The Dragonfly Nearby Galaxies Survey. III. The Luminosity Function of the M101 Group

We obtained follow-up HST observations of the seven low surface brightness galaxies discovered with the Dragonfly Telephoto Array in the field of the massive spiral galaxy M101. Out of the seven galaxies, only three were resolved into stars and are potentially associated with the M101 group at $D=7\text{ Mpc}$. Based on HST ACS photometry in the broad F606W and F814W filters, we use a maximum likelihood algorithm to locate the Tip of the Red Giant Branch (TRGB) in galaxy color-magnitude diagrams. Distances are $6.38^{+0.35}_{-0.35}, 6.87^{+0.21}_{-0.30}$ and $6.52^{+0.25}_{-0.27} \text{ Mpc}$ and we confirm that they are members of the M101 group. Combining the three confirmed low luminosity satellites with previous results for brighter group members, we find the M101 galaxy group to be a sparsely populated galaxy group consisting of seven group members, down to $M_V = -9.2 \text{ mag}$. We compare the M101 cumulative luminosity function to that of the Milky Way and M31. We find that they are remarkably similar; In fact, the cumulative luminosity function of the M101 group gets even flatter for fainter magnitudes, and we show that the M101 group might exhibit the two known small-scale flaws in the $\Lambda\textrm{CDM}$ model, namely `the missing satellite' problem and the `too big to fail' problem. Kinematic measurements of M101$'$s satellite galaxies are required to determine whether the `too big to fail' problem does in fact exist in the M101 group.