The history of star formation and mass assembly in early-type galaxies

Authors

  • Clemens, M. S. , INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio, 5, 35122 Padova, Italy
  • Bressan, A., INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio, 5, 35122 Padova, Italy; and, SISSA-ISAS, International School for Advanced Studies, via Beirut 4, 34014 Trieste, Italy
  • Nikolic, B., Astrophysics Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
  • Rampazzo, R., INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio, 5, 35122 Padova, Italy

Abstract

We define a volume limited sample of over 14,000 early-type galaxies (ETGs) selected from data release six of the Sloan Digital Sky Survey. The density of environment of each galaxy is robustly measured. By comparing narrow band spectral line indices with recent models of simple stellar populations (SSPs) we investigate trends in the star formation history as a function of galaxy mass (velocity dispersion), density of environment and galactic radius. We find that age, metallicity and a-enhancement all increase with galaxy mass and that field ETGs are younger than their cluster counterparts by about 2 Gyr. We find negative radial metallicity gradients for all masses and environments, and positive radial age gradients for ETGs with velocity dispersion over 180 kmsâ1. Our results are qualitatively consistent with a relatively simple picture for ETG evolution in which the low-mass halos accreted by a proto-ETG contained not only gas but also a stellar population. This fossil population is preferentially found at large radii in massive ETGs because the stellar accretions were dissipationless.We estimate that the typical, massive ETG should have been assembled at z less than 3.5.

Catalogues

The catalogues are available in a plain ASCII format (compressed with bzip2), with one row for each galaxy in the catalogue. Please reference the catalogues by citing the paper above.

The galaxies in the catalogues below may be cross-correlated with other SDSS data products through the use of the (mjd, plateid, fiberid) tuples.

Descriptions of the columns in the catalogues

Column Name Units Description
Pmjd Days MJD of the SDSS observation of the galaxy
Pplate none SDSS plate number of the observation of the galaxy
Pfid none SDSS fibre ID of the observation of the galaxy
Pz_helio none Redshift of the galaxy (heliocentric reference frame)
PC_r none Concentration index calculated in the r-band
Phalfl arcsec The Petrosian half-light radius measured in r-band
PDDist MPc Luminosity distance to the galaxy
PDensCr   Density measure
Pvel_dis km/s The galaxy velocity dispersion as measured by the SDSS spectroscopic pipeline