The study of stellar populations can provide the critical test needed to
understand the basic mechanism driving bulge formation. However, a clear
picture about the stellar population in bulges and their possible
correlations with other parameters is still not obvious. We have collated
the main results obtained from the study of colors and spectral
characteristics and the comparison with stellar population models and
can thus summarize them:
Bulges show a wide range of SSP-equivalent ages and metallicities.
There is a trend for which more massive bulges have, on average, older
stellar populations and higher values of metallicity and
[α / Fe].
- Bulges with high Sérsic index n tend
to be old and have high [α / Fe] but it is not clear if this trend
is due to the existing trend between mass and n.
- There are not strong differences in the stellar
populations of bulges and elliptical galaxies at the same mass, but the
details are of this comparison are still not clear and conclusions
differ between studies.
- Bulges are, in many cases, composite systems,
with disky, boxy/peanut and classical bulges coexisting in the same
galaxy. They usually host different stellar populations. Young stars,
when present, are located in central disks and rings. These young
components are not seen in edge-on samples which has lead studies
differing in the orientation of the sample to obtain different results.
- In general terms, there are not very clear
differences in the stellar population properties of bulges with and
without bars, neither in the central values nor in the variations with
radius. If there are differences, those are only present in massive
galaxies, and they show up as an excess of young populations when
compared to unbarred galaxies.
- When the mass-weighted mean values of age are
bulges, independent of their mass, seem to be dominated by old stars.
The trends between age, metallicity, and mass become much flatter and
almost non-existent. This is true for bulges with a variety of structural
parameters, such as different Sérsic indices or surface brightness
- Bulges show mild, negative, metallicity gradients
and almost null [α / Fe] and age gradients. The distribution of
the slopes is similar to that found in elliptical galaxies. At present,
there is no agreement about the possible correlation of these
gradients and other parameters.
- There is a correlation between the metallicity of
the bulge and the disk but not between the ages.