2026/02/27 by Abigail J. Lee, Daniel R. Weisz, Andrew E. Dolphin +1 · 1 voice
Physics and Astronomy · #astro-ph.GA
We measure the star formation history (SFH) of Local Group dwarf galaxy WLM using wide-area (∼4 half-light radii) ground-based near-infrared (NIR) imaging of bright (MJ<-4.9 mag) asymptotic giant branch (AGB) stars. From our NIR color-magnitude diagram (CMD) of 825 stars, we find that our recovered SFH is in excellent agreement with literature SFHs of WLM measured from much deeper CMDs (MF090W∼+4.3 mag) based on JWST imaging. We find good agreement in the qualitative shape of the SFHs as well as quantitative metrics such as the timescales for which 50% and 90% of the stellar mass formed with τ_50,\rm AGB=5.16-0.50+2.07 Gyr ago and τ_90,\rm AGB=1.33-0.09+0.11 Gyr ago versus τ_50,\rm JWST=5.94-0.31+0.37 Gyr ago and τ_90, \rm JWST=1.55-0.13+0.03 Gyr ago. The coarser precision of the AGB star-based values is driven by the low number of AGB stars. We derive an age-metallicity relation (AMR) from the AGB star CMD fitting that is similar to the JWST-based AMR and is consistent with other reported metallicities in WLM (i.e., spectroscopy, RR Lyrae). From our wide-area AGB star map and SFH, we identify a stellar over-density (M_*∼2.0×106 M\odot, rh∼340 pc) in WLM's northwestern outer disk. Despite WLM having long been considered an isolated galaxy, the mass, size, and age of this over-density are highly suggestive of an accreted dwarf galaxy. Overall, our findings illustrate the power of NIR observations of AGB stars for efficiently and accurately measuring SFHs and for identifying and characterizing substructures in nearby galaxies.