The Wolf-Rayet population of the nearby barred spiral galaxy NGC 5068 uncovered by VLT 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.

17 pages, 12 figures, undergone iteration of referee's comments and re-submitted to MNRAS

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