9.80 | 9.08 | 21.24 | 2.57 | 2.34
Hotspot3 | 9.56 | 8.80 | 20.98 | 2.59 | 2.34

Columns: (1): Part of NGC 891, (2): IR luminosity at 8 ${\mu}$m - 1000
${\mu}$m from SED fit, (3): 60 ${\mu}$m flux computed from the SED fit, (4):
radio continuum flux from the continuum map by Oosterloo et al. (2007), (5)
$q_{\rm IR}={\rm log}\left({\rm IR}\over{3.75\times 10^{12}~{}{\rm
W~{}m^{-2}}}\right)-{\rm log}\left({S_{\rm 1.4~{}GHz}\over{\rm
W~{}m^{-2}~{}Hz^{-1}}}\right)$ where IR is the FIR flux and $S_{\rm
1.4~{}GHz}$ is flux density at 1.4 GHz. (6): For $q_{\rm 60\mu m}$, the flux
at 60 ${\mu}$m is used. This definition is adopted from Condon et al. (1991);
Yun et al. (2001); Bell (2003).

##  5 Discussion

###  5.1 The Dusty Superbubble

We found an extraplanar, dusty region that is significantly warmer than the
dusty halo. We called it the dusty superbubble because its location is
coincident with thermal X-ray emission ($kT=0.71$ keV) likely associated with
an outflow. In Section 5.1.1 below, we show that the thermal energy of this
superbubble is sufficient for the superbubble to punch through the disk and
blowout into the halo. Blowout in NGC 891 is of particular interest because
the SFR surface density is a factor of three below the commonly assumed
threshold of 0.1 $~{}{\rm M_{\odot}}$ yr-1kpc-2. The fate of this outflow
depends largely on the halo gas density profile, which is reasonably well
constrained in NGC 891. We present this evolution in Section 5.1.2 and discuss
its relationship to the recent detection of cosmic ray advection in the halo
of NGC 891.

####  5.1.1 A Galactic Wind at SFR Surface Density $<0.1$ $~{}{\rm M_{\odot}}$
yr-1kpc-2

The first numerical simulations of superbubble blowout predicted that a
superbubble would grow to heights of one to two vertical scale heights before
the extraplanar shell accelerated and broke up (Mac Low & McCray, 1988; Mac
Low et al., 1989). Given the scale height measurements for NGC 891 (Bocchio et
al., 2016), the height of the superbubble (2$\aas@@fstack{\prime}$7) is 8
times the scale height of the dust/gas thick disk (0$\aas@@fstack{\prime}$34
in 24 ${\mu}$m) and 68 times that of the dust/gas thin disk
(0$\aas@@fstack{\prime}$04 in 24 ${\mu}$m).

We will therefore discuss whether the NGC 891 superbubble will develop into a
wind. The star formation rate surface density is elevated in the disk of NGC
891, similar to that in a typical disk galaxy several Gyr ago. Whereas the
galaxy M82 has become the prototypical starburst outflow, we suggest that the
galaxy NGC 891 provides a more typical example of star-formation feedback in
well-developed galactic disks. To better understand how winds are launched,
sophisticated simulations that include multiphase gas, cosmic rays, and
turbulence should be tested against the observed properties of NGC 891 as well
as starburst galaxies.

The pioneering work of Mac Low & McCray (1988); Mac Low et al. (1989) predicts
that when a superbubble grows to scales comparable to the gas scaleheight, the
expansion velocity $V(r)$ of the shell determines its fate. Shells moving
faster than the sound speed of the ambient medium will accelerate as they
break through the disk; hydrodynamic instabilities then disrupt the shell, and
a free-flowing galactic wind develops (Chevalier & Clegg, 1985). Shells with
$V(H)\leq\ c_{s}$ will continue to decelerate and will eventually collapse due
to disruption caused by differential rotation and ISM turbulence. The critical
rate of energy injection for blowout to occur depends on the vertical gas
density distribution.

Eqn. 5 of Strickland et al. (2004b) gives the critical power for an
exponential gas distribution, $\rho(z)=\rho_{0}\exp(-z/H)$, in terms of the
gas scale height $H$. The scale height of the thin, molecular disk in NGC 891
is just 83 pc (Scoville et al., 1993). The H I disk has a thickness $H\approx
2.6$ kpc (Oosterloo et al., 2007), and warm ionized gas extends 4.5 kpc above
the disk plane (Dettmar, 1990). Scaling to the thick H I disk, the critical
mechanical power increases to

$\displaystyle L_{crit}=2.3\times
10^{39}{\rm~{}ergs~{}s}^{-1}\left({1{\rm~{}cm}^{-3}}\over{n_{0}}\right)^{1/2}\times$
(2)
$\displaystyle\left({P_{0}/k}\over{10^{4}{\rm~{}K~{}cm}^{-3}}\right)^{3/2}\left({H}\over{2.6{\rm~{}kpc}}\right)^{2}.$

