1999A&A...351.1051T

We have selected four galaxies lying behind Galactic molecular clouds in order to probe the small scale structure of the latter. Using broad-band (B, V, R and I) CCD images, we searched for surface brightness fluctuations which might be attributable to spatial structure in the foreground extinction. Towards the three galaxies occulted by local clouds, we fail to detect such variations: from the lack of ``granularity'' and/or high degree of symmetry displayed by brightness profiles, upper limits on δA_V/A_V as low as 5% are inferred for scales of about 0.002pc (400AU). In front of Maffei 1, the extinction is also smooth even if significant fluctuations are seen in addition to a prominent fragment, presumably located at 3.3kpc in the Perseus arm. ¹²CO and ¹³CO (J = 1-0 and 2-1) observations of these four quiescent clouds have been made using the 30m IRAM telescope in order to perform a correlated study of these tracers and dust. In agreement with previous studies, ¹²CO and ¹³CO emission appear to display a much higher degree of variability at small spatial scales than dust extinction. Our study reveals no evidence for ubiquitous fragments with size l > 400 AU in the distribution of dust grains and confirms that the good correlation seen between extinction and CO and ¹³CO emission on large scales no longer holds at small scales. The local dust-to-gas ratio is then likely to vary locally which may in turn affect the physical and chemical evolution of the gas. We argue that the small scale structure in the density field (i.e. n(H₂)) is neither traced accurately by minor molecules like CO, ¹³CO or C¹⁸O (due e.g. to self-shielding and excitation effects) nor by visible extinction (due to the inertia of solid particles). Therefore, direct observations of H₂ itself are needed to decide whether or not a significant fraction of the mass is really located within dense cells occupying a small part of the cloud volume.