2015MNRAS.451.1713S

We present Karl G. Jansky Very Large Array (VLA) observations of the CO (J = 2 -> 1) line emission towards the z = 6.419 quasar SDSS J114816.64+525150.3 (J1148+5251). The molecular gas is found to be marginally resolved with a major axis of 0.9 arcsec (consistent with previous size measurements of the CO (J = 7 -> 6) emission). We observe tentative evidence for extended line emission towards the south-west on a scale of ∼ 1.4 arcsec, but this is only detected at 3.3σ significance and should be confirmed. The position of the molecular emission region is in excellent agreement with previous detections of low-frequency radio continuum emission as well as [C ii] line and thermal dust continuum emission. These CO (J = 2 -> 1) observations provide an anchor for the low-excitation part of the molecular line spectral energy distribution. We find no evidence for extended low-excitation component, neither in the spectral line energy distribution nor the image. We fit a single kinetic gas temperature model of 50 K. We revisit the gas and dynamical masses in light of this new detection of a low-order transition of CO, and confirm previous findings that there is no extended reservoir of cold molecular gas in J1148+5251, and that the source departs substantially from the low-z relationship between black hole mass and bulge mass. Hence, the characteristics of J1148+5251 at z = 6.419 are very similar to z ∼ 2 quasars, in the lack of a diffuse cold gas reservoir and kpc-size compactness of the star-forming region.