The UV spectrum of a giant extragalactic HII region is compared here with the synthetic spectrum generated by a fitting procedure which uses a combination of stellar spectra to match the observed spectrum.

Observations

The ultraviolet observations of giant extragalactic HII regions provide very valuable information on the stellar population embedded in these complexes.

The UV spectra of HII regions in the IUE short-wavelength range, SWP (1152–1950Å), show a stellar continuum which rises steeply towards the shorter wavelengths. Superposed over this continuum, emission lines and P-Cygni profiles are often observed, as an indication of the presence of hot luminous early-type stars with mass loss.

NGC 604, the brightest HII complex in the nearby galaxy M33, is, because of its size and distance, a good candidate for a detailed study of a giant extragalactic HII region in the UV based on IUE data.

NGC 604 has been observed with IUE on different occasions in low resolution (6 Å mm−1) with the large aperture slit (10 × 20 arcsec). A total of 16 SWP and 8 LWR, or LWP (1950–3200 Å), has been obtained. The spectra correspond to observations of different zones within the region.

The general characteristics of the IUE spectra of NGC 604 in the short wavelength range are similar to the ones mentioned above. In the near UV, the observed continuum is practically constant, no emission lines are detected, there is no 2200-Å absorption feature, and only interstellar absorption lines are present.

We report the results of new spectral and photometric observations of a sample of northern polar-ring galaxies (PRGs). We found that the nuclei of galaxies with a forming polar ring have both higher Hα equivalent width and luminosity and show enhanced star formation. We also find an excess of active nuclei (Sy or LINER) among forming PRGs. From the optical colours and Hα luminosities of the rings, we infer that the rate and IMF of star formation in the polar rings are similar to those in normal late-type spiral galaxies.

Introduction

Polar-ring galaxies (PRGs) are among the most interesting and spectacular relics of gravitational interactions between galaxies. The existence of prominent rings of gas, dust and stars aligned roughly in a perpendicular orientation with respect to the major axis of the main galaxy has provided a new opportunity to study the dynamical structure and evolution of galaxies. According to the most popular point of view, the observed structure of PRGs is the result of galaxy interactions ranging from a gas accretion to a complete merger.

With the goal of a detailed study of PRGs we undertook observations of a restricted sample of northern PRGs from the catalogue by Whitmore et al. (1990) using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences for the CCD photometry in the B, V, and Rc bands (this work is now in progress) and the 1.93-m telescope of the Observatoire de Haute-Provence (spectral observations of 16 PRGs in the range 6200–7170 Å).

I present here some preliminary results of a study of the spectroscopic properties of a group of star-forming dwarf galaxies that have been selected in order to sample the range in physical conditions imposed by extreme density environments. This investigation is part of an ongoing project intended to evaluate the relative influences of the environment, and the initial conditions, on the evolution of galaxies with active star formation. It has been found that, on average, starforming dwarf galaxies located in nearby low density regions appear to present spectra with higher excitation, higher Hβ equivalent widths and larger total Hβ luminosities than similar objects located within high-density environments.

Introduction

The influence of the environment on the mechanisms that control star formation is one of the most important subjects concerning the study of the origin and evolution of galaxies. Spiral galaxies in clusters, a high-density environment, have been used to trace the present-day star formation rate (SFR) in order to compare with field galaxies. Some studies of spirals in clusters suggest a reduced SFR with respect to field galaxies of the same morphological type, while others tend to favour a similar or higher SFR in cluster spirals; this question still remains open (Moss & Whittle, 1993). Current environmental effects which can be operating in galaxies as a consequence of the interaction with companions and with the intergalactic medium include, among others, tidal shaking, tidal stripping, ram pressure sweeping and evaporation.

Critical to the understanding of several fundamental problems in astronomy (among which the determination of the primordial helium is of foremost importance), extremely metal-poor galaxies have been almost impossible to find. In the past few years we have been successful in discovering them.

