First JWST Observations of JAGB Stars in the SN Ia Host Galaxies: NGC 7250, NGC 4536, NGC 3972

Determining a consistent and accurate value for the expansion rate of the Universe, as parameterized by the Hubble constant (H₀), has proven to be an extraordinarily difficult endeavor. Measurements of H₀ from local distance ladders compared with indirectly inferred measurements from the cosmic microwave background (Aiola et al. 2020; Planck Collaboration et al. 2020) currently disagree at the 5σ level, potentially indicating new physics and theories (Di Valentino et al. 2021). However, significant evidence still points to the possibility that systematic uncertainties in the local distance scale may be the reason for this tension. For example, the current two most precise local distance ladder measurements of H₀ from the tip of the red giant branch (TRGB) by the CCHP (Freedman 2021) and from the Cepheid Leavitt Law by the SH0ES group (Riess et al. 2022) themselves differ by about 2σ. It would seem prudent to resolve this local problem (involving stars) before invoking new physics (involving the foundations of contemporary cosmology).

One means of uncovering systematic errors in the local (stellar) distance ladder is by comparing the individual distances measured by the Cepheids and TRGB stars with a third equally precise and accurate distance indicator. Pertinently, two independent studies in the recent literature have presented a novel means of measuring distances to galaxies using carbon-rich, asymptotic giant branch stars (Freedman & Madore 2020; Madore & Freedman 2020; Ripoche et al. 2020), denominated the J-region Asymptotic Giant Branch (JAGB) method. JAGB stars are a well-defined class of carbon-rich, thermally pulsating AGB stars whose narrowly defined, constant luminosities in the near-infrared (NIR) have been shown to be constant from galaxy to galaxy. Easily identified on the basis of their NIR colors, the average luminosity of a galaxy's JAGB stars provide us with an excellent and simple, empirical standard candle (Nikolaev & Weinberg 2000; Weinberg & Nikolaev 2001).

Carbon stars' narrow range of luminosities have been predicted by stellar modeling for more than half a century (Iben 1973; Iben & Renzini 1983). JAGB stars are also so photometrically distinctive due to their extremely red color, a product of their carbon-enhanced atmospheres. This phenomenon results from the AGB stars' third dredge-up event, which transports carbon from the helium-burning shells up to the stellar surface through convective pulses (Habing & Olofsson 2003). The third dredge-up event is only effective for a small range of AGB star masses (2–5M_⊙) and thus a small range of luminosities (e.g., Marigo et al. 2008; Madore & Freedman 2020), therefore explaining the JAGB stars' low intrinsic scatter in their NIR luminosities (±0.3 mag).

The JAGB method has been shown to have comparable accuracy and precision in measuring distances to nearby galaxies with the Leavitt Law and TRGB (Freedman & Madore 2020; Lee et al. 2021; Parada et al. 2021; Zgirski et al. 2021; Lee et al. 2022; Parada et al. 2023), and therefore will be a useful cross-check for both of these distance indicators. The JAGB method also has several clear advantages as a distance indicator: first, JAGB stars are brighter in the near-infrared than both the TRGB and Cepheids with periods of 10 days. Second, JAGB stars are mostly ubiquitous in all galaxy morphologies ⁴ and inclinations, unlike the Cepheid Leavitt Law, which can only be applied to late-type spiral galaxies with low to moderate inclinations. Third, only one epoch of observing is needed to measure a JAGB distance, unlike Cepheids and Miras for which more than a dozen observations are required to measure their periods, amplitudes, mean magnitudes, and colors.

In this paper we show results for the first JWST NIRCam (Gardner et al. 2023; Rieke et al. 2023; Rigby et al. 2023) observations of JAGB stars in the first three galaxies of our JWST Cycle 1 program 1995 (PI: W. Freedman): NGC 7250, NGC 4536, and NGC 3972. The purpose of this program was to measure distances to 10 SN Ia host galaxies with three independent distance indicators, using the same imaging. Each of these methods, the Cepheids, the TRGB method, and the JAGB method, have been shown to be high-precision extragalactic distance indicators. Our research program, the CCHP, will provide a path to H₀ based on these three independent methods, aiming to decrease the systematic uncertainties below the 2% level for each method. This paper in particular marks the beginning of a future measurement of H₀ via the JAGB stars.

