Imaging diffuse clouds: Bright and dark gas mapped in CO
  We wish to relate the degree scale structure of galactic diffuse clouds to
sub-arcsecond atomic and molecular absorption spectra obtained against
extragalactic continuum background sources. To do this, we used the ARO 12m
telescope to map J=1-0 CO emission at 1' resolution over 30' fields around the
positions of 11 background sources occulted by 20 molecular absorption line
components, of which 11 had CO emission counterparts. We compare maps of CO
emission to sub-arcsec atomic and molecular absorption spectra and to the
large-scale distribution of interstellar reddening. The main results are: 1)
Typical covering factors of individual features at the 1 K.km/s level were 20%.
2) CO-H2 conversion factors as much as 4-5 times below the mean value N(H2)/Wco
= 2e20 H2 cm^-2 /(K.km/s) are required to explain the luminosity of CO emission
at/above the level of 1 K.km/s. Small conversion factors and sharp variability
of the conversion factor on arcminute scales are due primarily to CO chemistry
and need not represent unresolved variations in reddening or total column
density. Hence, like FERMI and PLANCK we see some gas that is dark in CO and
other gas in which CO is overluminous per H2. A standard CO-H2 conversion
factor applies overall owing to balance between the luminosities per H2 and
surface covering factors of bright and dark CO., but with wide variations.
