11institutetext:  Argelander-Institut für Astronomie (AIfA), Universität Bonn,
Auf dem Hügel 71, 53121 Bonn, Germany
11email<EMAIL_ADDRESS>22institutetext: Department of Astronomy
and Astrophysics, University of Chicago, Chicago, IL 60637, USA
33institutetext: Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstrasse, 85748 Garching, Germany 44institutetext: Hamburger
Sternwarte, University of Hamburg, Gojenbergsweg 112, D-21029, Hamburg,
Germany 55institutetext: Leiden Observatory, Leiden University, PO Box 9513,
NL-2300 RA Leiden, The Netherlands 66institutetext: SRON Netherlands Institute
for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, the Netherlands

# The SRG/eROSITA All-Sky Survey

View of the Virgo Cluster

Hannah McCall 11 2 2 Thomas H. Reiprich 11 Angie Veronica 11 Florian Pacaud 11
Jeremy Sanders 33 Henrik W. Edler 44 Marcus Brüggen 44 Esra Bulbul 33
Francesco de Gasparin 44 Efrain Gatuzz 33 Ang Liu 33 Andrea Merloni 33
Konstantinos Migkas 115566 Xiaoyuan Zhang 33

(Received January 30, 2024; accepted XXXX XX, 2024)

###### Abstract

Context. As the closest galaxy cluster, the Virgo Cluster is an exemplary
environment for the study of large-scale filamentary structure and physical
effects that are present in cluster outskirts but absent from the more easily
studied inner regions.

Aims. Here, we present an exploration of the SRG/eROSITA data from five all-
sky surveys.

Methods. eROSITA allows us to resolve the entire Virgo cluster and its
outskirts on scales between 1 kpc and 3 Mpc, covering a total area on the sky
of about 25∘ by 25∘. We utilize image manipulation techniques and surface
brightness profiles to search for extended emission, surface brightness edges,
and features in the outskirts. We employ a method of comparing mean and median
profiles to measure gas clumping beyond the virial radius.

Results. Surface brightness analysis of the cluster and individual sectors of
the cluster reveal the full extent of previously identified cold fronts to the
north and south. The emissivity bias due to gas clumping, which we quantify
over three orders of magnitude in radial range, is found to be mild,
consistent with previous findings. We find an estimated virial gas mass of
$M_{\mathrm{gas},r<r_{200}}=(1.98\pm 0.70)\times 10^{13}$ M⊙. Through imaging
analysis we detect the presence of extended emission spanning 320 kpc to the
southwest of M49. The extension has a significance of 3.15$\sigma$ and is
coincident with radio emission detected with LOFAR, which could be evidence of
an accretion shock or turbulent re-acceleration as the M49 group or background
W’ cloud falls into the cluster and interacts with the ICM.

###### Key Words.:

galaxies: clusters: individual: Virgo – galaxies: clusters: intracluster
medium – X-rays: galaxies: clusters – galaxies: clusters: general

##  1 Introduction

The Virgo Cluster is the closest ($\sim 16.1$ Mpc, Tonry et al. 2001) and
among the best-studied clusters of galaxies. In the X-ray band, where the
strongest emission processes are that of thermal bremsstrahlung and line
emission in the hot intracluster medium (ICM), the interior of Virgo and its
brightest cluster galaxy (BCG) M87 have been the subject of much previous
study (Böhringer et al., 1994, 1995; Schindler et al., 1998; Churazov et al.,
2001; Young et al., 2002; Forman et al., 2005; Werner et al., 2006; Forman et
al., 2007; Simionescu et al., 2008; Million et al., 2011; Arévalo et al.,
2016; Gatuzz et al., 2022, 2023). Other works have characterized portions of
the outskirts of Virgo (Urban et al., 2011; Simionescu et al., 2017; Mirakhor
& Walker, 2021). The outskirts of clusters, which Reiprich et al. (2013)
defines as between $r_{500}$ and $3r_{200}$, can be a location of physical
effects that are no longer observable in the more relaxed central regions.
These include breakdown of hydrostatic, thermal, equipartition, and ionization
equilibrium states, as well as structure formation effects, such as clumpy gas
distribution or accretion shocks (see Reiprich et al. 2013, Walker et al. 2019
for a review of these processes in cluster outskirts). Limited by high
particle backgrounds, however, previous X-ray instruments have been largely
unable to observe significant emission in the outskirts, or else have had a
field-of-view (FOV) too narrow to cover the entirety of the closest clusters.

The extended Roentgen Survey and Imaging Telescope Array (eROSITA) was
launched aboard the Spektrum Roentgen Gamma (SRG) mission (Sunyaev et al.,
2021) in 2019 with a main science goal of detecting the hot intracluster
medium of galaxy clusters and groups to enhance the study of cosmic structure
evolution (Predehl et al., 2021). eROSITA is expected to detect $10^{5}$
galaxy clusters and groups through its all-sky survey (Merloni et al., 2012;
Liu et al., 2022; Bulbul et al., 2022). eROSITA’s combination of soft-band
sensitivity and a wide FOV makes it the ideal instrument for the study of
faint emission in cluster outskirts which may have previously gone undetected.
Other current generation X-ray instruments, such as Chandra and XMM-Newton,
have a relatively small FOV and are thus inefficient in mapping large volumes
of the Universe; only through mosaics have they been able to study out to and
beyond $r_{200}$. The only imaging X-ray all-sky survey to precede eROSITA is
ROSAT, which was performed over 6 months in 1990. By the completion of the
eight all-sky surveys planned for eROSITA, its survey results will be 25 times
more sensitive than ROSAT in the soft band (0.2-2.3 keV Merloni et al. 2012;
Predehl et al. 2021).

The Virgo Cluster’s distance, at which 1 arcminute corresponds to 4.65 kpc,
provides the opportunity to study the ICM on smaller scales than is possible
with any other cluster. Recent studies with other X-ray instruments have taken
advantage of Virgo’s distance, modest physical size, and dynamism to better
analyze its outskirts. These include a study by Urban et al. (2011), which
uses a series of thirteen XMM-Newton pointings out to the virial radius in the
north to characterize the outskirts with spectral analysis; observations with
Suzaku to study gas clumping by Mirakhor & Walker (2021); and a Suzaku mosaic
along four arms out to the virial radius, which Simionescu et al. (2017) used
to analyze the thermodynamics of the outskirts. All of these studies were
limited to analysis along a single radial line to $r_{200}$ due to the narrow
fields of view of the instruments used.

Here we present an eROSITA imaging analysis of the Virgo Cluster produced from
the cumulative data of its first four complete surveys and partial fifth
survey. This is the first comprehensive X-ray imaging analysis of the cluster
since ROSAT and it reveals significantly more detail. This article is
organized as follows: in Section 2, we describe the data reduction steps and
analysis strategy. In Section 3 we present what the imaging analysis and
surface brightness analysis reveal about the large scale structure of the
cluster. We describe the results of deprojection analysis in Section 4.
Section 5 describes a region to the southwest of M49 where eROSITA soft-band
X-ray emission is found to be coincident with LOFAR radio emission. We
conclude with a summary in Section 6.

