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https://orcid.org/0000-0002-6124-1196


https://orcid.org/0000-0002-6124-1196


https://orcid.org/0000-0002-6124-1196


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https://orcid.org/0000-0002-5259-2314


https://orcid.org/0000-0002-5259-2314


https://orcid.org/0000-0002-5259-2314


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https://orcid.org/0000-0001-9420-6525


https://orcid.org/0000-0001-9420-6525


https://orcid.org/0000-0001-9420-6525


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https://orcid.org/0000-0002-1775-4859


https://orcid.org/0000-0002-1775-4859


https://orcid.org/0000-0002-1775-4859


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https://orcid.org/0000-0003-3889-7709


https://orcid.org/0000-0003-3889-7709


https://orcid.org/0000-0003-3889-7709


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https://orcid.org/0000-0002-4934-5849


https://orcid.org/0000-0002-4934-5849


https://orcid.org/0000-0002-4934-5849


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https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


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https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


mailto:ariess@stsci.edu


http://astrothesaurus.org/uat/758


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https://doi.org/10.3847/2041-8213/ad1ddd


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13

JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an
Explanation for the Hubble Tension at 8σ Confidence
Adam G. Riess1,2
, Gagandeep S. Anand1
, Wenlong Yuan2
, Stefano Casertano1, Andrew Dolphin3, Lucas M. Macri4
,
Louise Breuval2
, Dan Scolnic5
, Marshall Perrin1
, and Richard I. Anderson6
1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; ariess@stsci.edu
2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
3 Raytheon, 1151 E. Hermans Road, Tucson, AZ 85706, USA
4 NSF’s NOIRLab, 950 N Cherry Avenue, Tucson, AZ 85719, USA
5 Department of Physics, Duke University, Durham, NC 27708, USA
6 Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
Received 2023 December 21; revised 2024 January 2; accepted 2024 January 3; published 2024 February 6
Abstract
We present high-definition observations with the James Webb Space Telescope (JWST) of >1000 Cepheids in a
geometric anchor of the distance ladder, NGC 4258, and in five hosts of eight Type Ia supernovae, a far greater
sample than previous studies with JWST. These galaxies individually contain the largest samples of Cepheids, an
average of >150 each, producing the strongest statistical comparison to those previously measured with the Hubble
Space Telescope (HST) in the near-infrared (NIR). They also span the distance range of those used to determine the
Hubble constant with HST, allowing us to search for a distance-dependent bias in HST measurements. The superior
resolution of JWST negates crowding noise, the largest source of variance in the NIR Cepheid period–luminosity
relations (Leavitt laws) measured with HST. Together with the use of two epochs to constrain Cepheid phases and
three filters to remove reddening, we reduce the dispersion in the Cepheid P–L relations by a factor of 2.5. We find no
significant difference in the mean distance measurements determined from HST and JWST, with a formal difference
of −0.01± 0.03 mag. This result is independent of zero-points and analysis variants including metallicity
dependence, local crowding, choice of filters, and slope of the relations. We can reject the hypothesis of unrecognized
crowding of Cepheid photometry from HST that grows with distance as the cause of the “Hubble tension” at 8.2σ, i.e.,
greater confidence than that of the Hubble tension itself. We conclude that errors in photometric measurements of
Cepheids across the distance ladder do not significantly contribute to the tension.
Unified Astronomy Thesaurus concepts: Hubble constant (758)
Supporting material: machine-readable table
1. Introduction
In the past decade, an intriguing and persistent discrepancy
referred to as the “Hubble tension”7 has been apparent at high
significance (>5σ) between the Hubble constant (H0) directly
measured from redshifts and distances, which are independent
of cosmological models, and the same parameter derived from
the ΛCDM model calibrated in the early Universe (for a recent
review, see Verde et al. 2023).
The most significant disparity arises from the strongest
constraints. These come from measurements of 42 local
Type Ia supernovae (SNe Ia) calibrated by Cepheid variables,
yielding H0 = 73.0± 1.0 km s
−1 Mpc (SH0ES Collaboration;
Riess et al. 2022, hereafter R22), compared to the analysis of
Planck observations of the cosmic microwave background
(Planck Collaboration et al. 2020), predicting H0 = 67.4±
0.5 km s −1 Mpc −1 in conjunction with ΛCDM. Cepheids are
the preferred primary distance indicators in these studies due to
the Leavitt law (P–L relation; Leavitt 1912), their extraordinary
luminosity (MH ∼− 7 mag at a period of 30 days), intrinsic
precision (approximately 3% in distance per star),
reliable
identification based on periodicity and light-curve shape
(Hertzsprung 1926), and comprehensive understanding (since
Eddington 1917). They also serve as the best-calibrated
distance indicators accessible in the largest volume of SN Ia
hosts (D∼ 50 Mpc), thanks to the consistent use of a single
stable instrument, Hubble Space Telescope (HST) WFC3 UVIS
+IR, by the SH0ES team in measurements within SN hosts and
in several independent geometric anchors: the megamaser host
NGC 4258
(Reid et al. 2019),
the Milky Way
(through
parallaxes, now including Gaia EDR3; Gaia Collaboration et al.
2021), and the Magellanic Clouds (via detached eclipsing
binaries; Pietrzyński et al. 2019). Near-infrared (NIR) observa-
tions are crucial to mitigating the impact of dust, a challenge
faced by many cosmic probes. Yet, the modest NIR resolution of
HST, ∼0 1, has limited the inherent precision of individual
Cepheid measurements due to the effects of crowding in nearby
galaxies. As noted by Freedman et al. (2019), “[p]ossibly the
most significant challenge for Cepheid measurements beyond
20Mpc is crowding and blending from redder (RGB and AGB)
disk stars, particularly for near-infrared H-band measure-
ments [...].”
The James Webb Space Telescope (JWST) provides new
capabilities to scrutinize and refine the strongest observational
evidence contributing to the tension. Specifically, the significantly
The Astrophysical Journal Letters, 962:L17 (13pp), 2024 February 10
