H I study of isolated and paired galaxies: the MIR SFR-M⋆ sequence

Using mid-infrared star formation rate and stellar mass indicators in WISE (Wide-field Infrared Survey Explorer), 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 (Analysis of the interstellar Medium in Isolated GAlaxies; isolated galaxies) and pairs of ALFALFA (Arecibo Legacy Fast ALFA) galaxies with H I detections such that we can examine the relationship between H I content (gas fraction, H I deficiency) and galaxy location on the main sequence (MS) in these two contrasting environments. We derive for the first time an H I scaling relation for isolated galaxies using WISE stellar masses, and thereby establish a baseline predictor of H I content that can be used to assess the impact of environment on H I content when compared with samples of galaxies in different environments. We use this updated relation to determine the H I 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 H I deficiency), the AMIGA sample of isolated galaxies is found to have the lower dispersion: σ_AMIGA = 0.37 versus σ_PAIRS = 0.55 on the MS, σ_AMIGA = 0.44 versus σ_PAIRS = 0.54 in gas fraction, and σ_AMIGA = 0.28 versus σ_PAIRS = 0.34 in H I deficiency. We also note fewer isolated quiescent galaxies, 3 (0.6 ${{\ \rm per\ cent}}$ ), compared to 12 (2.3 ${{\ \rm per\ cent}}$ ) 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.