Throughout this work, we assume a $\Lambda$CDM cosmology with
$\Omega_{m}=0.27$, $\Omega_{\Lambda}=0.73$, and H${}_{0}=70$ km/s/Mpc. The
distance to the Virgo Cluster is taken to be that of its central galaxy, M87,
at 16.1 Mpc (Tonry et al., 2001), or $z\approx 0.00377$. For consistency with
other works (Urban et al., 2011; Simionescu et al., 2017), we consider
$r_{200}$, the radius within which the mean density is 200 times the critical
density of the Universe, to be the virial radius. The virial radius of the
Virgo Cluster is taken to be 1.08 Mpc (Urban et al., 2011), or a projected
radius of $\sim 3.9^{\circ}$. Other radii (i.e. $r_{500}$) are calculated with
the relationships given in Reiprich et al. (2013). Abundances are given
relative to solar abundance of Asplund et al. (2009). Unless otherwise stated,
all uncertainties are given at a 68 percent confidence level.

##  2 Data reduction and analysis

Observations of the Virgo Cluster from the first five eROSITA all-sky surveys
(eRASS:5) were used in this analysis, although the fifth survey only covers
approximately half of the area (see Merloni et al. (2024) for all-sky survey
information, sky tile definition, etc.). A comprehensive list of the 62
utilized sky tiles, which span an approximate area of $25^{\circ}$ by
$25^{\circ}$, similar to the area covered by ¿2500 XMM-Newton EPIC pointings,
can be found in Appendix A. The processing configuration c020, extended
Science Analysis Software (eSASS, Brunner et al. (2022)) version
eSASSusers_211214, and Heasoft version 6.29 were used throughout the data
reduction.

###  2.1 Data preparation

Upon retrieving the data, the first action was to merge the eRASS:5 event
lists of all seven111Not all TMs could be used for every eRASS or tile. See
Appendix A for a list of the excluded TMs. telescope modules (TMs) across the
full energy range 0.2-10 keV for each tile using the eSASS task evtool. In
this work, we use the notation that TM 0 is the combination of all seven TMs.
The parameters pattern=15 and flag=0xe00fff30 were applied to select single,
double, triple, and quadruple patterns and remove bad pixels and strongly
vignetted corners of CCDs. The eSASS task flaregti was then run with
parameters source_size = 150, which is in units of arcseconds and dictates the
diameter of the extraction area for dynamic threshold calculation, pimin=5000,
which is in units of electron volts and controls the lower energy range used,
and gridsize=26, which determines the number of grid points per dimension for
the dynamic threshold calculation. This task produces updated good time
intervals (GTIs) which exclude times during which there is flare
contamination. In addition to removing several short flares, flaregti aided in
the removal of a flare that spanned the entire area of analyzed sky during
eRASS 2.

Following flare filtering, the eSASS task radec2xy was applied to recenter all
tiles around a common center, R.A. 187.7042∘, Dec. +12.3911∘. Updated GTIs
were applied using evtool with the parameters gti=“FLAREGTI” and rebin=124,
which results in a pixel size of 6.2 arcseconds. This choice allowed the
entire sky area to fit into the maximum eSASS image size. The combined event
file was then split into its seven individual TMs for further image
processing, with the ultimate goal of a cleaned and corrected TM 0 image. The
energy band for the analysis was restricted to 0.3-2.0 keV for TMs with on-
chip filter (TMs 1, 2, 3, 4, and 6; combined they are referred to as TM 8) and
0.8-2 keV for TMs without on-chip filter (TMs 5 and 7; referred to as TM 9).
The reason for this differing lower limit is the optical light leak
contamination of TM 9, which is further detailed in Predehl et al. (2021).
Both vignetted and non-vignetted exposure maps were created for every image
using the task expmap.

###  2.2 Image corrections

Figure 1: eROSITA TM 0 particle-induced background subtracted, exposure
corrected, absorption corrected count rate image in the $0.3-2.0$ keV energy
band. The image is displayed in logarithmic scaling with a Gaussian smoothing
of radius 18 pixels, or 112 arcseconds. The white bar in the lower left spans
1 degree, or 279 kpc. Prominent groups and galaxies within Virgo are labeled
in white, while non-members of the cluster are labeled in yellow.

In order to better isolate the ICM emission visible in the eROSITA image, it
is necessary to account for the particle background, spatially variable
absorption, and variable coverage of the field by the different TMs.

####  2.2.1 Particle-induced background

The particle-induced background (PIB) subtraction steps are based on knowledge
of the eROSITA Filter-Wheel-Closed (FWC) data (Freyberg et al., 2020; Yeung et
al., 2023) and were performed as described in Reiprich et al. (2021).
Important TM-specific values for this calculation can be found in Table 3.
This process results in PIB maps for each TM, which can then be added to
create a TM 0 PIB map that can be subtracted from the photon image.

####  2.2.2 Relative absorption correction

The next step was to account for the spatially variable absorption, since
$N_{\mathrm{HI}}$ could vary across the large sky area and potentially
introduce a bias in soft emission detection. The $N_{\mathrm{HI}}$ map
covering the sky area around the Virgo Cluster (see Figure 14) is based on the
21 cm HI 4PI (HI4PI) survey (HI4PI Collaboration: et al., 2016). Its values
span $(0.71-5.95)\times 10^{20}\,\mathrm{cm}^{-2}$. Thus, in order to
calculate the $N_{\mathrm{HI}}$ correction factor across the entire image,
spectra for $525$ $N_{\mathrm{HI}}$ values in this range were simulated to
quantify the effect of varying absorption on the X-ray spectra and overall
count rates. The simulations were conducted in xspec version $12.12.0$
(Arnaud, 1996). The fakeit command was used with the model

$\displaystyle apec_{1}+tbabs\times\left(apec_{2}+pow\right),$ (1)

where $apec_{1}$ accounts for the X-ray emission from the unabsorbed Local Hot
Bubble, $tbabs$ corresponds to the absorption along the line-of-sight,
$apec_{2}$ accounts for the X-ray emission of the Milky Way Halo, and $pow$ is
the power law component originating from unresolved active galactic nuclei
(AGN). The temperatures, metal abundances, and redshifts of the LHB and MWH
are fixed, along with the power law photon index. Due to the origin within the
galaxy, both metal abundances are set to $1\,\mathrm{Z}_{\odot}$, while both
redshifts are set to zero. All values can be found in Table 4.

The simulations were run for TMs with and without on-chip filter. This
required the use of their separate response files and soft energy bands. The
result of the simulated spectra was count rates for every $N_{\mathrm{HI}}$
value. From these, a ratio of the count rate for a given $N_{\mathrm{HI}}$ to
the count rate of the reference $N_{\mathrm{HI}}$ could be calculated. The
reference $N_{\mathrm{HI}}$ is the median $N_{\mathrm{HI}}$ across the total
sky area and in this instance was $3.33\times 10^{20}\,\mathrm{cm}^{-2}$. The
correction factor map is then acquired by distributing the ratio corresponding
to a given $N_{\mathrm{HI}}$ value to its area on the sky. The correction maps
for TM 8 and TM 9 are applied by multiplying them by the exposure maps.

The variation of the correction factors is 18.6% for TM 8 and 7% for TM 9.
Since absorption is weaker at higher energies, the smaller variation of TM 9
is expected.

