# Proceedings of IAU Symposium 376
Richard de Grijs, Patricia Whitelock and Marcio Catelan, eds.

The Cepheid Extragalactic Distance Scale: Past, Present and Future

Wendy L. Freedman  Dept. of Astronomy & Astrophysics, University of Chicago,
5640 S. Ellis Ave., Chicago, IL 60637, USA and
Kavli Institute for Cosmological Physics, University of Chicago, 5640 S. Ellis
Ave., Chicago, IL 60637, USA  Barry F. Madore  Carnegie Observatories, 813
Santa Barbara St., Pasadena CA 91101, USA and
Dept. of Astronomy & Astrophysics, University of Chicago, 5640 S. Ellis Ave.,
Chicago, IL 60637, USA

###### Abstract

Cepheids have been the cornerstone of the extragalactic distance scale for a
century. With high-quality data, these luminous supergiants exhibit a small
dispersion in their Leavitt (period–luminosity) relation, particularly at
longer wavelengths, and few methods rival the precision possible with Cepheid
distances. In these proceedings, we present an overview of major observational
programs pertaining to the Cepheid extragalactic distance scale, its progress
and remaining challenges. In addition, we present preliminary new results on
Cepheids from the James Webb Space Telescope (JWST). The launch of JWST has
opened a new chapter in the measurement of extragalactic distances and the
Hubble constant. JWST offers a resolution three times that of the Hubble Space
Telescope (HST) with nearly 10 times the sensitivity. It has been suggested
that the discrepancy in the value of the Hubble constant based on Cepheids
compared to that inferred from measurements of the cosmic microwave background
requires new and additional physics beyond the standard cosmological model.
JWST observations will be critical in reducing remaining systematics in the
Cepheid measurements and for confirming if new physics is indeed required.
Early JWST data for the galaxy, NGC 7250 show a decrease in scatter in the
Cepheid Leavitt law by a factor of two relative to existing HST data and
demonstrate that crowding/blending effects are a significant issue in a galaxy
as close as 20 Mpc.

Cepheids, HST, JWST, Extragalactic Distance Scale, Hubble constant

##  1 Introduction

No review of the Cepheid distance scale would be complete without recognition
of the contribution by Henrietta Swan Leavitt. Leavitt’s 1908 seminal paper
(Leavitt 1908) on variable stars in the Large (LMC) and Small (SMC) Magellanic
Clouds established the relationship between the period and luminosity of
Cepheids. Subsequently, Leavitt and Pickering (2012) published a
period–luminosity relation for 25 Cepheids in the SMC, finding a nearly linear
relationship between the logarithm of the luminosity and the logarithm of the
period. Leavitt and Pickering recognized that the Magellanic Cloud Cepheids
were likely at the same distance from Earth, “… their periods are apparently
associated with their actual emission of light, as determined by their mass,
density and surface brightness.” These critical insights laid the foundation
for the Cepheid distance scale, facilitating the measurement of distances to
astronomical objects. Leavitt passed away in 1921, and sadly did not live to
see the impact of her pioneering work. In 2008, a meeting was held at Harvard
University to commemorate the 100-year anniversary of Leavitt’s Cepheid
discovery paper. An outcome of the meeting was the recommendation to name the
Cepheid period–luminosity relation the ‘Leavitt Law’ (Freedman & Madore 2010).

Beginning in 1923, Edwin Hubble discovered Cepheids (Hubble 1925a,b, 1926)
galaxies M33, M31, and NGC 6822, and estimated the distances to these galaxies
employing Leavitt’s period–luminosity relation. These and subsequent
measurements culminated in his discovery of the relationship between galaxy
distances and their recessional velocities (Hubble 1929), and set the
foundation for modern cosmology with its implications for the expansion of the
Universe.

Building upon a body of work characterizing the nature of variable stars in
Milky Way clusters (Bailey 1902, Joy 1949, Hogg 1955) Baade (Baade 1956, Baade
& Swope 1963, 1965) unambiguously established the existence of two populations
of Cepheid variables, resulting in a recalibration of Hubble’s magnitude scale
by 1.5 mag, or a factor of two in distance.

The next major advance in Cepheid studies came with Wisniewski & Johnson’s
(1968) photoelectric $UBVRIJKL$ observations of Milky Way Cepheids. Their
high-precision photometry exhibited two major characteristics of Cepheid light
curves: (1) the decrease in amplitude moving from the ultraviolet to the near-
infrared, and (2) the shift in the relative phase of maximum light, also with
increasing wavelength.

