Molecular Gas in Three z ∼ 7 Quasar Host Galaxies

Abstract

We present ALMA band 3 observations of the CO(6-5), CO(7-6), and [C I] 369 μm emission lines in three of the highest-redshift quasar host galaxies at 6.6< z< 6.9. These measurements constitute the highest-redshift CO detections to date. The target quasars have previously been detected in [C II] 158 μm emission and the underlying FIR dust continuum. We detect (spatially unresolved, at a resolution of >2″, or ≳14 kpc) CO emission in all three quasar hosts. In two sources, we detect the continuum emission around 400 μm (rest-frame), and in one source we detect [C I] at low significance. We derive molecular gas reservoirs of (1-3) × 1010 {M}☉ in the quasar hosts, i.e., approximately only 10 times the mass of their central supermassive black holes. The extrapolated [C II]-to-CO(1-0) luminosity ratio is 2500-4200, consistent with measurements in galaxies at lower redshift. The detection of the [C I] line in one quasar host galaxy and the limit on the [C I] emission in the other two hosts enables a first characterization of the physical properties of the interstellar medium in z ∼ 7 quasar hosts. In the sources, the derived global CO/[C II]/[C I] line ratios are consistent with expectations from photodissociation regions, but not X-ray-dominated regions. This suggest that quantities derived from the molecular gas and dust emission are related to ongoing star-formation activity in the quasar hosts, providing further evidence that the quasar hosts studied here harbor intense starbursts in addition to their active nucleus.