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

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    17 Citations (Scopus)


    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
    Article number9
    JournalAstrophysical Journal
    Issue number1
    Publication statusPublished - 1 Jul 2017


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