The Wolf-Rayet population of the nearby barred spiral galaxy NGC 5068 uncovered by the Very Large Telescope and Gemini

Abstract

We present a narrow-band VLT/FORS1 imaging survey of the SAB(rs)cd spiral galaxy NGC 5068, located at a distance of 5.45Mpc, from which 160 candidate Wolf-Rayet sources have been identified, of which 59 cases possess statistically significant 4686 excesses. Follow-up Gemini/GMOS spectroscopy of 64 candidates, representing 40% of the complete photometric catalogue, confirms Wolf-Rayet signatures in 30 instances, corresponding to a 47% success rate. 21 out of 22 statistically significant photometric sources are spectroscopically confirmed. Nebular emission detected in 30% of the Wolf-Rayet candidates spectrally observed, which enable a re-assessment of the metallicity gradient in NGC 5068. A central metallicity of log(O/H)+12 ~ 8.74 is obtained, declining to 8.23 at R_25. We combine our spectroscopy with archival Halpha images of NGC 5068 to estimate a current star formation rate of 0.63(+0.11/-0.13)Msun/yr, and provide a catalogue of the 28 brightest HII regions from our own continuum subtracted Halpha images, of which ~17 qualify as giant HII regions. Spectroscopically, we identify 24 WC and 18 WN-type Wolf-Rayet stars within 30 sources since emission line fluxes indicate multiple Wolf-Rayet stars in several cases. We estimate an additional ~66 Wolf-Rayet stars from the remaining photometric candidates, although sensitivity limits will lead to an incomplete census of visually faint WN stars, from which we estimate a global population of ~170 Wolf-Rayet stars. Based on the Halpha-derived O star population of NGC 5068 and N(WR)/N(O)~0.03, representative of the LMC, we would expect a larger Wolf-Rayet population of 270 stars. Finally, we have compared the spatial distribution of spectroscopically confirmed WN and WC stars with SDSS-derived supernovae, and find both WN and WC stars to be most consistent with the parent population of Type Ib SNe.