Wolf Rayet populations in M M and beyond

Within the Local Group, M31 (Andromeda) is the only other candidate metal-rich galaxy, although studies are hindered by its orientation on the sky. Wolf-Rayet populations in M31 were studied by Moffat & Shara (1983, 1987) and Massey et al. (1986). As in the Milky Way, WC7-8 stars were located at smaller galactocentric distances (7 ± 3 kpc) than WC5-6 stars (11 ± 3 kpc). Representative examples of M31 WC stars obtained with WHT/ISIS are presented in Figure 29.3. These results suggest a weak metallicity gradient for M31, albeit a rather less metal-rich situation than in the Milky Way, according to its observed WC population; see also Trundle et al. (2002).

Further afield, the WR population of M83 (NGC 5236) has been studied by Hadfield et al. (2005). The galaxy M83 is well suited to optical imaging surveys for

Figure 29.4. A composite 12' x 12' VLT FORS2 image (A4685) of M83 (NGC 5236), indicating the locations of regions containing WN (squares), WC (plus symbols) and both WN and WC (crosses) stars. North is up and east is to the left. Regions to the southeast are masked to avoid saturation by bright foreground stars (Hadfield et al. 2005).

Figure 29.4. A composite 12' x 12' VLT FORS2 image (A4685) of M83 (NGC 5236), indicating the locations of regions containing WN (squares), WC (plus symbols) and both WN and WC (crosses) stars. North is up and east is to the left. Regions to the southeast are masked to avoid saturation by bright foreground stars (Hadfield et al. 2005).

WR stars at high metallicity (though see Chapter 17 in these proceedings), since it is face-on, nearby (4.5 Mpc distant) and possesses a high star-formation rate.

VLT/FORS2 revealed a total of 280 (non-nuclear) regions for which the presence of WR stars was inferred from an excess of A4685 (He ii 4686) narrow-band imaging versus A4781 (continuum) - see Figure 29.4. Spectroscopic follow-up of 198 regions confirmed a total of 132 sources, hosting in excess of 1000 WR stars according to the standard calibration of Schaerer & Vacca (1998). For M83

Vid Xyz Mpc

Wavelength (A) Wavelength (A)

Figure 29.5. Optical VLT/FORS2 spectrosocpy of typical regions of M83 hosting late WC8-9 stars, together with individual Milky Way WC stars (WR135 and WR103) scaled to a distance of 4.5 Mpc (Hadfield et al. 2005).

Wavelength (A) Wavelength (A)

Figure 29.5. Optical VLT/FORS2 spectrosocpy of typical regions of M83 hosting late WC8-9 stars, together with individual Milky Way WC stars (WR135 and WR103) scaled to a distance of 4.5 Mpc (Hadfield et al. 2005).

N(WC)/N(WN) ~ 1.2, confirming the trend towards a higher ratio at high metal-licity as indicated in Figure 29.5. Notably, the WC population of M83 is totally dominated by late subtypes, i.e. N(WC8-9)/N(WC4-7) - 9, in contrast with -1 for the Solar neighbourhood, for which representative examples are presented in Figure 29.5.

Wolf-Rayet populations are unresolved in more-remote metal-rich galaxies, but data also confirm the presence of late-type WC stars, as originally discovered by Phillips & Conti (1992) for NGC 1365 within Fornax and confirmed by Pindao et al. (2002) for NGC 4254 (M88) within Virgo.

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