HI study of isolated and pair galaxies: the MIR SFR-M$\star$ sequence

TL;DR

This study compares star formation and HI content in isolated versus paired galaxies using WISE data, revealing that isolated galaxies have more predictable properties and less scatter in their star formation and gas content than paired galaxies.

Contribution

First derivation of an HI scaling relation for isolated galaxies using WISE stellar masses, providing a baseline to assess environmental effects on HI content.

Findings

Isolated galaxies show lower dispersion in star formation and HI properties.

Fewer isolated galaxies are quiescent compared to paired galaxies.

Environmental effects increase stochasticity in galaxy properties.

Abstract

Using mid-infrared star formation rate and stellar mass indicators in $\textit{WISE}$, we construct and contrast the relation between star formation rate and stellar mass for isolated and paired galaxies. Our samples comprise a selection of AMIGA (isolated galaxies) and pairs of ALFALFA galaxies with HI detections such that we can examine the relationship between HI content (gas fraction, HI deficiency) and galaxy location on the main sequence (MS) in these two contrasting environments. We derive for the first time an HI scaling relation for isolated galaxies using $\textit{WISE}$ stellar masses, and thereby establish a baseline predictor of HI content that can be used to assess the impact of environment on HI content when compared with samples of galaxies in different environments. We use this updated relation to determine the HI deficiency of both our paired and isolated galaxies. Across all the quantities examined as a function of environment in this work (MS location, gas fraction, and HI deficiency), the AMIGA sample of isolated galaxies is found to have the lower dispersion: $\sigma_{\rm{AMIGA}} = 0.37$ versus $\sigma_{\rm{PAIRS}} = 0.55$ on the MS, $\sigma_{\rm{AMIGA}} = 0.44$ versus $\sigma_{\rm{PAIRS}} = 0.54$ in gas fraction, and $\sigma_{\rm{AMIGA}} = 0.28$ versus $\sigma_{\rm{PAIRS}} = 0.34$ in HI deficiency. We also note fewer isolated quiescent galaxies, 3 (0.6$\%$), compared to 12 (2.3$\%$) quiescent pair members. Our results suggest the differences in scatter measured between our samples are environment driven. Galaxies in isolation behave relatively predictably, and galaxies in more densely populated environments adopt a more stochastic behaviour, across a broad range of quantities.