The quenching time scale and quenching rate of galaxies

TL;DR

This study constrains galaxy quenching timescales and rates by analyzing color distributions and number density profiles, revealing that star formation decline accelerates before galaxies become quiescent, with quenching rates increasing with galaxy mass.

Contribution

It introduces a simple toy-model framework to estimate quenching timescales and rates from color-color diagrams, providing new quantitative constraints on galaxy quenching processes.

Findings

Quenching timescale in the second phase is about 0.5 Gyr.

Quenching rates are 19%, 25%, and 33% per Gyr for different mass bins.

Star formation decline accelerates before galaxies become quiescent.

Abstract

The average star formation rate (SFR) in galaxies has been declining since redshift of 2. A fraction of galaxies quench and become quiescent. We constrain two key properties of the quenching process: the quenching time scale and the quenching rate among galaxies. We achieve this by analyzing the galaxy number density profile in NUV-u color space and the distribution in NUV-u v.s. u-i color-color diagram with a simple toy-model framework. We focus on galaxies in three mass bins between 10 to 10 and 10 to 10.6 solar mass. In the NUV-u v.s. u-i color-color diagram, the red u-i galaxies exhibit a different slope from the slope traced by the star-forming galaxies. This angled distribution and the number density profile of galaxies in NUV-u space strongly suggest that the decline of the SFR in galaxies has to accelerate before they turn quiescent. We model this color-color distribution with a two-phase exponential decline star formation history. The models with an e-folding time in the second phase (the quenching phase) of 0.5 Gyr best fit the data. We further use the NUV-u number density profile to constrain the quenching rate among star-forming galaxies as a function of mass. Adopting an e-folding time of 0.5 Gyr in the second phase (or the quenching phase), we found the quenching rate to be 19%/Gyr, 25%/Gyr and 33%/Gyr for the three mass bins. These are upper limits of quenching rate as the transition zone could also be populated by rejuvenated red-sequence galaxies.