https://doi.org/10.3847/2041-8213/ad1ddd
© 2024. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title
of the work, journal citation and DOI.
7 Tension may refer to the discrepancy between measures or to the feeling it
produces as expressed by Verde et al. 2023: “The research community has been
actively looking for deviations from ΛCDM for two decades; the one we might
have found makes us wish we could put the genie back in the bottle.”
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greater resolution of JWST over HST has greatly reduced—in
practical terms, almost eliminated—the main source of noise in
NIR photometry of Cepheid variables observed in the hosts of
nearby SNe Ia. The resolution of JWST provides the ability to
cleanly separate these vital standard candles from surrounding
photometric “chaff.” This study extends the scope of JWST
measurements of Cepheids along the distance ladder, building upon
measurements in one SN Ia host (NGC5584) and the distance scale
anchor NGC 4258 (Riess et al. 2023, hereafter R23). Here we
present a greatly expanded sample of such measurements that
doubles its distance range to span the full range of distances of
nearby SN calibrators (the second rung of the distance ladder) and
triples the Cepheid sample size, while raising the number of SN
hosts studied from one to five and the number of SN Ia calibrated
from one to eight. In Section 2 we present the observations, in
Section 3 their analysis, and in Section 4 we discuss their
interpretation.
2. Data
The targets of this program, GO-1685, were selected at those
richest in Cepheids and SN Ia from the SH0ES host sample of
37. The first two to execute, NGC 4258 and NGC 5584, were
presented in R23 and the four presented here executed after, in
mid 2023. The observational parameters of our JWST NIRCam
imaging campaigns for four SN Ia hosts (NGC 1448, NGC 1559,
NGC 5468, and NGC 5643) are provided in Table 1 and shown
Table 1
Observation Log
Date
MJD
Epoch
Exposurea
Filter1
Filter2
Exp. Time (s)
R.A. (J2000)
Decl. (J2000)
Orientation
2023-06-30
60125.43
N1559e1
001001_02101_*
F090W
F277W
418.7 × 4
64.39918
−62.78429
216.0
2023-06-30
60125.46
N1559e1
001001_04101_*
F150W
F277W
526.1 × 4
64.39918
−62.78429
216.0
2023-07-15
60140.66
N1559e2
002001_03101_*
F090W
F277W
418.7 × 4
64.39906
−62.78429
221.0
2023-07-15
60140.69
N1559e2
002001_03103_*
F150W
F277W
526.1 × 4
64.39906
−62.78429
221.0
2023-07-07
60132.13
N5643e1
011001_02101_*
F090W
F277W
311.4 × 4
218.16850
−44.17334
91.7
2023-07-07
60132.15
N5643e1
011001_04101_*
F150W
F277W
418.7 × 4
218.16850
−44.17334
91.7
2023-07-22
60147.17
N5643e2
012001_03101_*
F090W
F277W
311.4 × 4
218.18105
−44.12627
96.7
2023-07-22
60147.19
N5643e2
012001_05101_*
F150W
F277W
418.7 × 4
218.18105
−44.12627
96.7
2023-08-02
60158.89
N1448e1
013001_02101_*
F090W
F277W
418.7 × 4
56.16443
−44.61610
251.0
2023-08-02
60158.91
N1448e1
013001_04101_*
F150W
F277W
526.1 × 4
56.16443
−44.61610
251.0
2023-08-18
60174.19
N1448e2
014001_03101_*
F090W
F277W
418.7 × 4
56.17600
−44.66334
256.0
2023-08-18
60174.21
N1448e2
014001_05101_*
F150W
F277W
526.1 × 4
56.17600
−44.66334
256.0
2023-06-28
60123.27
N5468e1
007001_02101_*
F090W
F277W
204.0 × 6
211.66669
−5.40969
114.6
2023-06-28
60123.29
N5468e1
007001_02103_*
F150W
F277W
472.4 × 6
211.66669
−5.40969
114.6
2023-07-14
60139.55
N5468e2
008001_02101_*
F090W
F277W
311.4 × 5
211.66745
−5.41007
115.6
2023-07-14
60139.57
N5468e2
008001_02103_*
F150W
F277W
526.1 × 5
211.66745
−5.41007
115.6
Note.
a All exposures start with jw01685.
Figure 1. NIRCam fields superimposed on Digitized Sky Survey color images for four hosts (top) and NIRCam RGB images (F090W/F150W/F277W) showing
positions of Cepheids (cyan circles) (bottom). North is up and east is to the left.
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in Figure 1 (see R23 for the same information for NGC 4258 and
NGC 5584). As in R23, the four new hosts were imaged with
three filters (F090W, F150W, and F277W, centered at 0.9, 1.5,
and 2.8 μm, respectively; see Figure 2 in R23 for filter curves) in
two epochs separated by 15–16 days, with one short wavelength
module of the NIRCam instrument covering the host and the
other placed on a far halo field as shown in Figure 1. The epoch
separation was selected by the JWST schedulers subject to the
requirements of a 15–30 days spacing and an orientation
difference within 5°. For the closer two hosts, NGC 1448 and
NGC 5643, we swapped the A and B modules on the host on
consecutive visits and thus doubled the spatial coverage of the far
halo field on opposite sides.
We analyzed the wave front history of JWST over the time span
of the observations from the telescope monitoring data, 2023 June–
August, as shown in Figure 2 to determine the photometric
stability of our data set. There was a moderately large tilt of mirror
segment C5 detected on July 16 (not present on July 14) that was
corrected July 22 and we had no observations between these two
dates. The wave front modeling indicates no large changes at the
time of the observations and that variations in encircled energy at
small radii (similar to point-spread function, PSF, photometry)
would be <0.005 mag (less at longer wavelengths). Based on this
we judged the epochs to be photometrically consistent and
proceeded with our measurements. We note the impact to
photometry due to changes in the shape of the PSF between
epochs is further negated by the determinations of aperture
corrections from stars within each frame.
We used the STScI NIRCam reduction pipeline, version
1.12.0, to calibrate the data frames. There has been only one
significant update to the reference files since those used in R23,
1125.pmap and 1126.pmap, which included zero-point updates
for each sensor chip assembly and flat field improvements. The
present work uses this update (and we have remeasured
NGC 4258 and NGC 5584 from R23). As a result of these
updates, the mean photometry of sources became brighter by
∼0.03 mag in F090W and ∼0.01 mag in F150W, where the
latter band was used for our baseline distance measurements.
Much of this change cancels when comparing Cepheids
between NGC 4258 and NGC 5584. Updates after 1126.pmap
through 2023 December do not affect NIRCam photometry.
We performed photometry on the images using the DOLPHOT
package (Dolphin 2000, 2016) and its NIRCam module (Weisz
et al. 2023; D.R. Weisz et al. 2024, in preparation), with the same
procedures described in R23 with the same cuts on the crowding,
sharpness, object type, signal-to-noise ratio, and error flags as
reported by DOLPHOT8 (see also Warfield et al. 2023). The
