Molecular gas in three z~7 quasar host galaxies
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(2017)cite arxiv:1707.05238Comment: 12 pages, 6 figures. Accepted for publication in ApJ.

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.
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