At the present time, early-type brightest cluster galaxies (BCGs) in the Sloan Digital Sky Survey (SDSS) MaxBCG and C4 catalogues have larger sizes than early-type galaxies of similar luminosity, ...whether these other objects are in the field or are satellites in clusters. BCG sizes are also stronger functions of luminosity Re∝L than are the sizes of the bulk of the population; this remains true if one restricts attention to narrow bins in velocity dispersion. At fixed stellar mass and formation time, objects at lower redshift are larger and have smaller velocity dispersions – i.e. the sizes increase and velocity dispersions decrease with age. In addition, at any given redshift, younger BCGs have slightly larger sizes than older BCGs of the same stellar mass; however, they have similar velocity dispersions. As a result, at redshifts ∼0.25, corresponding to lookback times of order 3 Gyr, BCGs are smaller than their lower redshift counterparts by as much as ∼70 per cent for the brightest BCGs: the sizes evolve as . Qualitatively, similar but weaker evolution in the sizes is also seen in the bulk of the early-type population: at Mr < −22 the sizes evolve as , while at Mr > −22 the evolution is approximately (1 +z)−0.7, independent of Mr. The velocity dispersion–luminosity correlation also evolves: at Mr < −22 (as for the BCGs) and (1 +z)0.2 for fainter galaxies. The size– and velocity dispersion–stellar mass correlations yield consistent results, although, in this case, accounting for selection effects is less straightforward. These trends, in particular the fact that the velocity dispersions at fixed stellar mass decrease with age, are most easily understood if early-type BCGs grew from many dry minor mergers rather than a few major mergers. Only in such a scenario can BCGs be the descendents of the super-dense galaxies seen at z∼ 2; major dry mergers, which increase the size in proportion to the mass, cannot bring these galaxies on to the BCG Re–M* relation at z∼ 0. We also compared the ages and sizes of our early-type BCGs with other cluster galaxies (satellites). BCGs are larger than satellites of similar luminosity or stellar mass at the same redshift. Although both satellites and BCGs trace the same weak age–L or age–M* relation, this can be understood by noting that BCGs are typically about 1 Gyr older than the satellites in their group, and they are about 0.5 mag more luminous. Finally, we find that the mean satellite luminosity is approximately independent of BCG luminosity, in agreement with recent predictions based on the luminosity dependence of clustering.
We compare the set of local galaxies having dynamically measured black holes with a large, unbiased sample of galaxies extracted from the Sloan Digital Sky Survey. We confirm earlier work showing ...that the majority of black hole hosts have significantly higher velocity dispersions σ than local galaxies of similar stellar mass. We use Monte Carlo simulations to illustrate the effect on black hole scaling relations if this bias arises from the requirement that the black hole sphere of influence must be resolved to measure black hole masses with spatially resolved kinematics. We find that this selection effect artificially increases the normalization of the M
bh–σ relation by a factor of at least ∼3; the bias for the M
bh–M
star relation is even larger. Our Monte Carlo simulations and analysis of the residuals from scaling relations both indicate that σ is more fundamental than M
star or effective radius. In particular, the M
bh–M
star relation is mostly a consequence of the M
bh–σ and σ–M
star relations, and is heavily biased by up to a factor of 50 at small masses. This helps resolve the discrepancy between dynamically based black hole–galaxy scaling relations versus those of active galaxies. Our simulations also disfavour broad distributions of black hole masses at fixed σ. Correcting for this bias suggests that the calibration factor used to estimate black hole masses in active galaxies should be reduced to values of f
vir ∼ 1. Black hole mass densities should also be proportionally smaller, perhaps implying significantly higher radiative efficiencies/black hole spins. Reducing black hole masses also reduces the gravitational wave signal expected from black hole mergers.