The 1970s through 1990s saw the ‘factor-of-two’ debate over the value of H0.
In a series of ten papers entitled “Steps Toward the Hubble Constant”,
published between 1974 and 1995, Allan Sandage and Gustav Tammann (2006)
presented evidence generally favoring a value of H${}_{0}\sim 50$ km s-1
Mpc-1. In contrast, de Vaucouleurs (1993) carried out his own calibration of
the distance scale, finding values closer to 100 km s-1 Mpc-1. An early
detailed review of these programs was given by Hodge (1981) Hodge
optimistically summarized the controversy: “Eventually, I assume, when
sufficiently reliable measurements are possible … there will be some number,
such as H0, that everyone will agree on, within some reasonable and understood
limits.” It would be another 20 years before the Hubble Space Telescope (HST)
Key Project team (Freedman et al. 2001) measured a value of H0 to 10%
accuracy; a number that has stood the test of time.

The accurate measurement of Cepheid distances remains non-trivial. One of the
main challenges is distinguishing Cepheids from fainter stars, both resolved
and unresolved, that contribute to the overall background of the host galaxy.
Avoiding/minimizing the effects of crowding and confusion is crucial for the
successful identification and measurement of Cepheids in galaxies. A second
challenge is the correction for the presence of dust, which both scatters and
absorbs radiation. Cepheids are relatively young stars that have not had time
to diffuse far from the locations where they were formed; hence they are found
in the dusty disks of spiral (or irregular) galaxies. Third, the luminosity of
a Cepheid may have a dependence on the abundance or metallicity of the star.

##  2 Advantages of the Near-Infrared

The recognition of the applicability of near-infrared observations of Galactic
Cepheids to the distance scale did not occur immediately, despite the early
near-infrared observations made by Wisniewski & Johnson (1968) The first
application was by McGonegal et al. (1982) (see Figure 1, left), who
elucidated the advantages of the near-infrared, most specifically the
decreased sensitivity to reddening and the decrease in the observed width of
the Leavitt law, even with only single-epoch observations. The decrease in the
width of the period–luminosity relation is primarily due to the decrease in
sensitivity of the infrared surface brightness to the intrinsic temperature
width of the instability strip. Near-infrared observations allowed for
accurate distance determinations even with single, random-phase observations
of Cepheids whose periods had already been measured. These single-phase
measurements were comparable in accuracy to complete time-averaged magnitudes
derived from many nights of observations obtained at blue wavelengths.

Figure 1:  Left panel: Leavitt laws at JHK wavelengths (McGonegal et al. 1983)
demonstrating the power of infrared photometry for the extragalactic distance
scale. Right panel: Leavitt laws at BVRI wavelengths based on CCD photometry
(Freedman 1988) (open circles); filled circles illustrate photoelectric
photometry for LMC Cepheids.

##  3 The Era of CCDs and Infrared Arrays

With a wide wavelength sensitivity range, spanning from the $B$ band (0.45
$\mu$m) to the $I$ band (0.8 $\mu$m), CCD detectors allowed, for the first
time, the possibility of high-resolution, panoramic, digital data for the
extragalactic distance scale (Freedman 1984). With CCDs one could perform
local sky subtraction and point-spread-function fitting to obtain accurate
magnitudes and colors. In addition, CCDs enabled the handling of crowding and
confusion errors at the one-square-arcsecond level. Furthermore, the large
wavelength coverage of CCDs allowed the explicit and direct determination of
reddening, by applying the interstellar extinction law as a function of
wavelength, and eliminating the need to rely solely on foreground estimates
and/or assuming negligible additional reddening (in the host galaxy), as many
earlier studies had done. The approach, introduced by Freedman (1988) for
determining total line-of-sight extinctions and true distance moduli of
extragalactic Cepheids’ multiwavelength data (e.g., see Figure 1, right)
applied to single-epoch observations of Cepheids in IC 1613, M31, M33, and NGC
300, among others. In brief, the method involves determining differential
apparent moduli by scaling them with respect to the corresponding LMC
period–luminosity relations and assuming that the (observed) variation with
inverse wavelength can be attributed to selective absorption.

