S. Pedraz

Evidence of fast rotation in dwarf elliptical galaxies

In this letter we investigate the kinematical properties of early-type dwarfs by significantly enlarging the scarce observational sample so far available. We present rotation curves and mean velocity dispersions for four bright dwarf ellipticals and two dwarf lenticular galaxies in the Virgo cluster. Most of these galaxies exhibit conspicuous rotation curves. In particular, five out of the six new galaxies are found to be close to the predictions for oblate spheroids flattened by rotation. Therefore, and contrary to the previous observational hints, the present data suggest that an important fraction of dwarf early-type galaxies may be rotationally supported.

Empirical calibration of the near-infrared Ca II triplet - II. The stellar atmospheric parameters

We present a homogeneous set of stellar atmospheric parameters (T-eff, log g, [Fe/H]) for a sample of about 700 field and cluster stars which constitute a new stellar library in the near-IR developed for stellar population synthesis in this spectral region (lambda 8350-9020). Having compiled the available atmospheric data in the literature for field stars, we have found systematic deviations between the atmospheric parameters from different bibliographic references. The Soubiran, Katz & Cayrel sample of stars with very well determined fundamental parameters has been taken as our standard reference system, and other papers have been calibrated and bootstrapped against it. The obtained transformations are provided in this paper. Once most of the data sets were on the same system, final parameters were derived by performing error weighted means. Atmospheric parameters for cluster stars have also been revised and updated according to recent metallicity scales and colour-temperature relations.

Using spectroscopic data to disentangle stellar population properties

It is well known that, when analyzed in the light of current synthesis model predictions, variations in the physical properties of single stellar populations (e.g. age, metallicity, initial mass function, element abundance ratios) may have a similar effect in their integrated spectral energy distributions. The confusion is even worsened when more realistic scenarios, i.e. composite star formation histories, are considered. This is, in fact, one of the major problems when facing the study of stellar populations in star clusters and galaxies. Typically, the observational efforts have aimed to find the most appropriate spectroscopic indicators in order to avoid, as far as possible, degeneracies in the parameter space. However, from a practical point of view, the most suited observables are not, necessarily, those that provide more orthogonality in that parameter space, but those that give the best balance between parameter degeneracy and sensitivity to signal-to-noise ratio per Angstrom, S/N(Angstrom). In order to achieve the minimum combined total error in the derived physical parameters, this work discusses how the functional dependence of typical line-strength indices and colors on S/N(Angstrom) allows to define a suitability parameter which helps to obtain more realistic combinations of spectroscopic data. As an example, we discuss in more detail the problem of breaking the well known age-metallicity degeneracy in relatively old stellar populations, comparing the suitability of different spectroscopic diagrams for a simple stellar population of solar metallicity and of 12 Gyr in age.

Empirical calibration of the λ4000 Å break

Empirical fitting functions, describing the behaviour of the \lambda 4000 \AA break, D_4000, in terms of effective temperature, metallicity and surface gravity, are presented. For this purpose, the break has been measured in 392 stars from the Lick/IDS Library. We have followed a very detailed error treatment in the reduction and fitting procedures, allowing for a reliable estimation of the break uncertainties. This calibration can be easily incorporated into stellar population models to provide accurate predictions of the break amplitude for, relatively old, composite systems.

Empirical calibration of the \lambda 4000 \AA break

Empirical fitting functions, describing the behaviour of the \lambda 4000 \AA break, D_4000, in terms of effective temperature, metallicity and surface gravity, are presented. For this purpose, the break has been measured in 392 stars from the Lick/IDS Library. We have followed a very detailed error treatment in the reduction and fitting procedures, allowing for a reliable estimation of the break uncertainties. This calibration can be easily incorporated into stellar population models to provide accurate predictions of the break amplitude for, relatively old, composite systems.

Towards an Understanding of the λ4000Å Break Behaviour in Old Stellar Populations

We present new empirical fitting functions describing the behaviour of the discontinuity at λ4000 Å in terms of the stellar atmospheric parameters: effective temperature, metallicity and surface gravity. These calibrations can be easily incorporated into stellar population models, providing, for the first time, accurate predictions of the break amplitud for, relatively old, composite systems.

IFU observations of the core of Abell 2218

We present a study of the morphologies and stellar content of the galaxies in the central ∼ 74” × 64” region of the galaxy cluster Abell 2218 (see Fig. 1). The spectroscopic data were obtained with the integral field unit PMAS (Roth et al. 2005) in the PPAK mode (Verheijen et al. 2004; Kelz et al. 2006), at the 3.5 m telescope of the Calar Alto Observatory. The covered wavelength range was 4650–8000 Å, with a spectral resolution FWHM ∼ 10 Å. These data were combined with with deep HST/ACS F475W, F555W, F625W and F850LP imaging, and additional data from the literature. In Fig. 2 we show the rest-frame B − V vs. MV colour-magnitude diagram for the identified cluster members. The data are segregated according to an objective morphological classification based on the Sérsic index. Contrary to previous results (e.g. Ziegler et al. 2001; Smail et al. 2001) there is a clear indication that the cluster population is not dominated by early-type galaxies, and that there exists an almost parity between earlyand late-type objects. In addition, the late-type galaxies spread over a wider range of colours and luminosities with respect to the early-types. This indicates that early-type objects are more massive and have older stellar populations, while late-type galaxies are less massive and exhibit a wider range of stellar population properties. The results agree with the two-steps scenario for the evolution of galaxies in clusters (e.g. Poggianti 2003), which explains the co-existence of a primordial population of early-type galaxies formed at early epochs, with an additional population of late-type galaxies that have been captured by the cluster, infalling, suffering a short enhancement of the star formation that is later quenched by the interaction with the environment, and that afterwards evolved passively, becoming redder and fainter. Finally, galaxy-galaxy interactions and dry merging processes were responsible for the building up of new massive spheroidal galaxies. Full details are given in Sánchez et al. (2007).

Stellar Populations in Virgo Bright Spheroidals

We present the results from new spectroscopic observations of a sample of bright spheroidal galaxies (also called dwarf ellipticals) in the Virgo cluster. The central line-strength indices of this sample have been compared with those of bright elliptical galaxies and the predictions of stellar population models. We present evidences for significant differences in the the stellar populations of these two kind of galaxies. In particular, the relative positions of the two galaxy families in the index-index diagrams suggests a steeper initial mass function slope for the star formation in spheroidal galaxies.

The near-IR Ca II Triplet: Empirical Calibration and Stellar Populations Synthesis Models

We present a new stellar library of 706 stars in the near-infrared spectral region (λλλ8348–9020 A, FWHM = 1.5 A) covering a wide range of atmospheric parameters. The library is developed for the empirical calibration of the Ca II triplet and stellar populations synthesis modeling. We conclude that the Can strength saturates for stellar populations with [Fe/H] > −0.5, showing a mild dependence on metallicity but a very strong sensitivity to the initial mass function.

The Near-IR Calcium Triplet: Empirical Calibration and Stellar Populations Models

We present a new stellar library at the near-IR spectral range(λ 8350–9020A, FWHM ~ 1.5 A) devoted to the empirical calibration of the Ca II triplet (λλ 8498, 8542 and 8662 A) which consists of 706 stars spanning a wide range in atmospheric parameters (2747 K < Teff < 38367 K, 0.00 < log g < 5.12, and −3.45 < [Fe/H] < +0.60). The atmospheric parameters were determined by bootstrapping literature data against a fix reference system (Soubiran, Katz and Cayrel, 1998), constituting then a highly homogeneous set.