The rate of mechanical power injection from supernovae and stellar winds can
be estimated from the SFR. We adopt the solar metallicity models from
Starburst 99 and the Salpeter initial mass function. For a star formation
history with a constant SFR, the steady-state feedback power is $L_{\rm
w}=10^{41.8}$ ergs s-1 for a SFR of 1$~{}{\rm M_{\odot}}$ yr-1(Leitherer et
al., 1999). This calibration includes only stars in the mass range $1\leq\
{\rm M_{*}}/\mbox{${\rm~{}M_{\odot}}$}\leq\ 100$. To be consistent with the
mass range of $0.1\leq\ {\rm M_{*}}/\mbox{${\rm~{}M_{\odot}}$}\leq\ 100$ used
to describe the SFR (Kennicutt, 1998), we divide by a factor of 2.55,
obtaining $L_{\rm w}=10^{41.4}$ ergs s-1 for a SFR of 1$~{}{\rm M_{\odot}}$
yr-1.

We have listed far-infrared luminosities for the entire galaxy and the inner
disk in Table 3. Applying $SFR(\mbox{$~{}{\rm
M_{\odot}}$~{}yr${}^{-1}$})=4.5\times 10^{-44}L_{\rm FIR}({\rm
ergs~{}s}^{-1}~{}{\rm Hz}^{-1})$ (Kennicutt, 1998), the rate of obscured star
formation for the entire galaxy is 4.8$~{}{\rm M_{\odot}}$ yr-1. This is
technically a lower limit on the SFR because not all of the UV light from NGC
891 is obscured by dust. However, we obtained the FUV and NUV magnitudes of
NGC 891 from the GALEX nearby galaxy catalog and found that the contribution
from unobscured SFR is a relatively small, roughly $0.2$$~{}{\rm M_{\odot}}$
yr-1.

The mechanical energy produced by massive stars in the inner disk is most
relevant to the evolution of the superbubble. We measure an inner disk SFR of
3.5$~{}{\rm M_{\odot}}$ yr-1 from the 24 ${\mu}$m emission. The available
mechanical power is then $L_{\rm w}=8.7\times 10^{41}$ ergs s-1, and $L_{\rm
w}/L_{\rm crit}\approx 380$. Even a few percent of the mechanical energy
produced in the inner disk is sufficient to power the expansion of a
superbubble that will blowout of the disk of NGC 891.

Theoretical studies have tried to predict the critical SFR surface density for
the formation of a galactic wind. Overcoming radiative losses has been a major
hurdle when star-forming regions are embedded in realistic gas clouds. Murray
et al. (2011) considered the role of radiation pressure from massive star
clusters in launching winds and suggested 0.1$~{}{\rm M_{\odot}}$ yr-1 per
kpc2 as a threshold. Scannapieco (2013) and Hayward & Hopkins (2017) both
connected blowout directly to the turbulence in the ISM.

Empirically, Heckman (2002) suggested 0.1$~{}{\rm M_{\odot}}$ yr-1 per kpc2,
based largely on nearby starburst galaxies. When they were younger, however,
essentially all disk galaxies drove massive winds (Martin et al., 2012; Rubin
et al., 2014). Kornei et al. (2012) measured SFR surface densities for
redshift $z\sim 1$ star-forming galaxies. Roughly 30% of the outflow galaxies
has SFR surface densities below the suggested threshold of 0.1$~{}{\rm
M_{\odot}}$ yr-1 per kpc2. We suggest that NGC 891 is an excellent nearby
analog, where a wind is developing at a SFR surface density (SFRSD) a factor
of a few lower than the threshold suggested by Heckman (2002). Normalizing the
24 ${\mu}$m luminosity of the inner disk by the area of a disk of diameter
4$\aas@@fstack{\prime}$1, we obtain a SFRSD within the molecular ring of
$\Sigma=0.031$$~{}{\rm M_{\odot}}$ yr-1 kpc-2 for NGC 891. In comparison, for
NGC 4631 (Paper I), the SFRSD is estimated to be $\Sigma=0.15$$~{}{\rm
M_{\odot}}$ yr-1 kpc-2 as the SFR is 2.9$~{}{\rm M_{\odot}}$ yr-1 within a
radius of 2.5 kpc where almost all of the star formation is taking place, or a
SFR surface density about 5 times higher than that for NGC 891.

Hayward & Hopkins (2017) describe the condition for blowout by a critical gas
fraction, which they find is 30%. We showed in Section 4.4 that the total gas
mass of the inner disk is $7.4\times 10^{9}$${\rm~{}M_{\odot}}$. We now place
bounds on the stellar mass. The IRAC 3.6 ${\mu}$m flux of the inner disk
requires $M_{*}\lower
2.0pt\hbox{$\buildrel{\scriptstyle>}\over{\scriptstyle\sim}$}4\times 10^{10}$
${\rm~{}M_{\odot}}$, a lower limit due to optical depth effects. The rotation
speed requires a dynamical mass $M_{\rm Dyn}\approx 6.9\times
10^{10}$${\rm~{}M_{\odot}}$ within a radius of $R=5.83$ kpc. The gas fraction
in the inner disk of NGC 891 is in the 10-16% range, a factor of two below the
Hayward & Hopkins (2017) threshold.

