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Register a new userQuenching and Morphological Transformation in Semi-Analytic Models and CANDELS
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We examine the spheroid growth and star formation quenching experienced by galaxies from z~3 to the present 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 bulge-to-total stellar mass ratios to Sersic indices. We then subdivide our population into the four quadrants of the sSFR-Sersic 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, at about 10% and 15-20% respectively. Our model is qualitatively successful at reproducing the evolution of the two main populations (star forming disk-dominated galaxies and quiescent spheroid-dominated galaxies), 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 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 may grow only through mergers. We study evolutionary tracks of some individual galaxies as they experience morphological transformation and quenching, and examine the importance of different physical drivers of this transformation (major and minor mergers and disk instabilities). We find that complex histories with multiple transformative events are the norm.
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We study the morphological transformation from late types to early types and the quenching of galaxies with the seventh Data Release (DR7) of the Sloan Digital Sky Survey (SDSS). Both early type galaxies and late type galaxies are found to have bimodal distributions on the star formation rate versus stellar mass diagram ($lg SFR - lg M_*$). We therefore classify them into four types: the star-forming early types (sEs), the quenched early types (qEs), the star-forming late types (sLs) and the quenched late types (qLs). We checked many parameters on various environmental scales for their potential effects on the quenching rates of late types and early types, as well as the early type fractions among star-forming galaxies and those among quenched galaxies. These parameters include: the stellar mass $M_*$, and the halo mass $M_{halo}$; the small-scale environmental parameters, such as the halo centric radius $R_p/r_{180}$ and the third nearest neighbor distances ($d_{3nn}$); the large-scale environmental parameters, specifically whether they are located in clusters, filaments, sheets, or voids. We found that the morphological transformation is mainly regulated by the stellar mass. Quenching is mainly driven by the stellar mass for more massive galaxies and by the halo mass for galaxies with smaller stellar masses. In addition, we see an overall stronger halo quenching effect in early type galaxies, which might be attributed to their lacking of cold gas or earlier accretion into the massive host halos.
It is now possible for hydrodynamical simulations to reproduce a representative galaxy population. Accordingly, it is timely to assess critically some of the assumptions of traditional semi-analytic galaxy formation models. We use the Eagle simulations to assess assumptions built into the Galform semi-analytic model, focussing on those relating to baryon cycling, angular momentum and feedback. We show that the assumption in Galform that newly formed stars have the same specific angular momentum as the total disc leads to a significant overestimate of the total stellar specific angular momentum of disc galaxies. In Eagle, stars form preferentially out of low specific angular momentum gas in the interstellar medium (ISM) due to the assumed gas density threshold for stars to form, leading to more realistic galaxy sizes. We find that stellar mass assembly is similar between Galform and Eagle but that the evolution of gas properties is different, with various indications that the rate of baryon cycling in Eagle is slower than is assumed in Galform. Finally, by matching individual galaxies between Eagle and Galform, we find that an artificial dependence of AGN feedback and gas infall rates on halo mass doubling events in Galform drives most of the scatter in stellar mass between individual objects. Put together our results suggest that the Galform semi-analytic model can be significantly improved in light of recent advances.
We study the correlation between the specific star formation rate of central galaxies and neighbour galaxies, also known as galactic conformity, out to 20 Mpc/h using three semi-analytic models (SAMs, one from L-GALAXIES and other two from GALFORM). The aim is to establish whether SAMs are able to show galactic conformity using different models and selection criteria. In all the models, when the selection of primary galaxies is based on an isolation criterion in real space, the mean fraction of quenched galaxies around quenched primary galaxies is higher than that around star-forming primary galaxies of the same stellar mass. The overall signal of conformity decreases when we remove satellites selected as primary galaxies, but the effect is much stronger in GALFORM models compared with the L-GALAXIES model. We find this difference is partially explained by the fact that in GALFORM once a galaxy becomes a satellite remains as such, whereas satellites can become centrals at a later time in L-GALAXIES. The signal of conformity decreases down to 60% in the L-GALAXIES model after removing central galaxies that were ejected from their host halo in the past. Galactic conformity is also influenced by primary galaxies at fixed stellar mass that reside in dark matter haloes of different masses. Finally, we explore a proxy of conformity between distinct haloes. In this case the conformity is weak beyond ~ 3 Mpc/h (<3% in L-GALAXIES, <1-2% in GALFORM models). Therefore, it seems difficult that conformity is directly related with a long-range effect.
We present a direct comparison between the observed star formation rate functions (SFRF) and the state-of-the-art predictions of semi-analytic models (SAM) of galaxy formation and evolution. We use the PACS Evolutionary Probe Survey (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) data-sets in the COSMOS and GOODS-South fields, combined with broad-band photometry from UV to sub-mm, to obtain total (IR+UV) instantaneous star formation rates (SFRs) for individual Herschel galaxies up to z~4, subtracted of possible active galactic nucleus (AGN) contamination. The comparison with model predictions shows that SAMs broadly reproduce the observed SFRFs up to z~2, when the observational errors on the SFR are taken into account. However, all the models seem to under-predict the bright-end of the SFRF at z>2. The cause of this underprediction could lie in an improper modelling of several model ingredients, like too strong (AGN or stellar) feedback in the brighter objects or too low fall-back of gas, caused by weak feedback and outflows at earlier epochs.
Several mechanisms for the transformation of blue star-forming to red quiescent galaxies have been proposed, and the green valley (GV) galaxies amid them are widely accepted in a transitional phase. Thus, comparing the morphological and environmental differences of the GV galaxies with early-type disks (ETDs; bulge dominated and having a disk) and late-type disks (LTDs; disk dominated) is suitable for distinguishing the corresponding quenching mechanisms. A large population of massive ($M_* geqslant 10^{10}M_odot$) GV galaxies at $0.5 leqslant z leqslant 1.5$ in 3D-HST/CANDELS is selected using extinction-corrected $(U-V)_{rm rest}$ color. After eliminating any possible active galactic nucleus candidates and considering the mass-matching, we finally construct two comparable samples of GV galaxies with either 319 ETD or 319 LTD galaxies. Compared to the LTD galaxies, it is found that the ETD galaxies possess higher concentration index and lower specific star formation rate, whereas the environments surrounding them are not different. This may suggest that the morphological quenching may dominate the star formation activity of massive GV galaxies rather than the environmental quenching. To quantify the correlation between the galaxy morphology and the star formation activity, we define a dimensionless morphology quenching efficiency $Q_{rm mor}$ and find that $Q_{rm mor}$ is not sensitive to the stellar mass and redshift. When the difference between the average star formation rate of ETD and LTD galaxies is about 0.7 $M_odot rm ;yr^{-1}$, the probability of $Q_{rm mor}gtrsim 0.2$ is higher than 90%, which implies that the degree of morphological quenching in GV galaxies might be described by $Q_{rm mor}gtrsim 0.2$.
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