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Young stellar populations in early-type galaxies in the SDSS

Published online by Cambridge University Press:  01 July 2007

Louisa A. Nolan
Affiliation:
School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK email: lan@star.sr.bham.ac.uk
Ata Kabán
Affiliation:
School of Computer Science, University of Birmingham, Birmingham, B15 2TT, UK
Markus Harva
Affiliation:
Laboratory of Computer and Information Science, Helsinki University of Technology, Helsinki, PO Box 5400, FI-02015 HUT, Finland
Andrew Benson
Affiliation:
California Institute of Tech., MC 130-33, 1200 E. California Blvd., Pasadena, CA 91125, USA
Somak Raychaudhury
Affiliation:
School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK email: lan@star.sr.bham.ac.uk
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Abstract

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We use a purely data-driven rectified factor analysis to identify early-type galaxies with young (≲ 4 Gyr) stellar populations in the Sloan Digital Sky Survey Spectroscopic Catalogue. We call these galaxies E+F galaxies, analogous to E+A galaxies. These galaxies lie in the ‘Green Valley’, between the blue cloud and the red sequence on the colour-magnitude diagram. As such, these galaxies may represent an important transient stage in the evolution of galaxies from blue and star-forming to red and passive. We investigate the distribution in projected local galaxy surface density of the E+F galaxies, and compare it with the environment of early-type and E+A galaxies. We find that i) the E+A distribution peaks strongly in projected local galaxy surface density, Σ5, at ~ 0.1−0.2 Mpc−2, ii) early-types have a flatter peak at ~ 0.06−0.2 Mpc−2, iii) the E+Fs lie somewhere in between, and iv) the distributions of the models do not agree well with the data, peaking at higher densities, and under-predicting the number of E+As at low (Σ5 < 0.3 Mpc−2) densities. The dearth of E+A and E+F galaxies in dense environments confirms that E+A and E+F galaxies are most likely the products of galaxy-galaxy merging or interactions, rather than star-forming galaxies whose star formation has been quenched by processes unique to dense environments, such as ram-pressure stripping or galaxy harassment. The similarity of the environments in which the E+F population and the E+A galaxy sample are found, together with the spectral evidence, suggests that E+F galaxies are E+A galaxies, which have evolved by a further ~ one to a few Gyr.

Type
Contributed Papers
Copyright
Copyright © International Astronomical Union 2008

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