How does black holes sette in galaxies

Every galaxy is believed to harbor a compact massive body-black hole- at its center. How it gets in? A natural way is through galaxy evolution based on the Big Bang model. However, there is the other way around, which is said being enigmatic [Nature,469:305].

Bulges and their black holes seem to be a natural consequence of structure formation in the hot Big Bang theory of the expanding Universe. According to this theory, galaxies grew by gravitational assembly of matter into clumps that gathered into larger clumps, and so on to galaxies. In galaxies with bulges, including ellipticals, which have bulges and no disks, the mass of the central black hole correlates not only with the mass of the bulge, but also, as Kormendy, Bender and Cornell¹ note ( page 374), with the average spread of velocities of the bulge stars (see Fig. 2a on page 375). The plausible explanation is that part of the gas out of which bulge stars formed settled instead near to the black hole, in part increasing its mass and in part fuelling explosions that blew the gas away and suppressed bulge-star formation. That is, the growth of bulge and black hole may have controlled each other. The timing looks right. Bulge stars are old: they formed when the expanding Universe was roughly a third of its present size (redshift about 2). This is when the rate of star formation per unit of matter was near its maximum (more than 10 times the present rate³). It is also when quasars — explosions powered by the central black holes — were most abundant (100 times more common than now⁴), probably an explosive result of overfeeding of the black holes as the early generations of stars were forming.

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In theory, galaxies both with and without bulges were growing by the gravitational collection of clumps of matter when the star-formation rate was near its peak. That would suggest that the clumps contained stars; a recent discussion puts roughly comparable masses in stars and gas⁶. So where are these early generations of stars? Not in disks, because there is nothing that would slow the motion of a star to allow it to settle onto a disk. Bulges contain old stars, and it has been suggested that this is where the early stars ended up. But we now see that this is not plausible: why would these old stars have avoided our bulgeless Galaxy and settled instead in the bulge of our neighbour M31? Maybe the old stars are in diffuse stellar haloes. If so, it seems curious that the stellar halo of our Galaxy is much less prominent than that of M31. But more studies of other nearby galaxies will be required to check for inventories of stars that are old enough and abundant enough to account for stars that formed before disks.