We found that the dusty superbubble is spatially coincident with hot gas
($kT=0.71\pm 0.04$ kev) that is concentrated above the star-forming regions in
NGC 891 (Hodges-Kluck et al., 2018). This soft X-ray emission very likely
comes from the interaction of a hotter wind with cold gas. The cold gas mass
is roughly $3\times 10^{8}$${\rm~{}M_{\odot}}$ based on the outflowing dust
mass, $3\times 10^{6}$${\rm~{}M_{\odot}}$, and the disk gas-to-dust ratio. Hot
winds have very low emission measure and have only been directly detected in a
couple of starburst galaxies (Strickland et al., 2004a, b). It is therefore of
significant interest that the central star-forming region in NGC 891 is
surrounded by diffuse hard X-ray emission (Hodges-Kluck et al., 2018),
possibly related to the hot wind. Regardless of the origin of this hard
component, however, NGC 891 shows all the signatures of a thermally driven
wind.

####  5.1.2 Interaction of Wind-Driven Bubble with CGM

As the thermal wind plows into the CGM, it does work on the halo gas and also
cools. The classic wind model widely applied to starburst galaxies Chevalier &
Clegg (1985) does not adequately capture this wind – halo interaction because
it neglects gravity, radiative losses, and the density profile of the CGM.
These processes ultimately determine whether the wind cools, whether a
galactic fountain forms, and whether outflowing gas escapes the gravitational
potential. Hydrodynamic simulations have not yet spatially resolved the
production and evolution of cool gas throughout a galactic disk and the
surrounding CGM, but the wind – halo interaction has been explored over a
broad parameter range, however, using semi-analytic models (Scannapieco et
al., 2002; Furlanetto & Loeb, 2003; Samui et al., 2008; Lochhaas et al.,
2018). We applied the Lochhaas et al. (2018) model to NGC 891 to gain further
insight about the outflow.

The Lochhaas et al. (2018) model extends the classic structure of interstellar
bubbles (Weaver et al., 1977) to circumgalactic scales. The hot wind drives an
expanding shock front that compresses the surrounding halo gas into a thin
shell. A contact discontinuity separates this shell from the wind. The wind
fluid is described by the shocked wind immediately behind the shell, an
interior region of unshocked cool wind, and the innermost region of unshocked
hot wind. The results suggest that whether adiabatic cooling of the hot wind
is accompanied by radiative losses depends on the star formation rate, halo
density profile, and wind density in a non-monotonic manner.

Figure 2 of Lochhaas et al. (2018) shows the evolution of the contact
discontinuity and indicates when the shocked wind begins to cool. The figure
cannot be directly applied to NGC 891, however, because the fiducial launch
radius, $r_{1}=1$ kpc, corresponds to a SFRSD $\geq\ 0.3$ $~{}{\rm M_{\odot}}$
yr-1 kpc-2, considerably higher than the 0.031 $~{}{\rm M_{\odot}}$ yr-1 kpc-2
in the central disk of NGC 891. Repeating the calculation with $r_{1}=7$ kpc
and SFR=4.5$~{}{\rm M_{\odot}}$ yr-1 shows that the NGC 891 outflow lies in a
sweet spot where the shocked wind cools radiatively; there is enough mass
going into the shocked wind, yet the expansion rate is not fast enough to
dropping the density rapidly (C. Lochhaas, private communication).

After 20 Myr, the contact discontinuity has reached a height of 14 kpc and
decelerated to $\approx 200$ km s-1. The mean advection speed is therefore
larger than the 150 km s-1fit to the spectral index profile of the halo radio
emission (Schmidt et al., 2019). Whether this discrepancy is significant is
unclear. The semi-analytic model does not specify the physical mechanism
launching the wind; it simply predicts how the superbubble evolves for
specified $\alpha$ and $\beta$. The parameter $\alpha$ describes the
efficiency at which supernova energy is transferred to the wind; the $\beta$
parameter describes the mass loading. Neither parameter is well constrained,
and both are assumed to be unity in the Lochhaas et al. (2018) calculations.
Values of $\alpha<1$ would lower the launch velocity, however, so it is not
obvious that small adjustments could produce a better match between the
velocities.

We have argued that the thermal pressure of the star-forming region is
sufficient for blowout, but cosmic ray electrons may also be important for
driving the NGC 891 outflow. The non-thermal radio spectral index steepens
nearly linearly with height just above the disk of NGC 891, a signature that
the cosmic ray electrons are transported by advection rather than diffusion
(Mulcahy et al., 2018; Schmidt et al., 2019). The non-thermal spectral index
flattens at heights above 2 kpc, consistent with adiabatic expansion
dominating synchrotron losses across the region we call the superbubble.
Schmidt et al. (2019) argue that the cosmic ray advection plausibly reaches
the halo escape velocity 9 to 17 kpc above the disk.

