Description
We examine the connection between diffuse ionized gas (DIG), HII 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 ionizing flux leaked from the nearby HII regions, which we reproduce well using simple CLOUDY simulations. While we also find a strong correlation between Hα 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 HII 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 SFR(Hα) < SFR(FUV) everywhere. Within HII regions, we find a constant SFR ratio of 0.44 to a limit of ~10^-5 M⊙/yr. This result is in tension with other studies of star formation in spiral galaxies, which typically show a declining SFR(Hα)/SFR(FUV) ratio at low SFR. We reproduce such trends only when considering spatially averaged photometry that mixes HII 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 HII regions, and non-ionizing FUV-emitting stellar populations in different regions within the galaxy.
