Searching for Spectroscopic Signatures of Ongoing Quenching in SDSS Galaxies

Abstract

In this paper, we estimate the “star formation change parameter,” SFR79, which characterizes the current SFR relative to the average during the last 800 Myr, for ∼300,000 galaxies selected from the Sloan Digital Sky Survey (SDSS). The goals are to examine, in a much larger and independent sample, the trends previously reported in a sample of star-forming (SF) MaNGA galaxies and also to search for spectroscopic signatures of ongoing quenching in the so-called “Green Valley,” which is generally believed to contain galaxies that are migrating from the SF population to the quenched population of galaxies. Measuring SFR79 for our large sample of SDSS galaxies, we first confirm the basic results of SF galaxies published by Wang & Lilly. We then discuss in detail the calibration and meaning of SFR79 for galaxies that are well below the SFMS and establish the expected statistical signature of systematic ongoing quenching from modeling the z ∼ 0 quenching rate of the SF population. We conclude that it is not possible at present to establish unambiguous observational evidence for systematic ongoing quenching processes, due to limitations both in the noise of the observational data, in particular in the measurements of Hδ absorption, and in the calibration of SFR79, as well as biases introduced by the necessity of selecting objects with significant Hα emission. We do however see plausible indications of ongoing quenching, which are quantitatively consistent with expectations from “growth+quenching” models of galaxy evolution and a typical e-folding timescale for quenching of ∼500 Myr.