The small scale distribution of interstellar dust from studies of obscuredgalaxies

We have selected four galaxies lying behind Galactic molecular clouds in or der 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 whi ch 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 delta A(V)/A(V) as low as 5% are inferred for scales of about 0.002 pc (400 AU). 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 3.3 kpc in the P erseus arm. (CO)-C-12 and (CO)-C-13 (J = 1 - 0 and 2 - 1) observations of these four qu iescent clouds have been made using the 30 m IRAM telescope in order to per form a correlated study of these tracers and dust. In agreement with previo us studies, (CO)-C-12 and (CO)-C-13 emission appear to display a much highe r 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 (CO)-C-13 emission on large scales no lo nger holds at small scales. The local dust-to-gas ratio is then likely to v ary 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(H-2)) is neither traced accurately by minor molecules like CO, (CO)-C-13 Or (CO)- O-18 (due e.g. to self-shielding and excitation effects) nor by visible ext inction (due to the inertia of solid particles). Therefore, direct observat ions of H-2 itself are needed to decide whether or not a significant fracti on of the mass is really located within dense cells occupying a small part of the cloud volume.