2024/04/29 by A. E. Watkins, J. Christopher Mihos, Watkins, A. E. +5 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Stellar, planetary, and galactic studies
paper · doi:10.48550/arxiv.2404.19003
openalex publication_date 2024/04/29 · openalex created_date 2024/05/03 · openalex updated_date 2026/07/28
We examine the connection between diffuse ionised 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 ionising 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 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 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_\rm Hα < SFR\rm FUV everywhere. Within HII regions, we find a constant SFR ratio of 0.44 to a limit of ∼10-5 M\odot~yr-1. This result is in tension with other studies of star formation in spiral galaxies, which typically show a declining SFR_\rm Hα/SFR\rm 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-ionising FUV-emitting stellar populations in different regions within the galaxy.