Star formation and gas kinematics of quasar host galaxies at z ∼ 6: New insights from ALMA

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C II] 158 μm fine structure line and dust continuum emission from the host galaxies of five redshift 6 quasars. We also report complementary observations of 250 GHz dust continuum and CO (6-5) line emission from the z = 6.00 quasar SDSS J231038.88+185519.7 using the IRAM facilities. The ALMA observations were carried out in the extended array at 0.″7 resolution. We have detected the line and dust continuum in all five objects. The derived [C II] line luminosities are 1.6 × 10⁹ to 8.7 × 10 ⁹ L _⊙ and the [C II]-to-FIR luminosity ratios are 2.9-5.1 × 10^-4, which is comparable to the values found in other high-redshift quasar-starburst systems and local ultra-luminous infrared galaxies. The sources are marginally resolved and the intrinsic source sizes (major axis FWHM) are constrained to be 0.″3-0.″6 (i.e., 1.7-3.5 kpc) for the [C II] line emission and 0.″2-0.″4 (i.e., 1.2-2.3 kpc) for the continuum. These measurements indicate that there is vigorous star formation over the central few kpc in the quasar host galaxies. The ALMA observations also constrain the dynamical properties of the star-forming gas in the nuclear region. The intensity-weighted velocity maps of three sources show clear velocity gradients. Such velocity gradients are consistent with a rotating, gravitationally bound gas component, although they are not uniquely interpreted as such. Under the simplifying assumption of rotation, the implied dynamical masses within the [C II]-emitting regions are of order 10 ¹⁰-10¹¹ M _⊙. Given these estimates, the mass ratios between the supermassive black holes and the spheroidal bulge are an order of magnitude higher than the mean value found in local spheroidal galaxies, which is in agreement with results from previous CO observations of high redshift quasars.