Pregalactic Black Hole Formation with an Atomic Hydrogen Equation of State

Abstract

The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of Mh ~ 107-109 M☉. For roughly isothermal collapse around 104 K, we find that line trapping of Lyα (H I and He II) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H2 abundance and very modest pollution by metals (<10-4 solar), fragmentation is likely to be inhibited for such an equation of state. We argue on purely thermodynamic grounds that a single black hole of ~(0.02-0.003)Mh can form at the center of a halo for z = 10-20 when the free-fall time is less than the time needed for a resonantly scattered Lyα photon to escape from the halo. The absence of H2 follows naturally from the high temperatures, >104 K, that are attained when Lyα photons are trapped in the dense and massive halos that we consider. An H2-dissociating UV background is needed if positive feedback effects on H2 formation from X-rays occur. The black hole-to-baryon mass fraction is suggestively close to what is required for these intermediate-mass black holes, of mass MBH ~ 104-106 M☉, to act as seeds for forming the supermassive black holes of mass ~0.001Mspheroid found in galaxies today.