Plateau de Bure High-z Blue-Sequence Survey 2 (PHIBSS2): Search for
  Secondary Sources, CO Luminosity Functions in the Field, and the Evolution of
  Molecular Gas Density through Cosmic Time
  We report on the results of a search for serendipitous sources in CO emission
in 110 cubes targeting CO(2-1), CO(3-2), and CO(6-5) at z ~ 1-2 from the second
Plateau de Bure High-z Blue-Sequence Survey (PHIBSS2). The PHIBSS2 observations
were part of a 4-year legacy program at the IRAM Plateau de Bure Interferometer
aimed at studying early galaxy evolution from the perspective of molecular gas
reservoirs. We present a catalog of 67 candidate secondary sources from this
search, with 45 out of the 110 data cubes showing sources in addition to the
primary target that appear to be field detections, unrelated to the central
sources. This catalog includes the redshifts, line widths, fluxes, as well as
an estimation of their reliability based on their false positive probability.
We perform a search in the 3D-HST/CANDELS catalogs for the secondary CO
detections and tentatively find that ~64% of these have optical counterparts,
which we use to constrain their redshifts. Finally, we use our catalog of
candidate CO detections to derive the CO(2-1), CO(3-2), CO(4-3), CO(5-4), and
CO(6-5) luminosity functions over a range of redshifts, as well as the
molecular gas mass density evolution. Despite the different methodology, these
results are in very good agreement with previous observational constraints
derived from blind searches in deep fields. They provide an example of the type
of "deep field" science that can be carried out with targeted observations.
