We investigate the properties of galaxies as they shut off star formation over the 4 billion years surrounding peak cosmic star formation. To do this, we categorize ∼7000 galaxies from 1 < z < 4 into 90 groups based on the shape of their spectral energy distributions (SEDs) and build composite SEDs with R ∼ 50 resolution. These composite SEDs show a variety of spectral shapes and also show trends in parameters such as color, mass, star formation rate, and emission-line equivalent width. Using emission-line equivalent widths and strength of the 4000 Å break, D(4000), we categorize the composite SEDs into five classes: extreme emission line, star-forming, transitioning, post-starburst, and quiescent galaxies. The transitioning population of galaxies shows modest Hα emission (EWREST ∼ 40 Å) compared to more typical star-forming composite SEDs at log10(M/M⊙) ∼ 10.5 (EWREST ∼ 80 Å). Together with their smaller sizes (3 kpc vs. 4 kpc) and higher Sérsic indices (2.7 vs. 1.5), this indicates that morphological changes initiate before the cessation of star formation. The transitional group shows a strong increase of over 1 dex in number density from z ∼ 3 to z ∼ 1, similar to the growth in the quiescent population, while post-starburst galaxies become rarer at z ≲ 1.5. We calculate average quenching timescales of 1.6 Gyr at z ∼ 1.5 and 0.9 Gyr at z ∼ 2.5 and conclude that a fast-quenching mechanism producing post-starbursts dominated the quenching of galaxies at early times, while a slower process has become more common since z ∼ 2.
Bibliographical noteCopyright 2018 The American Astronomical Society. First published in the Astrophysical journal, 863(2), 131, 2018, published by IOP Publishing. The original publication is available at http://www.doi.org/10.3847/1538-4357/aad232. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.
- galaxies: evolution
- galaxies: high-redshift