Can Supermassive Black Holes Form in Metal-Enriched High-Redshift Protogalaxies ?
Abstract
Primordial gas in protogalactic dark matter (DM) halos with virial temperatures Tvir > 104 K begins to cool and condense via atomic hydrogen. Provided this gas is irradiated by a strong ultraviolet (UV) flux and remains free of H2 and other molecules, it has been proposed that the halo with Tvir ~104 K may avoid fragmentation, and lead to the rapid formation of a supermassive black hole (SMBH) as massive as M=105-106 Msun. This ``head--start'' would help explain the presence of SMBHs with inferred masses of several x 109 Msun, powering the bright quasars discovered in the Sloan Digital Sky Survey at redshift z>~6. However, high-redshift DM halos with Tvir~104K are likely already enriched with at least trace amounts of metals and dust produced by prior star-formation in their progenitors. Here we study the thermal and chemical evolution of low-metallicity gas exposed to extremely strong UV radiation fields. Our results, obtained in one-zone models, suggest that gas fragmentation is inevitable above a critical metallicity, whose value is between Zcr~3x10-4 Zsun (in the absence of dust) and as low as Zcr~ 5 x 10-6 Zsun (with a dust-to-gas mass ratio of about 0.01 Z/Zsun). We propose that when the metallicity exceeds these critical values, dense clusters of low--mass stars may form at the halo nucleus. Relatively massive stars in such a cluster can then rapidly coalesce into a single more massive object, which may produce an intermediate-mass BH remnant with a mass up to M <~102-103 Msun.