Data presented here were taken with the JWST's NIRCam. Imaging was simultaneously obtained in the short-wavelength channel (pixel scale of 0031 pixel⁻¹) and the long-wavelength (LW) channel (0063 pixel⁻¹). The F115W filter (J-band equivalent) was chosen for this program as the short-wavelength filter following Madore & Freedman (2020), who found that the JAGB stars have a constant mean luminosity in the J band. F444W was originally chosen as the long-wavelength filter to have a large (F115W–F444W) color baseline to easily separate O-rich AGB stars and C-rich AGB stars via their colors. The first two galaxies in our program to be imaged, NGC 7250 and NGC 4536, therefore have F115W and F444W data. However, it became clear that the pixel resolution of F444W was poorer than even the HST WFC3/IR camera. We elected to therefore change our LW filter to F356W for the remaining nine galaxies to take advantage of its increased resolution over F444W. We found that the (F115W–F356W) color was still excellent at separating the O-rich and C-rich AGB stars in NGC 3972.

Details on the calibration and the reduction of these data are extensively described in a companion paper, Owens et al. (2024). In short, point-spread function photometry was extracted from the images using the software package DOLPHOT (Dolphin 2000, 2016) via the the beta-testing JWST/NIRCAM module (Weisz et al. 2023).

The photometry used in this analysis remains preliminary for two reasons: photometry was extracted via the beta-testing DOLPHOT JWST/NIRCAM module (which is still being actively updated; D. Weisz, private communication), and the absolute photometric zero-points of NIRCam still remain relatively uncertain (Boyer et al. 2022). A JWST cycle 1 proposal aims to more accurately calibrate the NIRCam photometric zero-points (Gordon et al. 2022), after which we expect to be able to publicly provide final photometry closer to the requisite <2% precision needed for extragalactic distance scale measurements of H₀.

Nevertheless, even while using this preliminary photometry, the JAGB stars still exhibit low scatter and are clearly delineated from other stellar populations. However, we still cautiously emphasize that the magnitude axes should be taken as arbitrary. We have added the same additional random magnitude offset between −0.1 and 0.1 mag to both our F115W and F444W/F356W photometry. This offset will be removed once our photometric zero-points are finalized in the next paper in this series. Below, we now describe the data for the three galaxies analyzed in this paper.

2.1. NGC 7250

The irregular galaxy NGC 7250 (also known as PGC 68535 or UGC 11980) was the first SN Ia host galaxy in this program to be observed by JWST in 2022 November. Total exposures of 3769 s were simultaneously obtained in the F115W short-wavelength channel and the F444W long-wavelength channel. A color mosaic image of the observations is shown in Figure 1. In 2013, NGC 7250 was host to the type 1a SN 2013dy explosion, therefore uniquely positioning it as an SN Ia calibrator for measuring the Hubble constant. Indeed, Riess et al. (2016, 2022) used NGC 7250 as an SN Ia calibrator using 21 Cepheids observed with the Hubble Space Telescope (HST) as part of the SH0ES program, measuring a Leavitt law distance modulus of 31.628 ± 0.126 mag (or 21.2 ± 1.3 Mpc). Photometry for ∼514,000 sources was extracted from the images; after performing cleaning cuts designed to remove nonstellar sources (e.g., artifacts, cosmic rays, and extended sources) (DOLPHOT type = 1, SNR_F115W > 3, SNR_F444W > 1, sharp² < 0.04, crowd_F115W < 0.2, σ_F115W < 0.02 × 0.0025e^{F115W−24.05}), ∼112,000 sources remained in the final catalog. In particular, our chosen quality cuts do a reasonable job of eliminating background galaxies, but we plan to more thoroughly investigate the optimal quality cut parameters in our full sample of SN Ia host galaxies. We note, however, that the mode (used to measure the JAGB magnitude) is particularly robust to contamination from background galaxies, which are mostly ∼1 mag fainter than the JAGB stars (Madore et al. 2022).

