E. Toloba

Formation and evolution of dwarf early-type galaxies in the Virgo cluster - I. Internal kinematics

We present new medium resolution kinematic data for a sample of 21 dwarf early-type galaxies (dEs) mainly in the Virgo cluster, obtained with the WHT and INT telescopes at the Roque de los Muchachos Observatory (La Palma, Spain). These data are used to study the origin of the dwarf elliptical galaxy population inhabiting clusters. We confirm that dEs are not dark matter dominated galaxies, at least not up to the half-light radius. We also find that the observed galaxies in the outer parts of the cluster are mostly rotationally supported systems with disky morphological shapes. Rotationally supported dEs have rotation curves similar to those of star-forming galaxies of similar luminosity and follow the Tully-Fisher relation. This is expected if dE galaxies are the descendants of low-luminosity star-forming systems that have recently entered the cluster environment and lost their gas due to a ram-pressure stripping event, quenching their star formation activity and transforming into quiescent systems, but conserving their angular momentum.

KINEMATIC PROPERTIES AS PROBES OF THE EVOLUTION OF DWARF GALAXIES IN THE VIRGO CLUSTER

We present new observational results on the kinematical, morphological, and stellar population properties of a sample of 21 dEs located both in the Virgo Cluster and in the field, which show that 52% of the dEs (1) are rotationally supported, (2) exhibit structural signs of typical rotating systems such as disks, bars, or spiral arms, (3) are younger (~3 Gyr) than non-rotating dEs, and (4) are preferentially located either in the outskirts of Virgo or in the field. This evidence is consistent with the idea that rotationally supported dwarfs are late-type spirals or irregulars that recently entered the cluster and lost their gas through a ram pressure stripping event, quenching their star formation and becoming dEs through passive evolution. We also find that all, but one, galaxies without photometric hints for hosting disks are pressure supported and are all situated in the inner regions of the cluster. This suggests a different evolution from the rotationally supported systems. Three different scenarios for these non-rotating galaxies are discussed (in situ formation, harassment, and ram pressure stripping).

CARBON AND NITROGEN ABUNDANCES IN EARLY-TYPE GALAXIES

For the first time, we undertake a systematic examination of the nitrogen abundances using the NH3360 feature for a sample of 35 early-type galaxies spanning a range of masses and local environment. The nitrogen-sensitive molecular feature at 3360A has been employed in conjunction with a suite of atomic- and molecular-sensitive indices to provide unique and definitive constraints on the chemical content of these systems. By employing NH3360, we are now able to break the carbon, nitrogen, and oxygen degeneracies inherent to the use of the cyanogen (CN) index. We demonstrate that the NH3360 feature shows little dependency upon the velocity dispersion (our proxy for mass) of the galaxies, contrary to what is seen for carbon-and magnesium-sensitive indices. At face value, these results are at odds with conclusions drawn previously using indices sensitive to both carbon and nitrogen, such as CN. With the aid of stellar population models, we find that the N/Fe ratios in these galaxies are consistent with being mildly enhanced with respect to the solar ratio. We also explore the dependence of these findings upon environment, by analyzing the co-added spectra of galaxies in the field and the Coma cluster. We confirm the previously found differences in carbon abundances between galaxies in low-and high-density environments, while showing that these differences do not seem to exist for nitrogen. We discuss the implications of these findings for the derivation of the star-formation histories in early-type galaxies, and for the origin of carbon and nitrogen themselves.

NH AND Mg INDEX TRENDS IN ELLIPTICAL GALAXIES

We examine the spectrum in the vicinity of the NH3360 index of Davidge & Clark, which was defined to measure the NH absorption around 3360 A and shows almost no trend with velocity dispersion, unlike other N-sensitive indices, which show a strong trend. Computing the effect of individual elements on the integrated spectrum with synthetic stellar population integrated spectra, we find that, while being well correlated with nitrogen abundance, NH3360 is almost equally well anti-correlated with Mg abundance. This prompts the definition of two new indices, Mg3334, which is mostly sensitive to magnesium, and NH3375, which is mostly sensitive to nitrogen. Rather surprisingly, we find that the new NH3375 index shows a trend versus optical absorption feature indices that is as shallow as the NH3360 index. We hypothesize that the lack of a strong index trend in these near-UV indices is due to the presence of an old metal-poor component of the galactic population. Comparison of observed index trends and those predicted by models shows that a modest fraction of an old, metal-poor stellar population could easily account for the observed flat trend in these near-UV indices while still allowing substantial N abundance increase in the larger galaxies.

Disentangling Nitrogen and Carbon Abundances in Early-Type Galaxies

The near-UV NH3360 feature has been measured in a sample of 35 early-typen galaxies in a wide range of masses and environments. We found that, contrary ton what is seen with Mg- and C-sensitive Lick-style indices, NH3360 does not varyn significantly with galaxy velocity dispersion. When compared with solar-scaledn stellar population models N/Fe appears mildly enhanced. We do not detect anyn difference in the NH3360 index between galaxies in different environments. Wen outline the important implications of these conclusions for our understanding ofn the origin of carbon and nitrogen, and the star formation histories ofn early-type galaxies.

The CNO Puzzle in Elliptical Galaxies

The near‐UV NH3360 feature has been measured in a sample of 35 early‐type galaxies spanning a wide range of masses and environments. We found that, contrary to what is seen with Mg‐ and C‐sensitive Lick‐style indices, NH3360 does not vary significantly with galaxy velocity dispersion. When compared with solar‐scaled stellar population models N/Fe appears mildly enhanced. We do not detect any difference in the NH3360 index between galaxies in different environments. We outline the important implications of these conclusions for our understanding of the origin of carbon and nitrogen, and the star formation histories of early‐type galaxies.

Stellar populations of dwarf elliptical galaxies from high-resolution, optical spectroscopic data

Instituto de Astrof´isica de Canarias, V´ia L´actea s/n, 38200 La Laguna, Tenerife, Spainemail: cenarro@iac.esAbstract. We present some preliminary results obtained within the MAGPOP-ITP collabora-tion for a subsample of four dwarf elliptical galaxies. On the basis of high-resolution, opticalspectroscopic data obtained for these galaxies at the WHT, their ages, metallicities and singleelement abundance ratios have been derived from their line-strength indices and the improved,MILES-based, stellar population model predictions by Vazdekis