####  2.2.3 Exposure correction

Since TM 8 and TM 9 feature different soft responses and different energy
bands to account for the light leak, it is necessary to apply a correction
factor when combining all TMs and observations. This process, referred to as
exposure correction and described in larger detail in Reiprich et al. (2021),
involves using the TM 9-to-TM 8 ratio of the $N_{HI}$-corrected, PIB-
subtracted count rates. The correction factor for this analysis is $0.392$.
Exposure maps prior to and following absorption and exposure correction can be
seen in Figure 15.

The final image is found by adding the individual TM event files together to
create the TM 0 photon image, subtracting the PIB map, and dividing by the
corrected exposure map. It is shown in Figure 1. Several prominent groups,
galaxies, and clusters are labeled.

##  3 Larger Scale Structure

###  3.1 X-ray images

Figure 2: An RGB map of the Virgo Cluster out to three times the virial radius
of the cluster (red for $0.3-0.6$ keV, green for $0.6-1.0$ keV, and blue for
$1.0-2.3$ keV) is shown. The surface brightness for all channels has a dynamic
range from $10^{-5}$ to $1.8\times 10^{-3}$ counts/sec/arcmin2.

An RGB image can be seen in Figure 2. This was created following the steps
described in Section 2, except that different energy ranges were used. The
colors and corresponding bands are: red for $0.3-0.6$ keV, green for $0.6-1.0$
keV, and blue for $1.0-2.3$ keV. Note that TM 9 was not used to create the red
band, and was only used in the range $0.8-1.0$ keV for the green band. In the
image, the diffuse ICM emission around M87 is visible in a hazy white, while
many of the galaxies in Virgo, most obviously M86 and NGC 4636, appear green-
blue. This implies that these regions have a harder spectrum than the
surrounding medium, which could result from a higher temperature. Outside of
the virial radius, extended white-green emission is also visible surrounding
and to the southwest of M49. The point sources are likely a combination of
foreground stars and background AGN. The variety of point source colors
implies a plethora of spectral shapes. The northern eROSITA bubble (Predehl et
al., 2020), also known as the North polar spur, is bluer towards the Galactic
center (outside the analyzed area to the southeast), and then fades to green
and then red closer to the center of Virgo, indicating that the highest
energies are closest to the Galactic center. This observation is in agreement
with the assertion in Predehl et al. (2020) that the bubbles are likely due to
energy injections from the Galactic center.

We can infer from the color of the eROSITA bubbles that the soft Virgo Cluster
emission in the southeast is likely contaminated by bubble emission.
Comparison of the surface brightness in the lower left and upper right of the
image revealed that the bubble emission enhances the foreground by a factor of
$\sim 2$. In the blue band, however, there is almost no contribution from the
eROSITA bubbles within $\sim 3\times r_{200}$, and the bubbles are absent
inside $r_{200}$. This inspired the surface brightness analysis of the cluster
in the $1.0-2.3$ keV band, described in Subsection 3.2.

####  3.1.1 ICM enhancement

Figure 3: Point source excised, wavelet filtered eRASS:5 image of the Virgo
Cluster in the $0.3-2.0$ keV energy band. The $r_{200}$ is shown in white.
Features mentioned in the text are labeled.

As the focus of this study is the ICM emission, wavelet filtering was used for
point source detection and enhancement of diffuse emission. The steps
presented in Pacaud et al. (2006) and Reiprich et al. (2021) were followed to
apply the wavelet filtering algorithm. Different parameters were set to
manipulate the scales and thresholds depending on whether the goal of the
filtering run was to detect point sources or emphasize significant ICM
emission. For point source detection, the signal-to-noise threshold was lower
than for ICM detection, a smaller background cell size was used, the maximum
scale was lowered, and fewer iterations were required. Point source detection
was performed with the source detection tool SExtractor (Bertin & Arnouts,
1996), with some modifications made after manual inspection. Additionally,
background clusters were identified via the MCXC catalogue (Piffaretti et al.,
2011). Our approach to point source detection also identifies small extended
sources if they are above the signal-to-noise threshold of 4.0; therefore, we
have also excised additional background clusters that are not included in the
MCXC catalogue with this approach. Point sources and background clusters were
then masked, with the radius identified via SExtractor and out to $r_{500}$,
respectively, for all subsequent imaging manipulation and surface brightness
analysis steps. The $0.3-2.0$ keV wavelet filtered image can be seen in Figure
3, while the “blue band” $1.0-2.3$ keV wavelet filtered image is in Figure 16.

Adaptive smoothing produces an image where the signal-to-noise ratio (SNR) at
each pixel is approximately equivalent, meaning that fainter areas become more
smoothed than brighter areas in an image. In this work, the task asmooth from
the XMM-Newton Science Analysis System (xmmsas, SOC 2022) with parameter
smoothstyle=‘adaptive’ was used to apply adaptive smoothing to the PIB-
subtracted, absorption- and exposure-corrected image with point sources
masked. An SNR of $10$ was chosen, along with minimum and maximum values of
$0$ and $25$ respectively for the normalized Gaussian convolvers. Masked point
source areas were then effectively filled in with the signal level that
surrounds them. The point source excised, adaptively smoothed image is shown
in Figure 4.

Gradient filtering has previously been used to examine features in clusters
and simulations (Sanders et al., 2016; Walker et al., 2016). The Gaussian
gradient magnitude (GGM) filter was applied following Sanders et al. (2016)
using scipy (Virtanen et al., 2020) and $\sigma=1,2,4,8,16,32$ detector pixels
on an image with cosmetically removed point sources. A sample of the images
that best show edges is in Figure 5. The $\sigma=4$ map shows the well-known
M87 arms, while the $\sigma=8$ map reveals a spiral edge feature to the
northwest. On the largest scales, an extension to the southwest is prominent.

Looking at the wavelet filtered and adaptively smoothed images, faint X-ray
emission is visible out to $\sim 4-5^{\circ}$ from M87, beyond the virial
radius, with the most conspicuous emission to the south out to M49 and beyond.
An exception to this is the southwestern side, where there is a noticeable
emission drop-off prior to the virial radius, a feature which is especially
clear in the wavelet filtered image. To the south within the virial radius,
the ICM curves in an extension to the southwest, almost like a spiral. Further
south, the emission halo around M49 appears more extended to its southern edge
than to its northern, and is joined by an apparent extension, or tail, that
curves off of the halo to the southwest in the direction of NGC 4261. To the
north of the cluster, the emission appears more irregular than in the other
directions. The emission seems to clump into a number of extensions stretching
away from the cluster, with noticeable examples to the northeast and in the
radial direction beyond M86. This finger-like tail protruding from M86, which
appears out to a distance of 80 arcminutes or 370 kpc from the center of the
galaxy, has been observed previously with other instruments. Using Chandra
data, Randall et al. (2008) asserted that the tail is related to ram pressure
stripping as M86 falls into the Virgo Cluster and interacts with the ICM. To
the southeast, the eROSITA bubble emission dominates all images, contaminating
the ICM emission of Virgo. Overall, the cluster structure does not appear
spherical or symmetrical.

