1 Introduction

Wolf-Rayet stars (WR) are the descendants of the most massive stars. Although they live during a short time (Maeder & Conti 1994) these stars have been detected in young stellar systems, such as extragalactic HII regions (Kunth & Schild 1986) and the so-called WR galaxies (Conti 1991; Schaerer et al. 1999b). They are recognized by the presence of broad stellar emission lines at optical wavelengths, mainly at 4680 Å (known as the blue WR bump) and at 5808 Å (red WR bump). The blue bump is a blend of N  V  $\lambda\lambda$4604, 4620, N  III $\lambda\lambda$4634, 4641, C  III/IV  $\lambda\lambda$4650, 4658 and He  II $\lambda$4686 lines, that are produced in WR stars of the nitrogen (WN) and carbon (WC) sequences. In contrast, the red bump is formed only by C  IV $\lambda$5808 and it is mainly produced by WC stars. The detection of these features in the integrated spectrum of a stellar system provides a powerful tool to date the onset of the burst, and it constitutes the best direct measure of the upper end of the initial mass function (IMF). Thus, if WR features are found in the spectra of star forming systems, stars more massive than $M_{\rm WR}$, where $M_{\rm WR} \sim 25$ $M_{\odot }$ for solar metallicity, must be formed in the burst.

 

 

Table 1: Galaxy sample.

Galaxy	NED type and activity	$\alpha$ (J2000)	$\delta$ (J2000)	vr	distance
				[km s-1]	[Mpc]
NGC 3351	SB(r)b, HII Sbrst	10h 43m 57.8s	+11d 42m 14s	778	10.0
NGC 3521	SAB(rs)bc, LINER	11h 05m 48.6s	-00d 02m 09s	805	7.2
NGC 4254	SA(s)c	12h 18m 49.5s	+14d 24m 59s	2407	16.
NGC 4303	SAB(rs)bc, HII Sy2	12h 21m 54.9s	+04d 28m 25s	1566	16.
NGC 4321	SAB(s)bc, LINER HII	12h 22m 54.9s	+15d 49m 21s	1571	15.21

The IMF is one of the fundamental ingredients for studies of stellar populations, which has an important bearing on many astrophysical studies ranging from cosmology to the understanding of the local Universe. In particular the value of the IMF slope and the upper mass cut-off ( $M_{\rm up}$) strongly influences the mechanical, radiative, and chemical feedback from massive stars to the ISM such as the UV light, the ionizing radiation field, and the production of heavy elements.

A picture of a universal IMF has emerged from numerous works performed in the last few years (e.g. Gilmore & Howell 1998 and references therein). Indeed, these studies derive a slope of the IMF close to the Salpeter value for a mass range between 5 and 60 $M_{\odot }$. This result seems to hold for a variety of objects and metallicities from very metal poor up to the solar metallicity, with the possible exception of a steeper field IMF (Massey et al. 1995; Tremonti et al. 2002). However, the IMF in high metallicity ($12+\log$ (O/H) $\ga$ (O/H) $_\odot \approx$ 8.92) systems is much less well constrained. Different indirect methods to derive the slope and $M_{\rm up}$ give contradictory results.

The detection of strong wind resonance UV lines in the integrated spectrum of high metallicity nuclear starbursts clearly indicate the formation of massive stars (Leitherer 1998; Schaerer 2000; González Delgado 2001). In contrast, the analysis of the nebular optical and infrared lines of IR-luminous galaxies and high metallicity H  II regions indicates a softness of the ionizing radiation field that has beeninterpreted as due to the lack of stars more massive than $\sim$30 $M_{\odot }$ (Goldader et al. 1997; Bresolin et al. 1999; Thornley et al. 2000; Coziol et al. 2001). However, the interpretation of these indirect probes relies strongly on a combination of models for stellar atmospheres and interiors, evolutionary synthesis, and photoionisation, each with several potential shortcomings/difficulties (cf. García-Vargas 1996; Schaerer 2000; Stasinska 2002). For example, recently González Delgado et al. (2002) have shown that the above conclusion could be an artifact of the failure of WR stellar atmospheres models to correctly predict the ionizing radiation field of high metallicity starbursts (see also Castellanos 2001; Castellanos et al. 2002b).

A more direct investigation of the stellar content of metal-rich nuclear starbursts has been performed by Schaerer et al. (2000, hereafter SGIT00), using the detection of WR features to constrain $M_{\rm up}$. They found that the observational data are compatible with a Salpeter IMF extending to masses $M_{\rm up} \ga 40$ $M_{\odot }$. Most recently, a similar conclusion has been obtained by Bresolin & Kennicutt (2002, hereafter BK02) from observations of high-metallicity HII regions in M83, NGC 3351 and NGC 6384.

Here, we present a direct attempt to determine $M_{\rm up}$ based on the detection of WR features in metal-rich H  II regions of a sample of spiral galaxies. To obtain statistically significant conclusions about $M_{\rm up}$ and the slope of the IMF, a large sample of H  II regions needs to be observed. For coeval star formation with a Salpeter IMF and $M_{\rm up}=120$ $M_{\odot }$ at metallicities above solar, $\sim$60 to 80% (depending on the evolutionary scenario and age of the region) of the H  II regions are expected to exhibit WR signatures (Meynet 1995; Schaerer & Vacca 1998, hereafter SV98). Thus, to find $\ga$40 regions with WR stars (our initial aim) a sample of at least 5-7 galaxies with $\ga$10 H  II regions per galaxy needs to be observed. Spectra of high S/N (at least 30) in the continuum are also required to obtain an accurate measure of the WR features. For this propose, we have selected the nearby spiral galaxies NGC 3351, NGC 3521, NGC 4254, NGC 4303 and NGC 4321, which have have sufficient number of disk H  II regions of high-metallicity, as known from earlier studies.

Our observations have indeed allowed us to find a large number of metal-rich WR H  II regions. The analysis of their massive star content is the main aim of the present paper. Quite independently of the detailed modeling undertaken below, our sample combined with additional WR regions from Bresolin & Kennicutt (2002) allows us to derive a fairly robust lower limit on the upper mass cut-off of the IMF in these metal-rich environments (see Sect. 6).

 

 

Table 2: Log of the observations with meteorological conditions and exposure times for both grisms.

galaxy	date	weather	seeing ['']	exp. time blue [s]	exp. time red [s]
NGC 3351	19.04.2001	photometric	0.8-1.0	1700	1700
NGC 3521	25.04.2001	clear	1.6-2.0	1800	1800
NGC 4254	23.05.2001	clear	1.1-1.4	900	900
NGC 4303	23.05.2001	clear	0.8-1.1	750	750
NGC 4321	19.06.2001	photometric	1.3-1.5	1050	1050

The structure of the paper is as follows: The sample selection, observations and data reduction are described in Sect. 2. The properties of the H  II regions are derived in Sect. 3. Section 4 discusses the trends of the WR populations with metallicity. Detailed comparisons of the observed WR features with the evolutionary synthesis models are presented in Sect. 5. More model independent constraints on $M_{\rm up}$ are derived in Sect. 6. Our main results and conclusions are summarised in Sect. 7.