HI study of isolated and pair galaxies: the MIR SFR-Mstar sequence

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.