The bending of the star-forming main sequence traces the cold- to hot-accretion transition mass over 0 < <i>z</i> < 4
E. Daddi, I. Delvecchio, Paola Dimauro, B. Magnelli, Carlos Gómez-Guijarro, R. T. Coogan, D. Elbaz, Boris S. Kalita, Aurélien Le Bail, R. Michael Rich, Qinghua Tan
Abstract
We analyse measurements of the evolving stellar mass (ℳ 0 ) at which the bending of the star-forming main sequence (MS) occurs over 0 < z < 4. We find ℳ 0 ≈ 10 10 M ⊙ over 0 < z < 1 before ℳ 0 rises up to ∼10 11 M ⊙ at z = 2 and then stays flat or slowly increases towards higher redshifts. When converting ℳ 0 values into hosting dark matter halo masses, we show that this behaviour is remarkably consistent with the evolving cold- to hot-accretion transition mass, as predicted by theory and defined by the redshift-independent M shock at z < 1.4 and by the rising M stream at z ≳ 1.4 (for which we propose a revision in agreement with the latest simulations). We therefore argue that the MS bending is primarily due to a drop in cold accretion, causing a reduction in available cold gas in galaxies, which supports predictions of gas feeding theory. In particular, the rapidly rising ℳ 0 with redshift at z > 1 is evidence in favour of the cold-streams scenario. In this picture, a progressive fuelling reduction rather than its sudden suppression in halos more massive than M shock / M stream produces a nearly constant star-formation rate in galaxies with stellar masses larger than ℳ 0 , and not their quenching, which therefore requires other physical processes. Compared to the knee M * in the stellar mass function of galaxies, ℳ 0 is significantly lower at z < 1.5, and higher at z > 2, suggesting that the imprint of gas deprivation on the distribution of galaxy masses happened at early times ( z > 1.5–2). The typical mass at which galaxies inside the MS become bulge-dominated evolves differently from ℳ 0 , which is consistent with the idea that bulge formation is a distinct process from the phasing out of cold accretion.