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

Vivienne Wild*, Natalia Vale Asari, Kate Rowlands, Sara L. Ellison, Ho-Hin Leung, Christy Tremonti

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

In galaxies with significant ongoing star formation there is an impressively tight correlation between total infrared luminosity (LTIR) and Hα luminosity (L), when Hα 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*, where M* 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 LTIR/L increases steadily with decreasing Hα 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’, LTIR/L can be up to a factor of 30 larger than for star-forming galaxies. The smooth change in LTIR/L, irrespective of star formation history, ionisation or heating source, dust temperature or other properties, suggests that the value of LTIR/L is determined by the balance between star-forming regions and ambient interstellar medium contributing to both LTIR 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 > 3 Å (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 > 10 Å (sSFR≳ 10-10.5 /yr). We find no evidence for dust obscured star-formation in local post-starburst galaxies.
Original languageEnglish
Article number128125
Number of pages16
JournalThe Open Journal of Astrophysics
Volume8
DOIs
Publication statusPublished - 9 Jan 2025

Keywords

  • Starbust galaxies
  • Star formation
  • GAMA
  • Sloan Digital Sky Survey

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