We discuss the effects of a change of the mass loss rates of massive stars on the outputs of population synthesis models of starbursts. We find that models with high mass loss rates well account for the observed ratios of WNL to O-type stars in starburst galaxies.

Stellar evolutionary tracks and starburst models

The stellar populations as observed after a starburst episode depend on:

• The intensity as a function of time of the star formation rate (SFR) during the starburst. The observed properties of the starburst may also depend on the much lower SFR if this prevails between bursts of star formation. However, for very intense bursts, these “underlying” populations can be completely blurred out by the numerous new born stars.

• The slope of the initial mass function (IMF) and the lower and upper mass limits of the stars born in the burst.

• The time elapsed since the beginning of the burst (which we shall call here the age of the starburst).

• Some physical ingredients of the stellar models as, for instance, the mass loss rates by stellar winds and the metallicity.

• […]

Galactic bulges probably formed in starbursts such as those studied during this conference. We study the population contents and spatial structure of bulges to learn on the star formation history of these systems. From broadband optical CCD images we derive mean colours and colour gradients for a complete sample of ∼ 40 bulges of edge-on galaxies. After excluding objects with dust, the colours trace age and metallicity of the stellar populations. Metallicities inferred from the colours seldom reach solar values. This result is consistent with recent metallicity estimates for the Milky Way (MW) Bulge derived from spectroscopy of K giants. Colour profiles have negative slopes, i.e. colours become bluer outward. The derived gradients are similar to those observed in elliptical galaxies. Gradients of more luminous bulges are steeper than those of smaller bulges.

Introduction

Our notions on the stellar content of spiral bulges are largely derived from studies of a few nearby cases, notably our own MW Bulge. We know that the spectra of bulges often resemble more the strong-lined spectra of elliptical nuclei than those of metal-poor halo stars (Whitford 1978). It is also known that the MW Bulge, at Baade's window (BW) at least, contains stars of very high metallicity (Rich 1988).

Wolf-Rayet galaxies are a subset of starburst galaxies whose integrated spectra reveal the presence of hundreds to thousands of Wolf-Rayet stars. These galaxies exhibit a number of other properties indicative of a large “starburst” population of young, hot, massive stars. We have obtained optical ground-based and ultraviolet HST images of several Wolf-Rayet galaxies and present examples of the spatial morphologies observed at these wavelengths. Large star-forming regions which appear to be single units in the optical are resolved into numerous compact bright knots in the ultraviolet HST images. These multiple starburst knots are typically less than 100 pc in size and too small and closely spaced to be detected individually in the ground-based optical images. Yet they contain large numbers of hot stars and are typically several times as luminous as 30 Doradus, the giant HII region in the LMC. The intense bursts of star formation in these knots probably began only a few Myr ago and lasted less than about 1 Myr. It is possible that these knots represent proto-globular clusters which were formed as the result of recent galaxy mergers and/or interactions.

Introduction

Wolf-Rayet (W-R) galaxies are a subset of HII galaxies in whose integrated optical spectra a broad resolved HeII λ4686 emission feature has been detected (Conti 1991 and references therein).

We looked at galaxies with line ratios, which put them close to the borderline between starburst galaxies and LINERs/Seyferts in diagnostic diagrams. Comparison of the observed line ratios with line ratios from various models indicates the presence of a composite ionising mechanism: star formation in combination with either an active nucleus or with shocks. This is in agreement with the increasing number of observations of active nuclei with circumnuclear star formation and of starburst nuclei surrounded by shock-ionised shells. The far-infrared properties confirm the important role played by star formation in these galaxies.

Introduction

We searched the literature for galaxies, which, on the basis of their optical line ratios, could not be uniquely classified as either starburst galaxies or Seyferts/LINERs. The (arbitrary) selection criterion used was that the [NII]/Hα ratio should be within 0.3 from the empirical borderline between starbursts and Seyferts/LINERs in the log([NII] λ6583/Hα) vs. log([OIII] λ5007/Hβ) diagram (Veilleux & Osterbrock 1987). In all cases this also meant that the observed log([SII] λ6716+31/Hα) and log([OI] λ6300/Hα) ratios were close to their corresponding borderlines. In this way a total of 62 galaxies was selected. All selected galaxies are spiral galaxies, mostly late type: 54% are Sb and 27% Sc galaxies. The percentage of barred galaxies is 39%.