###  3.1 Metallicity Effects

In addition to reddening, nailing down the metallicity dependence of Cepheids
is also critical for application of the Leavitt law. Metallicity can affect
the internal structure of a Cepheid: a higher metallicity results in a higher
opacity, which can affect the temperature, radius, and luminosity of the star.
Line blanketing, controlled by the abundance of metals in the atmospheres of
Cepheids, can also result in a redistribution of light as a function of
wavelength. The atmospheric effects are predicted to be greater at optical
wavelengths than in the infrared.

The availability of CCDs also allowed the first empirical test of metallicity
on the Cepheid period–luminosity relation. Freedman & Madore (1990) obtained
$BVRI$ CCD observations of Cepheids in M31 and estimated the mean reddenings
and true distance moduli at three different radial positions (3, 10, and 20
kpc) in the disk of M31, for which a metallicity gradient of a factor of five
had been measured from Hii regions. They found evidence for a mild gradient of
$-0.2\pm 0.2$ mag dex-1. However, given the uncertainties, the value was also
consistent with no measurable effect.

Twenty-five years later, there still remains uncertainty in the metallicity
coefficient Ripepi et al. (2020) A number of recent studies have found a range
in the metallicity coefficient from $-0.5$ to 0 mag dex-1 (Udalski et al.
2001, Riess et al. 2016, Riess et al. 2021, Breuval et al. 2021, Ripepi et al.
2021, Ripepi et al. 2022). Generally, the effects of metallicity are found to
be larger in the optical than in the near-infrared, although in contrast, the
recent study by Breuval et al. (2022) finds nearly as large an effect in the
near-infrared as in the optical.

##  4 The Hubble Space Telescope Key Project

The goal of the Hubble Key Project (Freedman et al. 2001) was to extend the
Cepheid extragalactic distance scale to greater distances than possible from
the ground, tie in to several independent methods for measuring distances
beyond the Cepheids, and to ultimately measure the Hubble constant to an
overall accuracy of 10%. To achieve these goals, the Key Project team used the
HST to observe Cepheids in 18 galaxies spanning a range of distances. The
observations included both wide-field and deep imaging in the F555W and F814W
filters, with F555W ($V$ band) allowing for the discovery, characterization,
and measurement of the periods, amplitudes, and mean magnitudes of the
Cepheids; and using the longer-wavelength F814W ($I$ band) to form a color,
thereby providing the means to correct for the effects of extinction and
absorption by dust.

These measurements then enabled the calibration of secondary distance
indicators (including Type Ia and II supernovae, the Tully–Fisher relation,
surface brightness fluctuations, and the $D_{n}$–$\sigma$ relation; see Figure
2. The final value of the Hubble constant was measured as H${}_{0}=72\pm 3$
(statistical) $\pm 7$ (systematic) km s-1 Mpc-1, with an estimated uncertainty
of about 10%. This result provided early evidence for a Universe that is
accelerating, consistent with the presence of dark energy, and consistent with
the ages of globular clusters. And it resolved the longstanding factor-of-two
discrepancy in the distance scale.

Figure 2: Hubble diagram showing the final results from the HST Key Project
(Freedman et al. 2001) In the top panel velocity is plotted versus distance
for the five secondary distance methods labeled in the plot. The solid line is
for a slope of 72, flanked by dotted lines indicating a 10% uncertainty. The
bottom panel shows the residuals for a value of H${}_{0}=72\pm 3$ (stat) $\pm
7$ (sys) km s-1 Mpc-1.

##  5 NICMOS and Ground-Based Near-Infrared Observations of Cepheids

###  5.1 Early NICMOS Observations

Following on the HST project, near-infrared observations of 14 nearby galaxies
were obtained with HST/NICMOS (Macri et al. 2001). Distance moduli were
obtained by combining single-epoch $H$-band data with the $VI$ bands. These
data provided support for the assumption of the universality of the standard
(Milky Way) reddening law adopted by the Key Project (see Figure 3), by
demonstrating the excellent correspondence between $E(V-H)$ and $E(V-I)$.

An additional goal of the NICMOS program was to undertake another test for the
metallicity effect by observing two fields in the relatively face-on galaxy
M101, a test that had earlier been undertaken by the Key Project team. The
inner and outer M101 fields were found to have a large difference in distance
modulus (amounting to $+0.41\pm 0.11$ mag at $H$ and $+0.34\pm 0.09$ mag at
$J$, respectively). If interpreted in terms of a metallicity effect, this
implies a dependence of order 0.6 mag dex-1, much higher than theory predicts
for the near-infrared. Simulations, as well as artificial star tests showed
that crowding/blending effects were likely an issue for these data, explaining
some (but not all) of this large difference.

