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Register a new userThe velocity dispersion function of early-type galaxies and its redshift evolution: the newest results from lens redshift test
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The redshift distribution of galactic-scale lensing systems provides a laboratory to probe the velocity dispersion function (VDF) of early-type galaxies (ETGs) and measure the evolution of early-type galaxies at redshift z ~ 1. Through the statistical analysis of the currently largest sample of early-type galaxy gravitational lenses, we conclude that the VDF inferred solely from strong lensing systems is well consistent with the measurements of SDSS DR5 data in the local universe. In particular, our results strongly indicate a decline in the number density of lenses by a factor of two and a 20% increase in the characteristic velocity dispersion for the early-type galaxy population at z ~ 1. Such VDF evolution is in perfect agreement with the $Lambda$CDM paradigm (i.e., the hierarchical build-up of mass structures over cosmic time) and different from stellar mass-downsizing evolutions obtained by many galaxy surveys. Meanwhile, we also quantitatively discuss the evolution of the VDF shape in a more complex evolution model, which reveals its strong correlation with that of the number density and velocity dispersion of early-type galaxies. Finally, we evaluate if future missions such as LSST can be sensitive enough to place the most stringent constraints on the redshift evolution of early-type galaxies, based on the redshift distribution of available gravitational lenses.
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The distribution of early-type galaxy velocity dispersions, phi(sigma), is measured using a sample drawn from the SDSS database. Its shape differs significantly from that which one obtains by simply using the mean correlation between luminosity, L, and velocity dispersion, sigma, to transform the luminosity function into a velocity function: ignoring the scatter around the mean sigma-L relation is a bad approximation. An estimate of the contribution from late-type galaxies is also made, which suggests that phi(sigma) is dominated by early-type galaxies at velocities larger than ~ 200 km/s.
We analyze 40 cosmological re-simulations of individual massive galaxies with present-day stellar masses of $M_{*} > 6.3 times 10^{10} M_{odot}$ in order to investigate the physical origin of the observed strong increase in galaxy sizes and the decrease of the stellar velocity dispersions since redshift $z approx 2$. At present 25 out of 40 galaxies are quiescent with structural parameters (sizes and velocity dispersions) in agreement with local early type galaxies. At z=2 all simulated galaxies with $M_* gtrsim 10^{11}M_{odot}$ (11 out of 40) at z=2 are compact with projected half-mass radii of $approx$ 0.77 ($pm$0.24) kpc and line-of-sight velocity dispersions within the projected half-mass radius of $approx$ 262 ($pm$28) kms$^{-1}$ (3 out of 11 are already quiescent). Similar to observed compact early-type galaxies at high redshift the simulated galaxies are clearly offset from the local mass-size and mass-velocity dispersion relations. Towards redshift zero the sizes increase by a factor of $sim 5-6$, following $R_{1/2} propto (1+z)^{alpha}$ with $alpha = -1.44$ for quiescent galaxies ($alpha = -1.12$ for all galaxies). The velocity dispersions drop by about one-third since $z approx 2$, following $sigma_{1/2} propto (1+z)^{beta}$ with $beta = 0.44$ for the quiescent galaxies ($beta = 0.37$ for all galaxies). The simulated size and dispersion evolution is in good agreement with observations and results from the subsequent accretion and merging of stellar systems at $zlesssim 2$ which is a natural consequence of the hierarchical structure formation. A significant number of the simulated massive galaxies (7 out of 40) experience no merger more massive than 1:4 (usually considered as major mergers). On average, the dominant accretion mode is stellar minor mergers with a mass-weighted mass-ratio of 1:5. (abridged)
Determinations of the UV luminosity function of AGN at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of AGN data. We address this problem by constructing a sample of more than 80,000 colour-selected AGN from redshift z=0 to 7.5. While this sample is composed of multiple data sets with spectroscopic redshifts and completeness estimates, we homogenise these data sets to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift z=0 to 7.5. The luminosity function has a double power law form at all redshifts. The break magnitude $M_*$ of the AGN luminosity function shows a steep brightening from $M_*sim -24$ at z=0.7 to $M_*sim -29$ at z=6. The faint-end slope $beta$ significantly steepens from $-1.7$ at $z<2.2$ to $-2.4$ at $zsimeq 6$. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at $zsim 6$ is subdominant (< 3%), although it can be non-negligible (~10%) if these luminosity functions hold down to $M_{1450}=-18$. Under reasonable assumptions, AGN can reionize HeII by redshift z=2.9. At low redshifts (z<0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of HI absorption lines in the IGM. Our global analysis of the luminosity function also reveals important systematic errors in the data, particularly at z=2.2--3.5, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, luminosity function analysis codes, and homogenised AGN data publicly available.
