Issue |
A&A
Volume 407, Number 2, August IV 2003
|
|
---|---|---|
Page(s) | 423 - 435 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20030886 | |
Published online | 17 November 2003 |
Spatially resolved spectroscopy of Coma cluster early-type galaxies*
III. The stellar population gradients
1
Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany
2
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
3
Universitäts-Sternwarte München, Scheinerstraße 1, 81679 München, Germany
4
Department of Physics and Astronomy, 6127 Wilder Laboratory, Dartmouth College, Hanover, NH 03755-3528, USA
Corresponding author: D. Mehlert, dmehlert@lsw.uni-heidelberg.de
Received:
7
March
2003
Accepted:
27
May
2003
Based on Paper I of this series (Mehlert et al. [CITE]), we
derive central values and logarithmic gradients for the , Mg and Fe
indices of 35 early-type galaxies in the Coma cluster. We find that
pure elliptical galaxies have on average slightly higher velocity
dispersions, lower
, and higher metallic line-strengths than
galaxies with disks (S0). The latter form two families, one
comparable to the ellipticals and a second one with significantly
higher
, and weaker metallic lines. Our measured logarithmic
gradients within the effective radius are
,
,
and
. The gradients
strongly correlate with the gradients of σ, but only weakly
with the central index values and galaxy velocity dispersion.
Using stellar population models with variable element abundance ratios from
Thomas et al. ([CITE]) we derive average ages,
metallicities and [
] ratios in the center and at the effective
radius.
We find that the
ratio correlates with velocity dispersion
and drives 30% of the Mg–σ relation, the remaining 70% being
caused by metallicity variations.
We confirm previous findings that part of the lenticular galaxies in
the Coma cluster host very young (∼2 Gyr) stellar populations,
hence must have experienced relatively recent star formation
episodes. Again in accordance with previous work we derive negative
metallicity gradients (
dex per decade) that are
significantly flatter than what is expected from gaseous monolithic
collapse models, pointing to the importance of mergers in the galaxy
formation history. Moreover, the metallicity gradients correlate with
the velocity dispersion gradients, confirming empirically earlier
suggestions that the metallicity gradient in ellipticals is produced
by the local potential well. The gradients in age are negligible,
implying that no significant residual star formation has occurred
either in the center or in the outer parts of the galaxies, and that
the stellar populations at different radii must have formed at a
common epoch. For the first time we derive the gradients of the
ratio and find them very small on average. Hence,
enhancement
is not restricted to galaxy centers but it is a global phenomenon.
Our results imply that the Mg–σ local relation inside a galaxy,
unlike the global Mg–σ relation, must be primarily driven by
metallicity variations alone. Finally we note that none of the
stellar population parameters or their gradients depend on the density
profile of the Coma cluster, even though it spans 3 dex in density.
Key words: galaxies: clusters: individual: Coma / galaxies: elliptical and lenticular, cD / galaxies: stellar content
© ESO, 2003
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