  We present detections of the CO(J= 1-0) emission line in a sample of four
massive star-forming galaxies at z~1.5-2.2 obtained with the Karl G. Jansky
Very Large Array (VLA). Combining these observations with previous CO(2-1) and
CO(3-2) detections of these galaxies, we study the excitation properties of the
molecular gas in our sample sources. We find an average line brightness
temperature ratios of R_{21}=0.70+\-0.16 and R_{31}=0.50+\-0.29, based on
measurements for three and two galaxies, respectively. These results provide
additional support to previous indications of sub-thermal gas excitation for
the CO(3-2) line with a typically assumed line ratio R_{31}~0.5. For one of our
targets, BzK-21000, we present spatially resolved CO line maps. At the
resolution of 0.18 arcsec (1.5 kpc), most of the emission is resolved out
except for some clumpy structure. From this, we attempt to identify molecular
gas clumps in the data cube, finding 4 possible candidates. We estimate that
<40 % of the molecular gas is confined to giant clumps (~1.5 kpc in size), and
thus most of the gas could be distributed in small fainter clouds or in fairly
diffuse extended regions of lower brightness temperatures than our sensitivity
limit.
