The JWST Resolved Stellar Populations Early Release Science Program. IX. The RR Lyrae Population in WLM with HST and JWST

Abstract

RR Lyrae stars are a common, dependable Population II distance indicator, and provide an independent tracer of early star formation. Here, we utilize archival HST/ACS and JWST/NIRCam observations of the nearby dwarf star-forming galaxy WLM to study RR Lyrae in JWST filters. We independently identify RR Lyrae in HST and JWST imaging in order to evaluate JWST's efficacy at characterizing RR Lyrae in the near-IR. We use an MCMC template-fitting technique to obtain periods, amplitudes, and mean magnitudes from the RR Lyrae time-series data. The spatially overlapping HST and JWST observations allow us to directly compare the same sources observed with the instruments, and calibrate the NIRCam F090W and F150W RR Lyrae period-Wesenheit-metallicity (PWZ) relation to the Gaia-consistent HST PWZ. We additionally assess the epoch-to-epoch consistency of NIRCam photometry, and find evidence of burn-in. We conclude that the zero-point offset is negligible compared to the uncertainties from the template fitting. We conduct an MCMC fit of the PWZ with both HST and JWST data. Our results are three-fold. First, we find that we can reliably identify RR Lyrae in NIRCam data, but light-curve template fitting proves difficult on short-baseline observations. Second, the HST PWZ fit yields a distance modulus to WLM of $μ= 24.85\pm0.05$ ($0.93\pm0.02$ Mpc). This is closer than previous measurements, primarily attributed to consistency with the Gaia scale. Lastly, although the JWST PWZ fit has large uncertainties and a poorly-constrained slope, it represents a first-of-its-kind PWZ calibration in NIRCam filters.

Figures (21)

• Figure 1: Transmission curves for the four filters used in this study, overlaid on synthetic PHOENIX spectra (smoothed for readability purposes) for two broadly representative maximum (dashed, light gray) and minimum (solid, dark gray) RR Lyrae temperatures. For illustrative purposes, the difference in flux between these spectra in a given wavelength range serves as a rough proxy for the expected amplitude of an RR Lyrae for that filter. As such, we can see how a change in effective temperature corresponds to a much larger change in flux in the bluest filter (HST F475W), compared to the other three used in this study. Similarly, we expect relatively small amplitudes in the reddest filter (JWST F150W). Note the overlap in wavelength coverage for the HST F814W and JWST F090W filters.
• Figure 2: Ground-based optical image of WLM (image credit: ESO, VST/Omegacam Local Group Survey) with footprints of the NIRCam (solid), and HST ACS (dashed) fields overlaid. The outer NIRCam FOV overlaps with deep ACS F475W and F814W imaging. This overlap region of the deep HST and JWST observations enables a direct comparison of the RR Lyrae stars in both observations. Image adapted from Fig. 1 of McQuinn2024.
• Figure 3: A CMD of our HST observations centered on the horizontal branch. The RR Lyrae candidates (red) are plotted over the full photometric catalog (grayscale). Uncertainties on the RR Lyrae are shown, but are too small to see for most sources. The most dense regions of the photometric catalog are indicated by contours, for viewing ease. The other sources are shown as points (lightest gray). The color-magnitude cuts are shown as blue lines.
• Figure 4: An example output from the periodicity measurement step. The top panel shows the power spectrum of the hybrid algorithm (red). The most likely period (solid black line) and errors (dotted black lines) are indicated. The middle and lower panels show the HST F475W and F814W data, phase-folded according to the most likely period.
• Figure 5: An example MCMC template fit to phase-folded HST F475W (top) and F814W (bottom) data. Observations (with uncertainties) are shown in black, while the MCMC fit is shown in red. The pulsation mode (RRab), $\chi^2$ on the fit, and period are shown above. The filter-wise mean magnitudes and amplitudes are written in their respective panels.