###  5.2 The Origin of the Dust spur and the H I Filament

####  5.2.1 The Dust Spur

The Herschel imaging with PACS and SPIRE confirms the presence of a dust spur
discovered in the Spitzer/MIPS 24 ${\mu}$m image. The shape of the far-
infrared/sub-mm SED (see Figure 6) is consistent with emission from a
structure associated with NGC 891.

The relative locations of the dust spur to the southeast and the H I spur to
the northwest is perplexing in part because these two streams have distinctly
different composition. The dust-to-gas ratio (see Table 2) is high in the dust
spur but very low in the H I spur. The dust spur is therefore likely composed
of higher metallicity gas than the H I spur.

The dust spur contains roughly $7.6\times 10^{5}$${\rm~{}M_{\odot}}$ of dust.
If the dust-to-gas ratio is similar to the disk of NGC 891, then the gas mass
is $\approx 9.5\times 10^{7}$${\rm~{}M_{\odot}}$. This mass exceeds the upper
limit on the mass of neutral hydrogen, $M_{\text{H\,{I}}}<1.7\times
10^{7}$${\rm~{}M_{\odot}}$, in this region of the halo. This apparent
contradiction may be explained by additional gas mass in another phase, a
higher dust-to-gas ratio in the spur (compared to the NGC 891 disk), or some
combination of these properties.

Within the dust spur, the local maxima in the far-infrared maps coincide with
several knots in the radio continuum map. From inspection of Figure 3, we
defined four clumps (see Fig. 5) for which we provide photometry in Table 3.
The radio and 60 ${\mu}$m fluxes of these clumps lie on or just above the
radio-IR relation defined by star-forming galaxies (de Jong et al., 1985;
Condon, 1992; Yun et al., 2001). We therefore suggest that the dust spur
contains young, possibly obscured, star clusters. Since radio and 60 ${\mu}$m
flux both underestimate SFR at low luminosities (Bell, 2003), however, it is
difficult to make an accurate estimate of their SFR.

Could tidal forces exerted on the disk of NGC 891 by a satellite galaxy
produce the dust spur? We estimated the required size of a companion using the
Dahari parameter (Dahari, 1984) and the projected separation between the tip
of the dust spur and the center of NGC 891, $S=7\aas@@fstack{\prime}88$. The
tidal force on NGC 891 scales as the the mass of the companion, $F_{t}\propto
M_{c}/R^{3}$, where $R$ is the distance between the galaxies, but the
companion mass is unknown. Dahari (1984) introduced a dimensionless parameter
$Q\propto F_{t}$ which uses the diameter of the primary galaxy,
$D_{p}=14\aas@@fstack{\prime}46$ (de Vaucouleurs et al., 1991), as a scaling
parameter, obtaining

$F_{t}\propto Q\equiv\ \frac{(D_{c}D_{p})^{1.5}}{S^{3}},$ (3)

and empirically showing that a strong tidal interaction requires $Q\geq\ 1$.
This argument indicates that a satellite galaxy large enough to pull the dust
spur out of the NGC 891 disk would have a major axis $D_{c}\approx
4\aas@@fstack{\prime}30$ or larger. This size is remarkably similar to the
extent of the dust spur, which is roughly 2′ by 4′ in Figure 2. Yet the old
stellar population of this hypothetical satellite is not detected.

The dust spur may be the smoking gun of a dark satellite passing through the
disk of NGC 891. The Milky Way’s population of high velocity clouds (HVCs) is
thought to include a subpopulation that is confined by dark matter minihalos
(Blitz et al., 1999). The properties of compact, isolated HVCs are similar in
many respects to those of dwarf irreglular galaxies, but they lack a high
surface brightness stellar population (Braun & Burton, 2000). They have H I
masses of $10^{5}-10^{6}$${\rm~{}M_{\odot}}$, i.e., well below the upper
limits on the H I mass of the dust spur, but total masses of
$10^{7}-10^{8}$${\rm~{}M_{\odot}}$ which are as large as the masses of some
dwarf galaxies (Adams et al., 2013). Hydrodynamical simulations of a minihalo
colliding with a disk show that the interaction pulls a coherent gas cloud out
of the far side of the disk (Nichols et al., 2014; Galyardt & Shelton, 2016;
Tepper-Garcia & Bland-Hawthorn, 2018), a qualitatively different result than
the collision of a pure baryonic cloud with a disk. These structures persist
for $\sim 60$ Myr (Galyardt & Shelton, 2016) or perhaps even longer (Tepper-
Garcia & Bland-Hawthorn, 2018), trigger star-formation in the minihalo, but
eventually deposit the majority of the HVC’s gas mass to the disk of the
primary galaxy.

####  5.2.2 The H I Filament

Oosterloo et al. (2007) considered several interpretations of the H I spur.
They considered gas previously ejected from the disk and now returning to the
disk in a galactic fountain and found the recycled gas had to lose significant
angular momentum, perhaps to interaction from infall from the intergalactic
medium (IGM), to be consistent with the H I kinematics. Alternatively, they
suggested the stream could be cold gas directly accreted from the IGM or
condensing out of a hot, virialized halo. No star formation has been
associated with the H I spur.

