The Star Formation History of WLM from Asymptotic Giant Branch Stars and The Discovery of a Possible Accreted System in its Outer Disk

Abstract

We measure the star formation history (SFH) of Local Group dwarf galaxy WLM using wide-area ($\sim4$ half-light radii) ground-based NIR imaging of bright ($M_{J}<-4.9$ mag) AGB stars. From our NIR CMD of 825 stars, we find that our recovered SFH is in excellent agreement with literature SFHs of WLM measured from much deeper CMDs ($M_{F090W}\sim+4.3$ 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 $\tau_{50, {\rm AGB}}=5.16_{-0.50}^{+2.07}$ Gyr ago and $\tau_{90, {\rm AGB}}=1.33_{-0.09}^{+0.11}$ Gyr ago versus $\tau_{50, {\rm JWST}}=5.29_{-0.28}^{+0.34}$ Gyr ago and $\tau_{90, {\rm JWST}}=1.42_{-0.01}^{+0.16}$ Gyr ago. The coarser precision of the AGB star-based values is driven by the low number of AGB stars. We also recover an age gradient that is in good agreement with the age gradient measured from JWST data, where we find the outer regions of WLM are on average older than the inner regions. 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 ($M_*\sim2.0\times10^6~M_{\odot}$, $r_h\sim340$ pc) in WLM's northwestern outer disk. The over-density's SFH shows a burst of star formation $\sim8$ Gyr ago and its spatial location is near a known warp in WLM's H I. 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.