On the cosmic evolution of the specific star formation rate

Abstract

The apparent correlation between the specific star formation rate (sSFR) and total stellar mass (M ) of galaxies is a fundamental relationship indicating how they formed their stellar populations. To attempt to understand this relation, we hypothesize that the relation and its evolution is regulated by the increase in the stellar and gas mass surface density in galaxies with redshift, which is itself governed by the angular momentum of the accreted gas, the amount of available gas, and by self-regulation of star formation. With our model, we can reproduce the specific SFR− M relations at z ∼ 1-2 by assuming gas fractions and gas mass surface densities similar to those observed for z = 1-2 galaxies. We further argue that it is the increasing angular momentum with cosmic time that causes a decrease in the surface density of accreted gas. The gas mass surface densities in galaxies are controlled by the centrifugal support (i.e., angular momentum), and the sSFR is predicted to increase as, sSFR(z) = (1 + z) 3 /t H0 , as observed (where t H0 is the Hubble time and no free parameters are necessary). In addition, the simple evolution for the star-formation intensity we propose is in agreement with observations of Milky Way-like galaxies selected through abundance matching. At z > ∼ 2, we argue that star formation is self-regulated by high pressures generated by the intense star formation itself. The star formation intensity must be high enough to either balance the hydrostatic pressure (a rather extreme assumption) or to generate high turbulent pressure in the molecular medium which maintains galaxies near the line of instability (i.e. Toomre Q ∼ 1). We provide simple prescriptions for understanding these self-regulation mechanisms based on solid relationships verified through extensive study. In all cases, the most important factor is the increase in stellar and gas mass surface density with redshift, which allows distant galaxies to maintain high levels of sSFR. Without a strong feedback from massive stars, such galaxies would likely reach very high sSFR levels, have high star formation efficiencies, and because strong feedback drives outflows, ultimately have an excess of stellar baryons.

Spatially-resolved star formation histories of CALIFA galaxies

“…4. There is a remarkable similarity between the SFH of Sbc galaxies of M ∼ 7 × 10 10 M in our sample and the predictions for MW-like galaxies proposed by Haywood et al (2015) and Lehnert et al (2015) for the formation of the thick disk. In agreement with these models, we obtain that in the central (R < 0.5 HLR) Σ SFR shows a significant drop from 600M Gyr −1 pc −2 at z > 2 to 100M Gyr −1 pc −2 at z < 1, while the global SFR decreases from ∼ 10 to 2 M yr −1 over the same period.…”

“…Lehnert et al (2014Lehnert et al ( , 2015 and Haywood et al (2016) extended this analysis, showing that this scenario is useful to explain the formation of MW-like galaxies at high z. They propose that there is a drop of the SFI in the thick disk phase to a more quiescent phase at z < 1, a drop that is more significant than the decrease expected from the exhaustion of gas given by a Schmidt-Kennicutt relation.…”

“…In the right panel of Fig. 9 we compare the SFI derived by Lehnert et al (2015) with our estimations for the same Sbc and Sc galaxies averaging Σ SFR over the central 0.5 HLR, that for these spirals corresponds to the size of the thick disk of the MW (Bland-Hawthorn & Gerhard 2016).…”

“…Our sSFRs declines more slowly with time, following roughly (1 + z) 2 in the highest M bin, but this comparison is probably plagued by the resolution issues discussed above. For the same reason, we cannot possibly resolve the initial plateau or rising in sSFR between z = 2 and 6 (Feulner et al 2005;Magdis et al 2010;Stark et al 2013;Lehnert et al 2015).…”

A Statistical and Multi-wavelength Study of Star Formation in Galaxies

“…1.8. The idea behind the morphological quenching mechanism is that the large stellar mass in the center of the bulge generates additional turbulence that, in virtue of the Toomre criterion (Toomre 1964), should make it harder for the gas to fragment into clumps within the disk (see, e.g., Lehnert et al 2015), therefore inhibiting star formation without actually expelling the gas out of the galaxy. This is supported by the recent finding that star formation seems to stop gradually from inside out (Tacchella et al 2015), probably concurrent to the growth of the bulge with respect to the disk.…”