Galaxies with Wolf-Rayet signatures in the low-redshift Universe

Abstract

Context. The availability of large spectroscopic datasets has opened up the possibility of constructing large samples of rare objects in a systematic manner. Aims. The goal of this study is to analyse the properties of galaxies showing Wolf-Rayet features in their optical spectrum using spectra from the Sloan Digital Sky Survey Release 6. With this unprecedentedly large sample we aim to constrain the properties of the Wolf-Rayet phase and its impact on the surrounding interstellar medium. Methods. We carried out very careful continuum subtraction on all galaxies with equivalent widths of Hβ > 2 Å in emission and identify Wolf-Rayet features using a mixture of automatic and visual classification. We combined this with spectroscopic and photometric information from the SDSS and derive metal abundances using a number of methods. Results. We find a total of 570 galaxies with significant Wolf-Rayet (WR) features and a further 1115 potential candidates, several times more than even the largest heterogeneously assembled catalogues. We discuss in detail the properties of galaxies showing WolfRayet features with a focus on their empirical properties. We are able to accurately quantify the incidence of Wolf-Rayet galaxies with metal abundance and show that the likelihood of otherwise similar galaxies showing Wolf-Rayet features increases with increasing metallicity, but that WR features are found in galaxies of a wide range in morphology. The large sample allows us to show explicitly that there are systematic differences in the metal abundances of WR and non-WR galaxies. The most striking result is that, below EW(Hβ) = 100 Å, Wolf-Rayet galaxies show an elevated N/O relative to non-WR galaxies. We interpret this as a rapid enrichment of the ISM from WR winds. We also show that the model predictions for WR features strongly disagree with the observations at low metallicity; while they do agree quite well with the data at solar abundances. We discuss possible reasons for this and show that models incorporating binary evolution reproduce the low-metallicity results reasonably well. Finally we combine the WR sample with a sample of galaxies with nebular He ii λ4686 to show that, at 12 + log O/H < 8, the main sources of He ii ionising photons appears to be O stars, arguing for a less dense stellar wind at these metallicities, while at higher abundances WN stars might increasingly dominate the ionisation budget.

The evolution of superluminous supernova LSQ14mo and its interacting host galaxy system

“…There are two main WR features (see e.g. Brinchmann et al 2008) seen in optical spectra: the blue (4600-4680 Å) and red (5650-5800 Å) bumps. However, we do not find these features in our spectra for components A, B, or C. We followed the method used in Thöne et al (2015) to determine the upper limit of WR star populations.…”

“…However, we do not find these features in our spectra for components A, B, or C. We followed the method used in Thöne et al (2015) to determine the upper limit of WR star populations. We measured the 3σ detection limit of He ii λ4686 and C IV λ5808 lines assuming a full width at half maximum (FWHM) between 1000 and 3000 km s −1 (Brinchmann et al 2008). The luminosity of He ii line is < 9.29 × 10 38 -2.80 × 10 39 erg s −1 , and that of C IV line is < 1.29 × 10 39 -3.86×10 39 erg s −1 .…”

Massive star formation in Wolf-Rayet galaxies

“…The very rare occurrence of objects with a large N overabundance suggests the general idea of the short-time scales for the localized N pollution and its fast dispersal. Brinchmann et al (2008) used SDSS data to find that for |W(Hβ)| ≥ 100 Å, WR galaxies show a high N/O compared to non-WR galaxies. Quantitatively these authors found that on average Δ log(N/O) [ features, which likely is a reflection of WR stars being more abundant at higher metallicities (see Fig.…”

“…Although we do not dismiss the statistical analysis performed by Brinchmann et al (2008) comparing WR and non-WR galaxy data, we would like to warn about the use of data with low spectral resolution in order to derive an accurate nitrogen abundance in individual objects. This is commonly done via the analysis of the [N ii] λ5683 emission line, very close to Hα.…”

Gemini GMOS spectroscopy of HeII nebulae in M 33

“…Outside the Local Group, Brinchmann et al (2008) analysed spectra of 570 WR galaxies from SDSS6 and conclude that at 12 + log O/H < 8.0, the main sources of HeII ionizing photons appears to be O stars, arguing for a less dense stellar wind at these metallicities, while at higher abundances WN stars might increasingly dominate the ionization budget. De Mello et al (1998) showed that the observed equivalent widths of nebular HeII λ 4686 emission in the BCD IZw18 can be reproduced from an instantaneous burst model with the present WC/WO stars.…”