Implications on star-formation-rate indicators from HII regions and diffuse ionised gas in the M101 Group

TL;DR

This study investigates the sources of diffuse ionised gas in the M101 Group, finding that leakage from HII regions, rather than field stars, primarily ionizes the gas, and highlighting discrepancies in star formation rate indicators.

Contribution

It demonstrates that HII region leakage dominates DIG ionization in the M101 Group and explains the observed differences in SFR estimates from Hα and FUV data.

Findings

DIG Hα brightness correlates with ionising flux from HII regions.

SFR derived from Hα is systematically lower than FUV-based SFR in DIG.

Declining SFR ratios at low SFR are due to spatial averaging effects.

Abstract

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 H$\alpha$ narrow-band imaging and archival GALEX UV imaging. We find a strong correlation between DIG H$\alpha$ 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 H$\alpha$ 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 H$\alpha$- and FUV-derived star formation rates (SFRs) in the DIG, with SFR$_{{\rm H}\alpha} < $SFR$_{\rm FUV}$ everywhere. Within HII regions, we find a constant SFR ratio of 0.44 to a limit of $\sim10^{-5}$ M$_{\odot}$~yr$^{-1}$. This result is in tension with other studies of star formation in spiral galaxies, which typically show a declining SFR$_{{\rm H}\alpha}/$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 H$\alpha$ 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.