only relevant changes since R23 are the version of the
instrumental reference files provided by STScI stated above.
We measured photometry with DOLPHOT with the same,
recommended parameters on all of our images. The PSF fitting
procedure is automated and performs the photometry of all
frame stars simultaneously and blind to which are Cepheids,
thus it is completely unsupervised, removing any human
intervention from the Cepheid measurement procedure. The
Cepheids were identified by astrometrically matching the
JWST photometry catalogs to the Cepheid lists identified by
the SH0ES team from multiepoch, optical HST imaging (Yuan
et al. 2022b; Riess et al. 2022). The matching tolerance was set
to 0.7 NIRCam short wavelength pixels in distance (0 02) but
Figure 2. JWST wave front sensing data during the interval of these observations, 2023 June–August. The wave front sensing, drifts, rectifications, and wave front
errors are shown in the top three rows of panels. The bottom panel shows the change to encircled energy in the core of the PSF in the three filters used for this program.
The dates of our observations are indicated as vertical dashed lines and a star symbol.
8 A revised version of the DOLPHOT package became available after this
work was completed, which includes a new “-etctime” option to revise the
exposure times in the image headers. As a test we compared the Cepheid
photometry of NGC 5584 with the old and new version and measured a median
difference of 0.001 mag in F150W and 0.002 mag in F090W and an 0.3%
mean change in the noise. Further, using revised PSFs in DOLPHOT from
WebbPSF 1.2.1 from April 2 2024 resulted in a change of 0.003 mag, an
increase in sample size by 2%, and a reduction in PL noise by 1.2%. We judged
these differences too small to merit a change in versions for this work.
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matches were generally within 0 01 of the expected position.
This is the same result as seen in R23.
Among the 36 images we obtained for this program (three
filters in two epochs for six hosts), only one suffered an issue.
This was for the first epoch in F090W for NGC 5468 where the
number of samples “up the ramp” during accumulation (four)
resulted in incomplete cosmic-ray removal in the individual
cal files (dithers). This produced some scattered artifacts in
that single image. Although these artifacts are rare enough to
have little impact on the PSF-fitting region of the Cepheids,
they impacted the measurement of the aperture correction for
this image due their presence in source-subtracted sky annuli
(which extend to greater radii and thus cover more area than the
PSF-fitting region). To solve this problem, we determined the
appropriate aperture correction for this image and in each chip
by comparing the nonvariable stars between this epoch and
epoch 2 with F090W (which had more samples and no issues).
The result from comparing the nonvariable stars between these
two epochs produced an aperture correction consistent in value
with the set of aperture corrections seen for other images.
As in R23, we imposed additional quality requirements on
specific photometric parameters to ensure our sample contains
reliable Cepheid magnitudes
that yield
robust distance
measurements. The most important of these is based on the
value of χ2 (the quality of the scene modeling) reported by
DOLPHOT, which we generally require to be better (lower)
than 1.4 per degree of freedom; we allow a modest rise in this
limit, up to 1.7, as a linear function of log period, since long-
period Cepheids are very bright and their lower shot noise will
reveal more imperfections in the PSF model. This cut excluded
a median of ∼7% of sources among the set of hosts due to
subpar scene modeling. The full Cepheid sample forms a tight
locus when comparing log period to χ2, such that the poor
fits are readily apparent as a tail in the χ2 distribution toward
high values. The nature of poor χ2 objects is that they are
either confused even at JWST resolution or, more likely,
include a resolved source such as a cluster or background
galaxy, which is not well modeled with a set of PSFs by
DOLPHOT. We also employ the same color cut as R23,
0.3 < F090W−F150W < 1.15 (equivalent to 0.5 < V−I < 1.7,
a broad range for Cepheids) which excludes ∼5% of sources.
Nearly all of these are redder than the cut and are either highly
reddened or strongly blended with a red star. Not surprisingly,
this red boundary corresponds to the blue edge of the highly
populated red giant branch (RGB)/asymptotic giant branch
(AGB) (see Figure 3) so the odds of a direct blend or
misidentification will rise rapidly near this limit with hundreds
to thousands of red stars at similar brightness for every
Cepheid. The exception to the above is NGC 5643, the
only host with moderate Milky Way foreground extinction
(E(V−I) = 0.21 mag versus <0.05 mag for all others), shifting
all apparent colors redward including the RGB/AGB branch
and leading us to shift the accepted color range accordingly.
The values of χ2 for Cepheids in this host are also slightly
higher than the other hosts leading us to relax the χ2 limit by
0.3, resulting in the exclusion of a similar fraction as the other
hosts.
In Figure 3 we show the position of the Cepheids within the
color–magnitude diagrams for the field stars in each host. In
Figure 4 we show cutouts for two representative Cepheids with
P∼ 40 days from each of six host in three HST and three
JWST filters. Inspection of the JWST F150W stamps shows a
qualitative change relative to HST F160W thanks to the higher
angular resolution. The background is effectively resolved,
with the brightness fluctuations in the HST images transformed
to reveal individual stars and spatially constant backgrounds.
2.1. Artificial Stars
As in R23, we inject 200 artificial stars that bracket the range
of the Cepheid magnitudes, interpolating between these to
derive the background bias (i.e., the mean crowding correction)
based on the uncrowded magnitudes, estimated from Cepheid
periods and an iterative fit to the P–L relation. These crowding
corrections represent
the difference,
in units of source
magnitudes, between the measurement of the source on a
uniform background and on the speckled background,
Figure 3. Color–magnitude diagrams (CMD) for the modules covering Cepheids. Each star is plotted using a small black point, while Cepheids are indicated by cyan
symbols. A color cut in this space is included for NGC 1559, 0.3 < F090W−F150W < 1.15, corresponding to a broad range around the instability strip (0.5 < V
