On the optical counterpart of NGC300 X-1 and the global Wolf-Rayet content of NGC300

TL;DR

This study refines the Wolf-Rayet star population in NGC 300, identifies the optical counterpart of NGC 300 X-1 as a WN star, and estimates the system's black hole mass, contributing to understanding massive star evolution and X-ray sources.

Contribution

It provides revised counts of Wolf-Rayet stars in NGC 300, confirms the nature of the X-ray source counterpart, and estimates the black hole mass in the system.

Findings

Confirmed 9 new Wolf-Rayet stars in NGC 300.

Identified the optical counterpart of NGC 300 X-1 as a WN star.

Estimated the black hole mass in NGC 300 X-1 to be over 10 solar masses.

Abstract

(Conext:) 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 Msun based upon ground-based photometry, from which a black hole mass of > 10 Msun results from the 32.8 hr period of the system and WR wind velocity of 1250 km/s. 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 Halpha-derived star formation rate of the inner galaxy, we infer N(WR)/N(O)~0.04