* Astronomy

Astronomers have unmasked hundreds of black holes hiding deep inside dusty galaxies billions of light-years away.
The massive, growing black holes, discovered by NASA's Spitzer and Chandra space telescopes, represent a large fraction of a long-sought missing population. Their discovery implies there are hundreds of millions of additional black holes growing in our young universe, more than doubling the total amount known at that distance.

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Frenetic growth of supermassive black holes in the early universe

Linhua Jiang (Steward Observatory, University of Arizona) has led a United States/Germany team of astronomers in the study of some of the most distant and youngest known quasars. The six distant quasars they observed are at redshifts ranging between z = 5.8 to 6.3 and correspond to a period when the universe was only about one billion years old. Using the Gemini Near Infrared Spectrograph (GNIRS) at Gemini South and the Near-Infrared Imager (NIRI) at Gemini North, the team found these very young quasars to be already super-enriched in heavy elements. The quasars are also powered by extremely massive black holes. The Jiang et al. results cast a new light on the assembly of black holes and the chemical enrichment of the universe less than one billion years after the Big Bang.



Quasars are the most luminous and most energetic objects in the universe. The stellar-like objects are surmised to be powered by radiation from matter accreting onto supermassive black holes at the centre of host galaxies in the process of forming. Dense gas in the region surrounding the black hole moves at high velocities. It gives rise to a broad line region (BLR) in spectra that can be used as a diagnostic of several properties of the gas itself and of the central black hole.

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Recent calculations indicate that when two galaxies, and the supermassive black holes that lie at their centres, merge, these galactic `marriages` frequently produce gravitational forces strong enough to kick the new combined black hole right out of its merged galaxy. However, so far, none of the many `empty nest` galaxies predicted by such calculations have been found.
Now researchers at the University of Maryland say merged black holes probably are kicked out far less often than predicted because torques from the vast accretion disk of parental galactic material that spins around and feeds merging black holes act to align their spins in a way that reduces the kick force.
In findings presented today during a press briefing at the American Astronomical Society's meeting in Honolulu Hawaii, Maryland astronomers Tamara Bogdanovic and Christopher Reynolds propose that in the majority of gas-rich galactic mergers, torques from gas accretion align the spins of supermassive black holes and their orbital axis with a large-scale gas disk. This mechanism, they say, helps explain the ubiquity of black holes at the centre of galaxies despite the potentially large kicks from gravitational radiation recoil.

While we expect a black hole ejection to be uncommon in the aftermath of gas-rich mergers, it is still possible that it may happen, especially in merging galaxies that are relatively gas-poor. Future observations of such gas-poor mergers may point to a class of massive galaxies without a central supermassive black hole - Tamara Bogdanovic .

What we have uncovered here is a remarkable interaction between the galactic scale gas disk and the comparatively tiny black holes - Christopher Reynolds .

It has recently been shown using relativistic calculations that when galaxies -- and the supermassive black holes at their centre -- merge, a large pulse of gravitational radiation is produced. This radiation pulse, essentially ripples in the fabric of space itself, can kick the final black hole up to speeds of 3000 km/s. But these large kicks require the black holes to be rapidly spinning, and for the spin axes to be oppositely directed and tilted over into the plane of the orbit. The difficulty this poses, say Bogdanovic and Reynolds is that the escape speed from most galaxies is less that 1000 km/s. Therefore, if large recoil speeds are typical, one might expect that many galaxies that have undergone major mergers would be without a black hole.
Current estimates are that over the past 6 billion years, about a half of all galaxies have undergone mergers. And yet, in contradiction to what the models predict, all observed galaxies with bulges indicative of mergers appear to have central supermassive black holes.

 It therefore seems that there is an astrophysical avoidance of the types of supermassive black hole coalescences that would lead to kicks beyond galactic escape speeds, write Bogdanovic, Reynolds and fellow Maryland astronomer Coleman Miller in a paper (to be published in the June 1 2007 issue of Astrophysical Journal Letters) explaining their proposal.

The lower than expected kicks can be explained if (1) the spins of merging black holes are all small, (2) there is a large difference in mass between most merging black holes or (3) the spins tend to align with each other and be oriented perpendicular to the orbit of the pair of black holes.
Observations do not favour the low spin mechanism and evidence suggests strongly that coalescence of comparable-mass black holes (which would enable high kick out speeds) should be common.

The most likely solution therefore seems to be that astrophysical processes tend to align the spins of supermassive black holes with the orbital axis, write Bogdanovic, Reynolds and Miller.

This work was supported by the University of Maryland, College Park -Astronomy Centre for Theory and Computation Prize Fellowship program and by funding from the National Science Foundation.

Source University of Maryland