TURNING THE TIDES ON THE ULTRA-FAINT DWARF SPHEROIDAL GALAXIES: COMA BERENICES AND URSA MAJOR II

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 3 mag below the main-sequence turn-offs in these galaxies. We robustly calculate a total luminosity of MV = −3.8 ± 0.6 for ComBer and MV = −3.9 ± 0.5 for UMa II, in agreement with previous results and confirming that these galaxies are among the faintest of the known dwarf satellites of the Milky Way. ComBer shows a fairly regular morphology with no signs of active tidal stripping down to a surface brightness limit of 32.4 mag arcsec−2. Using a maximum likelihood analysis, we calculate the half-light radius of ComBer to be rhalf = 74 ± 4 pc (5.8 ± 0.′3) and its ellipticity ϵ = 0.36 ± 0.04. In contrast, UMa II shows signs of ongoing disruption. We map its morphology down to μV = 32.6 mag arcsec−2 and found that UMa II is larger than previously determined, extending at least ∼600 pc (1.°1 on the sky) and it is also quite elongated with an overall ellipticity of ϵ = 0.50 ± 0.2. However, our estimate for the half-light radius, 123 ± 3 pc (14.1 ± 0.′3) 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 ongoing tidal interaction.