A Very Large Telescope imaging and spectroscopic survey of the Wolf-Rayet population in NGC 7793*

Abstract

We present a Very Large Telescope/Focal Reducer and Low Dispersion Spectrograph #1 (VLT/FORS1) imaging and spectroscopic survey of the Wolf–Rayet (WR) population in the Sculptor group spiral galaxy NGC 7793. We identify 74 emission-line candidates from archival narrow-band imaging, from which 39 were observed with the Multi Object Spectroscopy mode of FORS1. 85 per cent of these sources displayed WR features. Additional slits were used to observe H ii regions, enabling an estimate of the metallicity gradient of NGC 7793 using strong line calibrations, from which a central oxygen content of log (O/H) + 12 = 8.6 was obtained, falling to 8.25 at R25. We have estimated WR populations using a calibration of line luminosities of Large Magellanic Cloud stars, revealing ∼27 WN and ∼25 WC stars from 29 sources spectroscopically observed. Photometric properties of the remaining candidates suggest an additional ∼27 WN and ∼8 WC stars. A comparison with the WR census of the LMC suggests that our imaging survey has identified ∼80 per cent of WN stars and ∼90 per cent for the WC subclass. Allowing for incompleteness, NGC 7793 hosts ∼105 WR stars for which N(WC)/N(WN) ∼ 0.5. From our spectroscopy of H ii regions in NGC 7793, we revise the global Hα star formation rate of Kennicutt et al. upward by 50 per cent to 0.45 M⊙ yr−1. This allows us to obtain N(WR)/N(O) ∼ 0.018, which is somewhat lower than that resulting from the WR census by Schild et al. of another Sculptor group spiral NGC 300, whose global physical properties are similar to NGC 7793. Finally, we also report the fortuitous detection of a bright (mV= 20.8 mag) background quasar Q2358-32 at z∼ 2.02 resulting from C ivλ1548-51 redshifted to the λ4684 passband.