Thursday, December 8, 2011

The Most Massive Black Holes

Simulation of gas falling into a supermassive BH. Credit: NASA/JPL-Caltech/JHU/UCSC
Two ten billion solar mass black holes were found. The first one is in the brightest galaxy cluster at a distance of 98 Mpc from Earth, NGC 3842. It weighs about 9.7 billion solar masses. The second supermassive black hole is at the center of NGC 4889 at a distance of 104 Mpc from us. It weighs about 21 billion solar masses. Until now the record for the most massive black hole was held by a black hole in the center of the M87 galaxy that weighs 6.3 billion solar masses. This post is mainly a summary of the Nature article.

How do scientists estimate black hole masses?
Empirical scaling relations between the black hole mass, galaxy bulge dispersion and luminosity are used to estimate the black hole masses in galaxies where a direct measurement is not possible due to large distance from Earth or low central stellar density.  These two black holes are special because scientists were able to directly measure their masses, i.e., they measured the stellar velocities of the central regions of the host galaxy accurately enough to determine the black hole mass. The two black hole masses did not agree with the predictions, which were off by several standard deviations (about a factor of 10 in both cases).

A rapidly growing BH in Markarian 231. Credit: NASA/ESA Hubble Telescope
What instruments did they use? 
The line of sight stellar velocities were measured using the Gemini North and Keck 2 telescopes, in Hawaii. The stellar luminosity distribution of each galaxy is provided by surface photometry from Hubble and ground based telescopes.

Possible conclusions?
The most massive black holes appear to be in the brightest cluster elliptical galaxies and not in brightest field elliptical galaxies. Clusters contain more objects and so more mergers are likely to occur. Many predictions are contradictory. Direct measurements of more black holes masses will help revise these relations. So far it appears that the mass vs. velocity dispersion relation either disappears or steepens at high masses. The steepening can occur due to the accretion of residual gas after star formation stops, which is likely in these very massive galaxies.

What are the progenitors of these black holes?
Black holes heavier than 10 billion solar masses are observed as quasars in the early universe (a few billion years after the big bag). Quasars contain young, rapidly accreting black holes. However, as time passes, the accretion slows down and they become regular galaxies. Quasars are still not well understood. Much more research needs to be done.

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