In our opinion, the substantial mass of the H I filament,
$M(\text{H\,{I}})\approx 1.6\times 10^{7}$${\rm~{}M_{\odot}}$, suggests a
discrete event. We suggest the ram pressure from the gas halo in NGC 891
pushed the gas out of a high-velocity cloud or satellite galaxy as it
approached the disk. For purposes of illustration, we have estimated the halo
density $n_{0}\approx 4\times 10^{-4}$ cm-3 at $z=12.5$ kpc and $R=22.7$ kpc
from the fitted H I surface density model (Oosterloo et al., 2007). Models
connecting the pressure to the ratio of molecular-to-atomic hydrogen, for
example, suggest $P_{0}/k\lower
2.0pt\hbox{$\buildrel{\scriptstyle<}\over{\scriptstyle\sim}$}\ 3\times 10^{3}$
cm-3 K for $\Sigma({\rm H_{2}})/\Sigma(\text{H\,{I}})\lower
2.0pt\hbox{$\buildrel{\scriptstyle<}\over{\scriptstyle\sim}$}\ 0.5$ (Yim et
al., 2011). To strip the ISM from the satellite, the satellite must approach
the disk at a velocity

$\displaystyle V_{sat}>210{\rm~{}km~{}s}^{-1}\left({P_{sat}}\over 3\times
10^{3}{{\rm~{}cm}^{-3}{\rm~{}K}}\right)^{1/2}$ (4)
$\displaystyle\left({4\times
10^{-4}{\rm~{}cm}^{-3}}\over{n_{0}}\right)^{1/2},$

which is comparable to the circular velocity of NGC 891 (Table 1) and
therefore a tenable explanation.

Figure 9: The color map shown in Figure 4 is shown with contours of 24
${\mu}$m (left) and 160 ${\mu}$m (middle and right) after image sharpening
(see text).

####  5.2.3 Recent Accretion

Our analysis suggests that recent accretion events produced both the dust spur
and the H I filament, but the nature of these events were different based on
the distinct compositions of the two structures. The interaction of an
infalling gas-rich satellite or gas stream with the CGM plausibly generated
the H I spur, but a collisionless component such as a dark matter minihalo was
required to pull the dust spur out the back side of the disk. These processes
deliver gas to the disk of NGC 891, thereby fueling further growth of the
stellar disk.

Studies of the stellar halo of NGC 891 show evidence that accretion of
satellite galaxies has formed giant streams much like those mapped around the
Milky Way (Ibata et al., 2003; Yanny et al., 2003; Belokurov et al., 2007).
Mouhcine et al. (2010) resolve structures that loop around the galaxy reaching
heights of $\approx 30$ kpc much like the rosette-shaped pattern generated by
a tidally disrupting dwarf galaxy. We note that the H I spur follows one of
the stellar streams where it plunges toward the disk. This alignment is most
easily seen in Figure 4 of Schulz (2014) who identify seven faint satellites
beyond this stellar stream. This satellite population provides more frequent
interactions with the disk than the more massive group member UGC 1807 (1:10
mass ratio) over $80$ kpc northwest of NGC 891. Interaction with a satellite
may also explain the lopsidedness of the disk of NGC 891 (Oosterloo et al.,
2007).

###  5.3 Origins of Halo Dust

We have detected thermal emission from $1.5\times 10^{7}$${\rm~{}M_{\odot}}$
of cool dust in the inner halo of NGC 891, i.e., the region within the ellipse
drawn in Figure 2. We estimate the total halo dust mass from an extrapolation
of the halo surface brightness profile. We use the 24 ${\mu}$m profile on the
west side of the disk and assume the halo extends vertically from a base of
radius 20.9 kpc. We equate the central surface brightness of 6.0 MJy sr-1 with
a mass surface density of 0.20${\rm~{}M_{\odot}}$ pc-2 based on Figure 7,
arriving at a total halo dust mass of $4\times 10^{7}$${\rm~{}M_{\odot}}$.

With this extrapolation, the halo dust mass increases from 15% to roughly 43%
of the dust mass in the thick disk. The estimated dust mass in the halo
therefore approaches the cosmic ratio of halo dust to disk dust. Ménard &
Fukugita (2012) measured the reddening of background quasars caused by dust in
Mg II absorbers and found that these absorbers account for most of the dust
inside the virial radii of galaxies and that the galaxy halos contain at least
52% as much dust as do galaxy disks. While we cannot rule out the possibility
of substantially more halo dust in a cooler component at $T\sim 7~{}\rm K$,
the similarity of these numbers suggests that the Herschel imaging has
detected the main component of halo dust in NGC 891.