−I < 1.7). For NGC 5643, which has large foreground extinction (E(V − I) = 0.21) the color range is shifted redward. The magnitudes and colors of non-Cepheids
are based on the first visit. If a Cepheid is observed in only the first or in both visits, its location on the CMD is based on the first visit. Otherwise, it is based on the
second visit.
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determined statistically from the level of nearby sources. The
results from these measurements were similar to the two
galaxies in R23 with a mean across all hosts of 0.04, 0.07, and
0.09 mag for F090W, F150W, and F277W, respectively. These
corrections are approximately 7 times smaller than the same
quantities for HST in F160W. The random errors derived
from the artificial stars ranged from 0.06 mag (NGC 1448) to
0.14 mag (NGC 5468) in F090W and 0.09 mag (NGC 1448) to
0.16 mag (NGC 5468) in F150W. These distributions are
slightly asymmetric in F090W and fairly symmetric in F150W
and F277W.
3. Analysis
The Cepheids we report on were discovered between 5 and
20 yr ago by the SH0ES team using ∼12 epochs of optical
imaging from HST, providing a measurement of their periods,
amplitudes, and photometry in WFC3 F555W, F814W, and
F160W (Hoffmann et al. 2016; Riess et al. 2022). However, the
typical period uncertainty is ∼2% (Yuan et al. 2021), so
knowledge of their phases elapses after just a few of years, and
certainly by the time of our JWST observations. In order to
recover their phases, we analyzed the change in the Cepheid
magnitudes between the two epochs following the methods
given in R23; we show an example of the relation between the
change in magnitude and phase for the Cepheids in one of our
hosts, NGC 5643, in Figure 5. The phase uncertainty for
individual Cepheids is a function of the difference in phase
(determined by the Cepheid period) for the two epochs. We
note that the difference photometry between two epochs has
less noise than the sum of the two epochs because some of the
sources of error, such as the crowding term, are correlated
between the two epochs. The determination of phase (discussed
in R23) also provides a magnitude uncertainty that takes into
account the quality of the phase constraint.
The phase-corrected photometry is provided in Table 2 and
includes the combined error terms (from artificial stars, shot
noise, empirical determination of the phase, and intrinsic width
of the instability strip). We show monochromatic P–L relations
for each measured filter in Figure 6.
3.1. Reddening-corrected Period–Magnitude Relation
Although reddening in the NIR is small compared to the
optical, it is not negligible. Therefore we make use of three
dereddened (or Wesenheit) magnitude systems chosen to make
the best use of the JWST data. These are derived from
combinations of six filters, three HST WFC3 bands, F555W,
F814W, and F160W (R22), and three JWST NIRCam bands,
F090W, F150W, and F277W. The relations are
1. JWST+HST NIR, (baseline): mH
W = F150W −0.41(F555W
−F814W);
2. JWST NIR: F150W −0.72(F090W−F150W);
3. JWST MIR: F277W −0.30(F090W−F150W).
By design these minimize extinction and temperature width
effects in the instability strip, aiding in distance measurements
by using a color term, R, which we derive from the Fitzpatrick
(1999) reddening law (RV = 3.3; see Brout & Riess (2023) for
further explanation). The SH0ES team previously used an
HST-only formulation of the baseline relation, mH
W= F160W
−0.39( F555W−F814W); the JWST+HST NIR system is very
Figure 4. Example HST and JWST image stamps around Cepheids with P ∼ 40 days in all hosts. The top row shows the location of each Cepheid. HST filters are
labeled in black while the JWST filters are in magenta.
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similar but crucially substitutes F150W for F160W. This
change allows for the most direct comparison with past HST
measurements, reducing NIR confusion noise in past HST data
while leaving the wavelengths measured constant. The color
F555W−F814W is well measured due to the high resolution of
HST WFC3-UVIS and strong contrast between Cepheids and
red giants in the optical as seen in Figure 4. The low value of R
demonstrates that F150W is subject to only modest extinction.
We also analyze two other filter combinations that are
independent of the HST measurements of color. The JWST
mid-infrared (MIR) system further reduces the impact of
extinction by referencing the Cepheids to 2.8 μm.
The mean slope of the mH
W P–L relation has been well measured
with HST to lie in the range of −3.26 to −3.30mag dex−1 (Riess
et al. 2016, 2019, 2022). The mean of these six hosts from the
HST measurements is −3.26±0.05. In principle we expect a
slightly shallower slope by ∼0.01mag dex−1 when substituting
HST F160W (λeff = 1.53 μm) for JWST F150W (λeff = 1.50μm)
due to the larger color term. The strongest constraints on the
slope come from the LMC due to its large measured period
range and low dispersion (Riess et al. 2019). For our baseline
we use −3.25, near the mean of the JWST sample, which is
−3.21±0.03, and the HST constraint and propagate an
uncertainty of 0.05mag dex−1 in the slope to the summary
results. We also include variants of the fits, which set the slope to
−3.20 and −3.30.
3.2. Baseline Results
In Table 3 we provide the fits to the Cepheid P–L relations in
each host for the baseline system with the NIR measurements
from JWST versus HST (SH0ES). We fit a common formulation,
zp
m
b
P
Z
log
1
O H ,
1
H
W
W
W
(
)
[
]
( )
=
-
- -
where the zero-point or intercept, zp
MH
W
0
,1
m
= +
, where μ0 is
the distance modulus and MH
W
,1 is the absolute magnitude of a
Cepheid with
P
log
1
=
. The term ZW is the Cepheid metallicity
dependence in this system (−0.21mag dex−1; Breuval et al.
2022; Riess et al. 2022) and we provide the product of this times
the difference from solar metallicity, [O/H], in Table 3 for each
host. Because the Cepheids have near-solar metallicity, with
measured [O/H]∼ 0, these metallicity corrections are very
small; their typical value is ∼0.01–0.02 mag. We include them
for consistency when comparing with previous results from HST.
We also include a variant where we set the metallicity term to
zero and with double the nominal term.
We determine the intercepts within the JWST+HST NIR
magnitude-system P–L relations from the weighted mean after
applying an iterative 3σ clip (set by Chauvenet’s criterion), which
removes ∼3% of sources (∼30 Cepheids out of ∼1000, a
comparable fraction to past studies such as R22). We note that the
empirical rejection is applied to the full Cepheid sample discovered
