Implications on star-formation-rate indicators from H ii regions and diffuse ionised gas in the M101 Group

ABSTRACT We examine the connection between diffuse ionized gas (DIG), H ii regions, and field O and B stars in the nearby spiral M101 and its dwarf companion NGC 5474 using ultra-deep H α narrow-band imaging and archival GALEX UV imaging. We find a strong correlation between DIG H α surface brightness and the incident ionizing flux leaked from the nearby H ii regions, which we reproduce well using simple cloudy simulations. While we also find a strong correlation between H α and co-spatial far-ultraviolet (FUV) surface brightness in DIG, the extinction-corrected integrated UV colours in these regions imply stellar populations too old to produce the necessary ionizing photon flux. Combined, this suggests that H ii region leakage, not field OB stars, is the primary source of DIG in the M101 Group. Corroborating this interpretation, we find systematic disagreement between the H α- and FUV-derived star formation rates (SFRs) in the DIG, with SFRH α SFRFUV everywhere. Within H ii regions, we find a constant SFR ratio of 0.44 to a limit of ∼10−5 M⊙ yr−1. This result is in tension with other studies of star formation in spiral galaxies, which typically show a declining SFRH α/SFRFUV ratio at low SFR. We reproduce such trends only when considering spatially averaged photometry that mixes H ii regions, DIG, and regions lacking H α entirely, suggesting that the declining trends found in other galaxies may result purely from the relative fraction of diffuse flux, leaky compact H ii regions, and non-ionizing FUV-emitting stellar populations in different regions within the galaxy.