Supermassive black hole–host galaxy relations are key to the computation of the expected gravitational wave background (GWB) in the pulsar timing array (PTA) frequency band. It has been recently ...pointed out that standard relations adopted in GWB computations are in fact biased-high. We show that when this selection bias is taken into account, the expected GWB in the PTA band is a factor of about 3 smaller than previously estimated. Compared to other scaling relations recently published in the literature, the median amplitude of the signal at f = 1 yr−1 drops from 1.3 × 10−15 to 4 × 10−16. Although this solves any potential tension between theoretical predictions and recent PTA limits without invoking other dynamical effects (such as stalling, eccentricity or strong coupling with the galactic environment), it also makes the GWB detection more challenging.
Size evolution of spheroids in a hierarchical Universe Shankar, Francesco; Marulli, Federico; Bernardi, Mariangela ...
Monthly notices of the Royal Astronomical Society,
01/2013, Letnik:
428, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Unveiling the structural evolution of spheroids, and in particular the origin of the tight size-stellar mass relation, has become one of the hottest topics in cosmology in the last years and it is ...still largely debated. To this purpose, we present and discuss basic predictions of an updated version of the latest release of the Munich semi-analytic hierarchical galaxy formation model that grows bulges via mergers and disc instabilities. We find that while spheroids below a characteristic mass M
s ∼ 1011 M grow their sizes via a mixture of disc instability and mergers, galaxies above it mainly evolve via dry mergers. Including gas dissipation in major mergers efficiently shrinks galaxies, especially those with final mass M
s 1011 M 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 per cent. 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 star forming. 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 M
star ∼ (1-2) × 1011 M 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 per cent 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 pseudo-bulges and the correlations with central black holes.
We select a sample of about 50 000 early-type galaxies from the Sloan Digital Sky Survey (SDSS), calibrate fitting formulae which correct for known problems with photometric reductions of extended ...objects, apply these corrections and then measure a number of pairwise scaling relations in the corrected sample. We show that, because they are not seeing corrected, the use of Petrosian-based quantities in magnitude-limited surveys leads to biases, and suggest that this is one reason why Petrosian-based analyses of brightest cluster galaxies have failed to find significant differences from the bulk of the early-type population. These biases are not present when seeing-corrected parameters derived from deVaucouleur fits are used. Most of the scaling relations we study show evidence for curvature: the most luminous galaxies have smaller velocity dispersions, larger sizes and fainter surface brightnesses than expected if there were no curvature. These statements remain true if we replace luminosities with stellar masses; they suggest that dissipation is less important at the massive end. There is curvature in the dynamical to stellar mass relation as well: the ratio of dynamical to stellar mass increases as stellar mass increases, but it curves upwards from this scaling both at small and at large stellar masses. In all cases, the curvature at low masses becomes apparent when the sample becomes dominated by objects with stellar masses smaller than 3 × 1010 M⊙. We quantify all these trends using second-order polynomials; these generally provide significantly better description of the data than linear fits, except at the least luminous end.
ABSTRACT
The mass and structural assembly of galaxies is a matter of intense debate. Current theoretical models predict the existence of a linear relationship between galaxy size (Re) and the host ...dark matter halo virial radius (Rh). By making use of semi-empirical models compared to the size distributions of central galaxies from the Sloan Digital Sky Survey, we provide robust constraints on the normalization and scatter of the Re−Rh relation. We explore the parameter space of models in which the Re−Rh relation is mediated by either the spin parameter or the concentration of the host halo, or a simple constant the nature of which is in principle unknown. We find that the data require extremely tight relations for both early-type and late-type galaxies (ETGs, LTGs), especially for more massive galaxies. These constraints challenge models based solely on angular momentum conservation, which predict significantly wider distributions of galaxy sizes and no trend with stellar mass, if taken at face value. We discuss physically motivated alterations to the original models that bring the predictions into better agreement with the data. We argue that the measured tight size distributions of SDSS disc galaxies can be reproduced by semi-empirical models in which the Re−Rh connection is mediated by the stellar specific angular momenta jstar. We find that current cosmological models of galaxy formation broadly agree with our constraints for LTGs, and justify the strong link between Re and jstar that we propose, however the tightness of the Re−Rh relation found in such ab initio theoretical models for ETGs is in tension with our semi-empirical findings.
