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Register a new userFormation and evolution of dwarf elliptical galaxies I. Structural and kinematical properties
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This paper is the first in a series in which we present the results of an ESO Large Program on the kinematics and internal dynamics of dwarf elliptical galaxies (dEs). We investigate the relations between the parameters that quantify the structure and internal dynamics of dEs such as the Faber_Jackson relation and the Fundamental Plane (FP). We show that the dE sequences in the various diagrams are disjunct from those traced by bright and intermediate-luminosity elliptical galaxies and bulges of spirals. It appears that semi-analytical models (SAMs) are able to reproduce the position of the dEs in those diagrams. While these findings are clearly a success for the hierarchical-merging picture of galaxy formation, they do not necessarily invalidate the alternative ``harassment scenario, which posits that dEs stem from perturbed and stripped late-type disk galaxies that entered clusters and groups of galaxies about 5 Gyr ago.
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We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using ULySS and STECKMAP, we derive radial profiles of the SSP-equivalent ages, metallicities and star-formation histories. The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1-5 Gyr) populations, and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dSph counterparts of the Local Group. Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star-formation histories. The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram-pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.
Elements of kinematical and dynamical modeling of elliptical galaxies (Es) are presented. In projection, NFW models resemble Sersic models, but with a very narrow range of shapes (m=3+/-1). The total density profile of Es cannot be NFW-like because the predicted local M/L and aperture velocity dispersion within an effective radius (Re) are much lower than observed. Stars must then dominate Es out to a few Re. Fitting an NFW model to the total density profile of Sersic+NFW (stars+dark matter [DM]) Es results in very high concentration parameters, as found by X-ray observers. Kinematical modeling of Es assuming an isotropic NFW DM model underestimates M/L at the virial radius by a factor of 1.6 to 2.4, because dissipationless LCDM halos have slightly different density profiles and slightly radial velocity anisotropy. In N-body+gas simulations of Es as merger remnants of spirals embedded in DM halos, the slope of the DM density profile is steeper when the initial spiral galaxies are gas-rich. The Hansen & Moore (2006) relation between anisotropy and the slope of the density profile breaks down for gas and DM, but the stars follow an analogous relation with slightly less radial anisotropies for a given density slope. Using kurtosis (h_4) to infer anisotropy in Es is dangerous, as h_4 is also sensitive to small levels of rotation. The stationary Jeans equation provides accurate masses out to 8 Re. The discrepancy between the modeling of Romanowsky et al. (2003), indicating a dearth of DM in Es, and the simulations analyzed by Dekel et al. (2005), which match the spectroscopic observations of Es, is partly due to radial anisotropy and to observing oblate Es face-on. However, one of the 15 solutions to the orbit modeling of Romanowsky et al. is found to have an amount and concentration of DM consistent with LCDM predictions.
We present the first set of cosmological baryonic zoom-in simulations of galaxies including dissipative self-interacting dark matter (dSIDM). These simulations utilize the Feedback In Realistic Environments (FIRE-2) galaxy formation physics, but allow the dark matter to have dissipative self-interactions analogous to Standard Model forces, parameterized by the self-interaction cross-section per unit mass, $(sigma/m)$, and the dimensionless degree of dissipation, $0<f_{rm diss}<1$. We survey this parameter space, including constant and velocity-dependent cross-sections, and focus on structural and kinematic properties of dwarf galaxies with $M_{rm halo} simeq 10^{10-11} {rm M}_{odot}$. Central density profiles of simulated dwarfs become cuspy when $(sigma/m)_{rm eff} gtrsim 0.1,{rm cm^{2},g^{-1}}$ (and $f_{rm diss}=0.5$ as fiducial). The power-law slopes asymptote to $alpha approx -1.5$ in low-mass dwarfs independent of cross-section, which arises from a dark matter cooling flow. Through comparisons with dark matter only simulations, we find the profile in this regime is insensitive to the inclusion of baryons. However, when $(sigma/m)_{rm eff} ll 0.1,{rm cm^{2},g^{-1}}$, baryonic effects can produce cored density profiles comparable to non-dissipative cold dark matter (CDM) runs but at smaller radii. Simulated galaxies with $(sigma/m) gtrsim 10,{rm cm^{2},g^{-1}}$ develop significant coherent rotation of dark matter, accompanied by halo deformation, but this is unlike the well-defined thin dark disks often attributed to baryon-like dSIDM. The density profiles in this high cross-section model exhibit lower normalizations given the onset of halo deformation. For our surveyed dSIDM parameters, halo masses and galaxy stellar masses do not show appreciable difference from CDM, but dark matter kinematics and halo concentrations/shapes can differ.
In this paper we compute new multi-zone photo-chemical evolution models for elliptical galaxies, taking into account detailed nucleosynthetic yields, feedback from supernovae and an initial infall episode. By comparing model predictions with observations, we derive a picture of galaxy formation in which the higher is the mass of the galaxy, the shorter are the infall and the star formation timescales. Therefore, in this scenario, the most massive objects are older than the less massive ones, in the sense that larger galaxies stop forming stars at earlier times. Each galaxy is created outside-in, i.e. the outermost regions accrete gas, form stars and develop a galactic wind very quickly, compared to the central core in which the star formation can last up to ~1.3 Gyr. In particular, we suggest that both the duration of the star formation and the infall timescale decrease with galactic radius. (abridged) By means of our model, we are able to match the observed mass-metallicity and color-magnitude relations for the center of the galaxies as well as to reproduce the overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass. Furthermore, we find that the observed Ca underabundance relative to Mg can be real, due to the non-neglibile contribution of type Ia SN to the production of this element. We predict metallicity and color gradients inside the galaxies which are in good agreement with the mean value of the observed ones. (abridged)
We present preliminary results of an extensive study of the fundamental properties of dwarf elliptical galaxies (dEs) in the Coma cluster. Our study will combine HST surface photometry with ground-based UBRIJK photometry and optical spectroscopy. The combined data set will be used to investigate the intrinsic correlations among global parameters in cluster dEs, including the Fundamental Plane, the color-magnitude relation, the Faber-Jackson and Kormendy relation, and velocity dispersion versus line strength indices. These empirical correlations have provided important constraints to theoretical models of galaxy formation and evolution for normal elliptical galaxies. Although dEs are the most abundant galaxy population in clusters their properties remain, however, largely unknown. Our study aims to provide an essential reference for testing current theories on the formation and evolution of dEs in clusters, and understanding their relation to more massive elliptical galaxies.
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