The average star formation rate (SFR) in galaxies has been declining since the redshift of 2. A fraction of galaxies quench and become quiescent. We constrain two key properties of the quenching process: the quenching timescale 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 versus ui color–color diagram with a simple toy-model framework. We focus on galaxies in three mass bins between 1010 and 1010.6 M . In the NUV−u versus ui color–color diagram, the red ui 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 the quenching rate as the transition zone could also be populated by rejuvenated red-sequence galaxies.

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Published in The Astrophysical Journal, v. 832, no. 1, 29, p. 1-9.

© 2016. The American Astronomical Society. All rights reserved.

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J. Lian gratefully acknowledges support from the China Scholarship Council. This work is supported by the Strategic Priority Research Program "The Emergence of Cosmological Structures" of the Chinese Academy of Sciences (No. XDB09000000), the National Basic Research Program of China (973 Program)(2015CB857004), and the National Natural Science Foundation of China (NSFC, Nos. 11225315, 1320101002, 11433005 and 11421303).

Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, NASA, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.

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