Bibcode
Proctor, Robert N.; Forbes, Duncan A.; Hau, George K. T.; Beasley, Michael A.; De Silva, G. M.; Contreras, R.; Terlevich, A. I.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 349, Issue 4, pp. 1381-1396.
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4
2004
Citations
87
Refereed citations
81
Description
Hickson compact groups (HCGs) constitute an interesting extreme in the
range of environments in which galaxies are located, as the space
density of galaxies in these small groups are otherwise only found in
the centres of much larger clusters. The work presented here uses Lick
indices to make a comparison of ages and chemical compositions of
galaxies in HCGs with those in other environments (clusters, loose
groups and the field). The metallicity and relative abundance of
`α-elements' show strong correlations with galaxy age and central
velocity dispersion, with similar trends found in all environments.
However, we show that the previously reported correlation between
α-element abundance ratios and velocity dispersion disappears when
a full account is taken of the abundance ratio pattern in the
calibration stars. This correlation is thus found to be an artefact of
incomplete calibration to the Lick system.
Variations are seen in the ranges and average values of age, metallicity
and α-element abundance ratios for galaxies in different
environments. Age distributions support the hierarchical formation
prediction that field galaxies are on average younger than their cluster
counterparts. However, the ages of HCG galaxies are shown to be more
similar to those of cluster galaxies than those in the field, contrary
to the expectations of current hierarchical models. A trend for lower
velocity dispersion galaxies to be younger was also seen. This is again
inconsistent with hierarchical collapse models, but is qualitatively
consistent with the latest N-body smoothed particle hydrodynamics models
based on monolithic collapse in which star formation continues for many
Gyr in low-mass haloes.