VLT observations of metal-rich extra galactic HIIregions

A&A 394, 443-457 (2002)
DOI: 10.1051/0004-6361:20021178

I. Massive star populations and the upper end of the IMF

M. Pindao¹, D. Schaerer², R. M. González Delgado³ and G. Stasinska⁴

¹ Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
² Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, 14, Av. E. Belin, 31400 Toulouse, France
³ Instituto de Astrofísica de Andalucía (CSIC), Apdo. 3004, 18080, Granada, Spain
⁴ LUTH, Observatoire de Meudon, 5 place Jules Jansses, 92150 Meudon, France

(Received 20 June 2002 / Accepted 7 August 2002 )

Abstract
We have obtained high quality FORS1/VLT optical spectra of 85 disk H II regions in the nearby spiral galaxies NGC 3351, NGC 3521, NGC 4254, NGC 4303, and NGC 4321. Our sample of metal-rich H II regions with metallicities close to solar and higher reveal the presence of Wolf-Rayet (WR) stars in 27 objects from the blue WR bump (~4680 Å) and 15 additional candidate WR regions. This provides for the first time a large set of metal-rich WR regions.

Approximately half (14) of the WR regions also show broad C IV $\lambda$ 5808 emission attributed to WR stars of the WC subtype. The simultaneous detection of C III $\lambda$ 5696 emission in 8 of them allows us to determine an average late WC subtype compatible with expectations for high metallicities. Combined with literature data, the metallicity trends of WR features and the WC/WN number ratio are discussed.

The WR regions show quite clear trends between their observed WR features and the H $\beta$ emission line. Detailed synthesis models are presented to understand/interpret these observations. In contrast with earlier studies of low metallicity WR galaxies, both $W({\rm WR})$ and $I({\rm WR})/I(\mathrm{H}\beta)$ are here found to be smaller than "standard" predictions from appropriate evolutionary synthesis models at corresponding metallicities. Various possibilities which could explain this discrepancy are discussed. The most likely solution is found with an improved prescription to predict the line emission from WN stars in synthesis models.

The availability of a fairly large sample of metal-rich WR regions allows us to improve existing estimates of the upper mass cut-off of the IMF in a robust way and independently of detailed modeling: from the observed maximum H $\beta$ equivalent width of the WR regions we derive a lower limit for $M_{\rm up}$ of 60-90 $M_{\odot}$ in the case of a Salpeter slope and larger values for steeper IMF slopes. This constitutes a lower limit on $M_{\rm up}$ as all observational effects known to affect potentially the H $\beta$ equivalent width can only reduce the observed $W({\rm H}\beta)$ .

From our direct probe of the massive star content we conclude that there is at present no evidence for systematic variations of the upper mass cut-off of the IMF in metal-rich environments, in contrast to some claims based on indirect nebular diagnostics.

Key words: galaxies: abundances -- galaxies: evolution -- galaxies: ISM -- galaxies: stellar content -- stars: luminosity function, mass function -- stars: Wolf-Rayet

Offprint request: D. Schaerer, schaerer@ast.obs-mip.fr