We are embarked on a programme for obtaining with linear detectors, very high S/N spectra of these objects, in order to derive He abundances to better than the 5% per object needed to constrain the Big Bang model of the origin of the universe. We will discuss some results and problems encountered in this quest.

Introduction

Three observational findings sustain the Big Bang model of the origin of the Universe: 1) the relic 3-K microwave background radiation; 2) the expansion of the Universe; and 3) the relative abundances of the light elements (H, D, 3He, 4He, Li); for a review, see e.g. Walker et al. (1991). Accurate measurements of these observables provide better constraints on the details of the model.

Even though an accurate measurement of the primordial light-element abundance and in particular of He, is critical to our understanding of the origin of the Universe, it has not been the target of an observational effort comparable to that for the other cosmological observables: the microwave background and the expansion of the universe.

Low-Ionization Nuclear Emission-Line Regions (LINERs) are a common constituent of galaxies, and are often regarded as a weak form of Seyfert activity. LINERs have emission-line luminosities that are similar to those of giant HII regions, however, and recent theoretical work suggests that their nebular properties can be reproduced in many cases with photoionization by normal O stars. In an extension of this scenario, energetic phenomena such as non-thermal radio emission, broad Hα features, and substantial X-ray luminosity seen in some LINERs might be attributable to supernovae. In this review I consider the empirical evidence bearing on an interpretation of LINERs as stellar-powered sources. While stellar phenomena appear capable of matching LINERs of modest luminosity in terms of broad-band energetics, some important differences remain in detailed spectral characteristics, particularly at X-ray energies. A certain amount of anomalous behavior on the part of stars within galaxy nuclei (e.g. in terms of the initial mass function) is required if LINERs result from stellar ionization.

Introduction

Spectroscopic surveys indicate that ∼ 30% or more of bright galaxies contain weak emission-line nuclei that are classified as LINERs (Low-Ionization Nuclear Emission-Line Regions; Heckman 1980b, hereafter H80b). The intensity ratios of low-ionization optical lines relative to recombination features are characteristically higher in LINERs than in HII regions, causing LINERs to be regarded as “active” nuclei subject to unusual energetic processes. The luminosities of LINERs and giant HII regions are comparable, however, with typical Hα luminosity ∼ 1040 erg s−1 or somewhat less.

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.

Recently, we have found evidence for an offset active nucleus in the Seyfert 1 galaxy NGC 3227 (Mediavilla & Arribas 1993). In fact, it was found that the BLR is offset with respect to the kinematical centre derived from the ionized gas. This kinematical centre has a heliocentric radial velocity which is in good agreement with previous CO and HI determinations for the systemic velocity of the galaxy, suggesting that the BLR is not at the galactic mass centroid. In addition, some kinematical distortions are observed around the BLR which are probably related to the activity. Here we will give some additional details on the fit of the observational data to a simple model, and the proper location of the kinematical centre.

Introduction: bidimensional spectroscopy with optical fibres

Bidimensional spectroscopy with optical fibres is a new technique which allows two-dimensional mapping of spectral features. Using this technique we are carrying out a programme to study the circumnuclear environment of Seyfert galaxies. To this end, we have developed several optical fibre systems for their use with the telescopes at the Observatorio del Roque de los Muchachos, on La Palma. In particular, the HEXAFLEX system (Arribas, Mediavilla & Rasilla 1991; hereafter Paper I) was conceived for the Nasmyth focus of the 4.2-m, William Herschel Telescope (WHT). The 2d-ISIS works at the auxiliary focus of the WHT, and HEXAFLEX-II with the Nordic Optical Telescope (NOT).

We present long-slit spectroscopy of the composite Seyfert 2 starburst nuclei of the galaxies NGC 5135, NGC 6221 and NGC 7130 (IC 5135). Extended emission is detected in all three galaxies, reaching about 1 kpc from the nuclei. We study the spatial variation of the stellar population and emitting gas properties over the central regions. We compare our observed emission-line ratios with those obtained using composite photoionization models, which include ionization by a power law and hot stars, to find the gaseous abundance and the HII region parameters.

