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

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