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2.2. NGC 4536

2.3. NGC 3972

First, the JAGB stars were identified by their near-infrared color. For NGC 7250 and NGC 4536, JAGB stars were selected as having colors of 2.6 < (F115W–F444W) < 3.2 mag. For NGC 3972, JAGB stars were selected as having colors of 2.4 < (F115W − F356W) < 3.0 mag. Tests of these color limits are described later in this section.

Then, the F115W magnitudes of the JAGB stars were finely binned using bin sizes of 0.01 mag. To control for Poisson noise, the binned luminosity function was smoothed using the GLOESS (Gaussian-windowed, LOcally weighted Scatterplot Smoothing) algorithm (introduced by Persson et al. 2004), a data-smoothing interpolating technique effective at suppressing false (noise-induced) edges and peaks in luminosity functions. We also describe our tests of varying the smoothing parameters used in the GLOESS algorithm later in this section. The mode of the smoothed luminosity function then marks the JAGB magnitude.

3.1. Defining the "Outer Disk"

Lee et al. (2022) partitioned the photometry of M33 into four concentric regions, showing that the measured mode of the JAGB star luminosity function stabilized to within 0.01 mag in the two outer regions, which covered the outer disk and halo of M33, respectively. The two distance moduli measured in the outer regions agreed with independent TRGB and Leavitt law distance moduli by 2% (Lee et al. 2022). On the other hand, the mode measured in the inner regions of M33 differed from the mode measured in the outer regions up to ∼0.7 mag. This test first demonstrated that the JAGB magnitude is less accurate when measured in the inner disks of galaxies due to a confluence of crowding, blending, and reddening errors. However, where the "outer disk" lies can be subjective.

We introduce a novel method for identifying the outer disk of galaxies for the purpose of measuring JAGB method distances, where reddening, crowding, and blending effects taper off. We separated the photometry of a given galaxy into eight concentric regions split by semimajor axis (SMA), and measured the change in JAGB magnitude from region to region. To compute the SMA, we obtained the galaxy's center coordinates from the NASA/IPAC Extragalactic Database, and its inclination angle and position angle from HyperLeda. The boundaries of the regions were chosen so that each region had an equal number of JAGB stars. These boundaries are shown in the left panels of Figures 3, 4, and 5, for NGC 7250, NGC 4536, and NGC 3972, respectively, overlaid onto color mosaics of our NIRCam imaging.

To measure relative changes in mode throughout the different regions, we computed Δm_JAGB, which was defined as the change in mode from the fiducial mode. In NGC 7250, the fiducial mode was measured using JAGB stars with colors between 2.6 < (F115W and F444W) < 3.2 mag and a luminosity function (LF) smoothed with a smoothing parameter of σ_s = 0.25 mag. In NGC 4536, the fiducial mode was measured using JAGB stars with colors between 2.6 < (F115W − F444W) < 3.2 mag and an LF smoothed with a smoothing parameter of σ_s = 0.30 mag. For NGC 3972, the fiducial mode was measured using JAGB stars with colors between 2.4 < (F115W − F444W) < 3.0 mag and an LF smoothed with a smoothing parameter of σ_s = 0.25 mag. Regarding the spatial selection, the fiducial JAGB modal magnitude was computed from the final selected "outer disk" region (i.e., the merged outer regions to the left of the black line in Figure A1). We now describe our process for selecting this outer disk region below. The color–magnitude diagrams with these chosen parameters are shown in Figure 2.

