ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey
  Description
  We present the rationale for and the observational description of ASPECS: The
ALMA SPECtroscopic Survey in the Hubble Ultra-Deep Field (UDF), the
cosmological deep field that has the deepest multi-wavelength data available.
Our overarching goal is to obtain an unbiased census of molecular gas and dust
continuum emission in high-redshift (z$>$0.5) galaxies. The $\sim$1$'$ region
covered within the UDF was chosen to overlap with the deepest available imaging
from HST. Our ALMA observations consist of full frequency scans in band 3
(84-115 GHz) and band 6 (212-272 GHz) at approximately uniform line sensitivity
($L'_{\rm CO}\sim$2$\times$10$^{9}$ K km/s pc$^2$), and continuum noise levels
of 3.8 $\mu$Jy beam$^{-1}$ and 12.7 $\mu$Jy beam$^{-1}$, respectively. The
molecular surveys cover the different rotational transitions of the CO
molecule, leading to essentially full redshift coverage. The [CII] emission
line is also covered at redshifts $6.0<z<8.0$. We present a customized
algorithm to identify line candidates in the molecular line scans, and quantify
our ability to recover artificial sources from our data. Based on whether
multiple CO lines are detected, and whether optical spectroscopic redshifts as
well as optical counterparts exist, we constrain the most likely line
identification. We report 10 (11) CO line candidates in the 3mm (1mm) band, and
our statistical analysis shows that $<$4 of these (in each band) are likely
spurious. Less than 1/3 of the total CO flux in the low-J CO line candidates
are from sources that are not associated with an optical/NIR counterpart. We
also present continuum maps of both the band 3 and band 6 observations. The
data presented here form the basis of a number of dedicated studies that are
presented in subsequent papers.
