Spectral gradients in central cluster galaxies: further evidence of star formation in cooling flows

We have obtained radial gradients in the spectral features D4000 and Mg2 for a sample of 11 central cluster galaxies (CCGs). The new data strongly confirm the correlations between line-strength indices and the cooling flow phenomenon found in our earlier study. We find that such correlations depend on the presence and characteristics of emission lines in the inner regions of the CCGs. CCGs in cooling flow clusters exhibit a clear sequence in the D4000-Mg2 plane, with a neat segregation depending on emission-line types and blue morphology. This sequence can be modelled, using stellar population models with a normal IMF, by a recent burst of star formation. In CCGs with emission lines, the gradients in the spectral indices are flat or positive inside the emission-line regions, suggesting the presence of young stars. Outside the emission-line regions, and in cooling flow galaxies without emission lines, gradients are negative and consistent with those measured in CCGs in clusters without cooling flows and giant elliptical galaxies. Index gradients measured exclusively in the emission-line region correlate with mass deposition rate. We have also estimated the radial profiles of the mass transformed into new stars which are remarkably parallel to the radial behaviour of the mass deposition rate. A large fraction (probably most) of the cooling flow gas accreted into the emission-line region is converted into stars. We discuss the evolutionary sequence suggested by McNamara (1997), in which radio triggered star formation bursts take place several times during the lifetime of the cooling flow. This scenario is consistent with the available observations.Comment: 19 pages, 18 PostScript figures, accepted for publication in MNRA

Reliable random error estimation in the measurement of line-strength indices

We present a new set of accurate formulae for the computation of random errors in the measurement of atomic and molecular indices. The new expressions are in excellent agreement with numerical simulations. We have found that, in some cases, the use of approximated equations can give misleading line-strength index errors. It is important to note that accurate errors can only be achieved after a full control of the error propagation throughout the data reduction with a parallel processing of data and error frames. Finally, simple recipes for the estimation of the required signal-to-noise ratio to achieve a fixed index error are presented.Comment: 9 pages, LaTeX file + 5 PostScript figures, psfig.sty and laa-s.sty required, to be published in Astronomy & Astrophysics Supplement Serie

Central Stellar Populations of S0 Galaxies in The Fornax Cluster

Based on FORS2-VLT long-slit spectroscopy, the analysis of the central absorption line indices of 9 S0 galaxies in the Fornax Cluster is presented. Central indices correlate with central velocity dispersions as observed in ellipticals. However, the stellar population properties of these S0s indicates that the observed trends are produced by relative differences in age and alpha-element abundances and not in metallicity ([Fe/H]) as previous studies have found in elliptical galaxies. The observed scatter in the line indices vs. velocity dispersion relations can be partially explained by the rotationally-supported nature of many of these systems. The presence of tighter line indices vs. maximum (circular) rotational velocity relations confirms this statement. It was also confirmed that the dynamical mass is the driving physical property of all these correlations and in our Fornax S0s it has to be estimated assuming rotational support.Comment: To appear in the Proceedings of IAU Symposium 241: "Stellar Populations as Building Blocks of Galaxies", 10-16 December, 2006 at La Palma, Canary Islands, Spai

Near-IR line-strengths in elliptical galaxies: evidence for IMF variations?

We present new relations between recently defined line-strength indices in the near-IR (CaT*, CaT, PaT, MgI, and sTiO) and central velocity dispersion (sigma_0) for a sample of 35 early-type galaxies, showing evidence for significant anti-correlations between CaII triplet indices (CaT* and CaT) and log(sigma_0). These relations are interpreted in the light of our recent evolutionary synthesis model predictions, suggesting the existence of important Ca underabundances with respect to Fe and/or an increase of the dwarf to giant stars ratio along the mass sequence of elliptical galaxies.Comment: 5 pages, 3 figures, LaTeX. Accepted for publication in MNRAS Pink Pages. See related information in http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.htm

Empirical calibration of the near-IR Ca II triplet - I. The stellar library and index definition

A new stellar library at the near-IR spectral region developed for the empirical calibration of the Ca II triplet and stellar population synthesis modeling is presented. The library covers the range 8348-9020 Angstrom at 1.5 Angstrom (FWHM) spectral resolution, and consists of 706 stars spanning a wide range in atmospheric parameters. We have defined a new set of near-IR indices, CaT*, CaT and PaT, which mostly overcome the limitations of previous definitions, the former being specially suited for the measurement of the Ca II triplet strength corrected for the contamination from Paschen lines. We also present a comparative study of the new and the previous calcium indices, as well as the corresponding transformations between the different systems. A thorough analysis of the sources of index errors and the procedure to calculate them is given. Finally, index and error measurements for the whole stellar library are provided together with the final spectra.Comment: 24 pages, 10 figures, LaTeX. For associated spectra, tables and software, see http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.html or http://www.nottingham.ac.uk/~ppzrfp/CATRIPLET.htm

Spectral Indices in Cooling Flow Galaxies: Evidence for Star Formation

Through the study of two absorption spectral features in the optical range (Mg2 and the 4000 A break), we find evidence for star formation in the inner regions of cooling-flow galaxies. The application of simple stellar population models reveals that the measured indices are explained if a relatively small fraction of the total mass flow (5-17%) is forming new stars with a normal IMF. However, we argue that this is only a lower limit, and conclude that a large fraction of the gas accreted inside the galaxy could be forming stars. In addition, the analysis of line-strength gradients in the inner galaxy regions reveals that, in the mean, they are lower than those of normal ellipticals, and exhibit a hint of correlation with the mass accretion rate. Simultaneously, the spectral indices in the outer regions of some galaxies, with and without cooling flow, attain extremely low values, suggesting that they could be hosting star formation with an origin not related to the cooling flows.Comment: PostScript file (compressed and encoded) containing 21 page

Evidence for intermediate-age stellar populations in early-type galaxies from K-band spectroscopy

The study of stellar populations in early-type galaxies in different environments is a powerful tool for constraining their star formation histories. This study has been traditionally restricted to the optical range, where dwarfs around the turn-off and stars at the base of the RGB dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, AGB stars are the main contributors. In this letter, we measure the near-infrared indices NaI and D$_{\rm CO}$ for a sample of 12 early-type galaxies in low density environments and compare them with the Fornax galaxy sample presented by Silva et al. (2008). The analysis of these indices in combination with Lick/IDS indices in the optical range reveals i) the NaI index is a metallicity indicator as good as C4668 in the optical range, and ii) D$_{\rm CO}$ is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low density environments show higher NaI and D$_{\rm CO}$ than those located in Fornax cluster, which points towards a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.Comment: 7 pages, 3 figures, accepted for publication in ApJ