Quenching and morphological transformation in semi-analytic models and CANDELS

Abstract

We examine the spheroid growth and star formation quenching experienced by galaxies since z ∼ 3 by studying the evolution with redshift of the quiescent and spheroid-dominated fractions of galaxies from the CANDELS and GAMA surveys. We compare the observed fractions with predictions from a semi-analytic model which includes prescriptions for bulge growth and AGN feedback due to mergers and disk instabilities. We facilitate direct morphological comparison by converting our model bulgeto-total stellar mass ratios to Sérsic indices. We then subdivide our population into the four quadrants of the sSFR-Sérsic index plane and study the buildup of each of these subpopulations. We find that the fraction of star forming disks declines steadily, while the fraction of quiescent spheroids builds up over cosmic time. The fractions of star forming spheroids and quiescent disks are both non-negligible, and stay nearly constant over the period we have studied. Our model is qualitatively successful at reproducing the evolution of the two "main" populations (star forming disks and quiescent spheroids), and approximately reproduces the relative fractions of all four types, but predicts a stronger decline in star forming spheroids, and increase in quiescent disks, than is seen in the observations. A model with an additional channel for bulge growth via disk instabilities agrees better overall with the observations than a model in which bulges can grow only through mergers. We also examine the relative importance of these different physical drivers of transformation (major and minor mergers and disk instabilities).

The Properties of the Massive Star-forming Galaxies with an Outside-in Assembly Mode

“…Further, Brennan et al (2015) confirmed that a model adding the disk instabilities for bulge growth agrees better with the observed galaxy distribution on the sSFR-Sérsic index plane than a model in which bulges grow only through mergers. By implementing the bulge growth model with the mergers and disk instabilities into the semi-analytic galaxy formation code, Tonini et al (2016) produced two distinct populations of bulges: merger-driven bulges, similar to classical bulges and instability-driven bulges, similar to pseudo-bulges.…”

“…The formation mechanisms of them are not clear. The disk instabilities in secular evolution and minor mergers may be another additional channels for bulge growth and morphological transformation (Porter et al 2014;Brennan et al 2015). In the near future, the on-going large IFS surveys will provide larger sample of galaxies with outside-in assembly mode, and help people to understand the origins of outside-in assembly mode.…”

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

“…On the other hand, galaxies with n>2 (implying more dominant bulges and higher central densities) have significantly lower sSFRs than the main ridgeline of the star formation sequence. Brennan et al (2015) presented a schematic diagram of how the various physical mechanisms could move galaxies around the sSFR-n plane, accounting for dry and wet mergers, disk instabilities, galaxy harassment, AGN feedback, and other slow gas-depletion processes. However, the relevant result from the present data is that both galaxy size and n alone are insufficient to isolate quiescent galaxies with low sSFRs.…”

The evolution of post-starburst galaxies from z=2 to 0.5

“…Choi et al 2014;Brennan et al 2015). Given the number of free parameters, studying the evolution of the distribution of galaxies in star-formation-rate/stellar-mass/morphology planes can only help to constrain the role of different processes in shutting down star formation in combination with detailed models which help to link cause and effect.…”