Embedded clusters in NGC 1808 central starburst - Near-infrared imaging and spectroscopy

Abstract

Context. In the course of a mid-infrared imaging campaign of close-by active galaxies, we discovered the mid-infrared counterparts of bright compact radio sources in the central star-forming region of NGC 1808. Aims. We aim at confirming that these sources are deeply embedded, young star clusters and at deriving some of their intrinsic properties.

Methods. To complement the mid-infrared images, we have collected a set of near-infrared data with ISAAC at the VLT: J, Ks, and L' images, as well as low-resolution, long-slit spectra for three of the sources.

Results. Surprisingly, the new images unveil a near-infrared counterpart for only one of the mid-infrared/radio sources, namely M 8 in the L' band. All the other sources are so deeply embedded that their emission does not pop out above the extended diffuse near-infrared emission. The near-infrared spectra of the sources look alike, with intense, ionised hydrogen lines. This supports the interpretation of these sources in terms of embedded young clusters. We derive extinctions and ionising photon production rates for two of the clusters.