SUPERMASSIVE BLACK HOLE
EVOLUTIONS
The recent discoverry of relations between supermassive black holes and their host galaxies indicate that the growth of the SMBHs in the centres of galaxies, and galaxies themselves, must be closely related.
The most massive black holes
THE MONSTER BLACK HOLES
Giant black holes, with masses of a few billion times of the Sun, have fascinated scientists since they were first proposed four decades ago. In 2011, McConell present the detection of the two most massive black holes ever found (NGC 3842 - 9.7 million solar masses; and NGC 4889 - 9.8 million solar masses). They argue that their discovering provides a key missing pieces of evidence to our understanding if how galaxies and black holes form.
Figure 1. Two most massive black holes ever found.
(Left). NGC 4889, the brightest galaxy in the Coma cluster (at a distance 103 Mpcs) - one of the richest nearby galaxy clusters - has a central black hole mass from 9.8 to 27 billion solar masses . (Right) NGC 3842 is the brightest galaxy in Abell 1367 cluster - a moderately rich galaxy cluster - has a supermassive black hole mass of 9.7 billion solar masses.
Upper limit to black hole masses
In 2009, Natarajan & Treister argued that there seem to exist a certain upper limit to the black hole growth via accretion of material which approximately 1e10 solar masses. The detection of the two most massive black holes indicated these black holes can not grow via material accretion, but the dry-mergers or forming by massive high red-shift seeds.
The co-evolution of massive black holes and their host galaxies
Simulations of mergers of gas-rich disk galaxies are able to produce remnant galaxies that follow the observed BH mass - sigma correlation in Figure 2, over the mass range 1e7-1e9 solar mass. By contrast, simulated mergers of elliptical galaxies with low-angular momentum progenitor orbits increase black hole mass and luminosity similar numerical factors, without increase the velocity dispersion. Because these mergers are a likely path to forming the most massive galaxies, the BH mass - sigma correlations may steepen or disappear altogether at the highest galaxy masses.
Interestingly, the two newly measured supermassive black holes are more massive than would be predicted from their velocity dispersion (BH - sigma correlation). This suggest that, unlike their smaller counterparts, these black holes did not grow most of their mass by gas accretion but instead grew by dry merging of gas-poor galaxies.
Figure 2. Correlations of dynamically measured black hole masses and bulk properties of host galaxies.
(Left). Black hole mass versus velosity dispersion. (Right) Black hole mass versus V-band bulge luminosity
A merger in NGC 4676 galaxy observed by Hubble Space Telescope (HST/ACS). The pair will eventually merge into a single giant galaxy. In this scenario, the central black hole mass will increase while velocity dispersion almost unchange
Two chanels of supermassive black hole growth
Neverthless, there are several arguments that offer a tantalizing indication that the BH mass - sigma scalign relation are not universal. In 2013, Michele Cappellari et al. found evidence for a change in the BH mass variation above the critical mass about 2e11 solar mass. The black holes in galaxies less massive than critical mass can be predicted by the BH mass-sigma relation, while black hole in more massive galaxies follow a modified relation. This is consistent with the senario where the majority of galaxies grow through star formation, while the most massive galaxies undergo the sequence of dissipation-less mergers.
To test this hyprothesis, we need to search for the records of growing dependence of black hole mass with galaxy properties change from velocity dispersion to galaxy stellar mass in the highest mass target. the most massive galaxies is the key for our understanding about galaxy evolution and in particular how ultramassive black holes can form. However, unfortunately, there are only 5 such galaxies that are studied. The reason is these galaxies are extremely rare in the local universe within 100 Mpc), and to find them, we have to reach to the required spatial resolutions and sensitivities go below the limits of existing ground-based adaptive optics (AO) assisted telescopes (like VLT or GEMINI).
In the future, ELT will be the solution to this problem. We define a sample survey of the most massive black holes with ELT in range of angular-size distance from 100 pc to 1 Gpc.
