Effect of cosmic ray/X-ray ionization on supermassive black hole formation

Kohei Inayoshi, Kazuyuki Omukai

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48 Citations (Scopus)


We study the 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 field photodissociates H2 and quenches its cooling. Even in such an environment, a massive cloud with virial temperature >rsim104K can contract isothermally at 8000K by hydrogen Lyman α cooling. This cloud collapses monolithically without fragmentation, and a supermassive star (>rsim105 M) is believed to form at the centre, which eventually evolves into a supermassive black hole (SMBH). However, candidates of FUV sources, including star-forming galaxies, are probably sources of strong CRs and X-rays too. We find that external ionization promotes H2 production and elevates the threshold FUV intensity Jcrit needed for SMBH formation for CR energy density UCR > rsim 10-14 ergcm-3 or X-ray intensity JX > rsim 10-24 ergs-1cm-2sr-1Hz-1 at 1keV. The critical FUV flux increases as Jcrit∝UCR1/2 (∝JX1/2) in the high CR (respectively X-ray) limit. With the same value of FUV intensity at the Lyman limit (13.6eV), the H- photodissociation rate, with a threshold of 0.755eV, increases and the H2 abundance decreases with decreasing effective temperature of the FUV sources T*. A lower value of T* thus results in a lower critical FUV flux Jcrit at the Lyman limit. Using an empirical relation between the intensities of FUV and CRs/X-rays from nearby star-forming galaxies, we find that the external ionization effect remarkably enhances the critical FUV flux for sources with T* as high as 105K and composed of stars with mass ≲100 M to a level that is not realized in any halo. This indicates that to induce SMBH formation the FUV sources must be either Population II/I galaxies with low brightness temperature (T*∼ 104K), Population III galaxies (T*∼ 105K) with a very top-heavy initial mass function or Population III galaxies too young to harbour sources of CRs/X-rays, for example supernova remnants or high-mass X-ray binaries.

Original languageEnglish
Pages (from-to)2748-2759
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 2011 Oct


  • Dark ages, reionization, first stars
  • Early Universe
  • Stars: Population III
  • Stars: formation


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