A Deep Census of Outlying Star Formation in the M101 Group

Abstract

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.