Predicting Quiescence: The Dependence of Specific Star Formation Rate on Galaxy Size and Central Density at 0.5 < z < 2.5

Abstract

In this paper, we investigate the relationship between star formation and structure, using a mass-complete sample of 27,893 galaxies at 0.5<z<2.5 selected from 3D-HST. We confirm that star-forming galaxies are larger than quiescent galaxies at fixed stellar mass (M  ). However, in contrast with some simulations, there is only a weak relation between star formation rate (SFR) and size within the star-forming population: when dividing into quartiles based on residual offsets in SFR, we find that the sizes of star-forming galaxies in the lowest quartile are 0.27±0.06 dex smaller than the highest quartile. We show that 50% of star formation in galaxies at fixed M  takes place within a narrow range of sizes (0.26 dex). Taken together, these results suggest that there is an abrupt cessation of star formation after galaxies attain particular structural properties. Confirming earlier results, we find that central stellar density within a 1 kpc fixed physical radius is the key parameter connecting galaxy morphology and star formation histories: galaxies with high central densities are red and have increasingly lower SFR/M  , whereas galaxies with low central densities are blue and have a roughly constant (higher) SFR/M  at a given redshift. We find remarkably little scatter in the average trends and a strong evolution of >0.5 dex in the central density threshold correlated with quiescence from z∼0.7-2.0. Neither a compact size nor high-n are sufficient to assess the likelihood of quiescence for the average galaxy; instead, the combination of these two parameters together with M  results in a unique quenching threshold in central density/velocity.

The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation

“…4 is inspired by Fig. 1 in Whitaker et al (2017) and shows very similar trends to the observations from the 3D-HST survey presented there. The full galaxy population (left column) shows a strong and continuous trend of decreasing sSFR with decreasing size, at a given stellar mass in the range 10 10−11 M .…”

“…The value corresponding to the lower line, 266 km s −1 , is adopted from van Dokkum et al (2015) as the value where the quenched fraction is about 50% at 1.5 < z < 3. For lower redshifts, the values are scaled following Franx et al (2008), as galaxies at lower redshifts are found observationally to quench at lower velocity dispersions (or densities; see also Woo et al 2015;Whitaker et al 2017). This scaling leads to a value of 199 km s −1 at z = 1 and 168 km s −1 at z = 0.…”

“…It has generally been found that: (i) more massive galaxies tend to be larger, (ii) late-type/star-forming galaxies are larger than early-type/quenched ones, and (iii) galaxies are smaller at higher redshifts, more so for high-mass than for low-mass galaxies, as well as more so for quenched than for star-forming galaxies 1 . There appears to be an intimate relation between star-formation activity and size, in that most star-formation occurs within a narrow ( 0.3 dex) size range (at a given stellar mass and redshift), and there exists a threshold in central density beyond which galaxies are quenched Franx et al 2008;van Dokkum et al 2015;Bluck et al 2016;Whitaker et al 2017;Mosleh et al 2017).…”

Structural and Star-forming Relations since z ∼ 3: Connecting Compact Star-forming and Quiescent Galaxies

“…A dichotomy is also present in the sizemass relations, where quiescent galaxies exhibit a much steeper slope than SFGs and a lower normalization, i.e., higher densities (Williams et al 2010;Newman et al 2012;Shibuya et al 2015). Although both the SFR-MS and the size-mass scaling relations evolve with time, the fundamental structural differences in SFGs and quiescent galaxies are always present, suggesting that having concentrated (denser) surface density profiles is a requisite for quenching (Kauffmann et al 2003(Kauffmann et al , 2006Schiminovich et al 2007;Bell 2008;Cheung et al 2012;Fang et al 2013;Lang et al 2014;Abramson et al 2016;Margalef-Bentabol et al 2016;Whitaker et al 2017). In other words, SFGs must grow dense cores before quenching.…”

“…Here we study the evolution since z 3 of the stellar mass surface density within a radius of 1 kpc, 1 S . We build upon previous results at lower redshift, which show that 1 S is closely related with quiescence and follows a much tighter correlation with stellar mass than the effective radius or the effective surface density, e S (e.g., Cheung et al 2012;Fang et al 2013;Tacchella et al 2015;Woo et al 2015;Whitaker et al 2017). We aim to answer whether this relation holds at high redshift, if it is a more fundamental quenching predictor, and if the global build up and quenching of SFGs can be described in simple terms using 1 S , i.e., if it can be used to track galaxies in transit from the star-forming to the quiescent phase.…”