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

In galaxies with significant ongoing star formation there is an impressively tight correlation between total infrared luminosity (L T I R ) and H α luminosity (L H α ), when H α 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/Mgal, where Mgal 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 L T I R /L H α increases steadily with decreasing H α equivalent width (W H α , 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’, L T I R /L H α can be up to a factor of 30 larger than for star-forming galaxies. The smooth change in L T I R /L H α , irrespective of star formation history, ionisation or heating source, dust temperature or other properties, suggests that the value of L T I R /L H α is determined by the balance between star-forming regions and ambient interstellar medium contributing to both L T I R and L H α . It is not a result of the differing timescales of star formation that these luminosities probe. While L H α can only be used to estimate the SFR for galaxies with W H α > 3 A (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 W H α >10 A (sSFR ≳ 10 − 10.5 /yr). We find no evidence for dust obscured star-formation in local post-starburst galaxies.