Deep observations of CO line emission from star-forming galaxies in a
  cluster candidate at z=1.5
  We report results from a deep Jansky Very Large Array (JVLA) search for CO
1-0 line emission from galaxies in a candidate galaxy cluster at z~1.55 in the
COSMOS field. We target 4 galaxies with optical spectroscopic redshifts in the
range z=1.47-1.59. Two of these 4 galaxies, ID51613 and ID51813, are nominally
detected in CO line emission at the 3-4 sigma level. We find CO luminosities of
2.4x10^10 K km/s pc^2 and 1.3x10^10 K km/s pc^2, respectively. Taking advantage
from the clustering and 2-GHz bandwidth of the JVLA, we perform a search for
emission lines in the proximity of optical sources within the field of view of
our observations. We limit our search to galaxies with K<23.5 (AB) and
z_phot=1.2-1.8. We find 2 bright optical galaxies to be associated with
significant emission line peaks (>4 sigma) in the data cube, which we identify
with the CO line emission. To test the reliability of the line peaks found, we
performed a parallel search for line peaks using a Bayesian inference method.
Monte Carlo simulations show that such associations are statistically
significant, with probabilities of chance association of 3.5% and 10.7% for ID
51207 and ID 51380, respectively. Modeling of their optical/IR SEDs indicates
that the CO detected galaxies and candidates have stellar masses and SFRs in
the range (0.3-1.1)x10^11 M_sun and 60-160 M_sun/yr, with SFEs comparable to
that found in other star-forming galaxies at similar redshifts. By comparing
the space density of CO emitters derived from our observations with the space
density derived from previous CO detections at z~1.5, and with semi-analytic
predictions for the CO luminosity function, we suggest that the latter tend to
underestimate the number of CO galaxies detected at high-redshift. Finally, we
argue about the benefits of future blind CO searches in clustered fields with
upcoming submm/radio facilities.