Several Seyfert galaxies exhibit the observational characteristics of vigorous star formation, either around the nucleus or in the galaxy disk, which is evident from extranuclear low excitation optical emission, diffuse non-thermal radio emission and very steep infrared spectra between 25 and 60 µm (Wilson 1988).

To investigate the connection of star formation and nuclear activity we have selected three Seyfert 2 galaxies with composite (Seyfert + HII) spectra – NGC 5135, NGC 6221 and NGC 7130. The nuclear spectrum of NGC 6221 presents a low excitation kinematical component and another, which is blueshifted, with high excitation (Pence & Blackman 1984). NGC 7130 also has 2 kinematical components, and the emission line ratios vary from values typical of active nuclei at the nucleus, to values characteristic of HII regions further out (Shields & Filippenko 1990).

Regions of violent star formation such as supergiant HII regions or superassociations often reveal a binary space structure: they contain two separate components within which very intensive star formation proceeds more or less simultaneously. This observational fact suggests an evolutionary scenario for the phenomenon, in which the key role is played by a strong collision of shock fronts produced by the energy release of the previous generation of massive stars in the region.

Binary stuctures and nonlinear gas dynamics

There are well observed giant regions of intensive star formation – superassociations – that consist of two (or three) components: Per-Cas, Sco-Cen and Car in the Milky Way, OB 78 in M31, Region IV = 30 Dor E + 30 Dor W in the LMC (Efremov 1988, 1989). A dust lane is observed in some cases between the two parts of the region that makes this composite structure especially obvious. In regions like OB 21 in M31, HII clouds give an even more contrasting picture when they concentrate at the two opposite sides of the dust lane.

Can this binary spatial structure be a clue to the physical nature of the violent star formation phenomenon? We assume that the answer to this question is positive and present it here in the form of an evolutionary scenario in which shock-shock collisions in the interstellar gas play a key role.

The age and location of stellar clusters and Wolf-Rayet stars in the third Galactic quadrant are analyzed. The cluster sample has been divided into three age groups: 1) younger than 107 yr, 2) between 107 and 3 × 107 yr and 3) between 3 × 107 and 108 yr. The mean z-locations of these samples in the central region of the Big Dent display a well defined z-age stratification. The existence of such an age gradient seems to corroborate previous hypotheses suggesting that the star formation activity was, probably, triggered by the same strong perturbation which originated the depression. A model in which the Big Dent originated by the collision of a high-velocity cloud with the Galactic disk is able to reproduce the observed gradient.

The analysis of the three-dimensional spatial distribution of a sample of young open clusters within 3 kpc around the Sun led Alfaro et al. (1991) to the discovery that the four nearest supercomplexes, previously detected by Efremov & Sitnik (1988), appear to be located below the formal Galactic plane. The one placed in the third Galactic quadrant (labelled III in ES88) shows the largest and deepest z-displacement and has been called the Big Dent.

The mechanisms able to explain the observed large z-departure as well as the different stages of star formation in the Big Dent need a source of mechanical energy able to inject in the Galactic disk a considerable amount of energy and momentum along the z-axis.

From HST observations we have detected a damped Lyα absorption and metal lines that originate from HI gas in front of a star cluster in the HII galaxy I Zw 18. We have found that this neutral gas is 1000 times more metal-deficient than solar and 30 times less than the O/H abundance of the HII region itself. We discuss the implications of these results for the scenario of star formation, the mixing of synthetised elements and the Population III hypothesis. Moreover no Lyman emission has been detected from the ionized gas. Such a surprising result is discussed in the context of the search for young HII galaxies at high redshift.

Introduction

We know the importance of blue compact dwarf galaxies in the context of galaxy evolution. These objects were first identified as a class by Searle & Sargent (1972) who recognized their unusual spectroscopical nature making them indistinguishable from giant extragalactic HII regions. Also named HII galaxies they are the closest objects we can find to being pure starburst galaxies (Melnick 1987). Their optical and ultraviolet continuum is dominated by the O and B star population. In one HII galaxy a broad emission typical for WR stars has been first detected at about 4650 Å by Allen, Wright & Goss (1976). Kunth & Sargent (1981) later discovered such a feature in the dwarf galaxy NGC 3115, estimated the number of WR stars (found to be large) and concluded that their presence was further strong evidence that star formation in HII galaxies did occur in bursts rather than continuously.

