Imaging diffuse clouds: bright and dark gas mapped in CO

Aims. 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. Methods. We used the ARO 12 m 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 compared maps of CO emission to sub-arcsec atomic and molecular absorption spectra and to the large-scale distribution of interstellar reddening. Results. 1) The same clouds, identified by their velocity, were seen in absorption and emission and atomic and molecular phases, not necessarily in the same direction. Sub-arcsecond absorption spectra are a preview of what is seen in CO emission away from the continuum. 2) The CO emission structure was amorphous in 9 cases, quasi-periodic or wave-like around B0528+134 and tangled and filamentary around BL Lac. 3) Strong emission, typically 4-5 K at EB-V <= 0.15 mag and up to 10-12 K at EB-V less than or similar to 0.3 mag was found, much brighter than toward the background targets. Typical covering factors of individual features at the 1 K km s(-1) level were 20%. 4) CO-H-2 conversion factors as much as 4-5 times below the mean value N(H-2)/WCO = 2 x 10(20) H-2 cm(-2) (K km s(-1))(-1) are required to explain the luminosity of CO emission at/above the level of 1 K km s(-1). 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. Conclusions. Like Fermi and Planck we see some gas that is dark in CO and other gas in which CO is overluminous per H-2. A standard CO-H-2 conversion factor applies overall owing to balance between the luminosities per H-2 and surface covering factors of bright and dark CO, but with wide variations between sightlines and across the faces of individual clouds.