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We then compared Δm_JAGB to the relative average surface brightness in each radial region. ⁵ In the right panels of Figures 3, 4, and 5, we show the change in Δm_JAGB as a function of the average sky_F115W ⁶ value in each radial bin, divided by the maximum sky_F115W value for the whole galaxy (all concentric regions). The sky_F115W parameter was derived from DOLPHOT for every star, and is a relative measure of the sky background at the star's position in a given image (Dolphin 2016). In the Appendix, we show plots of this sky_F115W parameter as a function of the SMA, with corresponding power-law fits. We emphasize the sky_F115W parameter is not an exact measure of surface brightness but is a simple parameter returned from DOLPHOT that can be used to track changes in Δm_JAGB. We also highlight that the choice of region separation (we used SMA here) is irrelevant to the final result, as long as Δm_JAGB converges in the outermost regions.

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The red points in Figures 3, 4, and 5 show the change in Δm_JAGB as a function of the sky_F115W parameter in each galaxy. The three panels show the effects of using different color cuts and smoothing parameters. In all cases, the JAGB magnitude leveled out in the outermost regions of the galaxy; therefore, the choice of color cuts and smoothing parameter did not significantly affect the final result. For example, in NGC 7250 and NGC 4536, where we used a color cut of 2.6 < (F115W–F444W) < 3.2 mag to measure the fiducial JAGB magnitude, we varied the blue color cut through {2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7} and the red color through {2.9, 3.0, 3.1, 3.2, 3.3, 3.4}. For NGC 3972, where we used a color cut of 2.4 < (F115W–F356W) < 3.0 mag to measure the fiducial JAGB magnitude, we varied the blue color cut through {2.2, 2.3, 2.4, 2.5, 2.6} and the red color through {2.8, 2.9, 3.0, 3.1, 3.2}. We also tested how different smoothing parameters σ_s in the GLOESS smoothing algorithm affected the mode. We varied σ_s from 0.15 to 0.40 mag in steps of 0.05 mag and quantified the change in the mode.

In all three galaxies, the mode m_JAGB stabilized in the outer regions of the galaxy. In NGC 7250, each region had 335 JAGB stars. The mode m_JAGB stabilized to within 0.01 in the three outermost regions. In NGC 4536, each region had 461 JAGB stars. The mode m_JAGB stabilized to within 0.03 in the five outermost regions. In NGC 3972, each region had 491 JAGB stars. The mode m_JAGB stabilized to within 0.04 in the four outermost regions. Therefore, we selected the outer bin of the innermost stable region to be the radial cutoff for that galaxy. In NGC 7250, this was 9.6 kpc; for NGC 4536, this was SMA = 10.3 kpc; for NGC 3972, this was SMA = 7.6 kpc. We recommend this procedure be used for measuring the JAGB magnitude for all future studies utilizing the JAGB method; the inner disk regions of galaxies can therefore be cut out in a procedural way, instead of visually.

3.2. Color–Magnitude Diagrams

The color–magnitude diagrams and smoothed luminosity functions for all three galaxies after performing the inner disk spatial cut are shown in Figure 2. The scatter on the mode m_JAGB of the JAGB stars between m_JAGB ± 0.75 mag was measured to be σ = 0.32 mag, σ = 0.34 mag, and σ = 0.35 mag for NGC 7250, NGC 4536, and NGC 3972, respectively. To measure the scatter on the mode, we used the following formula:

$$\begin{eqnarray}&&\sqrt{\displaystyle \frac{{\rm{\Sigma }}{({m}_{i}-{m}_{\mathrm{JAGB}})}^{2}}{{N}_{\mathrm{JAGB}}}},\end{eqnarray} \tag{ 1 }$$

where m_i are the F115W magnitudes of the JAGB stars and N_JAGB is the total number of JAGB stars.

These measured scatters from JWST are similar to the scatter measured from ground-based telescopes for the JAGB stars in the LMC, a galaxy 400 times closer than NGC 7250. Weinberg & Nikolaev (2001), the first study to use JAGB stars as standard candles in the near-infrared, measured a scatter of σ = 0.33 mag in their LMC sample using 2MASS data.

