Nuclear embedded star clusters in NGC 7582^*

Abstract

We report on the discovery of several compact regions of mid-infrared emission in the star-forming circumnuclear disc of the starburst/Seyfert 2 galaxy NGC 7582. The compact sources do not have counterparts in the optical and near-infrared, suggesting that they are deeply embedded in dust. We use the [NeII] 12.8-μm line emission to estimate the emission measure of the ionized gas, which in turn is used to assess the number of ionizing photons. Two of the brighter sources are found to have ionizing fluxes of ~2.5 × 10⁵², whereas the fainter ones have ~1 × 10⁵² photon s^-1. Comparing with a 1-Myr-old starburst, we derive stellar masses in the range 3-5 × 10⁵M_solar and find that the number of O stars in each compact source is typically 0.6-1.6 × 10³. We conclude that the compact mid-infrared sources are likely to be young, embedded star clusters, of which only a few are known so far. Our observation highlights the need for high-resolution mid-infrared imaging to discover and study embedded star clusters in the proximity of active galactic nuclei.