Structural and Photometric Properties of the Andromeda Satellite Dwarf Galaxy Lacerta I from Deep Imaging with WIYN pODI

We present results from WIYN pODI imaging of Lacerta I (And XXXI), a satellite dwarf galaxy discovered in the outskirts of the Andromeda galaxy (M31) in Pan-STARRS1 survey data. Our deep, wide-field $g,i$ photometry reaches ∼3 mag fainter than the photometry in the Pan-STARRS1 discovery paper and allows us to trace the stellar population of Lac I beyond two half-light radii from the galaxy center. We measure a Tip of the Red Giant Branch distance for Lac I of ${(m-M)}_0=24.44\pm 0.11$ mag (773 ± 40 kpc, or 264 ± 6 kpc from M31), which is consistent with the Pan-STARRS1 distance. We use a maximum-likelihood technique to derive structural properties for the galaxy, and find a half-light radius (r _h) of 3.24 ± 0.21 arcmin (728 ± 47 pc), ellipticity ($ϵ$) of 0.44 ± 0.03, total magnitude M _V = −11.4 ± 0.3, and central surface brightness ${\mu }_{V,0}=24.8\pm 0.3$ mag arcsec⁻². We find no H i emission in archival data and set a limit on Lac I's neutral gas mass-to-light ratio of $M_{HI}/L_V$ < 0.06 $M_{\odot }$/$L_{\odot }$, confirming Lac I as a gas-poor dwarf spheroidal galaxy. Photometric metallicities derived from Red Giant Branch stars within 2 r _h yield a median [Fe/H] of −1.68 ± 0.03, which is more metal-rich than the spectroscopically derived value from Martin et al. Combining our measured magnitude with this higher metallicity estimate places Lac I closer to its expected position on the luminosity–metallicity relation for dwarf galaxies.