Letter to the Editor

On the optical counterpart of NGC 300 X-1 and the global Wolf-Rayet content of NGC 300^*

¹ Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd, Sheffield, S3 7RH, UK e-mail: Paul.Crowther@sheffield.ac.uk
² XMM-Newton Science Operations Center, ESAC, 28080 Madrid, Spain

Received: 19 April 2007
Accepted: 9 May 2007

Abstract

Context.Surveys of Wolf-Rayet (WR) populations in nearby galaxies provide tests of evolutionary models plus type Ib/c supernova progenitors. This spectroscopic study complements the recent imaging survey of the spiral galaxy NGC 300 by Schild et al.

Aims.Revisions to the known WR content of NGC 300 are presented. We investigate the WR nature of candidate #41 from Schild et al. which is spatially coincident with the bright X-ray point source NGC 300 X-1.

Methods.VLT/FORS2 multi-object spectroscopy of WR candidates in NGC 300 is obtained.

Results.We establish an early-type WN nature of #41, i.e. similar to the optical counterpart of IC 10 X-1, which closely resembles NGC 300 X-1. We confirm 9 new WR stars, bringing the current WR census of the inner disk to 31, with N(WC)/N(WN) ~ 0.9.

Conclusions.If #41 is the optical counterpart for NGC 300 X-1, we estimate a WR mass of 38  based upon ground-based photometry, from which a black hole mass of ≥10  results from the 32.8 h period of the system and WR wind velocity of 1250 km s^-1. We estimate an 95% completeness among WC stars and 70% among WN stars, such that the total WR content is ~40, with N(WC)/N(WN) ~ 0.7. From the Hα-derived star formation rate of the inner galaxy, we infer N(WR)/N(O) ~ 0.04.