The mass and structural evolution of massive galaxies is one of the hottest topics in galaxy formation. This is because it may reveal invaluable insights into the still debated evolutionary processes governing the growth and assembly of spheroids. Ho
wever, direct comparison between models and observations is usually prevented by the so-called progenitor bias, i.e., new galaxies entering the observational selection at later epochs, thus eluding a precise study of how pre-existing galaxies actually evolve in size. To limit this effect, we here gather data on high-redshift brightest group and cluster galaxies, evolve their (mean) host halo masses down to z=0 along their main progenitors, and assign as their descendants local SDSS central galaxies matched in host halo mass. At face value, the comparison between high redshift and local data suggests a noticeable increase in stellar mass of a factor of >2 since z~1, and of >2.5 in mean effective radius. We then compare the inferred stellar mass and size growth with those predicted by hierarchical models for central galaxies, selected at high redshifts to closely match the halo and stellar mass bins as in the data. Only hierarchical models characterized by very limited satellite stellar stripping and parabolic orbits are capable of broadly reproducing the stellar mass and size increase of a factor ~2-4 observed in cluster galaxies since z ~1. The predicted, average (major) merger rate since z~1 is in good agreement with the latest observational estimates.
The stellar mass-halo mass relation is a key constraint in all semi-analytic, numerical, and semi-empirical models of galaxy formation and evolution. However, its exact shape and redshift dependence remain debated. Several recent works support a rela
tion in the local Universe steeper than previously thought. Based on the comparisons with a variety of data on massive central galaxies, we show that this steepening holds up to z~1, for stellar masses Mstar>2e11 Msun. Specifically, we find significant evidence for a high-mass end slope of beta>0.35-0.70, instead of the usual beta~0.20-0.30 reported by a number of previous results. When including the independent constraints from the recent BOSS clustering measurements, the data, independent of any systematic errors in stellar masses, tend to favor a model with a very small scatter (< 0.15 dex) in stellar mass at fixed halo mass, in the redshift range z < 0.8 and for Mstar>3e11 Msun, suggesting a close connection between massive galaxies and host halos even at relatively recent epochs. We discuss the implications of our results with respect to the evolution of the most massive galaxies since z~1.
We compare state-of-the-art semi-analytic models of galaxy formation as well as advanced sub-halo abundance matching models with a large sample of early-type galaxies from SDSS at z < 0.3. We focus our attention on the dependence of median sizes of c
entral galaxies on host halo mass. The data do not show any difference in the structural properties of early-type galaxies with environment, at fixed stellar mass. All hierarchical models considered in this work instead tend to predict a moderate to strong environmental dependence, with the median size increasing by a factor of about 1.5-3 when moving from low to high mass host haloes. At face value the discrepancy with the data is highly significant, especially at the cluster scale, for haloes above log Mhalo > 14. The convolution with (correlated) observational errors reduces some of the tension. Despite the observational uncertainties, the data tend to disfavour hierarchical models characterized by a relevant contribution of disc instabilities to the formation of spheroids, strong gas dissipation in (major) mergers, short dynamical friction timescales, and very short quenching timescales in infalling satellites. We also discuss a variety of additional related issues, such as the slope and scatter in the local size-stellar mass relation, the fraction of gas in local early-type galaxies, and the general predictions on satellite galaxies.
There is mounting evidence that a significant fraction of Black Holes (BHs) today live in late-type galaxies, including bulge-less galaxies and those hosting pseudobulges, and are significantly undermassive with respect to the scaling relations follo
wed by their counterpart BHs in classical bulges of similar stellar (or even bulge) mass. Here we discuss the predictions of two state-of-the-art hierarchical galaxy formation models in which BHs grow via mergers and, in one, also via disk instability. Our aim is to understand if the wealth of new data on local BH demography is consistent with standard models. We follow the merger trees of representative subsamples of BHs and compute the fractional contributions of different processes to the final BH mass. We show that the model in which BHs always closely follow the growth of their host bulges, also during late disk instabilities (i.e., bars), produces too narrow a distribution of BHs at fixed stellar mass to account for the numerous low-mass BHs now detected in later-type galaxies. Models with a looser connection between BH growth and bar instability instead predict the existence of a larger number of undermassive BHs, in better agreement with the observations. The scatter in the updated local BH-bulge mass relation (with no restriction on galaxy type) appears to be quite large when including later-type systems, but it can still be managed to be reproduced within current hierarchical models. However, the fuelling of BHs during the late bar-instability mode needs to be better quantified/improved to properly fit the data. We conclude discussing how the possibly large number of BHs in later type galaxies demands for an in-depth revision of the local BH mass function and its modelling.
We present basic predictions of an updated version of the Munich semi-analytic hierarchical galaxy formation model that grows bulges via mergers and disk instabilities. Overall, we find that while spheroids below Ms ~ 10^11 Msun grow their sizes via
a mixture of disk instability and mergers, galaxies above it mainly evolve via mergers. Including gas dissipation in major mergers, efficiently shrinks galaxies, especially those with final mass Ms < 10^11 Msun that are the most gas-rich, improving the match with different observables. We find that the predicted scatter in sizes at fixed stellar mass is still larger than the observed one by up to <40%. Spheroids are, on average, more compact at higher redshifts at fixed stellar mass, and at fixed redshift and stellar mass larger galaxies tend to be more starforming. More specifically, while for bulge-dominated galaxies the model envisages a nearly mass-independent decrease in sizes, the predicted size evolution for intermediate-mass galaxies is more complex. The z=2 progenitors of massive galaxies with mass around Ms and B/T>0.7 at z=0, are found to be mostly disc-dominated galaxies with a median B/T ~ 0.3, with only ~20% remaining bulge-dominated. The model also predicts that central spheroids living in more massive haloes tend to have larger sizes at fixed stellar mass. Including host halo mass dependence in computing velocity dispersions, allows the model to properly reproduce the correlations with stellar mass. We also discuss the fundamental plane, the correlations with galaxy age, the structural properties of pseudobulges, and the correlations with central black holes.