Figure 4: Point source excised, adaptively smoothed eRASS:5 image of the Virgo
Cluster in the $0.3-2.0$ keV energy band. The $r_{200}$ is shown in black. The
cold fronts are marked with arrows, white for the northern and yellow for the
southern cold front. The straight white line spans 1 degree. Figure 5: GGM
filtered images with $\sigma=4,8,16,$ and $32$ pixels. The line has a length
of 30 arcminutes, or 139.5 kpc.

###  3.2 X-ray surface brightness analysis

To acquire the surface brightness profiles, PyProffit (Eckert et al., 2020)
was used. For the full azimuthal profile, and subsequent profiles divided into
fourths and eighths, 656 concentric annuli centered at the X-ray surface
brightness peak, $(187.705^{\circ},+12.391^{\circ})$, and stretching to an
outermost radius of 700 arcminutes or $\sim 3r_{200}$ were utilized. Their
radial bin distribution is logarithmic and has a minimum bin size of 26
arcseconds, approximately the same as the survey average point spread function
(PSF) of eROSITA (Predehl et al., 2021).

To estimate the magnitude of the cosmic X-ray background (CXB), two box
regions of dimension $1.8^{\circ}$ by $3.1^{\circ}$ were extracted from
outside $3r_{200}$ to the northwest to avoid cluster emission and contribution
from the northern eROSITA bubble. The average value for 0.3-2.0 keV was found
to be $2.8\times 10^{-3}$ cts/s/arcmin2, and for $1.0-2.3$ keV, $1.4\times
10^{-3}$ cts/s/arcmin2.

A single $\beta$-model (Cavaliere & Fusco-Femiano, 1976; Voit, 2005):

$\displaystyle
S_{\mathrm{X}}(R)=S_{\mathrm{X}}(0)\left(1+\frac{R^{2}}{r_{\mathrm{c}}^{2}}\right)^{-3\beta+\frac{1}{2}}+K,$
(2)

was fit to the $0.3-2.0$ keV azimuthal profile out to 300 arcminutes, where
$R$ is the projected distance from cluster center, $S_{\mathrm{X}}(0)$ is the
normalization factor, $r_{c}$ is the core radius, $\beta$ determines the slope
of the surface brightness profile, and $K$ is the background value. The best-
fit $r_{c}$ and $\beta$ can be found in Table 1. This simple model was chosen
to allow for comparison with previous ROSAT publications, which also made use
of the single $\beta$-model and comprehensive Virgo data. The best-fit slope
of $0.43\pm 0.02$ is in good agreement with ROSAT (Nulsen & Böhringer, 1995),
which cites $\beta=0.46$, but the core radius of $47\pm 5$ arcseconds is about
half the ROSAT value of 115 arcseconds. This difference seems mainly due to
the poor fit of the single $\beta$-model to the innermost $\sim 2$ arcminutes
of the cluster combined with eROSITA’s much better PSF as compared to ROSAT;
additional factors could be different treatment of point sources, background
clusters, and the central AGN between this work and Nulsen & Böhringer (1995).

Although the spherical $\beta$-model allows us to directly compare to ROSAT
results, we can also achieve a better fit to cluster emission by allowing for
ellipticity in the model. This has the same form except that $R$ becomes

$R=\frac{(x_{e}^{2}(1-\epsilon)^{2}+y_{e}^{2})^{1/2}}{1-\epsilon}$ (3)

where $\epsilon$ is the eccentricity and the coordinates are defined as
$x_{e}=(x-x_{0})\cos(\theta)+(y-y_{0})\sin(\theta)$ and
$y_{e}=(y-y_{0})\cos(\theta)-(x-x_{0})\sin(\theta)$, where $\theta$ is the
ellipse’s angle of rotation.

The best-fit elliptical model out to $r_{200}$ (also in Table 1) was used to
create a flattened image of the cluster center, meaning the surface brightness
image was divided by the 2D model image. These data products can be seen in
Figure 6. In the flattened image, M87’s characteristic arms are visible, as
are several surface brightness edges. One is a weak shock identified in Forman
et al. (2007) with Chandra. Two have been identified as candidate cold fronts
in previous works and are explored further in the following section.

Figure 6: Series of eROSITA 0.3-2.0 keV images centered on M87 with half-size
65 arcminutes. Left: Surface brightness image smoothed with a sigma of 2
pixels, or 12 arcseconds. Point sources are present, but were removed for the
fitting of the model. Middle: Best-fit $\beta$-model. Right: Flattened image,
meaning the image divided by the model, smoothed with a sigma of 5 pixels, or
31 arcseconds.

####  3.2.1 Eight sectors

Figure 7: Image of the Virgo Cluster overlaid with the eight sector divisions
and their corresponding names and colors. The annuli used for surface
brightness profile extraction are not pictured. A number of radii are also
shown and labeled. Figure 8: Eight surface brightness profiles in the
$0.3-2.0$ keV energy band, one for each sector identified in Figure 7. The
other seven sectors for any given profile are plotted in gray for comparison.
The CXB measured to the northwest beyond $3r_{200}$ is shown in navy blue. A
selection of characteristic radii are shown as brown vertical dash-dotted
lines. The scaled difference to the median is the difference between each
sector and the median divided by the uncertainty (”in sigmas”).

To better study features in any given direction, eight $0.3-2.0$ keV surface
brightness profiles were constructed. The regions are shown in Figure 7 and
named for the eight half-wind compass points (north-northwest, east-northeast,
etc.). The resulting surface brightness profile for each sector is seen in
Figure 8. In this figure, each plot has a single profile for every sector,
which is plotted in the color it corresponds to in Figure 7. The other seven
sectors for any given profile are plotted in gray to offer a better idea of
how the profiles compare. Profiles for the eight sectors were also created in
the blue band of the RGB image, $1.0-2.3$ keV, which is approximately free
from eROSITA bubble contribution. These can be seen in Figure 17, and are
referred to as the “blue” profiles below.

The WNW sector contains a bump in the profile at $10$ arcminutes corresponding
to a surface brightness excess, i.e. in the flattened image, and is followed
by a drop-off at $20$ arcminutes. A prominent peak in the profile is visible
at $\sim 70$ arcminutes or $330$ kpc, corresponding to the X-ray halo around
M86. The profile then gradually flattens out into the outskirts past
$r_{200}$, where it shows a low level of surface brightness as compared to the
other sectors. In the blue band, M86 is still prominent.

In the NNW sector, which does not contain any of the brightest Virgo galaxies
or groups, the profile displays a number of interesting features. Between $10$
and $20$ arcminutes, there is an excess of brightness followed by a
discontinuity in the profile. This behavior is often attributed to shocks or
cold fronts, but to discern between the two, temperature and density profiles
of the region are necessary (Markevitch & Vikhlinin, 2007). This particular
feature was previously studied with XMM-Newton , Chandra, and Suzaku data
(Urban et al., 2011; Werner et al., 2016; Simionescu et al., 2017) and
identified as a cold front. Another excess as compared to the other sectors is
visible between $50$ and $100$ arcminutes. This could be associated with the
apparent extension of ICM emission in that direction. Past $r_{200}$, this
sector shows one of the lowest overall brightness when compared to the other
sectors. This is expected because NNW is the furthest from the eROSITA
bubbles. In the blue band, all of the same features are visible.

