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.

We present narrow-band Hα imaging and long-slit optical and near-infrared spectroscopy of the starburst galaxy NGC 7714. We have detected WR stars in the starburst region, which indicate an age for the burst of between 3 and 5 Myr. We have obtained the physical condition of the gas in the starburst region and in three HII regions. These have moderately low abundances, while the nucleus has half solar abundance, with an overabundance of N.

Introduction

A typical starburst galaxy can be defined as a spiral galaxy with a bright nucleus bluer than expected for its morphological type, which emits strong narrow emission lines similar to low-ionization HII region spectra, as a consecuence of the photoionization by the ultraviolet radiation of hot stars, with typical Hα luminosities ranging from 1040 to 1042 erg s−1. During this intense recent burst of star formation between 107 and 1010 M⊙ of massive stars are formed within a radius of a few hundred pc about its nucleus.

NGC 7714, the prototype of the starburst (henceforth SB) galaxies (Weedman et al, 1981) and classified as a SBb peculiar, is in interaction with the irregular galaxy NGC 7715. The X-ray luminosity (6 1040 erg s−1) is explained with about 104 supernova remmants in a volume of 280 pc radius (Weedman et al. 1981). The 6-cm radio map shows a weak double radio structure separated by about 1 arcsec at p.a = 30°.

The 30 Doradus nebula is the nearest example of a giant HII region, and as such it offers a unique laboratory for studying in detail the structure, stellar content, and dynamics of a starburst region. We begin with an overview of the 30 Doradus region on scales of 0.1–1000 pc, and then discuss two current problems of particular relevance to this conference, the stellar content and IMF in 30 Dor, and the violent dynamics of its interstellar medium.

Introduction

It is a pleasure to open a conference where 30 Doradus defines the bottom end of the star formation scale! The 30 Doradus region offers a most appropriate starting point for a conference on star formation in galaxies. It is the nearest example of a bona fide giant extragalactic HII region (GEHR), and it is the largest star forming region in the Local Group. It is large enough to exhibit many of the properties of the most luminous starbursts, yet close enough so that its physical structure and stellar content can be studied in detail. As such 30 Dor and other nearby GEHRs provide several crucial pieces of information about starbursts in general. They are the only regions where the embedded stellar population can be resolved and studied directly; this provides a unique stellar census of a starburst, which can be used to test the synthesis models which must be applied to more distant, unresolved GEHRs and starbursts.

The process of galaxy formation is one of the crucial problems of modern astronomy. Galactic alignments are important as a test of the various available scenarios for galaxy origin which predict different types of alignments. A method for investigating the galactic rotational axes is applied to two samples of galaxies chosen from the UGC, ESO and NGC catalogs for testing different models of galaxy formation. In the whole Supercluster the planes tend to be oriented perpendicularly to the Local Supercluster (LSC) plane. The effects strongly depend on the supergalactic coordinates. We compare the observed distribution of galactic rotation axes with theoretical models. Our results support the so-called “pancake” or “hedgehog” galaxy formation scenario and exclude the “turbulence” models. Moreover, we have some evidence on the importance of membership of clusters belonging to the LSC.

Introduction

Galactic alignments are a crucial problem for understanding the process of galaxy formation. Various scenarios of galaxy origin predict different types of galaxy alignments within superclusters. Analysis of LSC galaxies (Flin & Godlowski 1986; Godlowski 1991, 1992, 1993) has shown that the preferred orientation of the galactic plane is perpendicular to the LSC plane, and that the projection of the rotational axis on the LSC plane tends to be directed towards the Virgo Cluster center. The distributions of face-on and edge-on galaxies are different.

Central gaseous disks around the nuclei of flat galaxies continually increase their mass due to spiralling giant molecular clouds (GMCs) under the action of dynamical friction. The radius of the disk depends on a tidal condition in the central parts of a galaxy equal to GMC tidal disruption distance. A central part of the disk can become molecular and be able to undergo a subsequent spontaneous burst of star formation when the mean surface density of the disk becomes larger than the critical UV-opacity column density. In the outer parts of the disk, formation of gaseous clumps and stellar aggregates can be self-consistent with a characteristic clump mass of 107M⊙ and new-born stellar masses of 106M⊙ in each clump. This scenario is a good approximation to the observable characteristics of central molecular disks of normal galaxies like ours. However, the interaction of galaxies must modify the maximum cloud and stellar aggregate mass up to ∼ 108M⊙ and lead to stimulated bursts of star formation.

Introduction

Bursts of star formation are frequently observed in the nuclear regions of galaxies. Observations have revealed compact nuclei surrounded by an extended disk, or disk-like HII regions with radii of about 200–500 pc (Wilson et al. 1991). These regions are, as a rule, the brightest HII regions in such galaxies. One of the main reasons for such intense starbursts is the high star formation efficiency (SFE) in galactic nuclear regions (Planesas et al. 1989).

X-ray variability in the 0.1 – 2.4 keV ROSAT energy band with a doubling time scale of 800 s and a factor of 4 within a few hours has been detected in a 20 ksec pointing on the AGN IRAS 13224-3809. The optical spectrum indicates that IRAS 13224-3809 is a narrow-line Seyfert 1 galaxy with strong permitted Fell emission, a member of the unusual I Zw 1 class objects. IRAS 13224-3809 appears to be the most rapidly varying AGN known so far. This is the first time that variability on a time scale smaller than 1000 s is reported at such high X-ray luminosity [L(0.1 − 2.4 keV) = 3 · 1044 erg s−1] in Seyfert galaxies. It is also the first reported X-ray variability in I Zw 1 class objects. The Δt = 800 s variation indicates that the X-rays come from a compact region of about 17 light minutes in size. Our results from the X-ray spectral analysis favour a scenario in which a hard X-ray source irradiates the accretion disk which reemits at soft X-ray energies. The absence of broad HI wings can be explained only if a part of the BLR, far from the centre, is observed and the bulk of the region, which emits the wings, is hidden. We want to draw attention to the fact that rapid X-ray variability could also be connected with the absence of broad HI lines in IRAS 13224-3809.