We investigate the characteristic radiative efficiency epsilon, Eddington ratio lambda, and duty cycle P_0 of high-redshift active galactic nuclei (AGN), drawing on measurements of the AGN luminosity function at z=3-6 and, especially, on recent measu
rements of quasar clustering at z=3-4.5 from the Sloan Digital Sky Survey. The free parameters of our models are epsilon, lambda, and the normalization, scatter, and redshift evolution of the relation between black hole mass mbh and halo virial velocity V_vir. We compute the luminosity function from the implied growth of the black hole mass function and the quasar correlation length from the bias of the host halos. We test our adopted formulae for the halo mass function and halo bias against measurements from the large N-body simulation developed by the MICE collaboration. The strong clustering of AGNs observed at z=3 and, especially, at z=4 implies that massive black holes reside in rare, massive dark matter halos. Reproducing the observed luminosity function then requires high efficiency epsilon and/or low Eddington ratio lambda, with a lower limit (based on 2sigma agreement with the measured z=4 correlation length) epsilon> 0.7lambda/(1+0.7lambda), implying epsilon > 0.17 for lambda > 0.25. Successful models predict high duty cycles, P_0~0.2, 0.5, and 0.9 at z=3.1, 4.5 and 6, respectively, and they require that the fraction of halo baryons locked in the central black hole is much larger than the locally observed value. The rapid drop in the abundance of the massive and rare host halos at z>7 implies a proportionally rapid decline in the number density of luminous quasars, much stronger than simple extrapolations of the z=3-6 luminosity function would predict. (abridged)
We discuss how the effective radius Phi(Re) function (ERF) recently worked out by Bernardi et al. (2009) represents a new testbed to improve the current understanding of Semi-analytic Models of Galaxy formation. In particular, we here show that a det
ailed hierarchical model of structure formation can broadly reproduce the correct peak in the size distribution of local early-type galaxies, although it significantly overpredicts the number of very compact and very large galaxies. This in turn is reflected in the predicted size-mass relation, much flatter than the observed one, due to too large (~3 kpc) low-mass galaxies (<10^11 msun), and to a non-negligible fraction of compact (< 0.5-1 kpc) and massive galaxies (> 10^11 msun). We also find that the latter discrepancy is smaller than previously claimed, and limited to only ultracompact (Re < 0.5 kpc) galaxies when considering elliptical-dominated samples. We explore several causes behind these effects. We conclude that the former problem might be linked to the initial conditions, given that large and low-mass galaxies are present at all epochs in the model. The survival of compact and massive galaxies might instead be linked to their very old ages and peculiar merger histories. Overall, knowledge of the galactic stellar mass {em and} size distributions allows a better understanding of where and how to improve models.
We cross-correlate the SDSS DR3 quasar sample with FIRST and the Vestergaard et al. black hole (BH) mass sample to compare the mean accretion histories of optical and radio quasars. We find significant statistical evidence that radio quasars have a h
igher mean Eddington ratio Lambda at z > 2 with respect to optical quasars, while the situation is clearly reverse at z < 1. At z > 2 radio quasars happen to be less massive than optical quasars; however, as redshift decreases radio quasars appear in increasingly more massive BHs with respect to optical quasars. These two trends imply that radio sources are not a mere random subsample of optical quasars. No clear correlation between radio activity and BH mass and/or accretion rate is evident from our data, pointing to other BH properties, possibly the spin, as the driver of radio activity. We have checked that our main results do not depend on any evident bias. We perform detailed modelling of reasonable accretion histories for optical and radio quasars, finding that radio quasars grow by a factor of a few, at the most, since z ~ 4. The comparison between the predicted mass function of active radio quasars and the observed optical luminosity function of radio quasars, implies a significantly lower probability for lower mass BHs to be radio loud at all epochs, in agreement with what is observed in the local universe.
We utilize the local velocity dispersion function (VDF) of spheroids, together with their inferred age--distributions, to predict the VDF at higher redshifts (0<z<6), under the assumption that (i) most of the stars in each nearby spheroid formed in a
single episode, and (ii) the velocity dispersion sigma remained nearly constant afterward. We assume further that a supermassive black hole (BH) forms concurrently with the stars, and within ~1 Gyr of the formation of the potential well of the spheroid, and that the relation between the mass of the BH and host velocity dispersion maintains the form M_BH ~ sigma^{beta} with beta~4, but with the normalization allowed to evolve with redshift as ~(1+z)^{alpha}. We compute the BH mass function associated with the VDF at each redshift, and compare the accumulated total BH mass density with that inferred from the integrated quasar luminosity function (LF; the so--called Soltan argument). This comparison is insensitive to the assumed duty cycle or Eddington ratio of quasar activity, and we find that the match between the two BH mass densities favors a relatively mild redshift evolution, with alpha ~ 0.26, with a positive evolution as strong as alpha>1.3 excluded at the 99% confidence level. A direct match between the characteristic BH mass in the VDF--based and quasar LF--based BH mass functions also yields a mean Eddington ratio of lambda ~ 0.5-1 that is roughly constant within 0<z<3. A strong positive evolution in the M_BH-sigma relation is still allowed by the data if galaxies increase, on average, their velocity dispersions since the moment of formation, due to dissipative processes. If we assume that the mean velocity dispersion of the host galaxies evolves as sigma(z)=sigma(0)*(1+z)^{-gamma}, we find a lower limit of gamma>0.23 for alpha>1.5. abridged
