Molecular gas in three z~7 quasar host galaxies
  We present ALMA band 3 observations of the CO(6-5), CO(7-6), and [CI]
369micron 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 [CII] 158micron
emission and the underlying far-infrared (FIR) dust continuum. We detect
(spatially unresolved, at a resolution of >2", or >14kpc) CO emission in all
three quasar hosts. In two sources, we detect the continuum emission around
400micron (rest-frame), and in one source we detect [CI] at low significance.
We derive molecular gas reservoirs of (1-3)x10^10 M_sun in the quasar hosts,
i.e. approximately only 10 times the mass of their central supermassive black
holes. The extrapolated [CII]-to-CO(1-0) luminosity ratio is 2500-4200,
consistent with measurements in galaxies at lower redshift. The detection of
the [CI] line in one quasar host galaxy and the limit on the [CI] 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/[CII]/[CI] line ratios are consistent with expectations from
photodissociation regions (PDR), but not X-ray dominated regions (XDR). 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.