ABSTRACT
The origin of the quenching in galaxies is still highly debated. Different scenarios and processes are proposed. We use multiband (400–1600 nm) bulge–disc decompositions of massive galaxies ...in the redshift range 0 < z < 2 to explore the distribution and the evolution of galaxies in the $\log \, {\rm SFR-log}\: M_{*}$ plane as a function of the stellar mass weighted bulge-to-total ratio ($B/T_{M_{*}}$) and also for internal galaxy components (bulge/disc) separately. We find evidence of a clear link between the presence of a bulge and the flattening of the main sequence in the high-mass end. All bulgeless galaxies ($B/T_{M_{*}}$ < 0.2) lie on the main sequence, and there is little evidence of a quenching channel without bulge growth. Galaxies with a significant bulge component ($B/T_{M_{*}}$ > 0.2) are equally distributed in number between star forming and passive regions. The vast majority of bulges in the main-sequence galaxies are quiescent, while star formation is localized in the disc component. Our current findings underline a strong correlation between the presence of the bulge and the star formation state of the galaxy. A bulge, if present, is often quiescent, independently of the morphology or the star formation activity of the host galaxy. Additionally, if a galaxy is quiescent, with a large probability, is hosting a bulge. Conversely, if the galaxy has a discy shape is highly probable to be star forming.
We use a sample of about 48000 Sloan Digital Sky Survey early-type galaxies to show that older galaxies have smaller half-light radii Re and larger velocity dispersions σ than younger ones of the ...same stellar mass Mstar. We use the age-corrected luminosity Lcorrr as a proxy for Mstar to minimize biases: below Lcorrr∼ 1011L⊙, galaxies with age ∼11 Gyrs have Re smaller by 40 per cent and σ larger by 25 per cent, compared to galaxies that are 4 Gyr younger. The sizes and velocity dispersions of more luminous galaxies vary by less than 15 per cent, whatever their age, a challenge for current galaxy formation models. A closer check reveals that the lowering in the dispersion is caused by older galaxies that show a significant departure from the Re−Lcorrr and σ−Lcorrr relations at high Lcorrr. Such features might find an explanation in models where more massive galaxies undergo more minor mergers than less massive galaxies at late times, thus causing a break in the homology. In terms of the Fundamental Plane of early-type galaxies, the data indicate that all galaxies show a significant and similar increase in the dynamical-to-stellar mass ratio with increasing mass, independent of their age. However, older galaxies have smaller Mdyn/Mstar ratios than objects which formed more recently. These findings may suggest that lower mass galaxies and, at fixed stellar mass, higher redshift galaxies, formed from gas-richer progenitors, thus underwent more dissipation and contraction.
We carry out a systematic investigation of the total mass density profile of massive ( log M star M 11.3 ) early-type galaxies and its dependence on galactic properties and host halo mass with the ...aid of a variety of lensing/dynamical data and large mock galaxy catalogs. The latter are produced via semi-empirical models that, by design, are based on just a few basic input assumptions. Galaxies with measured stellar masses, effective radii, and Sérsic indices, are assigned, via abundance matching relations, host dark matter halos characterized by a typical ΛCDM profile. Our main results are as follows. (1) In line with observational evidence, our semi-empirical models naturally predict that the total, mass-weighted density slope at the effective radius γ′ is not universal, steepening for more compact and/or massive galaxies, but flattening with increasing host halo mass. (2) Models characterized by a Salpeter or variable initial mass function (IMF) and uncontracted dark matter profiles are in good agreement with the data, while a Chabrier IMF and/or adiabatic contractions/expansions of the dark matter halos are highly disfavored. (3) Currently available data on the mass density profiles of very massive galaxies ( log M star M 12 ), with M halo 3 × 10 14 M , favor instead models with a stellar profile flatter than a Sérsic one in the very inner regions (r 3-5 kpc), and a cored NFW or Einasto dark matter profile with median halo concentration a factor of ∼2 or 1.3, respectively, higher than those typically predicted by N-body numerical simulations.
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