These results provided a cautionary note regarding the difficulty of measuring
Cepheids in the near-infrared in all but the nearest galaxies, and the need to
avoid their inner (high-surface-brightness) regions. While the effects of dust
are reduced at increasing wavelengths, the background contamination by red
stars (red giants and even brighter asymptotic giants) increases
substantially. Thus, the advantages of moving to the near-infrared to mitigate
extinction effects were countered by the systematic disadvantages imposed by
crowding and blending.

Figure 3: A comparison of reddening values measured via $E(V-H)$ with those
from $E(V-I)$ from Macri et al. (2001). The good agreement provides evidence
that a standard extinction law is a reasonable approximation to correct for
the dust in these galaxies.

###  5.2 Ground-Based Near-Infrared Observations of the LMC

From the ground, an extensive and uniform near-infrared ($JHK$) study of 92
Cepheids in the LMC was undertaken by Persson et al. (2004), with $JHK$
sampling obtained at about 22 phase points for each star. The dispersion about
the Leavitt law was found to be on the order of $\pm$0.1 mag for all three
wavebands (see Figure 4). Fitting for a tilt to the LMC, the dispersions in
the near-infrared period–luminosity–color relations drop to $\sim$0.08 mag,
showing again the immense power of the infrared in cases where
crowding/blending effects were intentionally minimized and where the signal-
to-noise is high. The high quality of these data is apparent when cross-
comparing the positions of the individual Cepheids in these period–luminosity
relations. The deviations from the mean period–luminosity relations are highly
correlated across the various bandpasses, demonstrating intrinsic properties
of the Cepheids and/or location within the LMC, that are not blurred out by
photometric errors.

Figure 4: Leavitt laws for near-infrared ($JHK$) and mid-infrared (3.6 and 4.5
$\mu$m) for Cepheids in the LMC.

##  6 The Spitzer Space Telescope: Mid-infrared Observations of Cepheids

Using the IRAC camera on the Spitzer Space Telescope during its extended, warm
mission, well-sampled mid-infrared light curves were obtained for 80 long-
period LMC Cepheids (Scowcroft et al. 2011) falling in the period range
$0.8<\log(P)<1.8$ [days]. These observations provided a well-measured slope of
the period–luminosity relation based on the time-averaged 3.6 $\mu$m data.
Time-averaged 3.6 $\mu$m data were also obtained for 10 high-metallicity Milky
Way Cepheids with independently measured HST trigonometric parallaxes (Monson
et al. 2012). The 3.6 and 4.5 $\mu$m Leavitt laws for the LMC can be seen in
Figure 4. The 3.6 $\mu$m data resulted in a high-precision, reddening-
corrected distance to the LMC of $18.477\pm 0.033$ (systematic) mag. Freedman
et al. (2012) applied the Spitzer calibration to the Key Project galaxy
sample, yielding a value of H${}_{0}=74.3$ km s-1 Mpc-1, with a systematic
uncertainty of $\pm$ 2.1 (systematic) km s-1 Mpc-1. The Spitzer program
reduced the systematic uncertainty in H0 over that obtained by the Hubble
Space Telescope Key Project by more than a factor of three.

##  7 The Araucaria Project

For 20 years, the Araucaria Project has been carrying out a program to observe
a number of types of variable stars in nearby galaxies at both optical and
near-infrared wavelengths (Pietrzynski et al. 2019) They have monitored
Cepheid variables, RR Lyrae stars, and eclipsing binaries, as well as blue
supergiants, tip of the red-giant branch (TRGB) stars, and carbon stars. The
Araucaria Project team has measured Cepheid distances to galaxies in the Local
Group (LMC, SMC, IC 1613, NGC 6822, M33), the Sculptor Group (NGC 247, NGC
300, NGC 7793), providing fundamental distances and tests of Cepheid
metallicities (Udalski et al. 2001).

