A merger in the dusty, $z=7.5$ galaxy A1689-zD1?
  The gravitationally-lensed galaxy A1689-zD1 is one of the most distant
spectroscopically confirmed sources ($z=7.5$). It is the earliest known galaxy
where the interstellar medium (ISM) has been detected; dust emission was
detected with the Atacama Large Millimetre Array (ALMA). A1689-zD1 is also
unusual among high-redshift dust emitters as it is a sub-L* galaxy and is
therefore a good prospect for the detection of gaseous ISM in a more typical
galaxy at this redshift. We observed A1689-zD1 with ALMA in bands 6 and 7 and
with the Green Bank Telescope (GBT) in band $Q$. To study the structure of
A1689-zD1, we map the mm thermal dust emission and find two spatial components
with sizes about $0.4-1.7$\,kpc (lensing-corrected). The rough spatial
morphology is similar to what is observed in the near-infrared with {\it HST}
and points to a perturbed dynamical state, perhaps indicative of a major merger
or a disc in early formation. The ALMA photometry is used to constrain the
far-infrared spectral energy distribution, yielding a dust temperature ($T_{\rm
dust} \sim 35$--$45$\,K for $\beta = 1.5-2$). We do not detect the CO(3-2) line
in the GBT data with a 95\% upper limit of 0.3\,mJy observed. We find a slight
excess emission in ALMA band~6 at 220.9\,GHz. If this excess is real, it is
likely due to emission from the [CII] 158.8\,$\mu$m line at $z_{\rm [CII]} =
7.603$. The stringent upper limits on the [CII]/$L_{\rm FIR}$ luminosity ratio
suggest a [CII] deficit similar to several bright quasars and massive
starbursts.
