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  Current Research Highlight :: September 2009 all highlights

Supermassive black holes grow in different ways

Detailed structural analysis of galaxies from the Sloan Digital Sky Survey (SDSS) by scientists at the Max Planck Institute for Astrophysics (MPA) have uncovered signs that the growth of black holes at the centre of massive galaxies depends on the dominant process that regulates the formation of the host galaxy. The discovery is a crucial step towards a better understanding of the joint formation of supermassive black holes and their hosts.

Fig. 1: Through the last decade, astronomers have found compelling evidence that black holes with masses up to billions of solar masses reside in the centre of massive galaxies. The new study suggests that these supermassive black holes might grow in different ways, which depend on structural properties of the host galaxy. Credits: ESA / V. Beckmann (NASA-GSFC)

Fig. 2: Left: stellar masses of elliptical galaxies and bulges are plotted against central velocity dispersion, as indicated. Black hole masses in the right panels are derived through the relation between host mass and black hole mass. It is clear that ellipticals and bulges follow offset relations (with statistical significance of 99 per cent). Since direct black hole mass measurements show that it correlates with both host mass and velocity dispersion, ellipticals and bulges cannot follow single relations between black hole mass and host mass and between black hole mass and velocity dispersion. This points out that the growing of a black hole depends on the dominant formation process of its host, and is thus not the same to every black hole. This figure also shows that a galaxy hosting a pseudo-bulge has a higher probability of being off the relation set by galaxies with classical bulges if it is barred.

Fig. 3: One can obtain the supermassive black hole mass distribution function using either the relation between black hole mass and host mass, or that between black hole mass and velocity dispersion. Surprisingly, however, the results are different. This is at least partially due to the distinct relations between host mass and velocity dispersion for ellipticals and bulges. The answer to this riddle might likely reveal new details on the joint formation of galaxies and their central black holes.

There is little doubt now that the centres of massive galaxies are the habitat of black holes with masses ranging from hundreds of thousands to billions of solar masses. Over several years of investigation, astronomers have found that the masses of such supermassive black holes are tightly related to the velocity dispersion of stars near the centre of the galaxy. In addition, their masses are also related to the total stellar mass of the galaxy, when the host galaxy is an elliptical galaxy, or to the stellar mass of the central galaxy bulge, when the host galaxy has a disc component. These scaling relations prompted scientists to develop theoretical models in which the formation of an elliptical galaxy or a bulge is connected to the growth of the black hole. The rapid infall of baryonic matter onto the gravitational potential of dark matter haloes can in principle lead to the formation of stellar structures such as elliptical galaxies and galaxy bulges, as well as central supermassive black holes. In their early evolutionary stages, such black holes are believed to release enormous amounts of energy, which can regulate the formation of new stars in their hosts, a mechanism known as feedback. Nevertheless, recent developments indicate that elliptical galaxies and bulges might form through different ways. Furthermore, there are also signs that different formation processes dominate in different bulges. This leads us to question whether all black holes grow in a similar fashion, or if their growth depends on how their host galaxies form.

To tackle this problem, the new study uses results from detailed work on the structural properties of a large sample of local massive galaxies (linkPfeil.gifsee Research Highlight July 2009), which measures the stellar masses of elliptical galaxies and bulges, and distinguishes, in a objective fashion, pseudo-bulges and classical bulges, which are thought to form through different dominant processes. Velocity dispersion measurements for the same sample were obtained from the Sloan Digital Sky Survey (SDSS). With these data at hand, it was found that ellipticals and bulges follow offset relations between their stellar masses and velocity dispersion. Since black hole mass correlates with host stellar mass and velocity dispersion, it follows that ellipticals and classical bulges cannot follow single relations between black hole mass and host mass and between black hole mass and velocity dispersion. This in turn indicates that the way black holes grow within the two kinds of stellar systems is different. The new study also points out that galaxies with pseudo-bulges are displaced from the scaling relations defined by galaxies with classical bulges, and this is usually caused by the presence of a bar in the galaxy. Bars lead to larger measured central velocity dispersion, and this effect is important in galaxies hosting pseudo-bulges, because the true velocity dispersion of the bulge is low in these systems.

The MPA scientists also carried out a detailed census of the black holes present in the local Universe using the large sample of galaxies available to them. It was found that 55 per cent of the mass in black holes in the local Universe is in the centres of elliptical galaxies, 41 per cent in classical bulges and 4 per cent in pseudo-bulges. Unfortunately, however, the results depend quite sensitively on whether host mass or velocity dispersion is used to infer the black hole mass.

The next generation of very large ground-based telescopes with adaptive optics will allow scientists to measure the motions of the stars and gas in the central regions of galaxies, and obtain precise black hole mass measurements for thousands of nearby ellipticals and disc galaxies. Today we only have a few dozen such measurements. This will revolutionise the field, as we will no longer need to rely on indirect measures of black hole mass as we did in this study. This will bring new understanding to how supermassive black holes and their host galaxies have co-evolved across cosmic time.

Dimitri Gadotti and Guinevere Kauffmann


Dimitri Alexei Gadotti and Guinevere Kauffmann, "The growth of supermassive black holes in pseudo-bulges, classical bulges and elliptical galaxies", 2009, Monthly Notices of the Royal Astronomical Society, in press

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