Welcome to the Twilight Zone: The Mid-infrared Properties of Post-starburst Galaxies

Katherine Alatalo, Theodoros Bitsakis, Lauranne Lanz, Mark Lacy, Michael J.I. Brown, K. Decker French, Laure Ciesla, Philip N. Appleton, Rachael L. Beaton, Sabrina L. Cales, Jacob Crossett, Jesús Falcón-Barroso, Daniel D. Kelson, Lisa J. Kewley, Mariska Kriek, Anne M. Medling, John S. Mulchaey, Kristina Nyland, Jeffrey A. Rich, C. Meg Urry

Research output: Contribution to journalArticlepeer-review


We investigate the optical and Wide-field Survey Explorer (WISE) colors of "E+A" identified post-starburst galaxies, including a deep analysis of 190 post-starbursts detected in the 2 μm All Sky Survey Extended Source Catalog. The post-starburst galaxies appear in both the optical green valley and the WISE Infrared Transition Zone. Furthermore, we find that post-starbursts occupy a distinct region of [3.4]-[4.6] versus [4.6]-[12] WISE colors, enabling the identification of this class of transitioning galaxies through the use of broadband photometric criteria alone. We have investigated possible causes for the WISE colors of post-starbursts by constructing a composite spectral energy distribution (SED), finding that the mid-infrared (4-12 μm) properties of post-starbursts are consistent with either 11.3 μm polycyclic aromatic hydrocarbon emission, or thermally pulsating asymptotic giant branch (TP-AGB) and post-AGB stars. The composite SED of extended post-starburst galaxies with 22 μm emission detected with signal-to-noise ratio requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22 μm. The composite SED of WISE 22 μm non-detections (S/N < 3), created by stacking 22 μm images, is also flat, requiring a hot dust component. The most likely source of the mid-infrared emission of these E+A galaxies is a buried active galactic nucleus (AGN). The inferred upper limits to the Eddington ratios of post-starbursts are 10-2-10-4, with an average of 10-3. This suggests that AGNs are not radiatively dominant in these systems. This could mean that including selections capable of identifying AGNs as part of a search for transitioning and post-starburst galaxies would create a more complete census of the transition pathways taken as a galaxy quenches its star formation.

Original languageEnglish (US)
Article number9
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 1 2017
Externally publishedYes


  • galaxies: evolution
  • galaxies: star formation
  • galaxies: stellar content
  • infrared: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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