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

July 2017 • 2017ApJ...843....9A

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

Abstract • 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 ≥slant 3 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.


IPAC Authors


Phil Appleton

Senior Scientist