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(6) M(circle dot), black holes. These central black holes must have grown through accretion from less massive seeds in the early universe. The molecules CO and H 2 can be used to trace this young population of accreting massive black holes through the X-ray irradiation of ambient gas. The X-rays drive a low-metallicity ion-molecule chemistry that leads to the formation and excitation of CO and H(2) in 100 K < T <= 1000 K gas. H(2) traces very low metallicity gas, similar to 10(-3) solar or less, while some pollution by metals, similar to 10(-2) solar or more, must have taken place to form CO. Strong CO J > 15 and H(2) S(0) and 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 can be detected with ALMA, 5 10 M, over a redshift range of 5-20, provided that the black holes radiate close to Eddington.