Introduction

HII galaxies are dwarf emission-line galaxies undergoing violent star formation. They are characterized by having giant HII regions which dominate their observable properties at optical wavelengths. Most HII galaxies are contained as a subsample of Blue Compact Galaxies (BCGs), but due to the different selection criteria only a small percentage of BCGs are HII galaxies. We will stick to the name “HII galaxies” to refer to the systems selected by objective prism surveys and having strong emission lines.

Various studies of their spectroscopic properties in optical wavelengths have revealed systems of very low heavy-element abundance and high rates of star formation. Earlier morphological studies have suggested that a large proportion of the sample of HII galaxies observed are starlike and isolated (Melnick 1987). For these, no clear indication of the mechanisms which may have triggered the burst is apparent. This together with the spectroscopic properties have made workers in the field since their discovery pose the question of whether these systems may be truly young galaxies or periodic bursts followed by long quiescent periods in the lifetime of the galaxy. Reviews of the general statistical properties of HII galaxies can be found in the Spectrophotometric Catalogue of HII Galaxies (hereafter SCHG, Terlevich et al. 1991; Melnick 1992).

We have pursued the line of work of surface photometry to address the questions of morphology and dynamics (Telles & Terlevich 1993; Telles, Melnick & Terlevich 1994) of HII galaxies as well as the questions of the age of the underlying systems and possible importance of the immediate environment in the onset of the present burst of star formation in HII galaxies.

A discussion of the determination of the “functional parameters” of Giant Extragalactic HII Regions is made for a wide range of physical conditions of the ionized gas. These functional parameters which are found to be the effective temperature of the ionizing radiation, the ionization parameter and the gas metallicity, are then traslated into physical properties of the ionizing star clusters – age, total mass and metal content – through the use of evolutionary models. These models combine evolutionary synthesis techniques with a well-tested photoionization code, CLOUDY, to obtain emission line intensities that can be compared directly with observations. From this comparison some conclusions about the general properties of the ionizing clusters can be extracted.

Introduction

The term Giant Extragalactic HII Region (GEHR) usually refers to outstandingly large and luminous HII regions that are therefore easily observable in external galaxies, mainly on the discs of late-type spirals and in irregulars. Their size, characterized by mean diameters measured on Hα images, can be as large as 1 kpc. Their Hα emission luminosity, which can reach 1041 erg s−1, translates into a number of ionizing photons betbeen 1050 and 1053 s−1. This high number of ionizing photons cannot originate in a single star, which would provide about 1049 at most, but requires the presence of a young star cluster.

In the few instances in which GEHR can be spatially resolved, observations reveal a core-halo structure with the core showing detailed substructure.

There is mounting evidence that type transitions are a common property of AGN: the broad lines in at least eleven Seyfert galaxies have appeared or disappeared, leading to the reclassification of their nuclei from type 1–1.5 to type 1.8–2 or vice versa. We show that these phenomena find a natural explanation in the starburst model for AGN as transient phases without supernova activity in a 10–60 Myr old metal-rich massive stellar cluster with a low supernova rate (νSN ≲ 3 yr−1).

Type transient AGN: casuistry

Spectroscopic observations of Seyfert galaxies established early on that the broad permitted lines can experience strong variations in time scales of a few weeks–months. There is a growing number of extreme cases in which the broad components have temporarilly disappeared or become so weak that a reclassification of the objects has been allowed: from Seyfert nuclei of type 1–1.5 to type 1.8–2/LINER. Among them we can find the prototypes NGC 4151 and NGC 5548, along with NGC 1566, NGC 3516, NGC 6814, NGC 7603, Mrk 372 and 3C 390.3. In most of these cases, we know that the transitions took place while the nuclei were in deep photometric minima. Conversely, there are some narrow-line objects that have developed prominent broad components while brightening. Among them we can find Mrk 6, Mrk 993 and Mrk 1018.

