Early Black Holes Grew Up Quickly

Illustration of the early Universe. Image credit: NASA. Click to enlarge.
It all began a long time ago while the universe was very young. The earliest massive breeder stars frolicked in their youth – spinning and cavorting among rich green grasses of virgin matter. As their allotted time played out, nuclear engines boiled off expansive streams of hot hydrogen and helium gas – enrichening the interstellar media. During this phase, supermassive star clusters formed in small pockets near nascent galactic cores – each cluster a swim in small regions of primordial mini-halo matter.

Completing their cycle, the earliest breeder stars exploded, spewing forth heavy atoms. But before too much heavy matter accumulated in the Universe, the earliest black holes formed, grew rapidly through mutual assimilation, and accumulated enough gravitational influence to draw “Goldilocks” gases of precise temperatures and composition into large wide accretion disks. This supercritical phase of growth matured the earliest massive black holes (MBHs) rapidly to supermassive black hole (SMBH) status. Out of this the earliest quasars took residence within the fused mini-haloes of numerous protogalaxies.

This picture of early quasar formation emerged from a recent paper (published June 2, 2005) entitled “Rapid Growth of High Redshift Black Holes” written by Cambridge UK Cosmologists Martin J. Rees and Marta Volonteri. That study treats the possibility that a brief window of rapid SMBH formation opened after the time of universal transparency but before gases in the interstellar media fully re-ionized through stellar radiation and seeded with heavy metals by supernovae. The Rees-Volonteri model attempts to explain facts coming out of the Sloan Digital Sky Survey (SDSS) dataset. By 1 billion years after the Big Bang, many highly radiant quasars had already formed. Each with SMBHs having masses exceeding 1 billion suns. These had arisen out of “seed black holes” – gravitational cinders left behind after the earliest cycle of supernovae collapse among the first massive galactic clusters. By one billion years post Big Bang, it was all but over. How could so much mass condense so quickly into such small regions of space?

According to Volontari and Rees, “To grow such seeds up to 1 billion solar masses requires an almost continuous accretion of gas…” Working against such a high accretion rate, is the fact that radiation from matter falling into a black hole typically offsets rapid “weight gain”. Most models of SMBH growth show that about 30% of the mass falling toward an intermediate (massive – not supermassive) black hole is converted to radiation. The effect of this is two-fold: Matter that would otherwise feed the MBH is lost to radiation, and outward radiation pressure stifles the march of additional matter inward to feed rapid growth.

The key to understanding rapid SMBH formation lies in the possibility that early accretion disks around MBH’s were not as optically dense as they are today – but “fat” with tenuously distributed matter. Under such conditions, radiation has a wider mean free path and can escape beyond disks without impeding inward motion of matter. Fuel driving the entire SMBH growth process is delivered copiously into the black hole event horizon. Meanwhile, the type matter present in the earliest epoch was mainly monatomic hydrogen and helium – not the kind of heavy metal rich accretion disks of a later era. All of this suggests that early MBH’s grew up in a hurry, ultimately accounting for the many fully mature quasars seen in the SDSS dataset. Such early MBHs must have had mass-energy conversion ratios more typical of fully mature SMBH’s than the MBH’s of today.

Volontari and Rees say that earlier investigators have shown that fully developed “quasars have a mass-energy conversion efficiency of roughly 10%…” The pair cautions however that this mass-energy conversion value comes out of studies of quasars from a later period in Universal expansion and that “nothing is known about the radiative efficiency of pregalactic quasars in the early Universe.” For this reason “the picture we have of the low redshift Universe may not apply at earlier times.” Clearly the early Universe was more densely packed with matter, that matter was at a higher temperature, and there was a higher ratio of non-metals to metals. All these factors say that it?s almost anyone’s best guess as to the mass-energy conversion efficiencies of early MBHs. Since we now must account for why so many SMBHs exist among early quasars, it makes sense that Volontari and Rees use what they know of today’s accretion disks as a means to explain how they such disks may have been different in the past.

And it is the earliest times – before radiation from numerous stars re-ionized gases within the inter-stellar media – that offered conditions ripe for rapid SMBH formation. Such conditions may well have lasted less than 100 million years and required an adept balance in the temperature, density, distribution, and composition of matter in the Universe.

To get the complete picture (as painted in the paper), we start with the idea that the early universe was populated by innumerable mini-halos comprised of dark and baryonic matter with highly massive but exceedingly dense star clusters in their midst. Due to the density of these clusters – and the massiveness of the stars comprising them – supernovae quickly developed to spawn numerous “seed black holes”. These seed BHs coalesced into massive black holes. Meanwhile gravitational forces and real motions rapidly brought the various mini-halos together. This created ever more massive halos capable of feeding MBHs.

In the early Universe, matter surrounding MBHs took the form of huge metal-poor spheroids of hydrogen and helium averaging some 8,000 degrees Kelvin in temperature. At such high temperatures, atoms remain ionized. Due to ionization, there were few electrons associated with atoms to act as photon traps. The effects of radiation pressure diminished to the point where matter fell more readily into a black holes event horizon. Meanwhile free electrons themselves scatter light. Some of that light actually re-radiates back toward the accretion disk and another source of mass – in the form of energy – feeds the system. Finally a dearth of heavy metals – such as oxygen, carbon, and nitrogen – means that monotomic atoms remain hot. For as temperatures fall below 4,000 degrees K, atoms de-ionize and again become subject to radiation pressure reducing the flux of fresh matter falling into the BH event horizon. All these purely physical properties tended to push mass-energy efficiency ratios down – allowing MBHs to put on weight rapidly.

Meanwhile as mini-halos coalesced, hot baryonic matter condensed into huge “thick” disks – not the thin rings seen around the SMBH’s today. This came about because halo matter itself completely surrounded the rapidly growing MBHs. This spheroidal distribution of matter provided a constant source of fresh, hot, virgin matter to feed the accretion disk from a variety of angles. Thick disks meant greater amounts of matter at lower optical density. Once again, matter managed to avoid being “solar-sailed” outward away from the looming maw of the MBH and mass-energy conversion ratios fell.

Both factors – fat disks and ionized, low mass atoms – say that during the golden age of an early green Universe, MBHs grew up fast. Within one billion years of the Big Bang they had settled down into a relatively quiet maturity efficiently converting matter into light and casting that light across vast reaches of time and space into a potentially ever-expanding Universe.

Written by Jeff Barbour