We present evidence that there is a significant correlation between the fraction of a galaxy's mass that lies in its central black hole and the age of the galaxy's stellar population. Since the ...absorption line indices that are used to estimate the age are luminosity weighted, they essentially measure the time since the last significant episode of star formation in the galaxy. The existence of this correlation is consistent with several theories of galaxy formation, including the currently-favoured hierarchical picture of galaxy evolution, which predicts just such a relation between black hole mass and the time since the last burst of merger-induced star formation. It is not consistent with models in which the massive black hole is primordial, and hence uncoupled from the stellar properties of the galaxy.
We use the $C_{2}4668$, $Fe4383$, $H\gamma_A$ and $H\delta_A$ spectral
absorption line indices, together with U- and V-band photometry of 101 galaxies
in the Coma cluster, to investigate how mean age ...and metal abundance correlate
with galaxy luminosity. In particular, we use the line index measurements to
address the origin of the colour-magnitude relation (CMR). We find that the CMR
in Coma is driven primarily by a luminosity-metallicity correlation. We
additionally show evidence for a relation between age and luminosity, in the
direction predicted by the semi-analytic hierarchical clustering models of
Kauffmann & Charlot, but this is only present in the $C_24668$ index models,
and could be an effect of the lack of non solar abundance ratios in the Worthey
models used.
By comparing deviations from the CMR, with deviations in absorption index
from analogous `index-magnitude' relations, we find that colour deviations
bluewards of the mean relation are strongly correlated with the hydrogen Balmer
line series absorption. We show that the properties of these blue galaxies are
consistent with the presence of a young stellar population in the galaxies,
rather than with a reduced metallicity.
We use the \(C_{2}4668\), \(Fe4383\), \(H\gamma_A\) and \(H\delta_A\) spectral absorption line indices, together with U- and V-band photometry of 101 galaxies in the Coma cluster, to investigate how ...mean age and metal abundance correlate with galaxy luminosity. In particular, we use the line index measurements to address the origin of the colour-magnitude relation (CMR). We find that the CMR in Coma is driven primarily by a luminosity-metallicity correlation. We additionally show evidence for a relation between age and luminosity, in the direction predicted by the semi-analytic hierarchical clustering models of Kauffmann & Charlot, but this is only present in the \(C_24668\) index models, and could be an effect of the lack of non solar abundance ratios in the Worthey models used. By comparing deviations from the CMR, with deviations in absorption index from analogous `index-magnitude' relations, we find that colour deviations bluewards of the mean relation are strongly correlated with the hydrogen Balmer line series absorption. We show that the properties of these blue galaxies are consistent with the presence of a young stellar population in the galaxies, rather than with a reduced metallicity.
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 `\(\alpha\)-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 \(\alpha\)-element abundance ratios and velocity dispersion disappears when a full account is taken of the the abundance ratio pattern in the calibration stars. This correlation is thus found to be an artifact of incomplete calibration to the Lick system. Variations are seen in the ranges and average values of age, metallicity and \(\alpha\)-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-SPH models based on monolithic collapse in which star formation continues for many Gyr in low mass halos.