We study the behaviour of the dynamical and stellar mass inside the effective radius (re) of early-type galaxies (ETGs). We use several samples of ETGs -ranging from 19 000 to 98 000 objects- from the ninth data release of the Sloan Digital Sky Survey. We consider Newtonian dynamics, different light profiles and different Initial Mass Functions (IMF) to calculate the dynamical and stellar mass. We assume that any difference between these two masses is due to dark matter and/or a non Universal IMF. The main results for galaxies in the redshift range 0.0024 < z < 0.3500 and in the dynamical mass range 9.5 < log(M) < 12.5 are: i) A significant part of the intrinsic dispersion of the distribution of dynamical vs. stellar mass is due to redshift. ii) The difference between dynamical and stellar mass increases as a function of dynamical mass and decreases as a function of redshift. iii) The difference between dynamical and stellar mass goes from approximately 0% to 70% of the dynamical mass depending on mass and redshift. iv) These differences could be due to dark matter or a non Universal IMF or a combination of both. v) The amount of dark matter inside ETGs would be equal to or less than the difference between dynamical and stellar mass depending on the impact of the IMF on the stellar mass estimation. vi) The previous results go in the same direction of some results of the Fundamental Plane (FP) found in the literature in the sense that they could be interpreted as an increase of dark matter along the FP and a dependence of the FP on redshift.
We present an investigation about the shape of the initial mass function (IMF) of early-type galaxies (ETGs), based on a joint lensing and dynamical analysis, and on stellar population synthesis models, for a sample of 55 lens ETGs identified by the Sloan Lens ACS (SLACS) Survey. We construct axisymmetric dynamical models based on the Jeans equations which allow for orbital anisotropy and include a dark matter halo. The models reproduce in detail the observed textit{HST} photometry and are constrained by the total projected mass within the Einstein radius and the stellar velocity dispersion ($sigma$) within the SDSS fibers. Comparing the dynamically-derived stellar mass-to-light ratios $(M_*/L)_{rm dyn}$, obtained for an assumed halo slope $rho_{rm h}propto r^{-1}$, to the stellar population ones $(M_*/L)_{rm pop}$, derived from full-spectrum fitting and assuming a Salpeter IMF, we infer the mass normalization of the IMF. Our results confirm the previous analysis by the SLACS team that the mass normalization of the IMF of high $sigma$ galaxies is consistent on average with a Salpeter slope. Our study allows for a fully consistent study of the trend between IMF and $sigma$ for both the SLACS and ATLAS samples, which explore quite different $sigma$ ranges. The two samples are highly complementary, the first being essentially $sigma$ selected, and the latter volume-limited and nearly mass selected. We find that the two samples merge smoothly into a single trend of the form $logalpha =(0.38pm0.04)timeslog(sigma_{rm e}/200,mathrm{km~s}^{-1})+(-0.06pm0.01)$, where $alpha=(M_*/L)_{rm dyn}/(M_*/L)_{rm pop}$ and $sigma_{rm e}$ is the luminosity averaged $sigma$ within one effective radius $R_{rm e}$. This is consistent with a systematic variation of the IMF normalization from Kroupa to Salpeter in the interval $sigma_{rm e}approx90-270,mathrm{km~s}^{-1}$.
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