Shocked POststarbust Galaxy Survey I: Candidate Poststarbust Galaxies with Emission Line Ratios Consistent with Shocks (original) (raw)

There are many mechanisms by which galaxies can transform from blue, star-forming spirals to red, quiescent early-type galaxies, but our current census of them does not form a complete picture. Recent studies of nearby case studies seem to have identified a population of galaxies that quench “quietly.” Traditional poststarburst searches seem to catch galaxies only after they have quenched and transformed, and thus miss any objects with additional ionization mechanisms exciting the remaining gas. The Shocked POststarburst Galaxy Survey (SPOGS) aims to identify galaxies in an earlier phase of transformation, in which the nebular lines are excited via shocks instead of through star formation processes. Utilizing the Oh-Sarzi-Schawinski-Yi (OSSY) measurements on the Sloan Digital Sky Survey Data Release 7 catalog, we applied Balmer absorption and shock boundary criteria to identify 1,067 SPOG candidates (SPOGs*) within z = 0.2. SPOGs* represent 0.7% of emission line galaxies (and 0.2% of the OSSY sample galaxies, similar to the poststarburst fraction). SPOGs* colors suggest that they are in an earlier phase of transition than traditionally selected poststarburst galaxies. Quenching timescales are consistent with shock dissipation timescales. SPOGs* have a 13% 1.4 GHz detection rate from the Faint Images of the Radio Sky at Twenty centimeters survey, higher than most other subsamples, and comparable only to low-ionization nuclear emission line region hosts, suggestive of the presence of AGNs. SPOGs* also have stronger Na I D absorption than predicted from the stellar population, suggestive of interstellar winds. It appears that SPOGs* represent an earlier phase in galaxy transformation than traditionally selected poststarburst galaxies, and that a large proportion of SPOGs* also have properties consistent with disruption of their interstellar media, a key component to galaxy transformation. It is likely that many of the known pathways to transformation undergo a SPOG phase. Studying this sample of SPOGs* further, including their morphologies, active galactic nuclei properties, and environments, has the potential for us to build a more complete picture of the initial conditions that can lead to a galaxy evolving by finding galaxies previously not identified as transforming.