In the NNE, a similar discontinuity as in NNW is visible at the same distance,
$\sim 20$ arcminutes, indicating that the cold front extends along the entire
northern direction. There is a bump in the profile just past $r_{500}$ that
could be due to the finger-like extension of the ICM observed in that
direction. The outskirts past $r_{200}$ are at a similarly low level as in NNW
with a similar shape in the full and blue profiles, indicating that there may
not be substantial contribution by the eROSITA bubble in this region.

To the ENE, contribution from the eROSITA bubble becomes apparent both in the
images and in the surface brightness profile, where the brightness increases
from $r_{200}$ to $3r_{200}$. The blue profile instead flattens out in this
range.

The ESE sector shows a slow increase from $r_{200}$ to $3r_{200}$, a segment
in which it registers the highest surface brightness of all sectors due to the
eROSITA bubble. The blue profile for this region is not completely flat, but
instead also begins to increase around 400 arcminutes, implying some
contribution from the eROSITA bubble in the $1.0-2.3$ keV band to the ESE.
However, the blue surface brightness in this range is still a factor 3 below
the $0.3-2.0$ keV surface brightness when normalized to the same central
value, implying that most of the contribution from the eROSITA bubble is in
the $0.3-1.0$ keV band.

The SSE sector is brighter on average than other sectors around $40-50$
arcminutes, and then drops off gradually. The eROSITA bubble clearly also
affects the outer regions of this sector, which increases between $r_{200}$
and $3r_{200}$ and shows a prominent bump just after $r_{200}$ which becomes
even more prominent in the blue band. This bump is prominent even following
the removal of extended sources, or clumps, before production of the surface
brightness profiles.

The SSW sector contains M49 and its extended ICM tail, features which appear
in the profile in the form of a bump between $200$ and $300$ arcminutes. A
discontinuity, again possibly indicative of either a shock or cold front
(Markevitch & Vikhlinin, 2007), is present in this profile at the same
location as in the SSE around $40-50$ arcminutes, except that it appears more
pronounced in this sector.

Finally, to the WSW, the profile records the lowest values of all segments
between $10-20$ arcminutes, indicating that the cluster emission is not
stretched along this direction, which matches our imaging analysis. There is a
minor bump and drop around $50$ arcminutes, perhaps indicative of the same
feature present in the SSW and SSE.

Overall, if we compare the blue and full surface brightness profiles
normalized to the same central values, all of the same features observed in
the full profile are present in the blue band. This confirms that these
features are not due to the eROSITA bubble contribution.

####  3.2.2 Cold fronts

Figure 9: Surface brightness profiles for the north, south, east, and west
sectors in the $0.3-2.0$ keV energy band. The average western CXB measured by
eROSITA is shown as a yellow horizontal dashed line on each plot. A selection
of radii are shown as brown vertical dash-dotted lines.

Although dividing the surface brightness profile into eighths provides the
most in-depth view of the Virgo Cluster, previous pointings covered strips
radially outwards which lie at the intersections of the sectors used here.
Urban et al. (2011) studied the cluster in a strip of pointings directly north
$4.5^{\circ}$ from M87 with XMM-Newton , while Simionescu et al. (2017) used a
mosaic of Suzaku points that extend north, south, east, and west. To compare
to previous work, the surface brightness profile was divided into quarters
that encompass the prior north, south, east, and west mosaics. The eROSITA
surface brightness profiles can be seen in Figure 9. The north, east, south,
and west sectors are calculated by combining NNW and NNE, ENE and ESE, SSE and
SSW, and WSW and WNW respectively.

At the location of the discontinuity found with eROSITA in the NNE and NNW
subsectors, XMM-Newton , Suzaku, Chandra, and eROSITA all record the same
feature in the north quarter of the azimuthal surface brightness profile. The
level of brightness excess prior to the discontinuity differs slightly, but
the location of the discontinuity, at $19.1$ arcminutes or 90 kpc, is
identical across instruments. Although spectral analysis is outside the scope
of this work, Simionescu et al. (2010) used spectral analysis to derive the
temperature, density, and pressure profiles for this region, finding that the
discontinuity is evidence of a cold front rather than a shock. The signature
of cold fronts is a discontinuous drop in temperature paired with a jump in
density, while shocks show an increase in temperature, density, and pressure
(Zinger et al., 2018). Werner et al. (2016) analyzed the cold front with
Chandra and found that it is sharper to the north than the west, which is in
agreement with our surface brightness findings.

Simionescu et al. (2010) also showed that the cold front is not centered
spherically around M87, a conclusion supported by using the GGM and flattened
images and experimentation with the eROSITA surface brightness of different
regions around the location of the cold front to best determine its location.
The cold front location is marked by a series of white arrows in Figure 4. The
front is closer to M87 in the west than the east, matching the spiral
morphology of gas sloshing in simulations (Markevitch & Vikhlinin, 2007). A
single $\beta$-model with a discontinuity can be fit to the eROSITA data in
order to quantify the cold front. This model is found by scaling a standard
single $\beta$-model by a sigmoid function. This takes the form

$\displaystyle\left(1-\frac{a}{1.0+\exp(b(R-R_{0}))}\right)\times
S_{\mathrm{X}}(0)\left(1+\frac{R^{2}}{r_{\mathrm{c}}^{2}}\right)^{-3\beta+\frac{1}{2}},$
(4)

where $a$ is the relative decrease of the surface brightness at the
discontinuity, $b$ is the steepness of the jump, and $R_{0}$ is the location
of the discontinuity. The values for the best fit can be found in Table 1 and
the northern data plotted with this model can be found in Appendix C. We
measure a surface brightness decrease at the discontinuity of $41\%$.

| Total Sph. | Total Ell. | North | South
---|---|---|---|---
$r_{\mathrm{c}}$ | 47 $\pm$ 5 | 94 | - | -
$\beta$ | 0.43 $\pm$ 0.02 | 0.42 | - | -
$\theta$ | - | 1.035 | - | -
$\epsilon$ | - | 0.07 | - | -
$a$ | - | - | 0.41 $\pm$ 0.01 | 0.24 $\pm$ 0.02
$b$ | - | - | 1.29 $\pm$ 0.14 | 0.68 $\pm$ 0.22
$R_{0}$ | - | - | 19.5 $\pm$ 0.2 | 47.8 $\pm$ 0.5
Table 1: The best-fit parameters to various eROSITA surface brightness
profiles. The total is fit to both spherical and elliptical $\beta$-models,
while the “north” and “south” profiles are across the cold fronts and fit to a
$\beta$-model with a discontinuity. The core radius $r_{\mathrm{c}}$ is in
units of arcseconds and location of the discontinuity $R_{0}$ is in units of
arcminutes. The surface brightness profile and best fit can be found in
Appendix C.

To the south, the feature between $20-60$ arcminutes, $\sim$220 kpc, looks
similar to the cold front feature in the north and is present in both eROSITA
and Suzaku data. Simionescu et al. (2017) performed spectral analysis of two
discontinuities, one at 233 kpc in the western arm and one at 280 kpc to the
south, and found that their temperature profiles are also indicative of a cold
front. These two discontinuities are along the single discontinuity that we
find, which is not spherical with respect to M87 but is closer to it in the
west than the south. Future work should confirm that the discontinuity is
consistent with a cold front along its entire extent.