UGCA 86 (= VII Zw 009) is a companion of IC 342 and is one of the nearest starburst galaxies. It contains at least two starbursts, of which the central one is heavily obscured by dust. The IR radiation (IRAS has a relatively steep (cool) spectrum. The X-radiation (ROSAT pointed observation) seems to come from supernovae.

Introduction

In our optical observations of UGCA 86 (Richter et al. 1991) we found it to be a low surface brightness dwarf galaxy which contains two star formation regions, a central one and one near the southern border. Whereas the southern burst seems relatively normal, the central one is heavily reddened and has also a “softer” appearance (cf. Saha & Hoessel 1991). Together with the finding of an IR source in the IRAS catalog these facts fit very well the assumption that the central burst contains a large amount of dust.

Because the catalogued IRAS position did not fit the optical position well, we reprocessed the IRAS observation and observed the galaxy with a ROSAT pointing.

Observations

We have reprocessed the IRAS data using the GIPSY-IRAS system developed by the Laboratory for Space Research at Groningen (see Wesselius et al. 1992). High-resolution IRAS images were processed in Groningen using a maximum entropy method. While the IRAS Point Source Catalog shows only one source at the area of UGCA 86, the high-resolution images clearly reveal four sources.

Detailed Hα line investigations of the gas kinematics in the supershell around the Cyg OBI association were carried out. The supershell contains nebulae and CO-cavities around WR and Of stars which form a hierarchical system of mutually embedded gaseous dust shells. The nebulae around WR 134, 135, 141 and 142 and the SNR G73.9+0.9 are shown to be located at the far edge of the parent molecular cloud at Vlsr ∼ 5 − 10 km s−1. We found high negative velocities up to 70 km s−1 and [OIII]-Hα emission stratification typical for shocks. Both could be associated with stellar wind and SNe. The collective wind and ionizing radiation of the Cyg OBI stars (especially WR) and supernova explosions must play some role in forming the supershell. There are some reasons to suppose that the gas at the sound velocity Vlsr ∼ 12 km s−1 is flowing dowmstream of the ionization front.

Six years ago Lozinskaya & Sitnik, 1988 discovered a hierarchical system of mutually embedded gaseous-dust shells in the Cygnus arm (73 < l < 78°, −0°· 5 < b < 3°). In the sky plane this system consists of several small-size shells around WR and Of stars (NGC 6888 among them) inside the supershell around Cyg OBI association (Figure 1). The supershell (diameter about 100 pc) and inner shells of different sizes are seen as optical ring nebulae, radio-shells, CO-cavities (Lozinskaya & Sitnik, 1988) and IR supershells (Lozinskaya & Repin,1991; Saken et.al, 1992).

The propagating star formation model with anisotropic probability distribution is investigated. In each star-forming site we define the probability ellipse and show that its two parameters, the excentricity and the orientation relative to the galactic rotation, are closely related to the thickness and inclination of the resulting spiral arms. The relative size of a star-forming region with respect to the whole galaxy is also discussed. Simulations are compared to the observed galactic morphologies and we mimic the differences between the two groups of galaxies of types M101 and NGC 7217.

Propagating star formation

The idea that star formation at one place in a galaxy can initiate star formation in its neighbourhood was first suggested by Öpik (1953) and Oort (1954). Since then, a possible chain of physical processes that joins two regions of subsequent star formation has been proposed in which ionizing radiation from massive stars in a cluster leads to the disruption of the parental molecular cloud via supersonic champagne flows halting further star formation. The mechanical energy input from stellar winds and supernova explosions causes the agglomeration of gas in expanding shells. Their fragmentation, the building of molecules in high opacity areas, and large-scale gravitational instabilities may produce molecular clouds, where the next generation of star formation occurs.

The star-forming cycle described above is the basis of deterministic PSF models (Palouš et al. 1994). However, the physical parameters such as density, metallicity and cooling times of the ISM, are only partly known.

Starbursts in four galaxy locations are discussed: on the periphery and in tidally ejected debris, in the main disk, in inner Lindblad resonance rings, and in the nucleus. Starbursts in dwarfs are also briefly mentioned. Possible reasons for the starbursts are summarized, mostly in the context of two theoretical models, one where star formation is initiated spontaneously by gravitational instabilities in disks, spiral arms or rings, and another where star formation is stimulated by high-pressure star clusters. The observed rates, efficiencies, and durations of star formation in all five regions follow from the models. We emphasize the importance of a critical density for star formation, which is approximately κ2/G for epicyclic frequency κ, and the importance of large-scale radial gas flows. Star formation tends to occur wherever the density exceeds the critical value. The rate of star formation is very large in inner rings and nuclear regions because the critical density is very high there. Normal galaxy disks have lower rates because of their lower κ. This difference in rates implies that inner rings and nuclear regions of galaxies can maintain their star formation for much shorter times than the main disks following an episode of gas accretion that makes the density exceed the critical value. Thus only the inner regions will have major fluctuations in the star formation rate. Normal galaxy disks probably have fluctuations too, but with lower amplitudes and longer durations.