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

• Published in: arXiv.org
• Main Authors: Wild, Vivienne, Natalia Vale Asari, Rowlands, Kate, Ellison, Sara L, Ho-Hin Leung, Tremonti, Christy
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 13.09.2024
• Subjects: Cosmic dust; Infrared stars; Interstellar matter; Luminosity; Sky surveys (astronomy); Star & galaxy formation; Star formation rate; Starburst galaxies; Starbursts; Stars & galaxies; Stellar mass
• ISSN: 2331-8422

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/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\(_{TIR}\)/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 given by the balance between star-forming regions and ambient interstellar medium contributing to both L\(_{TIR}\) and L\(_{H\alpha}\). 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, and in particular only for galaxies with W\(_{H\alpha}\) >10 A (sSFR \(\gtrsim 10^{-10.5}\)/yr). We find no evidence for dust obscured star-formation in post-starburst galaxies.