We fit Equation 4 to the southern profile and found a less steep and less
drastic decrease than the northern cold front. Best-fit values can be seen in
Table 1 with the profile shown in Appendix C. The off-set cold fronts in both
the northern and southern directions could together be evidence of large-scale
gas sloshing in the Virgo Cluster, a phenomenon caused by the infall of
subclusters to the cluster core that displace the cool gas (Markevitch &
Vikhlinin, 2007). Similar evidence for this type of gas sloshing in the Virgo
Cluster was found in Simionescu, A. et al. (2007); Simionescu et al. (2010);
Gatuzz et al. (2022).

####  3.2.3 Median surface brightness profile

Based on Zhuravleva et al. (2013), which showed that the median surface
brightness in simulated galaxy clusters is robust against gas inhomogeneities,
Eckert et al. (2015) proposed a method for recovering unbiased density
profiles using the azimuthal median of the surface brightness in radial
annuli. They applied the method to 31 clusters observed with ROSAT/PSPC. In
this work, we applied the method, which involves making use of a 2D binning
algorithm based on Voronoi tesselation (Cappellari & Copin, 2003), to eROSITA
data. The VorBin python package was used (see Section 3.3 of Eckert et al.
(2015) for further detail on methods). We computed the median surface
brightness profile from the binned image and compared it with the surface
brightness profile obtained by averaging. The comparison of the two methods
can be seen in Figure 20. From these profiles, we computed the emissivity
bias,

$b_{X}=\frac{S_{\rm X,mean}}{S_{\rm X,median}},$ (5)

a measure used in Eckert et al. (2015) as a projected 2D surface brightness
distribution proxy for the true 3D clumping factor of the gas. The emissivity
bias obtained using the median and mean surface brightness profiles is plotted
as a function of radius in Figure 10, out to 1$\sigma$ detection significance.
Our results match the trend found in Eckert et al. (2015); the emissivity bias
is low ($\sim 1.1$) in the inner regions of the cluster, and increases with
radius in the outskirts ($\sim 1.2$). Mirakhor & Walker (2021) studied the
clumping in Virgo with XMM-Newton in the northern direction, and also found
mild clumping with a slight increasing trend. They report that their point
source detection algorithm was likely able to exclude a significant fraction
of clumps; this could well be the case in this work. As in that study, we also
find that the exclusion of extended sources (primarily M86, M49, and M58)
decreases the clumping factor significantly. After the exclusion of the
emission around these three Virgo member galaxies, the measurement is
consistent with the findings of Mirakhor & Walker (2021). When the emissivity
bias is calculated in the $1.0-2.3$ keV band, the same trend exists, with the
bias increasing to ($\sim 1.2$) past $r_{200}$, implying that the trend is due
to true cluster emission rather than foreground contribution from the eROSITA
bubble.

Overall, the effect of clumping in the Virgo Cluster appears to be mild.
Moreover, with the full and, at the same time, detailed eROSITA view, we are
not only able to measure the bias out to $\sim$$r_{200}$, but are also able to
trace back the causes for stronger bias to individual sources. That is, the
largest values of the emissivity bias occur at the locations of prominent
galaxies and groups within Virgo, such as M86 at $\sim 100^{\prime}$, M58 just
beyond it, and M49 at $\sim 300^{\prime}$.

Figure 10: Emissivity bias $b_{X}$ as a function of distance from cluster
center. The top panel is the result after removing point sources and
background clusters. The bottom panel is after additional removal of the
extended gas halos around M86, M58, and M49 to show the upward trend still
exists after removal of prominent sources.

##  4 Deprojected gas density and mass

Using PyProffit, deprojection was performed in order to extract gas density
and gas mass estimates. We followed Eckert et al. (2020), which gives details
of the application of the deprojection to eROSITA simulations. To calculate
the conversion factor between count rate and emissivity, we assume an
abundance of $0.2\,\mathrm{Z}_{\odot}$222Simionescu et al. (2017) used the
abundance table of Lodders (2003), but this difference played a negligible
role in determining conversion factor.and a temperature of $2.3\,$keV based on
Simionescu et al. (2017), and the median $N_{\mathrm{H}}$ across the analyzed
area from Section 2. The conversion factor from count rate to emission measure
is highly insensitive to temperature and abundance in this case. The
multiscale decomposition deprojection was then performed. This allowed for the
extraction of the gas density profile, and subsequent calculation of the gas
mass profile. Note that the deprojection process requires the assumption of
spherical symmetry, which we have shown in the rest of this work is a poor
assumption in the case of Virgo.

We applied these steps to the mean surface brightness profile. The gas density
profile is plotted in Figure 11, and the gas mass in Figure 12.

The hydrogen number density at 0.5 arcminutes is estimated to be $0.06\pm
0.01$ cm-3 and the density at $r_{200}$ = 1.08 Mpc is $(4.37\pm 0.51)\times
10^{-5}$ cm-3. The density profile approximately follows a power-law model
$n_{H}\propto r^{-k}$ where $k=1.25\pm 0.05$ between 1.5 and 100 arcminutes.
The slope is flatter inside 1.5 arcminutes. The overall shape of the density
profile matches typical profiles measured by ROSAT in that most clusters have
a steepening profile beyond $\sim r_{500}$ (Eckert, D. et al., 2012).

The deprojected mean profile yields gas masses
$M_{\mathrm{gas},r<r_{500}}=(1.05\pm 0.32)\times 10^{13}$ M⊙ and
$M_{\mathrm{gas},r<r_{200}}=(1.98\pm 0.60)\times 10^{13}$ M⊙. The gas mass
profile has a best-fit power-law model with slope $k=-1.75\pm 0.04$. Scaling
relations from Arnaud et al. (2005) predict a total gravitating mass of
$M_{200}=1.4\times 10^{14}$ M⊙ for a cluster temperature of $2.3$ keV, while
Simionescu et al. (2017) calculates an estimated virial mass of $(1.05\pm
0.02)\times 10^{14}$ M⊙. This places our $M_{\mathrm{gas},r<r_{200}}$ gas mass
estimate at approximately $14-19\%$ of the predicted total mass. Typical
estimates of gas fraction in clusters are on the same order as the cosmic
baryon fraction, $\sim 15\%$ (Aghanim et al., 2020), though Simionescu et al.
(2017) found increasing gas fraction with radius in Virgo, greater than $20\%$
for some radial directions beyond $r_{500}$. When the deprojection is
performed only for portions of the cluster that are not affected by the
eROSITA bubble, i.e. northern and western segments, estimated masses are $\sim
10\%$ lower, still within reported uncertainties of the mean profile. Our
findings with regard to the emissivity bias suggest that mild clumping in the
outskirts of the cluster could lead to similarly mild mass overestimates.

Figure 11: Deprojected gas density profile of the cluster out to 300
arcminutes.  Figure 12: Deprojected gas mass profile of the cluster out to 300
arcminutes.

