A Deep Census of Outlying Star Formation in the M101 Group

Abstract

We present deep, narrowband imaging of the nearby spiral galaxy M101 and its group environment to search for star-forming dwarf galaxies and outlying H II regions. Using the Burrell Schmidt telescope, we target the brightest emission lines of star-forming regions, Hα, Hβ, and [O III], to detect potential outlying star-forming regions. Our survey covers ~6 deg² around M101, and we detect objects in emission down to an Hα flux level of 5.7 × 10^-17 erg s^-1 cm^-2 (equivalent to a limiting star formation rate of 1.7 × 10^-6 M_⊙ yr^-1 at the distance of M101). After careful removal of background contaminants and foreground M stars, we detect 19 objects in emission in all three bands and 8 objects in emission in Hα and [O III]. We compare the structural and photometric properties of the detected sources to Local Group dwarf galaxies and star-forming galaxies in the 11HUGS and SINGG surveys. We find no large population of outlying H II regions or undiscovered star-forming dwarfs in the M101 Group, as most sources (93%) are consistent with being M101 outer-disk H II regions. Only two sources were associated with other galaxies: a faint star-forming satellite of the background galaxy NGC 5486 and a faint outlying H II region near the M101 companion NGC 5474. We also find no narrowband emission associated with recently discovered ultradiffuse galaxies and starless H I clouds near M101. The lack of any hidden population of low-luminosity star-forming dwarfs around M101 suggests a rather shallow faint-end slope (as flat as α ~ -1.0) for the star-forming luminosity function in the M101 Group. We discuss our results in the context of tidally triggered star formation models and the interaction history of the M101 Group.