We have identified two structures, a dusty superbubble and a dust spur, that
may pollute the CGM with dust. To add further insight into exactly how dust is
transported into the CGM, we applied unsharp masking techniques to highlight
the high spatial frequency structure in the dust emission. Regardless of the
exact procedure used, the results in Figure 9 show dust filaments emanating
from the hotspots associated with the molecular ring as well as from the
central hotspot. These filaments are detected over the full diameter of the
molecular ring and extend roughly 4 kpc from the midplane. These dust
filaments appear to be associated with the feedback processes that are mixing
dust into the halo; they are found over a larger region of the disk than the
base of the superbubble. It should be possible (in future work) to compare the
properties of such filaments directly to wind simulations that take the
interactions of the bubbles driven by multiple star-forming regions into
account (Tanner et al., 2016).

The halo dust clearly traces a fluid that is not pristine gas. While this
material most likely originated in galaxies, the question remains whether it
was primarily blown out of the disk by feedback processes or whether tidal
streams and ram pressure stripping of satellites have contributed a
substantial fraction of the mass. The newly discovered dust spur clearly shows
that interactions with satellites do play some role. The mass in the dust spur
is about 25% of that in the superbubble, however; and the filaments are also
lifting dust about 4 kpc off the disk midplane. Hence our results favor a
dominant role for stellar feedback in enriching the halo with dust.

To determine whether it is energetically plausible for feedback to lift most
of the gas associated with the halo dust out of the disk, we conservatively
estimate the total mass of gas lifted into the halo along with the dust. The
dust-to-gas mass ratio of the galaxy as a whole is $M(dust)/M(gas)\approx
1/110$. We estimate $M(gas)\approx 1.7\times 10^{9}$${\rm~{}M_{\odot}}$ just
above the disk and, very roughly for the entire halo, $M(gas)\approx 4\times
10^{9}$${\rm~{}M_{\odot}}$. A lower dust-to-gas ratio would yield more halo
gas associated with the dust, but the average galactic value is most
appropriate for testing the hypothesis that the dust was lifted out of the
disk by feedback. We note that $4\times 10^{9}$${\rm~{}M_{\odot}}$ is a small
fraction of the total mass found in the cool CGM of galaxies (Werk et al.,
2014, $7-12\times 10^{10}~{}\mbox{${\rm~{}M_{\odot}}$}$).

Following Howk & Savage (1997), we assume an isothermal sheet model for the
vertical distribution of light and mass in NGC 891. Based on the similarity of
the rotation curves for NGC 891 and the Milky Way, we adopt the Milky Way
midplane mass density $\rho_{0}$ (Bahcall, 1984) as a proxy for NGC 891; the
mass scale height $z_{0}$ is taken from infrared imaging of NGC 891 (Aoki et
al., 1991). The potential energy, $W$, of a cloud of mass $M_{\rm cl}$ a
height $z$ above the midplane is then

$\displaystyle W=10^{52}{\rm~{}ergs}\left({M_{\rm
cl}}\over{10^{5}~{}\mbox{${\rm~{}M_{\odot}}$}}\right)\left({z_{\rm
0}}\over{700\rm~{}pc}\right)^{2}$ (5) $\displaystyle\left({\rho_{\rm
0}}\over{0.185~{}\mbox{${\rm~{}M_{\odot}}$}~{}\rm pc^{-3}}\right){\rm
ln}\left[{\rm cosh}\left({z}\over{z_{\rm 0}}\right)\right].$

The energy required to lift the $4\times 10^{9}$${\rm~{}M_{\odot}}$ of H I,
$\rm H_{2}$, and dust in the superbubble to a height of 4.3 kpc is roughly

$\displaystyle W\approx 1.2\times 10^{58}{\rm~{}ergs}\left({M_{\rm
cl}}\over{4\times 10^{9}~{}\mbox{${\rm~{}M_{\odot}}$}}\right)\left({z_{\rm
0}}\over{700\rm~{}pc}\right)^{2}$ (6) $\displaystyle\left({\rho_{\rm
0}}\over{0.185~{}\mbox{${\rm~{}M_{\odot}}$}~{}\rm pc^{-3}}\right).$

For a constant SFR of 4.8$~{}{\rm M_{\odot}}$ yr-1, supernovae and stellar
winds provide a mechanical power, $L_{\rm w}\approx 1.2\times 10^{42}$ ergs
s-1. Assigning an efficiency $\epsilon$ for transferring this energy to the
cool gas and dust, the time required to enrich the halo with dust is roughly

$\displaystyle t\approx 3.1\times 10^{8}\epsilon^{-1}{\rm~{}yr}.$ (7)

The current rate of star formation in the disk provides enough feedback energy
to lift the dust and associated gas into the halo in just a few rotational
periods only if the feedback efficiency is very high. However, if we picture
the CGM as continually recycling disk material over a longer timescale, then
feedback can easily account for most of halo dust in NGC 891. For example, if
the disk of NGC 891 was assembled at $z\lower
2.0pt\hbox{$\buildrel{\scriptstyle<}\over{\scriptstyle\sim}$}\ 2$, and the SFR
has steadily declined over the last 10.3 Gyr, then the dust can be lifted into
the halo with inefficient feedback characterized by $\epsilon\approx\ 0.03$.

