Extremely massive young clusters in NGC 1365

Abstract

Context. In a previous work, three bright mid-infrared/radio sources were discovered in the nuclear region of starburst/AGN galaxy NGC 1365. Aims. The present study aims to confirm that these sources are indeed young and massive "embedded" clusters, and derive their physical parameters, such as extinction, age and mass.

Methods. Using ISAAC and VISIR at the VLT we obtained maps and low resolution spectra in the near-and mid-infrared. The resulting datasets are first interpreted by comparing the observations with images and spectra of the close-by young cluster R136 in the Large Magellanic Cloud and then by using model predictions for both the nebular emission lines and the spectral energy distribution of the sources.

Results. We produce maps of the region containing the three sources in the R, J, Ks, L' bands and at 12.8 mu m and perform their accurate relative positioning. We also provide spectra in the ranges 1.8-2.4 mu m, 3.3-4.0 mu m, 8.1-9.3 mu m and 10.4-13.2 mu m. The spectral energy distribution of the three sources rises with wavelength. Emission lines from ionised hydrogen and molecular hydrogen are detected, as well as PAH emission. A conspicuous [NeII] 12.8 mu m line is also observed, while neither the [ArIII] 8.9 mu m nor the [SIV] 10.4 mu m lines are detected. This provides a stringent constraint on the age of the sources: we argue that they are relatively evolved young clusters (6-8Myr). Owing to their ionising photon emission rates and ages, they must be extremely massive clusters (of the order of 10(7) M-circle dot). Their mid-infrared spectral energy distribution suggests the presence of two components: (1) an optically thin component, with a continuum comparable to that of R136; and (2) an optically thick component which might be related to subsequent or on-going episodes of star formation. We anticipate that these sources are good candidates for evolution according to a bi-modal hydrodynamical regime, in which matter is trapped at the centre of a compact and massive cluster and generates further star formation.