Circumnuclear starbursts can occur in galaxies with no notable companions. One of the consequences of this mechanism is to sweep out the star-forming gas from the few inner kiloparsec around the nucleus causing annular structures. Repeated starbursts (and perhaps the initial starburst itself) require a mechanism to feed new gas towards the axis of rotation. A dynamical process for this refuelling is based on the effects of departures from axisymmetry in the gravitational potential of the bulge, which can cause a net gaseous inflow, seen at its densest along a bar or bar-like structure, but also present for more generalized oval distortion. In an initial phase of the present work we have analysed the bulge component of a sample of spiral galaxies obtained using the 4.2-m WHT and the 2.5-m INT at the Observatorio del Roque de los Muchachos (La Palma). These galaxies present a starburst signature or exhibit evidence of residual phenomena from a previous starburst (see Beckman et al. 1991; Varela 1992). Seeing values oscillated between 0″·5 to 1″·4. Data reduction was performed using FIGARO and IRAF standard programs. We carried out a photometric analysis which enable us to perform a bulge/disc emission decomposition, in order to reconstruct that associated only with the bulge (Prieto et al. 1992a,b; Varela 1992).

HI observations are an excellent tool to probe the conditions of the ISM, giving information on the distribution and velocity structure of the cool and warm atomic gas in galaxies, and how that is affected by violent star formation. Currently, more distant galaxies have come within reach, allowing HI studies of galaxies of different Hubble types. In this talk I will review some of the work on the nearest galaxies and draw comparisons with our own. I will use the example of Holmberg II, a dwarf galaxy, to show how much can be learnt regarding violent star formation and its effects on the ISM by going to low-mass systems. Lastly, I will introduce some of the new and exciting projects which are under way.

Introduction

Carl Heiles' pioneering work on the structure of the Interstellar Medium (ISM) of our Galaxy confirmed the picture of it being a violent environment. His HI observations showed a wealth of structure in the form of shells and supershells, many of them expanding (Heiles 1979, 1984). His observations agreed, in general, with the prediction by Cox and Smith (1974) who, argued that supernovae create cavities of hot, coronal gas and play a significant rôle in the shaping of the ISM. Although some work was done on the Magellanic Clouds, it wasn't until the mid-80's that Heiles' results were dramatically confirmed by high-resolution (aperture synthesis) HI maps of the nearest galaxies, M31 and M33, enabling studies of the distribution and characteristics of HI shells across entire galaxies.

Violent Star Formation from 30 Doradus to QSOs was the most recent international conference organized jointly by the Institute de Astrofísica de Canarias (IAC) and the Royal Greenwich Observatory (RGO). The meeting took place in Puerto Naos, La Palma (Canary Islands, Spain), in November 1993.

This volume contains most of the invited talks and papers presented at the conference. It deals with observations, analysis and theory of violent star formation in its full dimension: from giant HII regions, dwarf and HII galaxies, to starburst galaxies, IRAS ultraluminous galaxies and interacting pairs, all the way up to violent star formation in active galactic nuclei and QSOs. Several critical reviews look at the implications of violent star formation from a variety of angles: ionization structure, interaction with the ISM, hydrodynamics, triggering mechanisms, chemical evolution, luminosity functions and the starburst model for AGNs.

On behalf of the organizing committee I would like to thank all the staff and colleagues at the IAC and the RGO who contributed to the organization of the meeting. We are particularly grateful to Monica Murphy (IAC), Judith de Araoz (IAC), Silvia Figueroa (RGO-La Palma) and Rachel Miles (RGO-La Palma) for their efficiency and their keen collaboration to warrant a very pleasant and successful conference. I also want to express my gratitude to Terence Mahoney for his help with the preparation of each of the papers in this volume.