The confinement of star-forming galaxies into a main sequence through episodes of gas compaction, depletion and replenishment

Abstract

Using cosmological simulations, we address the properties of high-redshift starforming galaxies (SFGs) across their main sequence (MS) in the plane of star-formation rate (SFR) versus stellar mass. We relate them to the evolution of galaxies through phases of gas compaction, depletion, possible replenishment, and eventual quenching. We find that the high-SFR galaxies in the upper envelope of the MS are compact, with high gas fractions and short depletion times ("blue nuggets"), while the lower-SFR galaxies in the lower envelope have lower central gas densities, lower gas fractions and longer depletion times, consistent with observed gradients across the MS. Stellarstructure gradients are negligible. The SFGs oscillate about the MS ridge on timescales ∼ 0.4 t Hubble (∼ 1 Gyr at z ∼ 3). The propagation upwards is due to gas compaction, triggered, e.g., by mergers, counter-rotating streams, and/or violent disc instabilities. The downturn at the upper envelope is due to central gas depletion by peak star formation and outflows while inflow from the shrunken gas disc is suppressed. An upturn at the lower envelope can occur once the extended disc has been replenished by fresh gas and a new compaction can be triggered, namely as long as the replenishment time is shorter than the depletion time. The mechanisms of gas compaction, depletion and replenishment confine the SFGs to the narrow (±0.3 dex) MS. Full quenching occurs in massive haloes (M vir > 10 11.5 M ) and/or at low redshifts (z < 3), where the replenishment time is long compared to the depletion time, explaining the observed bending down of the MS at the massive end.

The relationship between star formation activity and galaxy structural properties in CANDELS and a semi-analytic model

“…The radial size of the disk that forms is roughly proportional to the angular momentum of the gas, which (on average) traces that of the dark matter halo. Relatively minor perturbations, such as minor mergers or disk instabilities, cause galaxies to oscillate around the SFMS as suggested in Tacchella et al (2016), and seen also in our models. As long as galaxies remain in this relatively smooth undisturbed growth phase, their structural properties do not show a strong correlation with their distance from the SFMS.…”

“…In another type of study, Zolotov et al (2015) and Tacchella et al (2016) analyzed the star formation rates and sizes of a set of high resolution "zoomin" simulations. These 26 moderately massive halos are not representative of a cosmological sample, and have not been run past z = 1, but they attain considerably higher resolution and contain arguably more physical "sub-grid" recipes for processes such as star formation and stellar feedback than large cosmological volumes like Illustris.…”

“…These 26 moderately massive halos are not representative of a cosmological sample, and have not been run past z = 1, but they attain considerably higher resolution and contain arguably more physical "sub-grid" recipes for processes such as star formation and stellar feedback than large cosmological volumes like Illustris. Zolotov et al (2015) and Tacchella et al (2016) emphasize that in their simulations, mergers and violent disk instabilities can lead to rapid gas inflow, building a compact, dense nucleus. They find that this "compactification" phase is in general soon followed by a rapid decrease in SFR due to the changing decreasing inflow rate relative to the stellar-driven outflows.…”

“…They found that their model, which includes feedback from accreting supermassive black holes, was able to produce the population of quiescent bulge-dominated galaxies at z ∼ 0 needed to reproduce the distribution of observed morphologies. Tacchella et al (2016) examined how galaxies in the VELA simulations (Ceverino et al 2014) oscillate around the main sequence due to clumpy inflows and violent disk instabilities, leading to compaction and minor quenching episodes. We compare our model predictions with galaxies observed with the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS; Grogin et al 2011;Koekemoer et al 2011) and the Galaxy and Mass Assembly Survey (GAMA; Driver et al 2011).…”

High Angular Momentum Halo Gas: A Feedback and Code-independent Prediction of LCDM

“…This model of radiative feedback includes radiation pressure from ionizing and infrared photons, photoheating, and photoionization from massive stars. Other recent papers using similar physics include Zolotov et al (2015), Snyder et al (2015a), Ceverino et al (2015Ceverino et al ( , 2016Ceverino et al ( , 2017, Goerdt & Ceverino (2015), Tacchella et al (2016aTacchella et al ( , 2016b, and Tomassetti et al (2016), Mandelker et al (2017).…”