3.3. Artificial Star Tests

Artificial stars were computed with DOLPHOT for the first galaxy in our program, NGC 7250, to assess the robustness of the JAGB star photometry. In Figure 6, we show results for the ∼4200 injected stars in the JAGB color range and having an SMA > 9.6 kpc. The range of the x-axis in Figure 6 corresponds to the same range of the y-axis in Figure 2 for NGC 7250. At the average magnitude of the JAGB stars, the median offset was measured to be F115W_in − F115W_out = − 0.01 mag, demonstrating that crowding is a negligible source of systematic error for the JAGB stars in the outer regions of NGC 7250.

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This paper demonstrates JWST's impressive capabilities for utilizing JAGB stars as standard candles. Our results exemplify we now have the potential to study JAGB stars in galaxies at distances ≳20 Mpc away with JWST, with comparable or greater resolution than galaxies 50 kpc away with ground-based telescopes.

The high sensitivity and resolution of JWST allows us to resolve JAGB and TRGB stars in the outer regions of galaxies, at much farther distances than what was previously possible with HST. For example, all of the Cepheid variable stars in NGC 7250 have been discovered in the inner region (Riess et al. 2016, 2022; Owens et al. 2024), i.e., within the white ellipse in Figure 1, where the JAGB measurement was excluded in this paper. Thus, a significant advantage of the JAGB and TRGB methods to the Cepheid P-L relation is that carbon stars can be found in the significantly less crowded halos of galaxies, whereas Cepheids are mainly found in the more crowded star-forming disks.

A future calibration of the JAGB stars absolute magnitude in the F115W filter will soon be possible via imaging of the JAGB stars in the water megamaser host galaxy NGC 4258. These images are now scheduled for early 2024 and will also provide a zero-point for the TRGB and Leavitt law. This calibration will allow us to determine the distance to NGC 7250, NGC 4536, and NGC 3972 via the JAGB stars, as well as the rest of the SN Ia host galaxies in our observing program, allowing us to independently measure H_o via the JAGB method.

Suggestions and comments from Saurabh Jha, Adam Riess, and Dan Scolnic at the 2023 MIAPbP extragalactic distance scale workshop, regarding the selection of the location for the JAGB measurement, are gratefully acknowledged. We thank Dan Weisz and Andy Dolphin for their insight and advice on the use of the latest version of DOLPHOT tailored to JWST imaging data, and Jane Rigby for updates on the calibration of the NIRCam filters. Finally, we thank the anonymous referee for their constructive and helpful suggestions that improved this work.

A.J.L. was supported by the Future Investigators in NASA Earth and Space Science and Technology (FINESST) award No. 80NSSC22K1602 during the completion of this work. A.J.L. thanks the LSSTC Data Science Fellowship Program, which is funded by LSSTC, NSF Cybertraining Grant No. 1829740, the Brinson Foundation, and the Moore Foundation; her participation in the program has benefited this work. A.J.L. was supported by the Munich Institute for Astro-, Particle and BioPhysics (MIAPbP) which is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy—EXC-2094—390783311. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We also thank the University of Chicago and the Observatories of the Carnegie Institution for their support of our long-term research into the calibration and determination of the expansion rate of the universe.

This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program 1995. This research has made use of NASA's Astrophysics Data System Bibliographic Services.

All the JWST data used in this paper can be found in MAST: doi:10.17909/ffv8-5279.

Facility: JWST (NIRCam) - James Webb Space Telescope.

Software: NumPy (Harris et al. 2020), Matplotlib (Hunter 2007), Pandas (McKinney 2010).

In this section, we show plots of the average sky_F115W parameter in each of the eight regions as a function of SMA for the three galaxies. Power-law functions were fit to the data points, and are shown as the black line in Figure A1.

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