2 Sample selection, observations and reduction

2.1 Selection of the HII regions

Our target galaxies (see Table 1) are selected among nearby spiral galaxies where a sufficient number of disk H  II regions of high metallicity are known from the previous studies of Shields et al. (1991), Oey & Kennicutt (1993), and Zaritsky et al. (1994). Inspection of spectra from the two latter studies kindly made available to us showed that the vast majority of their spectra are not deep enough to allow the detection of WR or other stellar signatures in the continuum.

Metallicities $12 + \log({\rm O/H})$ of all known regions were estimated from the published [O  II] $\lambda$3727 and [O  III]  $\lambda\lambda$4959, 5007 intensities using the standard R₂₃ "strong line'' method and various empirical calibrations. For the FORS1 multi-object spectroscopic observations described below H  II regions with metallicities above solar ( $\log R_{23} \la$ 0.6) were given first priority. Secondary criteria taken into account in the choice of the known H  II regions were a large ${\rm H}\beta$ equivalent width, and bright continuum flux at $\sim$4650 Å as determined from inspection of the spectra. This procedure lead to a first selection of 4 to 7 H  II regions per galaxy. Other regions with lower metallicities and/or lower ${\rm H}\beta$ equivalent widths were retained as secondary targets.

Up to 19 slitlets per exposure can be used for spectroscopy with FORS1. Our primary targets were first positioned using the R-band images (see below) and the remaining slitlets were filled whenever possible with secondary targets. If a slitlet was left without any of our selected regions, we attempted to target other H  II regions selected from the ${\rm H}\alpha$ images of Hodge & Kennicutt (1983). For each galaxy a nuclear spectrum, to be reported upon later, was also obtained.

2.2 Observations

R band imaging was obtained with FORS1/VLT in April 2000, and was used to determine the positions of our targeted regions with sufficient accuracy. Subtracting a local average emission from the host galaxy the R band magnitudes of our target H  II regions were determined; typical magnitudes of $m_R \sim19$-21 are found.

The spectroscopic observations of our sample of H  II regions were carried out with FORS1/VLT in the second 2001 trimester. Table 2 gives informations about the exact dates and meteorological conditions during the observations.

The spectral range from 3600 Å to 1 $\mu$m was covered with a "blue'' spectrum from 3600 to 6500 Å with grism 300V+10, and a "red'' spectrum from 6000 to 10 000 Å with grism 300I+11. The use of a 1 $^{\prime\prime}$ slit width allowed to get medium spectral resolution of around 6 Å in the blue and 12 Å in the red. Due to the limited slit size, a fraction of the total nebular emission of the regions may be lost. This effect is acounted for in our interpretation of the data (Sect. 5). Unless WR stars follow systematically a different spatial distribution than other stars responsible for the continuum emission, a possible loss of continuum light does not alter our analysis.

Exposure times for each galaxy (see Table 2) were adapted to obtain in the continuum $S/N \sim 30$ in the blue, (needed for a precise measure of the WR bump) and $\sim$10 in the red (needed to measure the [S  III]  $\lambda\lambda$9069, 9532 lines). Spectrophotometric standard stars data were also acquired.

2.3 Data reduction and analysis

Reduction was carried out using the IRAF and MIDAS packages. The first steps consisted in the usual bias subtraction, flatfield division, and 2D wavelength calibration. Flux calibration was done using a standard atmospheric extinction curve and spectrophotometric standard stars. Given that the spectrophotometric standards were not always obtained during the night of the observations, we estimate an absolute flux accuracy of $\sim$10%. In addition, due to the optimisation for a maximum multiplex, the observations were not taken at parallactic angle, leading to a slight mismatch between the blue and red spectra. A quantitative analysis of the effetcs of differential refraction has not been undertaken here. As the main diagnostics used in the present paper lie in a limited wavelength range, and the observations have been taken at small airmass, this should represent a negligible source of uncertainty.

For each H  II region, a background including sky emission and underlying emission from the galaxy was extracted from the slitlet sub-image. This procedure was non-trivial as this background spectrum had in most cases to be determined near the edges of the sub-image, where the wavelength calibration may slightly deviate from the one of the H  II region. Special care has been taken for the red spectra, since the sky emission was often several times brighter than the H  II region emission. We thus re-calibrated the background emission spectrum according to the H  II region by comparing the position (and sometimes the intensity) of the sky emission lines. This time-consuming operation gave very satisfying results and useable spectra up to 1 $\mu$m for almost all H  II regions. The final 1D spectra were generally extracted with a 4 $^{\prime\prime}$ wide aperture.

Line intensities and equivalent width were obtained by visually placing a continuum on both sides of the line and then integrating all over this range. Errors were estimated by moving the continuum upwards by half the value of the noise near the line position and re-computing the intensity and equivalent width.

Where possible the following nebular emission lines were measured: [O  II] $\lambda$3727, the H Balmer line series including  ${\rm H}\alpha$ to H9, He  I $\lambda$4471, [O  III] $\lambda$4959, 5007, [N  II] $\lambda$5201, He  I $\lambda$5876, [O  I] $\lambda$6300, [N  II] $\lambda$6548, 6584, He  I $\lambda$6678, [S  II]  $\lambda\lambda$6717, 6731, He  I $\lambda$7065, [Ar  III] $\lambda$7136, [O  II] $\lambda$7325, and [S  III]  $\lambda\lambda$9069, 9532. If present, broad emission lines at $\lambda \sim 4680$ Å (referred to subsequently as the (blue) WR bump), C  III $\lambda$5696, and C  IV $\lambda$5808 indicative of Wolf-Rayet (WR) stars were also measured. The spectra were also inspected for the presence of stellar absorption lines like the Ca  II triplet, the CH G band at $\sim$4300 Å, Mg lines at $\sim$5200 Å, or TiO bands.

The spectra were deredened using the Whitford et al. (1958) extinction law as parametrised by Izotov et al. (1994) assuming an underlying absorption of $W({\rm H}\beta)= 2$ Å and an intrinsinc Balmer decrement ratio of $I({\rm H}\alpha)/I({\rm H}\beta)=2.86$.

All detailed results including finding charts, line measurements, and a detailed analysis of the nebular properties will be published in a forthcoming paper.