The compact, ~1 kpc host galaxy of a quasar at z=7.1
  We present ALMA observations of the [CII] fine structure line and the
underlying far-infrared (FIR) dust continuum emission in J1120+0641, the most
distant quasar currently known (z=7.1). We also present observations targeting
the CO(2-1), CO(7-6) and [CI] 369 micron lines in the same source obtained at
the VLA and PdBI. We find a [CII] line flux of F_[CII]=1.11+/-0.10 Jy km/s and
a continuum flux density of S_227GHz=0.53+/-0.04 mJy/beam, consistent with
previous unresolved measurements. No other source is detected in continuum or
[CII] emission in the field covered by ALMA (~25"). At the resolution of our
ALMA observations (0.23", or 1.2 kpc, a factor ~70 smaller beam area compared
to previous measurements), we find that the majority of the emission is very
compact: a high fraction (~80%) of the total line and continuum flux is
associated with a region 1-1.5 kpc in diameter. The remaining ~20% of the
emission is distributed over a larger area with radius <4 kpc. The [CII]
emission does not exhibit ordered motion on kpc-scales: applying the virial
theorem yields an upper limit on the dynamical mass of the host galaxy of
(4.3+/-0.9)x10^10 M_sun, only ~20x higher than the central black hole. The
other targeted lines (CO(2-1), CO(7-6) and [CI]) are not detected, but the
limits of the line ratios with respect to the [CII] emission imply that the
heating in the quasar host is dominated by star formation, and not by the
accreting black hole. The star-formation rate implied by the FIR continuum is
105-340 M_sun/yr, with a resulting star-formation rate surface density of
~100-350 M_sun/yr/kpc^2, well below the value for Eddington-accretion-limited
star formation.
