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Wolf-Rayet galaxies
A Wolf-Rayet galaxy is a galaxy where the characteristic spectral features of Wolf-Rayet stars are seen in the integrated spectrum of the galaxy. This is a very special case where you can look at the spectrum of a galaxy and directly see the contribution of a very narrow class of stars. As the stars are very short-lived, the presence of Wolf-Rayet features in the spectrum can place strong constraints both on the recent burst of star formation, and on the physical nature of these stars. Wolf-Rayet galaxies are often considered to be a subset of Starburst galaxies, but the Wolf-Rayet features can sometimes also be seen in galaxies that are more traditionally classified as Active Galactic Nuclei.

History
Wolf-Rayet stars have been known in the Milky Way since 1867, but the detection of these stars in more distant galaxies took somewhat longer. The first reported detection in the integrated spectrum of a galaxy was by in the dwarf galaxy He 2-10. A galaxy that in fact had previously been classified as a Planetary nebula.

After this initial discovery, a number of these galaxies were found and it was soon realised that the presence of these stars provide strong constraints on the recent star formation in such galaxies.

Many of these discoveries were serendipitous but there were also some systematic searches through the 1980s. However the term "Wolf-Rayet galaxy" became common usage only after Peter Conti's 1991 paper. That paper contained the first catalogue of Wolf-Rayet galaxies culled from the literature and defined Wolf-Rayet galaxies, to be those galaxies whose integrated spectrum has detectable Wolf-Rayet broad stellar emission lines. There were 37 objects in that catalogue and by the time Daniel Schaerer, Thierry Contini & Maximilien Pindao produced their influential catalogue of Wolf-Rayet galaxies in 1999, the total number of known Wolf-Rayet galaxies had expanded to 139.

The number of galaxies and the quality of the data on them was now such that Wolf-Rayet galaxies could be used in systematic studies of massive star evolution. A notable effort to do this was work done by Nataliya Guseva and co-workers in 2000 which showed that models for massive star evolution matched observations in metal-rich galaxies but that there was some tension in low metal content galaxies.

The availability of very large samples of galaxy spectra from the Sloan Digital Sky Survey offered an opportunity to significantly enlarge the samples of Wolf-Rayet galaxies known. With the sample assembled by Brinchmann et al containing 570 securely identified spectra and many more less secure.

In parallel to this effort to create large samples, a lot of effort has been expended in studying individual Wolf-Rayet galaxies to elucidate the process of massive star formation in galaxies.