Massive galaxies are stellar systems that display varied structural features. These features provide us with clues on how such galaxies form and evolve. Elliptical galaxies are characterised by a main spheroidal component, constituted mostly by old stars in random motion. Disc galaxies show a number of structural components: usually a central bulge, a flat and extended disc of stars, including young ones, which rotate about the galaxy centre, and, quite often, other components, such as bars, spiral arms and rings. How these components form and evolve is one of the major astrophysical problems today. Past studies have found indications that different types of bulges seem to exist. Classical bulges, the first type discerned, were found to have properties comparable to elliptical galaxies, and thus it was proposed that these systems share similar formation processes. In fact, this could mean that classical bulges are just simply elliptical galaxies which happen to have a disc around them. Later, pseudo-bulges were found with properties similar to discs, which suggested a different formation process.

In this new study, sophisticated methods of galaxy structural analysis are for the first time applied to a homogeneous and statistically large sample, using galaxy images in different passbands from the Sloan Digital Sky Survey (SDSS). This provides accurate measurements of bulge structural parameters such as its mass and size. Another novel contribution is to convincingly show that pseudo-bulges can be distinguished from classical bulges using a fundamental relation between bulge size and luminosity. The different bulge types occupy different loci in this relation.

This powerful methodology provides evidence that indeed classical and pseudo-bulges are formed through different processes and that the formation of the latter is connected to the growth of disc instabilities such as bars. In addition, it is also shown that the similarity between classical bulges and elliptical galaxies is restricted. One way to reach these conclusions is to assess in what manner these different stellar systems grow. This can be achieved with the results from the structural analysis of the SDSS galaxies. The data indicate that, for all stellar systems studied (elliptical galaxies, classical and pseudo-bulges, discs and bars), more massive systems are also larger. However, the relation between mass and size is different for classical and pseudo-bulges. This indicates that they grow in different ways. Moreover, the only systems that appear to grow in a similar fashion are pseudo-bulges and bars, which points out an intimate connection between both. Furthermore, the mass-size relations of elliptical galaxies and classical bulges are offset, in a way that an elliptical galaxy with the same mass as a classical bulge is on average significantly larger. Putting a stellar disc around such elliptical galaxy would thus not result in a realistic, existing disc galaxy. In other words, classical bulges cannot be seen as just elliptical galaxies with surrounding discs.

Surprisingly, however, the data also reveal bulges with mixed properties. Such bulges resemble classical bulges from a structural viewpoint, but are similar to pseudo-bulges in terms of their stellar content, i.e. they show signatures of stars that have been only recently formed. This brings on a new concept, whereby the processes that lead to the formation of a classical bulge can occur concomitantly with those that lead to the formation of a pseudo-bulge.

Finally, another interesting and useful outcome from this work is the stellar mass budget in the local Universe, that is how the total mass in stars is distributed amongst different systems. After accounting for sample selection effects, which made the probability of including an elliptical galaxy in the sample slightly larger than that of picking a disc galaxy, it is found that 32 per cent of the stellar mass in massive galaxies is contained in elliptical galaxies, 36 per cent in discs, 25 per cent in classical bulges, 3 per cent in pseudo-bulges and 4 per cent in bars. These figures are valuable for theoretical studies on the formation and evolution of galaxies, which seek to reproduce observations through a fundamental understanding of Nature.

Dimitri Gadotti

Publications

Dimitri Alexei Gadotti, "Structural properties of pseudo-bulges, classical bulges and elliptical galaxies: a Sloan Digital Sky Survey perspective", 2009, Monthly Notices of the Royal Astronomical Society, Volume 393, Issue 4, pp. 1531-1552
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