in the optical, so this is lieu of that imposed by the HST NIR data
Figure 5. Magnitude differences of two epochs for Cepheids in NGC 5643 in F090W (red points, left) and simulated (gray left and indicating phase, right). Gray
density shows expected frequency of sampling based on random phase and template light curves. Red points are measurements from JWST and the errors do not
include the crowding error which effectively cancels in the difference. The asymmetry of Cepheid light curves in F090W produces structure in this diagram that can be
used to constrain the phase. The log of the time interval between epochs produces a negligible difference at a value of
P
log = 1.34.
Table 2
Photometric Data for Cepheids
Host
ID
R.A.
Decl.
log P
F090W
σ
F150W
σ
F277W
σ
V−Ia
σ
N5584
96196
215.58141
−0.38763
1.2434
26.175
0.115
25.549
0.111
25.367
0.140
0.957
0.117
N5584
114600
215.58182
−0.39029
1.2317
26.505
0.130
25.765
0.129
25.641
0.159
0.897
0.158
N5584
115209
215.58295
−0.38981
1.2015
26.598
0.138
25.940
0.173
25.783
0.198
0.817
0.157
N5584
134727
215.58605
−0.39115
1.2538
26.364
0.159
25.480
0.186
25.245
0.207
1.126
0.189
Notes.
a F555W-F814W. We note the provided magnitudes from JWST are phase corrected.
(This table is available in its entirety in machine-readable form.)
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in R22. Because our uncertainties are well determined and
nonuniform we apply rejection as an individual Cepheid
contribution to the P–L, χ2 > 32. We also provide results with
no rejection. Table 4 provides summary results for the full sample
including the mean P–L dispersion. The small dispersion for the
JWST relations makes the small number of outliers quite evident,
as on average they are ∼5σ off the P–L.
The fits are remarkably tighter than those measured with
HST as seen in Figure 7, an expectable (but still impressive!)
direct consequence of the improved telescope resolution
(see R23; their Figure 1 for sources of noise in P–L relations).
We see a consistent reduction in dispersion by a factor of 2.5 to
a mean of „0.18 mag, with the three closest at 0.16–0.17 mag.
Only NGC 5468 has a dispersion > 0.2 mag, a consequence of
its 1.5 mag greater distance and relatively shorter exposure
time. We determine distances to the SN hosts by using the
geometric distance determination of NGC 4258 (Reid et al.
2019) μ0,N4258 = 29.397 mag and the intercept difference
between the SN hosts and NGC 4258, i.e.,
zp
zp
,
2
N
0,SN
SN
4258
0,4258
( )
m
m
=
-
+
fit from the P–L relations. The measured distances from the
baseline systems are given in Table 3 with differences
between HST and JWST, also plotted in Figure 8. The baseline
mean difference, JWST−HST, seen in Table 4, is −0.011±
0.032 mag.9
This
uncertainty
receives
nearly
equal
Figure 6. P–L relations for four new SN Ia hosts—NGC 1448 (top left), NGC 1559 (top center), NGC 5468 (bottom left), and NGC 5643 (bottom right)—and for the
two galaxies presented in R23—NGC 4258 (bottom right) and NGC 5584 (bottom center). Relations are plotted from bluest (bottom) to least reddened (top). The
bottom two relations in each panel are from HST while the others are from JWST. The top relation in each panel, plotted in black, is a dereddened or Wesenheit
magnitude, F150W −0.72(F090W–F150W). Magnitude offsets are applied as indicated for ease of view and the dispersion for each P–L is given.
9
If this difference is interpreted as a mean error in HST measurements, the
sense would be of slightly overcorrecting HST Cepheid photometry for
crowding and underestimating H0. However, the difference is not significant.
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contributions from three terms of size ∼0.02 mag: the
intercepts of NGC 4258 measured with HST and with JWST,
and the mean of the SN host period-luminosity relations. The
latter term comes mostly from the HST intercept uncertainties,
which are 3 times the size of the JWST means. The similarity of
the NGC 4258 P–L error terms results from the combinations of
smaller dispersion for JWST (0.16 versus 0.44 mag) balanced by
the smaller JWST sample (N= 107 versusN= 442; more fields
observed with HST), which may be remedied in the future with
more JWST pointings for NGC 4258.
Regressing all JWST crowding corrections versus the
residuals from their best-fit P–L relations (see Figure 9), we
find no interdependence between these quantities. The best-fit
value of 0.06 ± 0.13 mag (residual) / mag (crowding) implies
that at the mean crowding of the JWST sample (∼0.07 mag)
the mean P–L bias is 0.004 ± 0.008 mag. This determination
provides a useful estimate of the level of error that would apply
when comparing this Cepheid photometry to that of Cepheids
in the closest hosts, such as the Milky Way and the LMC,
where crowding is absent.
We can attempt to gain additional leverage in the JWST
versus HST comparison by factoring in the distances of the
hosts. For this purpose it is useful to devise a hypothetical
model in which unrecognized crowding in Cepheid photometry
measured with HST resolution grows linearly with distance
(modulus) beyond the calibration from NGC 4258 to cause the
Hubble tension, i.e., a size of 5 log 73 67.5
0.17
(
) =
mag at the
mean distance modulus of
the SH0ES Cepheid sample
(μ0 = 31.7 mag), equivalent
to ∼0.07 mag of bias per
magnitude of distance modulus. This model is shown in
Figure 8. We can reject this model at 8.2σ with the highest
leverage from the farthest host, NGC 5468 (μ0 = 33.0), which
shows no evidence of such an effect. Indeed, the evidence we
see against crowding as the source of the tension is now greater
than the evidence of the tension itself. As is, we see no
evidence of a growing difference between HST and JWST with
distance as would be required for such a model.
3.3. Baseline Variants
In Table 4 we provide the previously discussed variants to
the measurement process as well as additional variants which
exclude the phase correction, use only Cepheids with HST
F160W observations (the WFC3-IR field of view is smaller
than the optical WFC3-UVIS field where the Cepheids were
Table 3
Baseline HST and JWST Cepheid P–L Fits
JWST+HST NIR
HST NIR (SH0ES)
JWST-HST
Host
Metal
Slope
N
zp
σ
SD
μ
σ c
N
ZP
σ
SD
μ
σ c
Δ
σ
n4258 −0.018 −3.25
107
26.739
0.017
0.160
L
L
442
26.739
0.017
0.442
L
L
L
L
n5584 −0.022 −3.25
214
29.178
0.011
0.170
31.838
0.020
185
29.168
0.032
0.449
31.828
0.037
0.010
0.042
n5643
0.029
−3.25
270
27.861
0.011
0.166
30.520
0.020
251
27.859
0.028
0.424
30.518
0.033
0.002
0.039
n1559
0.003
−3.25
158
28.712
0.015
0.191
31.371
0.023
