On the Detection of High Redshift Black Holes with ALMA through CO and H2 Emission

Many present-day galaxies are known to harbor supermassive, ≥ 10 M⊙, black holes. These central black holes must have grown through accretion from less massive seeds in the early universe. The molecules CO and H2 can be used to trace this young population of accreting massive black holes through the Xray irradiation of ambient gas. The X-rays drive a low-metallicity ion-molecule chemistry that leads to the formation and excitation of CO and H2 in 100 < T ≤ 1, 000 K gas. H2 traces very low metallicity gas, ∼ 10 −3 solar or less, while some pollution by metals, ∼ 10 solar or more, must have taken place to form CO. Strong CO J > 15 and H2 S(0) & S(1) emission is found that allows one to constrain ambient conditions. Comparable line strengths cannot be produced by FUV or cosmic ray irradiation. Weak, but perhaps detectable, H+3 (2,2)→(1,1) emission is found and discussed. The models predict that black hole masses larger than 10 M⊙ can be detected with ALMA, over a redshift range of 5-20, provided that the black holes radiate close to Eddington. Subject headings: cosmology: theory – galaxies: black holes –ISM: clouds – ISM: molecules – molecular processes