First Very Large Telescope/X-shooter spectroscopy of early-type stars outside the Local Group★

As part of the VLT/X-shooter science verification, we obtained the first optica l mediumresolution spectrum of a previously identified bright O-typ e object in NGC 55, an LMC-like galaxy at a distance of∼2.0 Mpc. Based on the stellar and nebular spectrum, we investigate the nature and evolutionary status of the central object(s) and its influence on the surrounding interstellar medium. We conclude that the source, NGC 55 C1 31, is a composite object, likely a stellar cluster, which contains one or several hot ( Teff ≃ 50 000 K) WN stars with a high mass-loss rate (∼3× 10 −5 M⊙ yr −1 ) and a helium-rich composition (NHe/NH = 0.8). The visual flux is dominated by OB-type (super)giant stars wi th Teff . 35 000 K, solar helium abundance (NHe/NH = 0.1), and mass-loss rate ∼2× 10 −6 M⊙ yr −1 . The surrounding Hii region has an electron density ne 6 10 2 cm −3 and an electron temperature T (Oiii) ≃ 11 500± 600 K. The oxygen abundance of this region is [O/H] = 8.18± 0.03 which corresponds to Z = 0.31± 0.04 Z⊙. We observed no significant gradients in T (Oiii), ne or [O/H] on a scale of 73 pc extending in four directions from the ionising source. The properties of the Hii region can be reproduced by a CLOUDY model which uses the central cluster as ionising source, thus providing a self-consistent interpreta tion of the data. We also report on the serendipitous discovery of Heii nebular emission associated with the nearby source NGC 55 C2 35, a feature usually associated with strong X-ray sources.