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 ionised 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 Hα narrow-band imaging and archival GALEX UV imaging. We find a strong correlation between DIG Hα surface brightness and the incident ionising flux leaked from the nearby H ii regions, which we reproduce well using simple Cloudy simulations. While we also find a strong correlation between Hα and co-spatial FUV surface brightness in DIG, the extinction-corrected integrated UV colours in these regions imply stellar populations too old to produce the necessary ionising 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 Hα- and FUV-derived star formation rates (SFRs) in the DIG, with SFRHα <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 SFRHα/SFRFUV ratio at low SFR. We reproduce such trends only when considering spatially averaged photometry that mixes H ii regions, DIG, and regions lacking Hα 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-ionising FUV-emitting stellar populations in different regions within the galaxy.