##  8 The SH0ES Program

The SH0ES (Supernovae, H0, for the Equation of State) project, initially aimed
at the study of dark energy, now has the objective of determining H0 through
the calibration of the extragalactic distance scale using Cepheid variables
(Riess et al. 2009, 2012, 2016, 2022) Riess and colleagues report HST F555W,
F814W, and F160W observations of Cepheid variables in host galaxies of Type Ia
supernovae (SNe Ia) used to calibrate H0. Reddening corrections are obtained
using a small number of (2 to 4) random-phase observations in the F814W and
F555W bands, with the distances coming mainly from $\sim$11 observations taken
in the F160W ($H$) band. The scatter in the F160W period–luminosity relations
is generally of order 0.4–0.5 mag, i.e., a factor of four or so greater than
the intrinsic dispersion observed in the uncrowded sample of Cepheids in the
LMC. The zero-point calibration is set by geometric Gaia early Data Release 3
(EDR3) parallaxes, masers in the galaxy NGC 4258, and detached eclipsing
binaries in the LMC. The most recent SH0ES paper quotes a 1% uncertainty with
H${}_{0}=73.04\pm 1.04$ km s-1 Mpc-1, based on a sample of 42 galaxies with
distances in the range from 7 out to 80 Mpc.

##  9 An Independent External Check of Cepheid Distances: Tip of the Red Giant
Branch

Few methods for measuring nearby distances rival the Cepheids in precision,
accuracy, or numbers of galaxies for which the measurement can be made. A
notable exception is the TRGB method (Lee et al. 1993, Mould & Sakai 2009,
Hoyt 2023). The TRGB method also has a number of advantages relative to the
Cepheid Leavitt law (Freedman et al 2019, Freedman 2021). It can be applied in
the halos of galaxies where the effects of dust are generally negligible and
where the surface brightness is about 100 times lower than in the disks, so
that crowding/blending effects are not severe. It is a much easier practical
issue to measure the luminosity of a halo TRGB star than it is to disentangle
Cepheids from nearby neighbors in the disk. In addition, metallicity effects
are much smaller for the TRGB (and can be directly calibrated), unlike in the
case of Cepheids. Thus, beyond providing an independent calibration of H0
(Freedman et al. 2019), the TRGB provides an excellent external constraint on
the combined systematics of both methods.

In Figure 5 we show a comparison of distance moduli for Cepheids and the TRGB.
The agreement between the two distance scales is excellent, with
$\Delta\mu$(Cepheid $-$ TRGB) $=-0.026\pm 0.014$ mag. In this case, the zero
point for the TRGB calibration is $M_{I}=-4.05$ mag. The scatter (expectedly)
increases with increasing distance and photometric errors.

Figure 5: Comparison of published Cepheid and TRGB distances in the sense of
(Cepheid $-$ TRGB). The blue/red solid lines show the mean offset for galaxies
with distance moduli closer/farther than 30.5 mag. The dashed lines indicate
the 1$\sigma$ scatter for the near and far samples, which are also labeled in
the plot. The agreement between the Cepheid and TRGB distances is excellent,
particularly for the nearby sample.

In Figure 6 we show an update to a recent comparison of the values of H0
obtained from Cepheids (blue) and the TRGB (red); both tied into measurements
of SNe Ia (Freedman 2021) and that inferred from observations of the cosmic
microwave background (CMB; Planck Collaboration et al. 2020, black). The
Cepheid and TRGB H0 values agree to within 2$\sigma$ of their quoted
uncertainties; the TRGB measurements are in better agreement with the CMB
value to within 1$\sigma$.

Figure 6: Measurements of H0 from the HST Key Project through to today based
on Cepheids (blue), the TRGB (red), and the CMB (Planck: yellow; ACT + WMAP:
turquoise).

##  10 Remaining Issues for the Cepheid Distance Scale

As previously noted, several challenges are still outstanding in measuring
Cepheid distances, resulting from the fact that Cepheids are located in
regions of star formation within the disks of galaxies, hence in dusty and
crowded regions, particularly in their inner disks. The amount (and even the
sign) and the wavelength dependence of the metallicity contributions to
Cepheid luminosities, within and between galaxies, has yet to converge (Ripepi
et al. 2020, Breuval et al. 2021, da Silva et al. 2022, Owens et al. 2023).

Both the HST Key Project and the SH0ES group approach the issue of
crowding/blending of Cepheids with the use of artificial star tests. Figure 7
(courtesy of I. Jang) shows the F160W crowding-induced bias found for Cepheids
observed in nine of the closest SNe Ia host galaxies (Riess et al. 2011). As
can be seen, the bias can be quite large, reaching over 1 mag, with a median
correction of $\sim$0.25 mag, which corresponds to a 10% difference in H0 of
$>7$ km s-1 Mpc-1 (as shown to the right of the plot). With the (unexplained)
scatter in the F160W period–luminosity relation being as large as 0.4–0.5 mag,
accurate crowding corrections are critical. And within that scatter, there
remains the potential for a hidden systematic effect that may be difficult to
identify and account for.

