The infrared luminosity of retired and post-starburst galaxies : a cautionary tale for star formation rate measurements

Abstract

Funding: VW acknowledges Science and Technologies Facilities Council (STFC) grants ST/V000861/1 and ST/Y00275X/1. NVA and VW acknowledge the Royal Society and the Newton Fund via the award of a Royal Society–Newton Advanced Fellowship (grant NAF\R1\180403). NVA acknowledges support from Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq). KR acknowledges support from NASA grant 80NSSC23K0495.In galaxies with significant ongoing star formation there is an impressively tight correlation between total infrared luminosity (LTIR) and Hα luminosity (LHα), when Hα is properly corrected for stellar absorption and dust attenuation. This long-standing result gives confidence that both measurements provide accurate estimates of a galaxy’s star formation rate (SFR), despite their differing origins. To test the extent to which this holds in galaxies with lower specific SFR (sSFR=SFR/M*, where M* is the stellar mass), we combine optical spectroscopy from the Sloan Digital Sky Survey (SDSS) with multi-wavelength (FUV to FIR) photometric observations from the Galaxy And Mass Assembly survey (GAMA). We find that LTIR/LHα increases steadily with decreasing Hα equivalent width (WHα, a proxy for sSFR), indicating that both luminosities cannot provide a valid measurement of SFR in galaxies below the canonical star-forming sequence. For both ‘retired galaxies’ and ‘post-starburst galaxies’, LTIR/LHα can be up to a factor of 30 larger than for star-forming galaxies. The smooth change in LTIR/LHα, irrespective of star formation history, ionisation or heating source, dust temperature or other properties, suggests that the value of LTIR/LHα is determined by the balance between star-forming regions and ambient interstellar medium contributing to both LTIR and LHα. It is not a result of the differing timescales of star formation that these luminosities probe. While LHα can only be used to estimate the SFR for galaxies with WHα > 3 Å (sSFR ≳ 10-11.5 /yr), we argue that the mid- and far-infrared can only be used to estimate the SFR of galaxies on the star-forming sequence, and in particular only for galaxies with WHα > 10 Å (sSFR≳ 10-10.5 /yr). We find no evidence for dust obscured star-formation in local post-starburst galaxies.Peer reviewe