##  5 M49 region

A bright extended region, which is, to our knowledge, first studied in X-ray
here with eROSITA, is located to the southwest of the M49 group, bent at a
$50^{\circ}$ angle below the R.A. axis. It spans a projected length of 320 kpc
($1.1^{\circ}$), is 140 kpc ($0.5^{\circ}$) wide, and is approximately 1.3 Mpc
from the center of the Virgo Cluster, outside $r_{200}$. A surface brightness
profile across the extension shows the morphology as singly peaked (in
contrast to double-peaked stripping tails, see e.g. Ge et al. 2023). To
determine the significance of this enhancement, the surface brightness and
propagated error was extracted from a number of $20^{\prime}\times
20^{\prime}$ square regions along the extension (so-called source boxes) and
just outside of it (so-called background boxes). A mean value for the source
boxes and for the background boxes was calculated. The error was found with

$\textrm{Error}=\sqrt{\sigma_{\mathrm{src}}^{2}+\sigma_{\mathrm{bkg}}^{2}},$
(6)

where $\sigma_{\mathrm{src}}$ is the standard error of the mean surface
brightness of the source boxes and $\sigma_{\mathrm{bkg}}$ is the standard
error of the mean surface brightness of the background boxes. The relative
difference between the extension and background is found to be 41.75%, or a
significance of 3.15$\sigma$.

We searched the projected area around this feature for clues as to its origin.
It is coincident with the so-called W’-cloud, made up of elliptical galaxy NGC
4365 and several member galaxies, which has been previously studied in the
optical and infrared (de Vaucouleurs, 1961; Binggeli et al., 1987; Mei et al.,
2007). This group is located at $\sim$ 23 Mpc (Mei et al., 2007) and is
proposed to be falling into Virgo from behind. NGC 4365 and fellow group
member NGC 4342 are coincident and thus likely responsible for the peaks in
X-ray emission within the extension. The extended X-ray emission could be the
W’ intra-group medium, extended due to its fall into the cluster. Nearby, to
the northeast of the extension, the M49 group is likely falling into the
cluster from the south (Su et al., 2019). Su et al. (2019) finds an M49
stripped tail of smaller dimensions (70 kpc long by 10 kpc wide), which they
argue is evidence of the brightest group galaxy (BGG) moving relative to the
M49 group gas. This smaller tail associated with the BGG is also visible with
eROSITA; the longer extension matches the morphology and direction of the
smaller one. The longer extension could be a stripped tail of intra-group gas,
as opposed to the smaller stripped tail of intergalactic gas.

A diffuse feature with a similar extent, location, morphology, and orientation
as the X-ray feature was also recently identified in the radio band with Low-
Frequency Array High-band Antenna (LOFAR HBA) as part of ViCTORIA (Virgo
Cluster multiTelescope Observations in Radio of Interacting galaxies and AGN)
project (Edler et al., 2023). The radio extension has a length of $\sim
1^{\circ}$, about 280 kpc, and shares an orientation with the radio tails
around M49, although no clear connection can be seen between structures.
Figure 13 shows a cut-out of the region in radio, and in X-ray with radio
contours overlaid, showing the similar structures of X-ray and radio emission.

Figure 13: An image displaying the region to the southwest of M49 in radio
with LOFAR (left) and in X-ray with eROSITA (right). Contours of the radio
image have been overlaid on the X-ray image for easier comparison, and images
are displayed with the same frame. The white line in the upper left is 100 kpc
in length.

Based on its morphology and shared location, we suggest that the X-ray
extension could be causally connected to the radio. The feature could be
related to the in-falling of diffuse gas associated with the W’ cloud group or
stripped gas of the M49 group and its interaction with the ICM. The radio
feature could possibly originate in an accretion shock or due to turbulence in
the hot plasma (van Weeren et al., 2019). A follow-up on its X-ray and radio
spectral properties is required to gain a better understanding of this
feature.

##  6 Conclusion

In this work, data collected during the four complete all-sky surveys and
partial fifth survey of eROSITA was used to study the Virgo Cluster in an area
spanning approximately 25∘ by 25∘. Data reduction and preparation included
flare removal and corrections for PIB, variation of $N_{HI}$ across the sky
area, and exposure differences between TM 8 and TM 9.

Different image manipulation techniques allowed for the exploration of larger
scale structure. The RGB image revealed extended white-green emission around
prominent galaxies and groups, as well as a gradient within the northern
eROSITA bubble, which is mainly projected onto Virgo Cluster emission in the
red and green bands (0.3-1.0 keV). The bubble fades from blue-green to green
($0.6-1.0$ keV) to red ($0.3-0.6$ keV) as the distance grows from Galactic
center. Wavelet filtering and adaptive smoothing revealed faint ICM emission
in several directions out to $4-5^{\circ}$ from M87 with a larger radius to
the north and south, and a notable drop-off in emission to the west prior to
the virial radius. The northern region of the Virgo Cluster contains a number
of ICM extensions stretching away from the cluster, while a spiral-like
extension can be seen to the south.

Surface brightness profiles were created for the full azimuth, eighths and
fourths of the overall cluster in the $0.3-2.0\,$keV and $1.0-2.3\,$keV bands.
The latter band choice arose from the apparent lack of contribution to the
Virgo Cluster emission by the northern eROSITA bubble in the blue emission of
the RGB image. To the north, in the NNW and NNE subsectors, a prominent
discontinuity at $\sim 19$ arcminutes marks a $41\%$ decrease in surface
brightness. This has been identified as a cold front in other studies that
used spectral analysis. To the south, most prominent in the SSW subsector and
less sharp in SSE and WSW, another discontinuity in the profile is present at
$\sim 48$ arcminutes, though with just a $24\%$ decrease. This was also
previously identified as a cold front at locations along it to the south and
west; however, this is the first time the full extent of the front can be
seen. The off-set cold fronts to the north and south of the cluster support a
picture of large-scale gas sloshing. The $0.3-2.0$ keV surface brightness
profile to the east is dominated by the eROSITA bubble past $r_{200}$, while
the $1.0-2.3$ keV profile shows only a minor contribution from the bubble in
this region. The surface brightness profiles to the west match the visual
evidence of the images, displaying few notable features except for an excess
at the location of the ICM surrounding M86. All reported features (with the
exception of the eROSITA bubble) are present and similarly significant in both
the $0.3-2.0\,$keV and $1.0-2.3\,$keV bands.

We make use of Voronoi tessellation to bin the data before extracting a median
surface brightness profile. One would expect the median surface brightness
profile to be robust against gas clumping down to the scale of the typical
Voronoi cell size, unlike the mean surface brightness profile. The comparison
of the two can be quantified by the emissivity bias $b_{X}$, which will be
larger in the case of gas clumping. We find a low emissivity bias, about 1.1,
in the inner regions, growing to about 1.2 in the outskirts of the Virgo
Cluster. Peaks in the emissivity bias can be traced back to M86, M58, and M49,
with an increasing trend still apparent following their removal. The effect of
gas clumping from this measure appears to be mild. This detailed measurement
is the first of its kind for the full azimuth of Virgo, and offers a high
resolution view of clumping that would be difficult to achieve in more distant
clusters.

A deprojection analysis was performed in order to determine gas density and
gas mass profiles. The result was a virial gas mass of
$M_{\mathrm{gas},r<r_{200}}=(1.98\pm 0.70)\times 10^{13}$ M⊙, which is on the
order of 14-19% of total mass estimates for Virgo, largely consistent with the
expected gas mass fraction.