Figure 7: A measure of the bias in Cepheid photometry from Table 2 as
published by Riess et al. (2011). The bias is determined from artificial star
tests for each Cepheid’s environment and is a statistical correction. The bias
can be quite significant and ranges from 0 to 1.5 mag (a factor of up to four
in luminosity). The median bias is 0.24 mag, corresponding to
$\Delta$H${}_{0}=7$ km s-1 Mpc-1. The galaxies in this earlier study are
amongst the closest in the more recent sample of 42 galaxies in the Riess et
al. (2022).

It is important to note that on average, for similar types of galaxies,
crowding effects will become more severe with increasing distance. Sixty
percent of the Riess et al. (2022) sample of galaxies in which Cepheids have
been discovered lie at greater distances than 20 Mpc, and 25% of the total
sample lies beyond 40 Mpc. At a distance of 40 Mpc, four times the area will
be contained within a given pixel. For the most distant SH0ES galaxy at 80
Mpc, 16 times the area will be covered. As the need for percent-level accuracy
has grown in importance, and given the level of crowding for Cepheids in a
galaxy at a distance of only 20 Mpc (see the JWST observations in the
following section), it remains important to demonstrate that crowding effects
do not produce a systematic bias in the photometry and, hence, the distance
measurements for these more distant galaxies observed with HST.

##  11 The Future: Cepheids and JWST

While measurements of the Cepheid distance scale have improved since the time
of the Key Project, great strides have been made in measurements of
fluctuations in the temperature and polarization of the microwave background
(Planck Collaboration 2018). The current controversy over the value of H0 now
requires pushing the determination of local values to limits approaching 1%
accuracy, in order to be of comparable accuracy to the CMB measurements. As we
have seen, systematic errors have played, and continue to play, a dominant
role, keeping more accurate measurement of H0 a challenge.

Much like the 1970s and 1980s, when local distance measurement differences
remained at an impasse, it is clear once again that improvements in technology
will be required to settle the current differences, at the few-percent level,
for the Cepheid extragalactic distance scale. We describe here a new program
using the James Webb Space Telescope (JWST), aimed at addressing the current
systematics in the HST measurements of Cepheid distances. The program is
actually three-pronged and includes also the measurement of distances to
nearby galaxies using not only Cepheids, but also TRGB stars and carbon/JAGB
stars. In this review, however, we describe only the Cepheid portion of this
program.

Because of the greater sensitivity of the surface brightness to temperature at
bluer (optical) wavelengths, the amplitudes of Cepheid variables are larger
than at longer wavelengths. Thus, its blue sensitivity and high spatial
resolution (compared to the ground) made HST an ideal facility for the
discovery of Cepheid variables (Freedman et al. 2001, Riess et al. 2016).

Two key features now make JWST the optimal telescope for addressing the
accuracy of measurements of H0: its red sensitivity and higher spatial
resolution (compared to HST, and certainly far exceeding the spatial
resolution and sensitivity of Spitzer). The extinction is significantly lower:
for example, $A_{J}$ is smaller by a factor of four relative to the visual
extinction, $A_{V}$ (Cardelli et al. 1989). For the F115W ($J$-band) filter on
JWST/Near-Infrared Camera (NIRCAM; Rieke et al. 2023), the sampling resolution
is four times that of HST/Wide-Field Camera 3 (WFC3) F160W ($H$ band), with a
FWHM of 0.04 arcsec versus 0.15 arcsec. With $\sim 4\times$ better resolution
than HST/WFC3, crowding effects are reduced by more than an order of magnitude
in flux. This is especially important because, in the near-infrared,
contamination of the Cepheids by red giant and asymptotic giant branch stars
exacerbates crowding effects relative to optical wavelengths.

As part of the Chicago Carnegie Hubble Program (CCHP), we have been awarded
time in JWST Cycle 1 (JWST-GO-1995: PI W. L. Freedman, Co-PI B. F. Madore) for
a program designed to address specifically known systematic effects:
extinction and reddening by dust, metallicity effects, and crowding/blending
of stellar images. We are obtaining NIRCAM $J$-band data and, simultaneously,
F356W (mid-infrared) observations of 10 nearby SNe Ia-host galaxies with known
Cepheids. For calibration purposes, we are also observing NGC 4258, a galaxy
with a geometric distance derived from the motions of nuclear H2O megamasers.
In addition to the JWST results, our group is undertaking a complete re-
analysis (Owens et al. 2023) of the archival optical and near-infrared HST
data obtained by the SH0ES group (Riess et al. 2016).

