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) and H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>α</mml:mi> </mml:math> luminosity (L), when H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>α</mml:mi> </mml:math> 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/Lincreases steadily with decreasing H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>α</mml:mi> </mml:math> equivalent width (W, 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/L can be up to a factor of 30 larger than for star-forming galaxies. The smooth change in L/L, irrespective of star formation history, ionisation or heating source, dust temperature or other properties, suggests that the value of L/L is determined by the balance between star-forming regions and ambient interstellar medium contributing to both L and L. It is not a result of the differing timescales of star formation that these luminosities probe. While L can only be used to estimate the SFR for galaxies with W &gt; 3 A (sSFR <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≳</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11.5</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> /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 &gt;10 A (sSFR <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≳</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10.5</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> /yr). We find no evidence for dust obscured star-formation in local post-starburst galaxies.