Background galaxies as reddening probes throughout the Magellanic Clouds

We study the spectral properties in the range 3600 A-6800 A of the nuclear region of galaxies behind the Magellanic Clouds. The radial velocities clarified the nature of the objects as background galaxies or extended objects belonging to the Clouds. For most galaxies behind the main bodies of the LMC and SMC, radial velocities were measured for the first time. In the present sample typical LMC background galaxies are nearby ($4000 < V({\rm km s^{-1}}) < 6000$), while SMC's are considerably more distant ($10 000 < V({\rm km s^{-1}}) < 20 000$). We determine the reddening in each line of sight by matching a reddening-free galaxy template with comparable stellar population. For the LMC main body we derive a combined Milky Way and internal reddening value $E(B-V)_{\rm MW+i} = 0.12 \pm 0.10$, while for the SMC $E(B-V)_{\rm MW+i} = 0.05 \pm 0.05$. By subtracting Milky Way reddening values for galaxies projected onto the surroundings of each Cloud, we estimate average internal reddening values $\Delta E(B-V)_{\rm i} = 0.06$ and 0.04, respectively for the main bodies of the LMC and SMC. The Clouds are optically thin, at least in the directions of the studied background galaxies which are often difficult to be identified as such on ESO/SERC sky survey images. Nevertheless, more reddened zones may occur where it is difficult to identify galaxies.