Here, we present some preliminary results from our program for the nearby
galaxy NGC 7250, host to SN2013DY, located at a distance of about 20 Mpc.
Figure 8 illustrates cut-out images of four Cepheids in NGC 7250 from (Owens
et al. 2023). Shown are images at F115W (from JWST; top panel) and F160W (from
HST; bottom panel), which illustrate the superb resolution and the power of
JWST. As can be seen in the HST data, many of the Cepheid candidates are
crowded by nearby neighbors.

Figure 8: Four Cepheids in NGC 7250 discovered as part of the SH0ES project
(bottom row: HST F160W/$H$-band exposures; top row: JWST F115W/$J$ band). Each
postage-stamp image is $2\times 2$ arcsec on a side. The red circles are
centered at the position of the Cepheid determined from the HST optical
(F350LP white light) photometry. It is immediately evident that the crowding
for these Cepheids is quite severe and the signal-to-noise ratio (SNR) for the
$H$-band data tends to be low, ranging from 1 to 23. In contrast, the SNR for
the $J$-band data ranges from 36 to 121. On average, the JWST data have almost
an order of magnitude greater SNR, and a four times better angular resolution,
allowing the Cepheids to be distinguished clearly from the background.  Figure
9: Period–luminosity relations for Cepheids in the LMC (top row) and NGC 7250
(bottom row). The left-hand panels are for ground-based $H$ band for the LMC
(top) and the similar F160W filter (from HST) for NGC 7250 (bottom). The
right-hand panels are similar, but for ground-based $J$ and the corresponding
F115W filter (from JWST). The JWST data are plotted on an arbitrary magnitude
scale. The scatter about the period–luminosity fit in each filter is labeled
in each plot.

In Figure 9, we show the Leavitt laws for a sample of Cepheids in the LMC
(Persson et al. 2004) and in NGC 7250 (Owens et al. 2023). The scatter in the
JWST F115W data for NGC 7250 is a factor of two smaller than in the SHoES
F160W data; i.e., the improved resolution and higher signal-to-noise ratio of
the JWST data results in a lower variance ($\sigma^{2}$) for the F115W
relation by almost a factor of four. This is all the more remarkable since the
$J$-band data are single-phase observations only, while the HST observations
have been corrected to mean light. The HST data exhibit more than three times
the scatter of the $H$-band data for the LMC, the latter of which reflects the
expected scatter for that band, as exemplified by the LMC data.

##  12 Summary

The accuracy of the Cepheid distance scale has continued to improve over the
century during which it has been used to measure the distances to nearby
galaxies and set the scale for the determination of H0. Still, challenges
remain in overcoming systematic uncertainties. Many of these challenges will
be overcome with new capabilities provided by the JWST.

New JWST data for the nearby galaxy NGC 7250 already demonstrate that (1) many
of the Cepheids observed with HST/WFC3 are significantly crowded (and biased
to brighter apparent magnitudes) by nearby neighbors. A re-analysis of the
SH0ES optical data, then coupled with the new high-resolution and higher
signal-to-noise JWST F115W data, leads to significantly reduced effects of
crowding and smaller photometric uncertainties. (2) These improvements result
in a factor of two lower scatter in the near-infrared Leavitt law for JWST
F115W compared with HST F160W, even with single-epoch F115W JWST photometry.

The galaxies in our JWST CCHP program sample have all been selected to have
with distances $\lesssim$20 Mpc, close enough to minimize crowding effects. As
for the case of NGC 7250 presented here, these data will be combined with a
re-analysis of the SH0ES HST optical data for the Cepheids. TRGB, carbon star,
and Cepheid distances to the same sample of galaxies being observed as part of
the CCHP will allow measurement of three independent distances to each of
these galaxies. These new results already illustrate the power of JWST to
improve the measurement of extragalactic distances, and specifically, to
address remaining systematics in the determination of H0.

Ultimately, these data will allow us to provide an answer to one of the most
important problems in cosmology today: Is there new fundamental physics
required beyond standard $\Lambda$ Cold Dark Matter?