110
28.813
0.041
0.471
31.473
0.045 −0.102
0.050
n5468 −0.020 −3.25
112
30.316
0.019
0.227
32.975
0.026
93
30.398
0.049
0.463
33.058
0.052 −0.083
0.058
n1448 −0.022 −3.25
76
28.630
0.017
0.168
31.289
0.024
72
28.577
0.029
0.348
31.236
0.034
0.053
0.042
Note. R22 Table 6 μN5584 = 31.772 ± 0.052 based on three anchors and P > 18 days; here we use only one anchor, NGC 4258, and P > 15 days to allow a direct
comparison. To allow a direct comparison with JWST+HST NIR, we applied a transformation of F150W–F160W = 0.033+0.036(F555W–F814W-1) which adds
0.03–0.04 for most hosts, and corrected here for CRNL by the subtraction of 0.035 mag for all hosts, the two corrections canceling to <0.01 mag with no impact on
distance.
a Error does not include geometric distance uncertainty for NGC 4258 of ± 0.032.
Table 4
Results
Sample
Comment
P–L
σ-
Cepheids
SD
2cn
JWST
σ
Slope
Clip
−HST
JWST+HST NIR
Baseline
−3.25
3
938
0.178
1.0
−0.011
0.032
JWST+HST NIR
σ-clip
−3.25
no
966
0.218
1.7
0.017
0.031
JWST+HST NIR
no min P or σ-clip
−3.25
no
1005
0.222
1.7
0.015
0.031
JWST+HST NIR
P > 15 days
−3.25
3
870
0.181
1.1a
−0.020
0.043
JWST+HST NIR
shallower slope
−3.20
3
938
0.179
1.1
−0.010
0.032
JWST+HST NIR
steeper slope
−3.30
3
937
0.179
1.1
−0.008
0.032
JWST+HST NIR
no metallicity cor.
−3.25
3
937
0.178
1.0
−0.005
0.032
JWST+HST NIR
double metal cor.
−3.25
3
939
0.179
1.0
−0.016
0.032
JWST+HST NIR
and in SH0ES F160W
−3.25
3
611
0.172
1.1a
0.014
0.035
JWST+HST NIR
no phase correction
−3.25
3
941
0.191
1.0a
−0.007
0.033
JWST+HST NIR
least JWST crowding half
−3.25
3
487
0.165
1.1a
−0.023
0.035
JWST+HST NIR
highest JWST crowding half
−3.25
3
451
0.201
0.9a
−0.006
0.040
JWST+HST NIR
first epoch only
−3.25
3
821
0.224
1.0a
−0.007
0.036
JWST NIR
F090W,F150W
−3.25
3
929
0.177
1.0
−0.012
0.032
JWST MIR
F090W,F150W,F277W
−3.25
3
864
0.197
1.0
−0.030
0.033
Note.
a These variants affect the errors as well as magnitudes or sample so
2cn is not directly comparable.
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found and which reduces the sample size), and split the sample
into the more and less crowded halves (where the DOLPHOT
crowding parameter is above or below the sample median).
This latter test is meant to mimic the subselection of Cepheids
by those that appear visually least crowded with JWST
resolution, as proposed in Freedman & Madore (2023) for
host NGC 7250 (see also the Appendix). This produces a
significant difference in sample mean dispersion, namely
∼0.17 mag for the less crowded half and ∼0.20 mag for the
more crowded half, but only a 0.015± 0.053 mag difference in
H0 between them. Although the difference is small,
this
selection has the potential to bias the remaining sample because
it is enacted on the appearance of the source, which may
include unresolved blending rather than the statistical proper-
ties of the scene. For example, a coincidental superposition of a
source with the Cepheid would appear to be uncrowded and yet
would suffer a bias whose size would be seen with artificial star
measurements. Another likely consequence of imposing a
selection is the skewing of distributions of measurements
relative to the expected errors as suggested in the values of χ2.
We caution that any subselection of the Cepheid sample needs
to be simulated and included in artificial star selection to avoid
bias in either measurements or errors (see further discussion
in R23).
We also determined the results for a single epoch (the first);
this provides useful information on the dispersion of the P–L
relation when no phase information is available. The single-
epoch solution yields standard deviation (SD) = 0.224 mag
versus SD = 0.178 for two epochs and phase corrected (plus an
additional 100 Cepheids with only a single epoch), a 25%
reduction in dispersion or equivalent weight to a 70% increase
in the sample size (58% from the scatter and 12% more
objects). With perfect measurements, a single random phase at
this wavelength will produce ∼0.15 mag scatter, two random
phases reduces this to ∼0.11 mag, and two phase-corrected
magnitudes to ∼0.075 mag. We note that most of these variants
are not statistically independent of each other, as they use the
same Cepheid samples and measurements. As all variants and
tests yield results consistent with the baseline, we consider the
baseline results robust.
We also provide the summary results from the other two
filter combinations based purely on JWST measurements,
JWST MIR and JWST NIR as shown in Figure 8. These yield
very similar results to the baseline.
Figure 7. Comparison between the standard (SH0ES: R22) dereddened magnitude mHW period–magnitude relation used to measure distances to SN Ia hosts. The red
points use JWST F150W (λeff = 1.50 μm) and the gray points are from HST F160W (λeff = 1.53 μm), including a small transformation F150W–F160W = 0.033 +
0.036[(V − I) − 1.0] to account for the differences in these passbands.
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4. Discussion
The sample of observations of ∼1000 Cepheids with JWST
in five hosts of eight SNe Ia and in NGC 4258 provides very
strong evidence that NIR HST Cepheid photometry is accurate,
albeit noisier than from JWST. While this may not be
surprising since past observations used artificial stars to correct
for crowding, the search for an explanation of the Hubble
Figure 8. Comparison of distances to the five SN Ia hosts measured with HST and JWST anchored by the same geometric distance reference, NGC 4258. The lower
plot shows the differences in the measurements from the two telescopes. Black shows the comparison for the baseline system used to measure H0,WV I
H
, and is the only
system plotted on the top panel. Green and red shows comparisons with two JWST-only magnitude systems. The bottom plot shows a hypothetical, linear model of
unrecognized crowding tuned to match the Hubble tension, 5log(73/67.5) = 0.17 mag at the mean distance of the SH0ES sample, μ = 31.7, a trend of 0.07 mag per
magnitude of distance modulus beyond NGC 4258. This model is ruled out at 8.2σ.
Figure 9. Relation between the F150W P–L fit residuals and crowding, as measured in JWST F150W frames as the difference between artificial star input and
extraction. We find no dependence between these quantities. Red points and errors shown the mean results if dividing the sample in half. The mean level of crowding
in NGC 4258 and the SN hosts are indicated by vertical lines. The scatter is evenly split as coming from phase uncertainty, crowding (at JWST resolution) with smaller
contributions from the width of the instability strip.