###  5.4 Comparison to NGC 4631

It is interesting to compare the feedback in NGC 891 to that in NGC 4631,
another edge-on galaxy which we recently studied with Herschel in Paper I. The
galaxies NGC 4631 and NGC 891 have comparable H$\alpha$, X-ray, and radio
luminosities. Linearly polarized radio continuum emission has been detected in
the halos of both galaxies and shows that magnetic field lines reach into the
halos of both galaxies (Hummel et al., 1991). The FIR to radio flux ratio for
NGC 891 is $q_{60\mu m}=2.19$ in Table 3. The FIR luminosity at 60 ${\mu}$m is
$9.64-9.70\mbox{${~{}\rm L_{\odot}}$}$ (Sanders et al., 2003; Surace et al.,
2004) and radio continuum luminosity is $21.85-21.76~{}\rm
W~{}m^{-2}~{}Hz^{-1}$ (Condon et al., 2002; Murphy et al., 2009). Thus, we
estimate $q_{60\mu m}=2.03-2.19$ for NGC 4631. This ratio is a sensitive probe
of age during a starburst phase (Bressan et al., 2002), and the comparable
values for NGC 4631 and NGC 891 indicate both galaxies are in a post-starburst
phase.

In spite of these similarities, the halos of the two galaxies show some
significant differences. The density and scale height of the warm ionized gas
are a bit larger in NGC 4631, and the magnetic field extends further into the
halo of NGC 4631 (Hummel et al., 1991). The direction of the magnetic field
lines is not well constrained for either galaxy due to poorly constrained
models for Faraday rotation, but after correction the foreground Faraday
rotation the field lines are quite different in the two galaxies. The magnetic
field lines point radially outward from the disk of NGC 4631 while no overall
orientation is visible in NGC 891 where the field appears to be drawn into the
halo by more local events (especially on the south side of the disk).444 The
radio continuum emission in Fig. 4 also shows an extension in total intensity
away from the disk plane to the southwest (opposite the dust spur). The linear
polarization of this southwest extension is larger than the polarization in
any other region of NGC 891, and the polarized emission extends further into
the halo here than it does above the superbubble (Hummel et al., 1991). Bulk
motion has convected the magnetic field outwards, and the adiabatic losses of
the electrons may explain the changes in spectral index reported by Hummel et
al. (1991). Whether there is a direct connection between this inferred
velocity gradient and the dust spur remains unclear. Overall, these results
suggest the galactic wind is stronger in NGC 4631 than in NGC 891.

Stronger feedback in NGC 4631 would be expected based on its higher SFR
surface density and gas fraction. The SFRSD of NGC 4631 is about 5 times
larger than that of NGC 891 as estimated in Section 5.1. It is also clear that
NGC 4631 has a higher gas fraction than NGC 891 based on its higher H I mass,
lower dynamical mass, and later spiral type (de Vaucouleurs et al., 1991).

It is therefore quite interesting to compare the dust properties of these two
halos. We find the measured dust mass of NGC 891 is 2.6 times higher than NGC
4631 in Paper I. We attribute this result to the larger size and mass of NGC
891. The superbubble in NGC 4631 is slightly smaller and contains a factor of
two less dust mass than the NGC 891 superbubble.

The temperature of the dust, however, differs significantly between the two
superbubbles. The temperature of the dusty superbubble is significantly higher
in NGC 4631, $T_{\rm dust}=24.46\pm 0.77$ K. When we include the 100 ${\mu}$m
data point from the HEDGES data, the superbubble temperature for NGC 891
increases by $\sim 1$ K which is still 3 K below the the temperature of the
superbubble in NGC 4631. When we take the 70 ${\mu}$m data point as an upper
limit, the temperature decreases slightly. Therefore, the temperature
difference of superbubble for NGC 4631 and NGC 891 cannot be alleviated by
different SED fitting processes.555 For the SED modeling, we used 5 bands, 70
${\mu}$m, 160 ${\mu}$m, 250 ${\mu}$m, 350 ${\mu}$m, and 500 ${\mu}$m while
Paper I adopted 100 ${\mu}$m in addition to the 5 bands. Also, in Paper I, the
70 ${\mu}$m data point was considered as an upper limit. The higher
temperature of the SED fit for NGC 4631 is driven by the data point at 100
${\mu}$m. When we include the 100 ${\mu}$m data point from the HEDGES data,
the temperature of all components of NGC 891 increases by $\sim 1$ K.

Dust in galaxy halos can be heated by the evolved stellar population or
processes associated with the stellar feedback. Using B and V band photometry,
Ann et al. (2011) measured a scale height for the thick stellar disk of NGC
4631 of $1.33\pm 0.25$ kpc and $1.25\pm 0.28$ kpc, and Morrison et al. (1997)
report an R band scale height of $1.5-2.5$ kpc for NGC 891. It therefore seems
unlikely that the stellar halo of NGC 891 is any less effective at heating
dust than that in NGC 4631. The SFR surface density of NGC 4631 is only
slightly higher than that of NGC 891, so an attempt to explain the difference
in superbubble temperatures would need to appeal to the lower dust mass and
smaller size of the superbubble in NGC 4631. It would be interesting to
explore whether these properties of the NGC 4631 superbubble reflect a younger
age or the more ordered magnetic field of the halo. The increased temperature
of the superbubble might reflect the larger pressure required to push these
field lines out of the superbubble’s path.

