Bulges

Bulges and disks are the main components of galaxies. More than half of the stellar luminosity in the universe comes from disks and a further 25% is due to bulges. It is therefore vital to understand how these two components form and evolve.

The bulges of about 45% of edge-on galaxies are box- or peanut- (B/P) shaped. Edge-on studies have established a connection between B/P-shaped bulges and bars, and may form via the action of the bar. These are then refered to as pseudo-bulges.

A classical example of a peanut-shaped bulge is that of NGC 4565 shown in figure 1.

I devised a way to recognize B/P-shaped pseudo-bulges kinematically at low inclinations even when gas is present. In the past these were identified by their box/peanut-shape when viewed edge-on; since edge-on galaxies cannot be uniquely deprojected, the ability to identify pseudo-bulges at low inclinations will improve our understanding of them. The diagnostic is based on the fact that peanut shapes are associated with a flat density distribution in the vertical direction. We showed that the kinematic signature corresponding to such a distribution is a minimum in the fourth-order Gauss-Hermite moment s₄.

We used high resolution N-body models to study the kinematic signatures of face-on B/P-shaped pseudo bulges. The simulations shown in figure 2 all started with classical bulges and formed bars which in the case of B2 and B3 then caused a B/P-shape to form. The middle row shows the Gauss-Hermite moment d₄ of the vertical density distribution while the bottom row shows s₄ for the kinematics.

Reference: Debattista, et al. (2005).

Disks

When gas can cool in a gas rich disk, the gaseous disk becomes violently gravitationally unstable and the gas fragments into clumps that sink to the center, dragging an associated stellar clump (figure 3). Such clump instabilities build central bulge-like objects directly. In order for the resulting bulge to sit on the M_•-σ, less than 0.3% of the central gas needs to collapse into a black hole.

Disk galaxies are often characterized by an exponential surface brightness profile. However, this is typically valid only over a limited range in radii. At large galacto-centric radii most disk galaxies reveal a truncation in their surface brightness profile. Originally these truncations were interpreted as marking the outer edges of the stellar disks. It has now become clear that they rather indicate a sharp break between an inner and outer exponential profile. About 70% of late-type galaxies have this type of double-exponential breaks. My simulations showed that angular momentum redistribution within galaxies leads to realistic breaks in the surface density of disks. The breaks that result in these simulations are in very good agreement with observations, not only in terms of the break radii in units of inner disk scale length but also outer scale-lengths and the difference between central surface brightnesses of the two exponentials. This is true both in the face-on view and in the edge-on view (figure 4).

Reference: Debattista, et al. (2006).

Dwarf Ellipticals

Dwarf elliptical galaxies are the most common type of galaxy observed. In a hierarchical model, dwarf galaxies are expected to form first and to later merge to form larger galaxies. Thus understanding how they form and evolve is crucial for understanding galaxy formation in general (indeed the study of dwarf galaxies has been called "near field cosmology".)

A sizeable fraction of dwarf elliptical (dE) galaxies contain nuclei. These nuclei are sometimes observed to be offset from the center of the galaxy. In collaboration with S. De Rijcke, I studied the counter-streaming instability as a source of lopsided nuclei in dwarf elliptical galaxies. This instability results when some fraction of a galaxy's stars are counter-rotating with respect to the rest of the galaxy. We considered the case of FCC 046 (figure 5), a prototypical dE with a lopsided nucleus. We showed that simulations of the counter-streaming instability produced systems which are in broad agreement with observations of this galaxy. We also showed that low resolution spectra of FCC 046 seem to suggest the presence of counter-streaming, as required by our model. Higher resolution spectra with less noise are required, however, to confirm this result.

Figure 6 shows the absorption-line spectrum of FCC 046 along the major axis (top 3 panels, left and right) with the prediction from the N-body model overlaid in the gray line. Despite the low S/N, there is an indication that the line-of-sight velocity distribution (LOSVD) is split. The bottom panel shows the spectrum along the minor axis, the split is now absent, exactly as the model predicts.

Reference: De Rijcke & Debattista (2004).