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tension merits a broad array of investigations including
independent checks on Cepheid photometry.
Although the sample of Cepheids remeasured with JWST
and presented here represents nearly a third of the full SH0ES
sample, we refrain from providing a value of H0 determined
exclusively from the JWST data because the sample size of
calibrated SN Ia and independent geometric anchors
is
substantially inferior to what is available in R22. The true
value of the JWST data provided here is to provide a high-
fidelity test of the HST Cepheid measurements.
Additional JWST observations of some Cepheids in the
SH0ES host sample are available from other Cycle 1 programs.
These observations are more difficult to compare directly with
HST observations because of the respective program design;
however, they appear broadly consistent with the conclusions
found here and in R23. Yuan et al. (2022a) presented a
comparison of Cepheids in NGC 1365 from program GO-2107
(PI Lee) with NIRCam F200W. They obtained a similar result
as the current program, albeit with lower significance, partly
because the observations were not ideal for this purpose due to
the very different wavelength than HST F160W and limited
depth. We analyzed the first set of observations by program
GO-1995 (PI Freedman), the only nonproprietary set at the
time of writing, of the SH0ES host NGC 7250 observed in
F115W and F444W and discussed in Freedman & Madore
(2023). NGC 7250 is one of the smallest hosts in the sample in
terms of physical size and has the lowest mass and the HST-
based analysis by Riess et al. (2022)
includes only 21
Cepheids, about 8 times less than the average host in this
work. Consequently, the comparison between HST and JWST
for NGC 7250 is limited in precision by the±0.13 mag error
from the HST intercept. The comparison is further limited by
the lack of more than one epoch to determine phase corrections
for JWST and the large difference between JWST and HST
filters
(λeff = 1.15 versus 1.53 μm) necessitating a
larger
photometric transformation of ∼0.4−0.5 mag, which at present
can only be obtained from model spectral energy distributions
(SEDs). Nevertheless, a comparison of
the JWST and
transformed HST observations of NGC 7250, presented in the
Appendix, are consistent at the ∼1.1σ
level, with JWST
brighter than HST (the direction of decreasing the distance to
the host and increasing H0).
4.1. Outlook
The Cepheid measurements from HST have passed a very
strong test of their accuracy provided by the resolution of
JWST. At this point, a solution to the Hubble tension is most
likely to exist elsewhere, because the evidence against a bias in
HST Cepheid photometry is greater than the evidence of the
tension itself.
We anticipate
related
improvements afforded by the
remarkable capabilities of JWST. Specifically, we expect
additional calibrations of SNe Ia from enhancements to the
primary distance indicators of TRGB, JAGB, and Miras, as
well as Surface Brightness Fluctuations. We suggest the most
effective manner of comparing these distance indicators is by
using them to measure the distance to the same set of hosts
rather than by comparing values of H0, which necessarily
involves additional rungs and potentially unrelated differences.
Tying all of these together by observing large samples in
common can lead to the calibration of ∼100 SNe Ia and a <1%
local measurement of H0, a landmark in our quest to understand
the expansion of the Universe.
Acknowledgments
We are indebted to all of those who spent years and even
decades bringing JWST to fruition. This research made use of
the NASA Astrophysics Data System. We thank Martha Boyer,
Yukei Murakami, and Siyang Li for helpful conversations
related to this work. We thank our PC Alison Vick. We thank
an anonymous referee for improving the draft. Please contact
us if you have any questions or if something does not make
sense.
Some of the data presented in this paper were obtained from
the Mikulski Archive for Space Telescopes (MAST) at the
Space Telescope Science Institute. The specific observations
analyzed can be accessed via doi:10.17909/17eb-qz46.
We acknowledge support from JWST GO-1685. R.I.A. is
funded by the SNSF via an Eccellenza Professorial Fellowship
PCEFP2_194638 and acknowledges support from the Eur-
opean Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation program (grant Agree-
ment No. 947660). This research was supported by the Munich
Institute for Astro-, Particle and BioPhysics (MIAPbP), which
is funded by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) under Germanyʼs Excellence
Strategy—EXC-2094—390783311.
Appendix
Cepheids in NGC 7250
We retrieved the JWST NIRCam observations of NGC 7250
from program 1995 (PI Freedman) from the MAST archive,
calibration version 1130.pmap. We found the Cepheid
observations in F444W (4.4 μm) to be of poor resolution due
to the broad PSF and poor contrast between the Cepheids and
red giants so we did not analyze them. Cepheids from the
sample from Hoffmann et al. (2016) and Riess et al. (2022)
were well detected in F115W and we measured their
photometry with DOLPHOT following
the same steps
discussed above. Unfortunately a second useful color from
JWST was not available to apply a similar color quality cut as
employed here and only a single epoch was obtained so we
could not measure phase corrections. Also, the JWST filter
F115W (λeff = 1.15 μm) is quite different than the HST filter
F160W (λeff = 1.53 μm) with no overlap, so unlike our
comparisons with JWST F150W presented above, quantifying
the difference requires a rather large, SED model-dependent
(and hence uncertain) color transformation between the filters,
equivalent to inferring the J − H colors of a Cepheid from
these models. We used the Padova isochrone SED models to
provide a synthetic relation between colors, F160W=F115W
+0.41 ± 0.05 + 0.49(F555W−F814W −1) for Cepheid like
SEDs, i.e., typical Cepheids with F555W−F814W∼1 must be
offset by ∼0.4–0.5± 0.05 mag to compare between the
telescopes. We identified 27 Cepheids in F115W from
the optical sample of 40
(with minimum completeness
LogP >1.25) from Hoffmann et al. (2016) using the same
procedure as in Section 3. We used a 2.5σ clip (appropriate for
this small sample size of N„ 40 by Chauvenet’s criterion) for
both the JWST and HST samples as shown in Figure 10. The
clip rejected 2 of the 27 in the JWST set and 1 of the 21
Cepheids in NGC 7250 provided in Riess et al. 2022,
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https://orcid.org/0000-0002-4934-5849


