Catching Quenching Galaxies: The Nature of the WISE Infrared Transition Zone

TL;DR

This study identifies a distinct infrared color bifurcation in galaxies, linking it to different galaxy types and transitional stages, especially highlighting the role of dust, PAH emission, and AGN activity in galaxy evolution.

Contribution

It reveals a prominent infrared transition zone in galaxies and explores its connection to galaxy evolution, dust, and AGN activity, providing new insights into transitional galaxy populations.

Findings

Infrared bifurcation separates early- and late-type galaxies.

AGN are prevalent within the infrared transition zone.

Transitional galaxies shed their interstellar media as they evolve.

Abstract

We present the discovery of a prominent bifurcation between early-type galaxies and late-type galaxies, in [4.6]-[12] micron colors from the Wide Field Infrared Survey Explorer (WISE). We then use an emission-line diagnostic comparison sample to explore the nature of objects found both within, and near the edges of, this WISE infrared transition zone (IRTZ). We hypothesize that this birfurcation might be due to the presence of hot dust and PAH emission features in late-type galaxies. Using a sample of galaxies selected through the Shocked Poststarburst Galaxy Survey (SPOGS), we are able to identify galaxies with strong Balmer absorption (EW(Hdelta)>5 Angstroms) as well as emission lines inconsistent with star formation (deemed SPOG candidates, or SPOGs*) that lie within the optical green valley. Seyferts and low ionization nuclear emission line regions, whose u-r colors tend to be red, are strongly represented within the IRTZ, whereas SPOGs* tend to sit near the star-forming edge. Although AGN are well-represented in the IRTZ, we argue that the dominant IRTZ population are galaxies that are in late stages of transitioning across the optical green valley, shedding the last of their remnant interstellar media.