Line-of-sight velocity distributions of low-luminosity elliptical galaxies

The shape of the line-of-sight velocity distribution (LOSVD) is measured for a sample of 14 elliptical galaxies, predominantly low-luminosity ellipticals. The sample is dominated by galaxies in the Virgo cluster but also contains ellipticals in nearby groups and low-density environments. The parametrization of the LOSVD given by Gerhard and van der Marel & Franx is adopted, which measures the asymmetrical and symmetrical deviations of the LOSVD from a Gaussian by the amplitudes h ₃ and h ₄ of the Gauss–Hermite series. Rotation, velocity dispersion, h ₃ and h ₄ are determined as a function of radius for both major and minor axes. Non-Gaussian LOSVDs are found for all galaxies along the major axes. Deviations from a Gaussian LOSVD along the minor axis are of much lower amplitude if present at all. Central decreases in velocity dispersion are found for three galaxies. Two galaxies have kinematically decoupled cores: NGC 4458 and the well-known case of NGC 3608.     

Crash-testing the cauldron code for joint lensing and dynamics analysis of early-type galaxies

We apply the joint lensing and dynamics code for the analysis of early-type galaxies, 'Combined Algorithm for Unified Lensing and Dynamics ReconstructiON ( cauldron )', to a rotating N -body stellar system with dark matter halo which significantly violates the two major assumptions of the method, i.e. axial symmetry supported by a two-integral distribution function. The goal is to study how cauldron performs in an extreme case, and to determine which galaxy properties can still be robustly recovered. Three data sets, corresponding to orthogonal lines of sight, are generated from the N -body system and analysed with the identical procedure followed in the study of real lens galaxies, adopting an axisymmetric power-law total density distribution. We find that several global properties of the N -body system are recovered with remarkable accuracy, despite the fact that the adopted power-law model is too simple to account for the lack of symmetry of the true density distribution. In particular, the logarithmic slope of the total density distribution is robustly recovered to within less than 10 per cent (with the exception of the ill-constrained very inner regions), the inferred angle-averaged radial profile of the total mass closely follows the true distribution, and the dark matter fraction of the system (inside the effective radius) is correctly determined within ∼10 per cent of the total mass. Unless the line-of-sight direction is almost parallel to the total angular momentum vector of the system, reliably recovered quantities also include the angular momentum, the   V /σ  ratio and the anisotropy parameter δ. We conclude that the cauldron code can be safely and effectively applied to real early-type lens galaxies, also providing reliable information for the systems that depart significantly from the method's assumptions.     

Feedback and late star formation in elliptical galaxies

We present a one-zone model of star formation in elliptical galaxies that includes thermal feedback from supernovae and a temperature dependent star formation efficiency. The modulation of feedback with the total mass results in the triggering of late episodes of star formation in low-mass galaxies. These small `bursts' can occur as late as at redshifts z ∼ 0.5 but they do not change significantly the optical and NIR color-magnitude relation (CMR) of cluster galaxies, both locally and out to moderate redshifts, in agreement with the observations. However, they introduce a large scatter at the faint end of the NUV-Optical CMR, as recently found in cluster Abell 851 (z = 0.41). This revised version was published online in August 2006 with corrections to the Cover Date.     

Structural evolution in elliptical galaxies: the age–shape relation

We test the hypothesis that the apparent axial ratio of an elliptical galaxy is correlated with the age of its stellar population. We find that old ellipticals (with estimated ages t >7.5 Gyr) are rounder on average than younger ellipticals. The statistical significance of this shape difference is greatest at small radii; a Kolmogorov–Smirnov test comparing the axial ratios of the two populations at R = R _e/16 yields a statistical significance greater than 99.96 per cent. The relation between age and apparent shape is linked to the core/power-law surface brightness profile dichotomy. Core ellipticals have older stellar populations, on average, than power-law ellipticals and are rounder in their inner regions. Our findings are consistent with a scenario in which power-law ellipticals are formed in gas-rich mergers, while core ellipticals form in dissipationless mergers, with cores formed and maintained by the influence of a binary black hole.     

Spectroscopic constraints on the stellar population of elliptical galaxies in the Coma cluster

Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). CO_sp (2.3-μm) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated.
It is found that the strength of CO_sp features in elliptical galaxies increases from the core ( r <02) to the outskirts ( r >02) of the Coma cluster. Using the Mg₂ strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster.
Using the CO_sp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 to 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal.
A relation is found between the CO_sp and V − K colours of ellipticals, corresponding to a slope of 0.036±0.016, significantly shallower than that from the Mg₂–( V − K ) relation. This is studied using stellar synthesis models.     

Dark matter in elliptical galaxies – I. Is the total mass density profile of the NFW form or even steeper?

Elliptical galaxies are modelled as Sérsic luminosity distributions with density profiles (DPs) for the total mass adopted from the DPs of haloes within dissipationless ΛCDM (cold dark matter) N -body simulations. Ellipticals turn out to be inconsistent with cuspy low-concentration NFW models representing the total mass distribution, neither are they consistent with a steeper −1.5 inner slope, nor with the shallower models proposed by Navarro et al., nor with NFW models 10 times more concentrated than predicted, as deduced from several X-ray observations – the mass models, extrapolated inwards, lead to local mass-to-light ratios that are smaller than the stellar value inside an effective radius ( R _e), and to central aperture velocity dispersions that are much smaller than observed. This conclusion remains true as long as there is no sharp steepening (slope < −2) of the dark matter DPs just inside 0.01 virial radii.
The very low total mass and velocity dispersion produced within R _e by an NFW-like total mass profile suggests that the stellar component should dominate the dark matter component out to at least R _e. It should then be difficult to kinematically constrain the inner slope of the DP of ellipticals. The high-concentration parameters deduced from X-ray observations appear to be a consequence of fitting an NFW model to the total mass DP made up of a stellar component that dominates inside and a dark matter component that dominates outwards.
An appendix gives the virial mass dependence of the concentration parameter, central density and total mass of the Navarro et al. model. In a second appendix are given single integral expressions for the velocity dispersions averaged along the line of sight, in circular apertures and in thin slits, for general luminosity density and mass distributions, with isotropic orbits.     

