Effect of cosmic ray/X-ray ionization on supermassive black hole
  formation
  We study effects of external ionization by cosmic rays (CRs) and X-rays on
the thermal evolution of primordial clouds under strong far-ultraviolet (FUV)
radiation. A strong FUV radiation dissociates H2 and quenches its cooling. Even
in such an environment, a massive cloud with Tvir>10^4 K can contract
isothermally at 8000 K by Lyman alpha cooling. This cloud collapses
monolithically without fragmentation, and a supermassive star (>10^5 Msun) is
believed to form at the center, which eventually evolves to a supermassive
black hole (SMBH). However, candidates of FUV sources, including star-forming
galaxies, are probably sources of strong CRs and X-rays, as well. We find that
the external ionization promotes H2 production and elevates the threshold FUV
intensity Jcr needed for the SMBH formation for CR energy density U_CR>10^-14
erg/cm^3 or X-ray intensity J_X>10^-24 erg/s/cm^2/sr/Hz at 1 keV. The critical
FUV flux increases in proportion to U_CR^{1/2} (J_X^{1/2}) in the high CR
(X-ray, respectively) limit. With the same value of FUV intensity at the Lyman
limit (13.6 eV), the H^- photodissociation rate, with threshold of 0.755 eV,
increases and the H2 molecules decrease with decreasing effective temperature
of the FUV sources T*. The lower value of T* thus results in the lower critical
FUV flux at 13.6 eV. Using an empirical relation between intensities of FUV and
CR/X-ray from nearby star-forming galaxies, we find that external ionization
effect remarkably enhances the critical FUV flux for sources with T* as high as
10^5 K and composed of stars with <100 Msun to a level that is not realized in
any halo. This indicates that, to induce SMBH formation, the FUV sources must
be either Pop II/I galaxies with low brightness temperature, Pop III galaxies
with a very top-heavy IMF, or Pop III galaxies too young to harbor sources of
CR/X-ray, e.g., supernova remnants or high-mass X-ray binaries.
