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 $τ_{50, {\rm AGB}}=5.16_{-0.50}^{+2.07}$ Gyr ago and $τ_{90, {\rm AGB}}=1.33_{-0.09}^{+0.11}$ Gyr ago versus $τ_{50, {\rm JWST}}=5.29_{-0.28}^{+0.34}$ Gyr ago and $τ_{90, {\rm JWST}}=1.42_{-0.01}^{+0.16}$ 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.

Figures (9)

• Figure 1: Optical image of WLM (image credit: ESO's VST/Omegacam) overlaid with footprints of the Magellan/FourStar (solid), JWST NIRCam (dotted), and NIRISS (dashed) fields. The pale yellow line denotes WLM's half-light radius. The half-light circle of Region 'A' is denoted by the pink circle and is discussed in § \ref{['sec:outer_burst']}.
• Figure 2: Ground-based NIR CMD of WLM which includes $\sim13,000$ stars, of which 825 are used in our CMD modeling analysis ($J<20$ mag, $-0.2<J-K<3$ mag). Representative photometric uncertainties are shown on the right-hand side as a function of magnitude. We indicate AGB stars (yellow shading), RHeB stars (red shading), and RGB stars (purple shading). Spatial maps of these populations are shown in Figure \ref{['fig:maps']}. The purple x's represent stars that were matched with Gaia and removed as foreground stars. The 80% and 50% completeness limits are marked with dashed lines. All sources have a signal-to-noise ratio (SNR) of $\ge7$ in the J band. Stars at the color and magnitude of the TRGB have an SNR $\approx85$ in the J band.
• Figure 3: The spatial distribution of RGB, AGB, and RHeB stars. The photometric criteria used to select each respective stellar population is shown in Figure \ref{['fig:cmd']}. The grey highlighted region labeled "A" is discussed in § \ref{['sec:outer_burst']}.
• Figure 4: Hess diagrams for our AGB star SFH fits for the observed data (left panel), best-fit linear combination of SSPs (middle panel), and residual CMDs (right panel) for the inner region ($r<r_h$, top) and outer region ($r>r_h$, bottom). The residuals are expressed in Poisson standard deviations.
• Figure 5: SFR as a function of time (top), cumulative SFH (middle), and AMR (bottom) derived from the AGB stars for the inner ($r<r_h$) and outer regions ($r>r_h$) of WLM.