Imaging the cold molecular gas in SDSS J1148 + 5251 at z = 6.4
  We present Karl G. Jansky Very Large Array (VLA) observations of the CO ($J =
2 \rightarrow 1$) line emission towards the $z = 6.419$ quasar SDSS
J$114816.64+525150.3$ (J$1148+5251$). The molecular gas is found to be
marginally resolved with a major axis of $0.9"$ (consistent with previous size
measurements of the CO ($J = 7 \rightarrow 6$) emission). We observe tentative
evidence for extended line emission towards the south west on a scale of
~$1.4"$, but this is only detected at $3.3\sigma$ 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 ($J = 2
\rightarrow 1$) observations provide an anchor for the low excitation part of
the molecular line SED. 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 J$1148+5251$, and that the source departs substantially from
the low $z$ relationship between black hole mass and bulge mass. Hence, the
characteristics of J$1148+5251$ 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.