An X-ray extension south of M49, outside the virial radius of Virgo, has a
length of $\sim 320$ kpc. The surface brightness of the extension has a
relative difference to the background of $41.75\%$ and a lower-bound
significance of $3.15\sigma$. These values were determined using extracted
surface brightness values along and just outside of the extension. The X-ray
emission is coincident with radio emission observed by LOFAR. The aligned
morphology to the M49 BGG stripped tail, shared location with the W’ cloud
galaxies behind Virgo, and the coincident radio emission could together be
indication of an accretion process such as a shock or turbulence.

###### Acknowledgements.

This work is based on data from eROSITA, the soft X-ray instrument aboard SRG,
a joint Russian-German science mission supported by the Russian Space Agency
(Roskosmos), in the interests of the Russian Academy of Sciences represented
by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und
Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL)
and its subcontractors, and is operated by NPOL with support from the Max
Planck Institute for Extraterrestrial Physics (MPE). The development and
construction of the eROSITA X-ray instrument was led by MPE, with
contributions from the Dr. Karl Remeis Observatory Bamberg & ECAP (FAU
Erlangen-Nuernberg), the University of Hamburg Observatory, the Leibniz
Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and
Astrophysics of the University of Tübingen, with the support of DLR and the
Max Planck Society. The Argelander Institute for Astronomy of the University
of Bonn and the Ludwig Maximilians Universität Munich also participated in the
science preparation for eROSITA. The eROSITA data shown here were processed
using the eSASS software system developed by the German eROSITA consortium. HM
thanks Dominique Eckert for his correspondence on the use of PyProffit. MB
acknowledges support from the Deutsche Forschungsgemeinschaft under Germany’s
Excellence Strategy - EXC 2121 ”Quantum Universe” - 390833306 and from the
BMBF ErUM-Pro grant 05A2023. AV acknowledges funding by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) – 450861021.

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##  Appendix A Observations

Tile | Excluded eRASS: TM
---|---
190066 | 3: 4, 5, 7; 4: 4; 5
186066 | 3: 4, 5, 7; 4: 4; 5
183066 | 3: 4, 5, 7; 4: 4; 5
190069 | 3: 4, 5, 7; 4: 4; 5
186069 | 3: 4, 5, 7; 4: 4; 5
183069 | 3: 4, 5, 7; 4: 4; 5
180069 | 3: 4, 5, 7; 4: 4; 5
196072 | 4: 4
180072 | 4: 4; 5
198075 | 4: 4
195075 | 4: 4
180075 | 3: 4, 5, 7; 4: 4; 5
177075 | 3: 4; 4: 4; 5
200078 | 4: 4
197078 | 4: 4
198081 | 4: 4
195081 | 4: 4
200084 | 4: 4
197084 | 4: 4
194084 | 4: 4
194087 | 4: 4
191087 | 4: 4
188087 | 4: 4
185087 | 4: 4
191090 | 4: 4
188090 | 4: 4
185090 | 4: 4
182087 | 3: 4, 5, 7; 4: 4; 5
179087 | 3: 4, 5, 7; 4: 4; 5
182084 | 3: 4, 5, 7; 4: 4; 5
179084 | 3: 4, 5, 7; 4: 4; 5
180081 | 3: 4, 5, 7; 4: 4; 5
177081 | 3: 4, 5, 7; 4: 4; 5
178078 | 3: 4, 5, 7; 4: 4; 5
175078 | 3: 4; 4: 4; 5
193072 | 4: 4
189072 | 4: 4
186072 | 3: 4, 5, 7; 4: 4; 5
183072 | 3: 4, 5, 7; 4: 4; 5
192075 | 4: 4
194078 | 4: 4
192081 | 4: 4
191084 | 4: 4
188084 | 4: 4
185084 | 4: 4
183081 | 3: 4, 5, 7; 4: 4; 5
191078 | 4: 4
189081 | 4: 4
189075 | 4: 4
188078 | 4: 4
186081 | 4: 4
186075 | 3: 4, 5, 7; 4: 4; 5
185078 | 3: 4, 5, 7; 4: 4; 5
183075 | 3: 4, 5, 7; 4: 4; 5
182078 | 3: 4, 5, 7; 4: 4; 5
177072 | 3: 4; 4: 4; 5
193069 | 4: 4
193066 | 4: 4
197087 | 4: 4
199072 | 4: 4
177069 | 4: 4; 5
196069 | 4: 4
Table 2: All observations of the Virgo Cluster by eROSITA used in this work.
TM 4 of eRASS 4 was excluded from every tile due to micrometeorite
impacts333This was discussed in internal communications at the eROSITA
consortium meeting in January 2022.. TMs 4, 5, and 7 were unavailable for
several tiles in eRASS 3.

##  Appendix B Data Reduction Products

TM | $R$ | $H_{\mathrm{obs}}$
---|---|---
1 | 1.05 | 203152
2 | 1.12 | 185851
3 | 1.08 | 199857
4 | 1.01 | 213538
5 | 0.69 | 252434
6 | 1.04 | 198713
7 | 0.66 | 255768
Table 3: Important TM-specific values for the PIB subtraction. $R$ is the ratio of the number of counts of the Filter-Wheel-Closed (FWC) data in the soft band to the hard band (6.7-9.0 keV) and $H_{\mathrm{obs}}$ are the hard band counts. Figure 14: $N_{HI}$ map from HI 4PI trimmed to match the area of the sky analyzed in this work. The color bar is in units of cm-2. $r_{200}$ and $3r_{200}$ are plotted. Figure 15: TM 0 exposure maps from various stages in the data reduction. Left: Following flaregti application, but before the application of any additional corrections. Right: Following flare filtering, absorption correction, and exposure correction steps. LHB Temp. (keV) | 0.099
---|---
MWH Temp. (keV) | 0.225
PL Photon Index | 1.4
LHB Norm. ($\mathrm{cm}^{-5}/\mathrm{deg}^{2}$) | 0.0019
MWH Norm ($\mathrm{cm}^{-5}/\mathrm{deg}^{2}$) | 0.0041
PL Norm ($\mathrm{photons}/\mathrm{keV}/\mathrm{cm}^{2}/\mathrm{s}/\mathrm{deg}^{2}$) | 0.0036
Table 4: The values used in the simulated spectra.

##  Appendix C Surface Brightness Analysis Products

Figure 16: 1.0-2.3 keV (blue band) wavelet filtered image with point sources
removed. $r_{200}$ and $3r_{200}$ are plotted. Figure 17: Eight surface
brightness profiles in the $1.0-2.3$ keV energy band, one for each sector
identified in Figure 7. The $0.3-2.0$ keV band for any given profile is
plotted in black for comparison. The CXB measured by eROSITA in the northwest
of the cluster is shown as a dashed black line on each plot. The blue band CXB
estimate is taken from the same regions as the full band CXB. A selection of
characteristic radii are shown as brown vertical dash-dotted lines.  Figure
18: 0.3-2.0 keV eROSITA surface brightness profile for the northern cold front
at $90$ kpc fitted to a single $\beta$-model with a discontinuity. Figure 19:
0.3-2.0 keV eROSITA surface brightness profile for the southern cold front at
$\sim\ 220$ kpc fitted to a single $\beta$-model with a discontinuity. Figure
20: Surface brightness profiles obtained by performing the azimuthal mean
(blue) and azimuthal median (red).