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 μ m and perform their accurate relative positioning. We also provide spectra in the ranges 1.8-2.4 μ m, 3.3-4.0 μ m, 8.1-9.3 μ m and 10.4-13.2 μ 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 μ m line is also observed, while neither the [ArIII] 8.9 μ m nor the [SIV] 10.4 μ m lines are detected. This provides a stringent constraint on the age of the sources: we argue that they are relatively evolved young clusters (6-8 Myr). Owing to their ionising photon emission rates and ages, they must be extremely massive clusters (of the order of 10^7~M_⊙). 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.