The JWST Resolved Stellar Populations Early Release Science Program. IV. The Star Formation History of the Local Group Galaxy WLM

Abstract

We present the first star formation history (SFH) and age–metallicity relation (AMR) derived from resolved stellar populations imaged with the JWST NIRCam instrument. The target is the Local Group star-forming galaxy WLM at 970 kpc. The depth of the color–magnitude diagram (CMD) reaches below the oldest main sequence turnoff with a signal-to-noise ratio = 10 at M F090W = + 4.6 mag. This is the deepest CMD for any galaxy that is not a satellite of the Milky Way. We use Hubble Space Telescope (HST) optical imaging that overlaps with the NIRCam observations to directly evaluate the SFHs derived based on data from the two great observatories. The JWST and HST-based SFHs are in excellent agreement. We use the metallicity distribution function measured from stellar spectra to confirm the trends in the AMRs based on the JWST data. Together, these results confirm the efficacy of recovering an SFH and AMR with the NIRCam F090W−F150W filter combination, and validate the sensitivity and accuracy of stellar evolution libraries in the near-infrared relative to the optical for SFH recovery work. From the JWST data, WLM shows an early onset to star formation, followed by an extended pause post-reionization before star formation reignites, which is qualitatively similar to what has been observed in the isolated galaxies Leo A and Aquarius. Quantitatively, 15% of the stellar mass formed in the first Gyr, while only 10% formed over the next ∼5 Gyr. The stellar mass then rapidly doubled in ∼2.5 Gyr, followed by constant star formation over the last ∼5 Gyr.

The JWST Resolved Stellar Populations Early Release Science Program. V. DOLPHOT Stellar Photometry for NIRCam and NIRISS

“…It is remarkable for its depth and precision. Many of the features in the SW CMD are similar to a previous analysis of WLM with HST/ACS (Albers et al 2019) and are discussed in more depth in our team's star formation history paper of WLM (McQuinn et al 2024). Here, we briefly summarize CMD features.…”

“…DOLPHOT is a well-tested, widely used, and publicly available package that already supports modules specific to several HST cameras (WFPC2; Advanced Camera for Surveys, hereafter ACS; WFC3/UVIS; and IR), has been a testing ground for the Roman Space Telescope, and includes general purpose routines that can be used on virtually any images of resolved stars. The addition of JWST modules will enable a wide array of JWST-specific and cross-facility (e.g., JWST and HST) science, some of which has already been demonstrated using early versions of our JWST DOLPHOT modules (e.g., Lee et al 2023Lee et al , 2024Chen et al 2023;Riess et al 2023;Van Dyk et al 2023;Warfield et al 2023;Li et al 2024;McQuinn et al 2024;Peltonen et al 2024).…”

What if the Universe Expands Linearly? A Local General Relativity to Solve the “Zero Active Mass” Problem

“…General relativity (GR) is commonly assumed to be valid at cosmic scales, but it might not be. After the breaking observations of "Hubble tension" (Poulin et al 2018(Poulin et al , 2019Kamionkowski & Riess 2023), "El Gordo's collision" (Zhang et al 2015;Asencio et al 2021Asencio et al , 2023, and the "impossible early galaxies" (z > 10) Ferreira et al 2022;Gupta 2023;McQuinn et al 2024), deep discussions on the standard ΛCDM model and modified cosmology theories have intensified. Thus, the need to explore alternative models should not be discarded, even if it is limited to a minority sector (Kroupa 2013;Merritt 2017).…”