https://orcid.org/0000-0002-4934-5849


https://orcid.org/0000-0002-4934-5849


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0002-3191-8151


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


https://orcid.org/0000-0001-8089-4419


http://arxiv.org/abs/2311.08253


https://doi.org/10.3847/1538-4357/ac97e2


https://ui.adsabs.harvard.edu/abs/2022ApJ...939...89B/abstract


http://www.ascl.net/1608.013


https://doi.org/10.1086/316630


https://ui.adsabs.harvard.edu/abs/2000PASP..112.1383D/abstract


https://ui.adsabs.harvard.edu/abs/1917Obs....40..290E/abstract


https://doi.org/10.1086/316293


https://ui.adsabs.harvard.edu/abs/1999PASP..111...63F/abstract


https://doi.org/10.1088/1475-7516/2023/11/050


https://ui.adsabs.harvard.edu/abs/2023JCAP...11..050F/abstract


https://doi.org/10.3847/1538-4357/ab2f73


https://ui.adsabs.harvard.edu/abs/2019ApJ...882...34F/abstract


https://doi.org/10.1051/0004-6361/202039657


https://ui.adsabs.harvard.edu/abs/2021A&A...649A...1G/abstract


https://ui.adsabs.harvard.edu/abs/1926BAN.....3..115H/abstract


https://doi.org/10.3847/0004-637X/830/1/10


https://ui.adsabs.harvard.edu/abs/2016ApJ...830...10H/abstract


https://ui.adsabs.harvard.edu/abs/1912HarCi.173....1L/abstract


https://doi.org/10.1038/s41586-019-0999-4


https://ui.adsabs.harvard.edu/abs/2019Natur.567..200P/abstract


12


13

ID = 65093 with P= 82.1 days. The 25 that remained for
JWST have a dispersion of 0.29 mag (see Figure 10). Given the
small Cepheid sample size, larger P–L dispersion, and large
filter difference, we can only conclude the P–L relations are
broadly consistent (JWST brighter than HST by 1.1σ) and that
the JWST dispersion is a factor of ∼2 smaller than HST. A
more conservative period completeness limit (see Hoffmann
et al. 2016) of Log P >1.4 yields an ever closer match with the
remaining JWST sample fainter by 0.3σ but leaves a very small
comparison sample of 11 and 16 Cepheids for HST and JWST,
respectively. The mean crowding for this irregular, star-
bursting Sdm type host is substantially higher due of its
compact nature, ∼2 times that of the large spiral hosts studied
here and comprising most of the R22 sample. We also
considered a “least crowded” sample selection, following
Freedman & Madore (2023) who sought to reduce scatter by
visual selection of the least crowded Cepheids according to the
JWST images. As we did in Section 3 for the data from our
program, a less crowded sample was selected as those with a
DOLPHOT “crowd” parameter that was below the full sample
median, 0.135. The reduced sample of 13 Cepheids have a
mean crowding of 0.07 mag (comparable to the mean of the six
hosts studied here without a crowding cut, estimated for the
same 1.15 μm wavelength to be ∼0.05 mag) have a reduced
scatter of 0.18 mag and the same intercept as the full sample
within ∼0.02 mag. This sample is very small so that minor
differences in its composition may produce large, stochastic
variations. However, we are skeptical that there is any real gain
to be made by subselecting less crowded Cepheids because
their
lower scatter
is already reflected in their smaller
uncertainties (measured with artificial stars) and their greater
weight. In Table 4, this same selection also yielded a similar
result, lowering the dispersion while yielding a difference in
mean intercept of 0.01 mag, and given the also larger and
independent samples, 40 times larger, the significance of this
test is substantially stronger.
ORCID iDs
Adam G. Riess
https://orcid.org/0000-0002-6124-1196
Gagandeep S. Anand
https://orcid.org/0000-0002-
5259-2314
Wenlong Yuan
https://orcid.org/0000-0001-9420-6525
Lucas M. Macri
https://orcid.org/0000-0002-1775-4859
Louise Breuval
https://orcid.org/0000-0003-3889-7709
Dan Scolnic
https://orcid.org/0000-0002-4934-5849
Marshall Perrin
https://orcid.org/0000-0002-3191-8151
Richard I. Anderson
https://orcid.org/0000-0001-
8089-4419
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