Young Massive Clusters in the Circumnuclear Ring of NGC1365
Galliano E., Alloin D., Pantin E., Granato G.L., Delva P., Silva L., Lagage P.O., Panuzzo P.

Abstract: Three bright circumnuclear mid-infrared-radio sources were discovered in the nuclear region of the starburst-AGN galaxy NGC136 in a mid-infrared (TIMMI2) AGN imaging campaign., The current analysis aims at confirming that these sources are young and massive embedded clusters, and at deriving their basic properties such as age, extinction and mass., Using ISAAC and VISIR at the VLT we obtained in the near-infrared and mid-infrared, maps and low resolution spectra for the three sources. The resulting dataset is interpreted using a model for these clusters., We produce maps of the region containing the three clusters in the R, J, Ks, L' bands and at 12.8 micron and perform their accurate relative positioning. We also provide spectra in the ranges 1.8-2.4, 3.3-4.0, 8.1-9.3 and 10.4-13.2 micron. The spectra of the three clusters show rising continua, and the presence of the silicate 9.7 micron absorption feature. Emission lines from ionised hydrogen and molecular hydrogen are detected, as well as PAH emission. Conspicuous HeI 2.06 and %5BNeII%5D12.8 micron lines are also present on the spectra, while neither the %5BArIII%5D 8.9 nor the %5BSIV%5D 10.4 micron lines were detected. For each cluster, we derive the extinction both from the near-infrared, nebular lines and from the mid-infrared silicate absorption band. A low figure is obtained for the extinction in the near-infrared, while a large figure is found for the extinction in the mid-infrared. This puzzling, but not uncommon, result requires that a two-component model be considered for each embedded cluster: (a) an optically thin component, about 7 Myr old, with total stellar mass of several 10^6 Msol, to which the ionising stars traced by the emission lines pertain, and (b) an optically thick -still deeply embedded- component responsible for the bulk of the mid-infrared emission, to which it is difficult to assign a precise age because of parameter degeneracy in the fitting procedure. We propose that the latter component might be associated with the still forming low-mass star population.,