First constraints of dense molecular gas at z~7.5 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=7.5419, obtained with the NOrthern Extended Millimeter Array (NOEMA). Pōniuā'ena belongs to the HYPerluminous quasars at the Epoch of ReionizatION (HYPERION) sample of 17 $z>6$ quasars selected to be powered by supermassive black holes (SMBH) which experienced the fastest mass growth in the first Gyr of the Universe. 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 $\mathrm{M}({\mathrm{H}}_2)=(2.2\pm 0.2)\times 10^{10}$ ${\mathrm{M}}_{\odot }$, assuming a CO spectral line energy distribution typical of high-redshift quasars and a conversion factor $\alpha =0.8$ ${\mathrm{M}}_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$. We model the cold dust spectral energy distribution (SED) to derive a dust mass of M${}_{\mathrm{dust}}=(2.1\pm 0.7)\times 10^8$ ${\mathrm{M}}_{\odot }$, and thus a gas to dust ratio $\sim 100$. Both the gas and dust mass are not dissimilar from the reservoir found for luminous quasars at $z\sim 6$. We use the CO detection to derive an estimate of the cosmic mass density of ${\mathrm{H}}_2$, ${\Omega }_{H_2}\simeq 1.31\times 10^{-5}$. 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 non-equilibrium molecular-chemistry cosmological simulations of cold gas at early times.

Submitted to ApJ Letters