First Constraints on Dense Molecular Gas at z = 7.5149 from the Quasar Pōniuā‘ena

Abstract

We report the detection of CO(6–5) and CO(7–6) and their underlying continua from the host galaxy of quasar J100758.264+211529.207 (Pōniuā‘ena) at z = 7.5149, obtained with the NOrthern Extended Millimeter Array. Pōniuā‘ena belongs to the HYPerluminous quasars at the Epoch of ReionizatION sample of 18 z > 6 quasars selected to be powered by supermassive black holes, which experienced the fastest mass growth in the first cosmic gigayear. The one reported here is the highest-redshift measurement of the cold and dense molecular gas to date. The host galaxy is unresolved, and the line luminosity implies a molecular reservoir of M (H _2 ) = (2.2 ± 0.2) × 10 ^10 M _⊙ , assuming a CO spectral line energy distribution typical of high-redshift quasars and a conversion factor α = 0.8 ${M}_{\odot }{({\rm{K}}\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{pc}}^{2})}^{-1}$ . We model the cold dust spectral energy distribution to derive a dust mass of M _dust = (1.7 ± 0.6) × 10 ^8 M _⊙ and thus, a gas-to-dust ratio ∼130. Both the gas and dust mass are remarkably similar to the reservoirs found for luminous quasars at z ∼ 6–7. We use the CO detection to derive an estimate of the cosmic mass density of H _2 , ${{\rm{\Omega }}}_{{{\rm{H}}}_{2}}\simeq 1.31\times {10}^{-5}$ . This value is in line with the general trend suggested by literature estimates at z < 7 and agrees fairly well with the latest theoretical expectations of nonequilibrium molecular-chemistry cosmological simulations of cold gas at early times.

Keywords

• Quasars