Spatially resolved spectroscopy of early-type galaxies over a range in mass

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Title:		Spatially resolved spectroscopy of early-type galaxies over a range in mass
Authors:		Sánchez-Blázquez, Patricia; Forbes, Duncan A.; Strader, Jay; Brodie, Jean; Proctor, Robert
Affiliation:		AA(University of Central Lancashire, Centre for Astrophysics, Preston PR1 2HE; Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland; Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia), AB(Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia), AC(UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA), AD(UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA), AE(Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 377, Issue 2, pp. 759-786. (MNRAS Homepage)
Publication Date:		05/2007
Origin:		MNRAS
MNRAS Keywords:		galaxies: abundances, galaxies: elliptical and lenticular, cD, galaxies: evolution, galaxies: formation, galaxies: kinematics and dynamics
Abstract Copyright:		(c) 2007 The Authors. Journal compilation © 2007 RAS
DOI:		10.1111/j.1365-2966.2007.11647.x
Bibliographic Code:		2007MNRAS.377..759S

Abstract

Long-slit spectra have been obtained with the Keck telescope for a sample of 11 early-type galaxies covering a wide range in luminosity and hence mass. Rotation velocity and velocity dispersions, together with 19 Lick line-strength gradients have been measured, to, on average, two effective radii. Stellar population models taking into account the effect of the non-solar chemical composition have been used to derive ages, metallicities and α/Fe abundances along the radius. We find that line-strength gradients are due mainly to variations of the total metallicity with the radius. One galaxy out of 11 shows very strong age gradients, with a young central component, while the age gradient for the rest of the sample is very shallow or consistent with zero. We also find small variations in the [α/Fe] ratio with radius. Contrary to what is expected in simple collapse models, galaxies show both positive and negative [α/Fe] profiles. This rules out a solely inside-out, or outside-in, formation mechanism for all early-type galaxies. We do not find a correlation between the metallicity and the [α/Fe] gradients, and the local metallicity is not correlated with the local velocity dispersion for all the galaxies of our sample, which rules out scenarios where the delay in the onset of the galactic winds is the only mechanism producing the metallicity gradients. We found that metallicity gradients are correlated with the shape of the isophotes and the central mean age and metallicity of the galaxies, for galaxies younger than ~10Gyr. We show that the correlation between the gradients and the central values is not due to the correlation of the errors and indicates that the same process that shaped the gradient, also modified the structural parameters of the galaxies and triggered star formation in their centres. This strongly supports the merger scenario for the formation of these systems, where the degree of dissipation during those mergers increases as the mass of the progenitor galaxies decreases. Finally, we also find a dichotomy in the plane grad [α/Fe]-[α/Fe] between galaxies with velocity dispersions below and above ~200kms^-1, which requires confirmation with larger samples.

The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W.M. Keck Foundation.

E-mail: psanchez-blazquez@uclan.ac.uk

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