Implications on star formation rate indicators from H II regions and diffuse ionized 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 SFR H α <SFR FUV 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 SFR H α/SFR FUV 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.