Unraveling the nature of bars and bulges
Classical bulges are traditionally understood as spheroids with an r1/4 surface brightness profile, in most ways identical to elliptical galaxies of the same luminosity. However, dynamicists have shown in their numerical simulations that disc instabilities can pump disc material above the plane, thus generating central structures that also bulge over the thin disc. The disky nature of bulges is usually discussed in relation to late-type, low-mass bulges. The apparent dichotomy in the way bulges are formed raises the question of whether bulges come in two flavours, early-types forming in mergers while late-types through disc instabilities (i.e. bars). A significant amount of information about their formation and evolution comes from the analysis of the internal dynamics and stellar populations. However, very little work has been devoted to this subject given their complicated star formation histories and also because of difficulties in the analysis due to extinction by the often large amounts of surrounding dust.
Our group is following an original approach to this long-standing problem by characterizing both the dynamical and stellar population state of bars and bulges using traditional and integral-field spectroscopy, in both optical and infrared wavelengths. This study will give us an accurate description not only of the bulge, but also of the disk and the bulge-to-disc transition regions. We are also investigating the role of single and double bars in the secular evolution of galaxies.
Outskirts of disks in nearby spirals
While the size evolution of the galaxies is now well established, we currently lack a detailed knowledge of how this evolutionary process works: we still do not know how and where the stars are created and how they are assembled into the galaxies. A key element to understand this process is located in the outskirts of galactic discs. In the inner region of the galaxies, the surface density (or stellar distribution) evolves very fast because of its own self-gravity and, consequently, the information about the processes that form the galaxies is quickly erased. At the galaxy periphery, however, the imprints of the galaxy formation mechanisms survive for longer. Some theoretical works claim that stars at the outskirts were formed in the dense regions of the galaxies and later migrated to the periphery as a consequence of mergers with smaller galaxy systems or gas displacement due to bars. Similarly, the stars could be there as a result of the interaction of the galaxies with their environment through tidal stripping and harassment from nearby encounters. Whatever the responsible mechanisms for placing (or creating) new stars in the galactic outskirts, the stellar population properties and the distribution of these stars are strongly linked to the processes involved in the growing and shaping of galaxies.
In order to address these questions, we are conducting a systematic study of the morphological and stellar populations properties of the outskirts of nearby spiral galaxies in the SDSS Stripe 82 data.
Formation and evolution of dwarf elliptical galaxies
Dwarf elliptical galaxies (dEs) are small, low-luminosity early-type galaxies which constitute the dominant population of nearby galaxy clusters. Indeed, dwarf ellipticals alone outnumber high luminosity galaxies by a factor of 6 in the Local Group, and they represent more than 50% of the galaxies in the Virgo cluster. As potential building blocks of massive galaxies in hierarchical frameworks of galaxy formation, dEs may provide important clues on the main processes involved in galaxy assembling and evolution. Unfortunately, the origin of dEs is still a matter of debate with various scenarios for their formation being proposed:
- They might be primordial objects which expelled their gas in early stages of their evolution because of supernova explosions.
- dEs could be the by-product of late-type disky galaxies that entered the cluster ~5 Gyr ago and evolved into a hot spheroid because of internal dynamical processes. Under this scenario, some dEs might still keep some memory of their origin. In fact, there exists a growing number of evidences for the existence of stellar disks, bars, spiral structures, and kinematically decoupled cores in the central regions (< 5 arcsec radius) of dEs.
- Tidal harassment within the cluster can also account for these substructures. Moreover, dEs are mostly found in clusters and groups of galaxies, while star forming dwarfs are predominantly found in the field. This very pronounced morphology density-relation for dwarfs shows that indeed the environment plays a very important role in their evolution.
We are currently studying integral-field spectrograph SAURON observations of dwarf galaxies in the field and cluster environments. These observations will allow us to study the presence of kinematic substructures, the internal variation of the distributions of metallicity and age, and the connection of the kinematics of the stars (and gas) with the local metal enrichment in these galaxies. These results, combined with dynamical models, will help us investigate the relation between the stellar populations and dynamical mass-to-light ratios of dwarf ellipticals and thus shade some light in their (potential) relationship with massive early-type galaxies.