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

Understand

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.