##  6 Conclusions

We present the results of our Spitzer/MIPS and Herschel/PACS GO observations
of NGC 891. The angular resolution and sensitivity allow us to resolve the
distribution of dust in the disk and halo. We place these components in the
context of the stellar component and multi-phase ISM using supplementary
observations from the radio to the X-ray. We obtain the following insight into
the disk - halo interaction.

  * •

The MIPS image at 24 ${\mu}$m shows a dust spur extending 20 kpc southeast of
the galaxy. The PACS and SPIRE images confirm the presence of this feature.
Fitting a MBB model to the SED indicates a dust mass of $7.6\times
10^{5}$${\rm~{}M_{\odot}}$, which is surprisingly large compared to the upper
limits on the H I gas mass. The high dust-to-gas ratio is inconsistent with a
primordial origin (e.g., a cold stream of infalling material) and indicates
the dust was likely pulled out of NGC 891 by a satellite interaction. Its
composition differs distinctly from the H I spur to the northwest where we do
not detect thermal dust emission. The local maxima in the far-infrared surface
brightness coincide with knots of radio continuum emission, perhaps a sign
that they are young star-forming regions. The absence of optical or near-
infrared emission from the dust spur suggests the colliding satellite had
little stellar mass, suggesting that the dust spur may be the remnant of a
minihalo collision with the disk.

  * •

The PACS 70/160 ${\mu}$m ratio map draws attention to a superbubble extending
over 7 kpc above the midplane on its western side. We find a close
correspondence between the superbubble and the thermal X-ray emission
suggesting that the thermal pressure of hot gas drives the expansion. We argue
that the superbubble is likely accelerating as it breaks through the disk
because its height exceeds the pressure scale height of not only the thin disk
but also the gaseous halo. The feedback associated with the star formation
activity within the inner disk appears to be sufficient to launch this
galactic wind. The SFR surface density of 0.03$~{}{\rm M_{\odot}}$ yr-1 kpc-2
and gas fraction ($\lower
2.0pt\hbox{$\buildrel{\scriptstyle<}\over{\scriptstyle\sim}$}\ 10\%$) suggest
somewhat lower threshold values for driving winds than do the values measured
previously in starburst galaxies.

  * •

This disk contains regions of hot dust associated with the galactic center and
the molecular ring. Image sharpening techniques reveal dust filaments rising
roughly 4 kpc above these regions. The dust temperature is higher in the inner
disk than the halo, and the mass surface density of the dust is centrally
concentrated.

  * •

The halo dust component is detected 10 kpc or more above the entire stellar
disk with the measured halo dust mass of $M_{\rm dust,halo}=1.5\times
10^{7}$${\rm~{}M_{\odot}}$. A rough extrapolation of the dust mass surface
density suggests $\approx 4\times 10^{7}$${\rm~{}M_{\odot}}$ of dust has been
lifted into the halo carrying with it a gas mass roughly 100 times larger.
This mass flux accounts for roughly 10% of the gas mass estimated to reside in
the CGM of similar galaxies.

Because NGC 891 is a very close analog of the Milky Way in respect to many of
its properties, we believe these results provide a viable picture of how dust
is deposited in the halos of typical galaxies. The elevated SFR and molecular
gas fraction with respect to the Milky Way would perhaps most closely describe
the Milky Way in its recent past when its SFR was higher. In this sense, NGC
891 provides a direct image of the emission from dust and multi-phase gas in
the inner halo of a galaxy where the baryon budget of the CGM is well
constrained observationally (Hodges-Kluck et al., 2018; Das et al., 2020).

## Acknowledgements

We thank Cassi Lochhaas for discussions about the circumgalactic bubble which
clarified how its structure depends on SFR surface density, and we thank an
anonymous referee for suggesting we apply the Dahari parameter. This work was
supported in part by the National Science Foundation (NSF) under AST-1817125
(CLM) and ASTR1009583 (SV), JPL Awards 1276783 and 1434779, and NASA grants
NHSC/JPL RSA 1427277, 1454738 (SV and MM), and ADAP NNX16AF24G (SV). Some of
the work was completed at the Aspen Center for Physics, and accompanying
support from the NSF through PHY-1066293 is gratefully acknowledged. We deeply
regret that the co-author (CE) who provided the H$\alpha$ image and made a
significant contribution to the analysis of the Spitzer data did not live to
see the article published. Herschel is an ESA space observatory with science
instruments provided by European-led Principal Investigator consortia and with
important participation from NASA.

## Data Availability

The data underlying this article will be shared on reasonable request to Dr.
Joo Heon Yoon.

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