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 HII regions. Using the Burrell Schmidt telescope, we target the brightest emission lines of star-forming regions, H$\alpha$, H$\beta$, and [OIII], to detect potential outlying star-forming regions. Our survey covers $\sim$6 square degrees around M101, and we detect objects in emission down to an H$\alpha$ flux level of $5.7 \times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ (equivalent to a limiting SFR of $1.7 \times 10^{-6}$ $M_\odot$ 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 H$\alpha$ and [OIII]. 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 HII regions or undiscovered star-forming dwarfs in the M101 Group, as most sources (93%) are consistent with being M101 outer disk HII regions. Only two sources were associated with other galaxies: a faint star-forming satellite of the background galaxy NGC 5486, and a faint outlying HII region near the M101 companion NGC 5474. We also find no narrowband emission associated with recently discovered ultradiffuse galaxies and starless HI 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 $\alpha \sim -1.0$) 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.