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$_{TIR}$) and H$\alpha$ luminosity (L$_{H\alpha}$), when H$\alpha$ 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$_{gal}$), we combine optical spectroscopy from the SDSS with multi-wavelength (FUV to FIR) photometric observations from GAMA. We find that L$_{TIR}$/L$_{H\alpha}$ increases steadily with decreasing H$\alpha$ equivalent width (W$_{H\alpha}$, 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$_{TIR}$/L$_{H\alpha}$ can be up to a factor of 30 larger than for star-forming galaxies. The smooth change in L$_{TIR}$/L$_{H\alpha}$, irrespective of star formation history, ionisation or heating source, dust temperature or other properties, suggests that the value of L$_{TIR}$/L$_{H\alpha}$ is determined by the balance between star-forming regions and ambient interstellar medium contributing to both L$_{TIR}$ and H$\alpha$. It is not a result of the differing timescales of star formation that these luminosities probe. While L$_{H\alpha}$ can only be used to estimate the SFR for galaxies with $W_{H\alpha}$ > 3A (sSFR $\gtrsim 10^{-11.5}$/yr), we argue that the mid- and far-infrared can only be used to estimate the SFR of galaxies on the star-forming sequence with W$_{H\alpha}$ >10A (sSFR $\gtrsim 10^{-10.5}$/yr). We find no evidence for dust obscured star-formation in local post-starburst galaxies.