Supermassive black holes, star formation and downsizing of elliptical galaxies

The overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass, poses a serious problem for all current models of galaxy formation. Here, we improve on the one-zone chemical evolution models for elliptical galaxies by taking into account positive feedback produced in the early stages of supermassive central black hole growth. We can account for both the observed correlation and the scatter if the observed anti-hierarchical behaviour of the AGN population couples to galaxy assembly and results in an enhancement of the star formation efficiency which is proportional to galactic mass. At low and intermediate galactic masses, however, a slower mode for star formation suffices to account for the observational properties.     

Ages of elliptical galaxies: single- versus multi-population interpretation

New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophotometric indices are presented, permitting estimates of mean stellar population ages and metallicities. These calibrations are based on evolutionary models including a two-phase interstellar medium, infall and a galactic wind. Free parameters were fixed by requiring that models reproduce the mean trend of data in the colour–magnitude diagram as well as in the plane of indices  Hβ–Mg₂  and  Mg₂–〈Fe〉  . To improve the location of faint ellipticals  ( M _B > −20)  in the  Hβ–Mg₂  diagram, downsizing was introduced. An application of our calibrations to a sample of ellipticals and a comparison with results derived from single stellar population models are given. Our models indicate that mean population ages span an interval of 7–12 Gyr and are correlated with metallicities, which range from approximately half up to three times solar.     

Constraints on the merger models of elliptical galaxies from their globular cluster systems

The discovery of protoglobular cluster candidates in many present-day mergers allows us to understand better the possible effects of a merger event on the globular cluster system of a galaxy, and to foresee the properties of the end-product. By comparing these expectations with the properties of globular cluster systems of elliptical galaxies at the present time we can constrain merger models. The observational data indicate that (i) every gaseous merger induces the formation of new star clusters, and (ii) the number of new clusters formed in such a merger increases with the gas content of the progenitor galaxies. Low-luminosity (about M _V  > −21), discy ellipticals are generally thought to be the result of a gaseous merger. As such, new globular clusters are expected to form but have not been detected to date. We investigate various reasons for the non-detection of subpopulations in low-luminosity ellipticals, i.e. absence of an old population, absence of a new population, destruction of one of the populations and, finally, an age–metallicity conspiracy that allows old and new globular clusters to appear indistinguishable at the present epoch. All of these possibilities lead us to a similar conclusion, namely that low-luminosity ellipticals did not form recently ( z  < 1) in a gas-rich merger, and might not have formed in a major merger of stellar systems at all. High-luminosity ellipticals do reveal globular cluster subpopulations. However, it is difficult to account for the two populations in terms of mergers alone and, in particular, we can rule out scenarios in which the second subpopulation is the product of a recent, gas-poor merger.     

Gravitational lensing by elliptical galaxies

The fraction of high-redshift sources which are multiply imaged by intervening galaxies is strongly dependent on the cosmological constant, and so can be a useful probe of the cosmological model. However its power is limited by various systematic (and random) uncertainties in the calculation of lensing probabilities, one of the most important of which is the dynamical normalization of elliptical galaxies. Assuming ellipticals' mass distributions can be modelled as isothermal spheres, the mass normalization depends on the velocity anisotropy, the luminosity density, the core radius and the area over which the velocity dispersion is measured. The differences in the lensing probability and optical depth produced by using the correct normalization can be comparable to the differences between even the most extreme cosmological models. The existing data are not sufficient to determine the correct normalization with enough certainty to allow lensing statistics to be used to their full potential. However, as the correct lensing probability is almost certainly higher than is usually assumed, upper bounds on the cosmological constant are not weakened by these possibilities.     

Recent star-forming activity in local elliptical galaxies

The formation and evolution of elliptical galaxies (EGs) are still an open question. In particular, recent observations suggest that EGs are not only simple spheroidal systems of old stars. In this paper, we analyse a sample of EGs selected from the Sloan Digital Sky Survey in order to study the star-forming activity in local EGs. Among these 487 ellipticals, we find that 13 EGs show unambiguous evidence of recent star formation activity betrayed by conspicuous nebular emission lines. Using the evolutionary stellar population synthesis models and Lick absorption line indices, we derive stellar ages, metallicities and α-element abundances, and thus reconstruct the star formation and chemical evolution history of the star-forming elliptical galaxies (SFEGs) in our sample.
We find that SFEGs have relative younger stellar population age, higher metallicity and lower stellar mass, and that their star formation history can be well described by a recent minor and short starburst superimposed on old stellar component. We also detect 11 E+A galaxies whose stellar population properties are closer to those of quiescent (normal) ellipticals than to star-forming ones. However, from the analysis of their absorption line indices, we note that our E+A galaxies show a significant fraction of intermediate-age stellar populations, remarkably different from the quiescent galaxies. This might suggest an evolutionary link between E+As and star-forming ellipticals. Finally, we confirm the relations between age, metallicity, α-element abundance and stellar mass for local EGs.