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

• Main Authors: Alatalo, Katherine, Bitsakis, Theodoros, Lanz, Lauranne, Lacy, Mark, Brown, Michael J. I., French, K. Decker, Ciesla, Laure, Appleton, Philip N., Beaton, Rachael L., Cales, Sabrina L., Crossett, Jacob, Falcón-Barroso, Jesús, Kelson, Daniel D., Kewley, Lisa J., Kriek, Mariska, Medling, Anne M., Mulchaey, John S., Nyland, Kristina, Rich, Jeffrey A., Urry, C. Meg
• Other Authors: Univ Arizona, Steward Observ
• Language: en
• Published: IOP PUBLISHING LTD 2017
• Subjects: galaxies: evolution; galaxies: star formation; galaxies: stellar content; infrared: galaxies
• Online Access: http://hdl.handle.net/10150/624929; http://arizona.openrepository.com/arizona/handle/10150/624929

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 mu m 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 mu m) properties of post-starbursts are consistent with either 11.3 mu m 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 mu m emission detected with signal-to-noise ratio >= 3 requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22 mu m. The composite SED of WISE. 22 mu m non-detections (S/N < 3), created by stacking 22 mu m 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.