Turning the Tides on the Ultra-Faint Dwarf Spheroidal Galaxies: Coma Berenices and Ursa Major II

Abstract

We present deep CFHT/MegaCam photometry of the ultra-faint Milky Way satellite galaxies Coma Berenices (ComBer) and Ursa Major II (UMa II). These data extend to r~25, corresponding to three magnitudes below the main sequence turn-offs in these galaxies. We robustly calculate a total luminosity of M_V=-3.8 +/- 0.6 for ComBer and M_V=-3.9 +/- 0.5 for UMa II, in agreement with previous results. ComBer shows a fairly regular morphology with no signs of active tidal stripping down to a surface brightness limit of 32.4 magarcsec^-2. Using a maximum likelihood analysis, we calculate the half-light radius of ComBer to be r_half=74 +/- 4 pc (5.8 +/- 0.3 arcmin) and its ellipticity e=0.36 +/- 0.04. In contrast, UMa II shows signs of on-going disruption. We map its morphology down to mu_V=32.6 mag arcsec^-2 and found that UMa II is larger than previously determined, extending at least ~700 pc (1.2 deg on the sky) and it is also quite elongated with an ellipticity of e=0.50 +/- 0.2. However, our estimate for the half-light radius, 123 +/- 3 pc (14.1 +/- 0.3 arcmin) is similar to previous results. We discuss the implications of these findings in the context of potential indirect dark matter detections and galaxy formation. We conclude that while ComBer appears to be a stable dwarf galaxy, UMa II shows signs of on-going tidal interaction.