As part of the GEFE collaboration, observations of star-forming regions with high spectral resolution and long-slit sampling are being undertaken. 2D maps of physical parameters like density, excitation, extinction…etc. have been produced with 1″ spatial resolution and 2″ spatial sampling. Some preliminary results on the giant HII Region NGC 5471 and the irregular galaxy NGC 4214 are presented. Very high velocity components have been detected at some particular positions on the nebulae, as well as other peculiar kinematical structures (redshifted secondary emission peaks, line splitting etc.). The whole emitting area of NGC 5471 behaves as a unique entity with respect to excitation, with no correlation with the emitting knots. On the other hand, differentiated star-forming regions can be identified on NGC 4214. Finally, the distribution of dust particles seems to be rather inhomogeneous and anticorrelated with the distribution of emission-line intensities.

Introduction: aim and targets

The ultimate aim of the GEFE collaboration is to determine which are the physical parameters that control the formation of a violent burst of star formation. Within this framework and in order to fulfil this main objective it is important to know the physical properties of star-forming regions with high enough spatial resolution as to determine variations of the measurable parameters within the emitting nebulae. We aim to use measurements of age, excitation degree, velocity dispersion and chemical composition to know whether we are dealing with single star-forming regions or with well differentiated physical entities within a patch of ionized gas, which cause misinterpretation in our understanding of the “physical object” (Muñoz-Tuñón et al. 1993).

Cross-correlation of the ROSAT All Sky Survey and the IRAS Point Source Catalog has provided a sample of 244 galaxies with strong emission at both far-infrared and soft X-ray ranges. IRAS 13224-3809 appeared as an outstanding object within this sample due to its extreme X-ray luminosity (Lx = 3 · 1044 erg s−1), steep X-ray spectrum and rapid X-ray variability, with a doubling timescale of only 800 s (Boiler et al. 1993). We have performed repeated IUE observations of this object in January, February and May 1993, looking for variable features in its spectrum, having detected a strong variability in the Lyα line. While a relatively broad Lyα component (FWHM ∼ 5000 km s−1) remains essentially constant over the three IUE observations, the initially strong and narrow core emission component vanishes completely becoming a strong absorption. A maximum variation of 50% has also been detected in the UV continuum level. IRAS 13224-3809 has a deficit of UV emission when compared to Seyfert 1 galaxies. The UV-X-ray energy distribution suggests that the UV bump frequently found in these galaxies might be present at higher energies, well within the ROSAT band (0.1–2.4 keV). If this bump is due to thermal emission of a heated accretion disk, as proposed by several authors, its temperature should be significantly higher than in other similar objects (blackbody temperature kT ∼ 100 eV).

Evolutionary stellar population synthesis models have been performed for several metallicities and two extreme star formation rates, instantaneous (IB) and extended bursts (EB). We discuss the dependence on metallicity of the population of Wolf-Rayet (WR) and Red Supergiant (RSG) stars. We show that both populations become more abundant for higher metallicities. We also show the effects of metallicity on the effective temperature and Hβ equivalent width. These effects are independent of the IMF slope and can account, at least in part, for the higher values of Teff and W(Hβ) systematically found in low-metallicity star formation episodes.

A more complete study can be found in Cerviño & Mas-Hesse (1994).

Wolf-Rayet population

In Figure 1a we compare observational values of WR bump over Hβ ratio taken from Kunth & Joubert (1985), Kunth & Schild (1986) and Vacca & Conti (1992) with the model predictions. Ages have been estimated by using the W(Hβ) computed for a Salpeter IMF slope (Figure lb). The average WR bump over Hβ intensities fall within the range predicted by our models for an IB regime and cannot be reproduced assuming an extended one. We can therefore reject the possibility of having extended star formation episodes in the majority of the cases, and can also constrain the age of the episodes to a short range between 3 and 5 Myr after the onset of the burst.

We present narrow-band Hα imaging and long-slit optical and near infrared spectroscopy of the giant HII region NGC 2363. We have found broad emission lines at 4686 Å and at 5810 Å attributed to WC stars at 6 arcsec to the East of the brightest core of the region. We confirm the existence of low-intensity broad components in Hα and [OIII] which extend some 500 pc. We have derived the physical conditions and chemical composition of the gas in 15 different zones in the region, and do not find significant variations in the abundances. The Paschen discontinuity has been found in emission. The Pa electron temperatures obtained are significantly smaller than those obtained from the [OIII] and [SIII] emission lines, indicating the presence of large temperature fluctuations.

Introduction

One of the targets of the GEFE programme is the giant HII region NGC 2363 located in the SW of the irregular galaxy NGC 2366. This is one of the largest extragalactic HII regions with high surface brightness. The object was observed in La Palma in narrow band Hα and long-slit spectrophotometry from [OII] λ3227 to [SIII] λ9532 at two positions, at the brighest core of the region (which we call knot A) and at 6 arcsec to the East (knot B).

Narrow-band images

The object was observed with the 1-m JKT telescope. We used a CCD with a spatial scale of 0.3 arcsec pixel−1.