Implications on star formation rate indicators from H II regions and diffuse ionized gas in the M101 Group

A. E. Watkins, J. Christopher Mihos, Paul Harding, Garner III Ray

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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.

Original languageEnglish
Article numberstae1153
Pages (from-to)4560-4577
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date30 Apr 2024
Publication statusPublished - 1 Jun 2024


  • astro-ph.GA
  • ultraviolet: galaxies
  • ISM: evolution
  • galaxies: star formation
  • ISM: clouds
  • ISM: H II regions


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