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Register a new userThe velocity dispersion function of early-type galaxies
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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.
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The majority of nearby early-type galaxies contains detectable amounts of emission-line gas at their centers. The emission-line ratios and gas kinematics potentially form a valuable diagnostic of the nuclear activity and gravitational potential well. The observed central gas velocity dispersion often exceeds the stellar velocity dispersion. This could be due to either the gravitational potential of a black hole or turbulent shocks in the gas. Here we try to discriminate between these two scenarios.
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)
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.
We study the structure of spatially resolved, line-of-sight velocity dispersion for galaxies in the Epoch of Reionization (EoR) traced by [CII] $158murm{m}$ line emission. Our laboratory is a simulated prototypical Lyman-break galaxy, Freesia, part of the SERRA suite. The analysis encompasses the redshift range 6 < z < 8, when Freesia is in a very active assembling phase. We build velocity dispersion maps for three dynamically distinct evolutionary stages (Spiral Disk at z=7.4, Merger at z=8.0, and Disturbed Disk at z=6.5) using [CII] hyperspectral data cubes. We find that, at a high spatial resolution of 0.005 ($simeq 30 pc$), the luminosity-weighted average velocity dispersion is $sigma_{rm{CII}}$~23-38 km/s with the highest value belonging to the highly-structured Disturbed Disk stage. Low resolution observations tend to overestimate $sigma_{rm CII}$ values due to beam smearing effects that depend on the specific galaxy structure. For an angular resolution of 0.02 (0.1), the average velocity dispersion is 16-34% (52-115%) larger than the actual one. The [CII] emitting gas in Freesia has a Toomre parameter $mathcal{Q}$~0.2 and a rotational-to-dispersion ratio of $v_{rm c}/sigma$~ 7 similar to that observed in z=2-3 galaxies. The primary energy source for the velocity dispersion is due to gravitational processes, such as merging/accretion events; energy input from stellar feedback is generally subdominant (< 10%). Finally, we find that the resolved $sigma_{rm{CII}} - {Sigma}_{rm SFR}$ relation is relatively flat for $0.02<{Sigma}_{rm SFR}/{{rm M}_{odot}} mathrm{yr}^{-1} {mathrm kpc}^{-2} < 30$, with the majority of data lying on the derived analytical relation $sigma propto Sigma_{rm SFR}^{5/7}$. At high SFR, the increased contribution from stellar feedback steepens the relation, and $sigma_{rm{CII}}$ rises slightly.
We investigate the Mg-sigma and <Fe>-sigma relations in a sample of 72 early-type galaxies drawn mostly from cluster and group environments using a homogeneous data-set which is well-calibrated onto the Lick/IDS system. The small intrinsic scatter in Mg at a given sigma gives upper limits on the spread in age and metallicity of 49% and 32% respectively, if the spread is attributed to one quantity only and if the variations in age and metallicity are uncorrelated. The age/metallicity distribution as inferred from the Hbeta vs <Fe> diagnostic diagram reinforces this conclusion, as we find mostly galaxies with large luminosity weighted ages spanning a range in metallicity. In our sample we do not find significant evidence for an anti-correlation of ages and metallicities which would keep the index-sigma relations tight while hiding a large spread in age and metallicity. As a result of correlated errors in the age-metallicity plane, a mild age-metallicity anti-correlation cannot be completely ruled out given the current data. Correcting the line-strengths indices for non-solar abundance ratios following the recent paper by Trager et al., leads to higher mean metallicity and slightly younger age estimates while preserving the metallicity sequence. The [Mg/Fe] ratio is mildly correlated with the central velocity dispersion and ranges from [Mg/Fe]=0.05 to 0.3 for galaxies with sigma > 100 km/s. Under the assumption that there is no age gradient along the index-sigma relations, the abundance-ratio corrected Mg-sigma, <Fe>-sigma and Hbeta-sigma relations give consistent estimates of Delta [M/H]/ Delta log sigma = 0.9 (+- 0.1). The slope of the Hbeta-sigma relation limits a potential age trend as a function of sigma to 2-3 Gyrs along the sequence.(abriged)
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