Nuclei, Supermassive Black Holes and Bulges

Supermassive Black Holes

Supermassive black holes (SMBHs) are the most extreme examples of gravity in action. The possibility that they can lead to new insights into general relativity, with broad impact on physics and cosmology, is part of the motivation for the LISA space mission. Moreover, SMBHs sit at privileged locations, the centers of galaxies, which themselves are extraordinary laboratories of galactic evolution. Empirical scaling relations, such as the M-σ and M-Mhalo relations, point to a tight coupling between small scales and the galaxy overall.

Figure 1 shows the WFPC2 F814W image of the galaxy VCC 128 (left panel, 32" x 37"). On the top right we zoom into the central 2.5" x 1.5", showing that the nucleus is resolved into two components. The bottom right shows the same zoom into the V-I color map, where V-I ranges between 1 and 2 mag, reaching a value of about 1 in the double nucleus. The presence of a double nucleus is usually interpreted as evidence for a supermassive black hole. This would be a rare detection of a supermassive black hole in a dwarf elliptical galaxy. You can read more about this discovery in our press release at the Winter 2007 AAS meeting in Seattle.

We found that equal-mass mergers always lead to the formation of a close SMBH pair at the center of the remnant with separations limited solely by the adopted force resolution of ∼100 pc. Instead, the final separation of the SMBHs in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the companion galaxy is entirely disrupted before the merger is completed, leaving the satellite SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. We showed that galaxies constructed to obey the MBH-σ relation, move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. In dissipational mergers, the interplay between strong gas inflows associated with the formation of massive nuclear disks and the consumption of gas by star formation may provide the necessary fuel to the SMBHs and allow their host galaxies to satisfy the relation.

Figure 2 shows the final separation of SMBHs in merger simulations. The large-scale (left) and small-scale (right) structure of the remnants projected onto the orbital plane is also shown. All frames correspond to remnants that were allowed to relax for several dynamical times after the merger was complete. The top and bottom rows of panels present results for the coplanar 1:1 and 4:1 mergers with gas cooling, respectively. The middle rows of panels corresponds to the coplanar collisionless 4:1 merger. The frames on the left show the logarithmic baryonic surface density maps and are 320 x 230 kpc. The limiting surface density is 1 Msun/pc2. Blue and red maps are used for the stellar and gaseous component, respectively, and adaptive smoothing is used to preserve details in high-density regions. The top and bottom frames on the right are enlarged by a factor of 100 and show the central nuclear gaseous disk. The middle frame on the right shows the stellar distribution of the merger remnant and is enlarged by a factor of 30.

Reference: Kazantzidis, et al. (2005).

Nuclear Clusters

We found 9 nuclear cluster candidates in a sample of 14 edge-on, late-type galaxies observed with HST/ACS (figure 3). Three of the nuclear clusters are significantly flattened and show evidence for multiple, coincident structural components. The elongations of these three clusters are aligned to within 10° of the galaxies' major axes. Structurally, the flattened clusters are well fit by a combination of a spheroid and a disk or ring, with the disk preferred in two of three cases. The nuclear cluster disks/rings have stellar ages < 1 Gyr. Based on our observational results we proposed an in situ formation mechanism for nuclear clusters in which stars form episodically in compact nuclear disks, and then lose angular momentum or heat vertically to form an older spheroidal structure. We estimated the period between star formation episodes to be about 0.5 Gyr.

Reference: Seth, et al. (2006).

Nuclear Bars

Some galaxies may have two bars, a large scale bar and a smaller nuclear bar, and are then referred to as double-barred. Nuclear bars have been known since the 1970s, but only with HST has it been possible to search for them systematically because of their small sizes. As a result, we now know that about one third of barred galaxies host a nuclear bar inside their main bar. Nuclear bars have been thought to be one way in which gas can be funneled down to the centers of galaxies to feed the monster black holes lurking there. We discovered a double-barred galaxy, GOODS J033230.93-273923.7 (figure 4), using the wealth of data collected by the Hubble Space Telescope's Advanced Camera for Surveys. It is the farthest known example of a double-barred galaxy, at a distance of 2.3 billion light-years. The nuclear bar, which is 5000 light-years across, is surrounded by small spiral arms and embedded within the main bar, which is 46,000 light-years from end to end.

Reference: Lisker, et al. (2006).

Figure 5 illustrates an N-body simulation of a double-barred galaxy showing the non-uniform relative rotation of the secondary in the frame of the primary bar, which remains horizontal. The panels are equally-spaced in time. The straight line marks the major axis of the secondary bar. The secondary bar rotates faster when the two bars are parallel than when they are perpendicular.

Animation 1 shows the non-rigid rotation of nuclear bars inside a large scale bar.

Reference: Debattista & Shen (2007).