7. SUMMARY

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 considered, all 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 profiles.
• 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.