We used the photoionisation code CLOUDY to see if the observed line ratios can be explained by a single ionising mechanism, i.e. by a starburst or by an active nucleus.

In this paper we discuss the differences between normal supernova remnants (SNRs) evolving in comparatively low-density ambient media (n0 ≪ 105 cm−3) and “compact” supernova remnants that evolve in the high-density (n0 > 104 cm−3) environment that one would expect to find in starburst regions of galaxies. For normal SNRs, radiative losses do not start to become important until time scales of the order of 104 yr, after the onset of thin shell formation. For compact SNRs, however, the evolution proceeds at a much quicker pace, with radiative losses due to free-free emission, given the high temperatures (≥ 107 K), being important because of the high densities. The onset of thin shell formation in this case occurs over time scales of the order of years, and most of the radiation is emitted in X-rays and the UV. We argue that the compact supernova activity associated with starburst regions in the centers of galaxies gives rise to most of the typical properties of the Broad Line Regions of active galactic nuclei.

Introduction

Starbursts are usually traced by their bright photoionized regions and large cluster luminosities, in either optical or IR wavelengths. The mass spectrum of the resulting stellar groups is difficult to derive but a large fraction of massive stars is usually implied. Stars with initial masses above ∼ 8 M⊙ have strong UV radiation fields and significant mass loss during their whole evolution, and they are also the progenitors of Type II and Ib supernovae (SNe).

We present a kinematical study of NGC 604 in the emission lines of Hα and [OIII] λ5007 Å, based on data obtained with TAURUS-2, a Fabry-Perot imaging spectrograph. The main result is that the global supersonic velocity value is dominated by emission coming from the two maximum brightness peaks covering an area of about 12 arcsec.

NGC 604, located in M33, is one of the most prominent giant extragalactic HII regions (GEHR). It was observed with TAURUS-2 at the 4.2-m William Herschel Telescope (La Palma), with the Image Photon Counting System (IPCS) as detector. Two data cubes, each with an integration time of 3600 sec, were obtained in the emission line of Hα and [OIII] λ5007. The 125-µm etalon was used, which gave a free spectral range for Hα of 17.6 Å (806.6 km s−1) and 10.0 Å (596.1 km s−1) for [OIII]. The velocity resolution (σ of the calibration line) was 23.1 km s−1 for Hα and 19.6 km s−1 for [OIII].

In order to determine the origin of the supersonic mass motion in NGC 604, the kinematics of the region were analyzed by means of individual spectra, applying automatic fitting routines with single Gaussian fits. The measured velocity dispersions were corrected for instrumental and thermal broadening. The thermal velocity dispersion correction was calculated assuming Te = 10,000 K.

Star formation in general and violent star formation in particular as observed in dwarf irregular galaxies are discussed. Emphasis is placed on those qualities of dwarf irregular galaxies that may be regarded as controversial. In particular, the conditions leading to star formation and the effects of star formation on the chemical and dynamical evolution of a dwarf irregular galaxy are discussed.

Introduction

The title of this talk reflects a relatively broad reach, and I have no aspirations of achieving an all-encompassing review. To find such material, I recommend the reviews of Elmegreen (1992), Franco (1992), Hunter (1992), Kennicutt (1992), and Melnick (1992) in the proceedings of the Third Canary Islands Winter School of Astrophysics on Star Formation in Stellar Systems. Altogether, I believe that these lectures will provide an excellent background from which to discuss the problem of star formation in dwarf galaxies.

Instead, I would like to take this opportunity to discuss some of what I consider to be the points of contention one might encounter in the more lengthy reviews. To get right to the point, my discussion will take the form of a presentation of my prejudices. In preparing this talk, I was able to assemble my prejudices and review the observations and theories that led to their development. To admit that these are prejudices, I think, allows them to be openly confronted by both myself and others.

ROSAT and optical observations of NGC 4861 and NGC 5408, two HII galaxies exhibiting both narrow Hell λ4686 emission line and luminous soft X-ray emission reveal that X-ray and line emitting regions are spatially distinct. This demonstrates that the X-ray ionization mechanism which accounts for the HeIII regions around luminous X-ray sources in the LMC is not at work on a large scale in these two HII galaxies. We Briefly review alternative explanations for the formation of the narrow Hell λ4686 line. The X-ray spectrum of NGC 4861 is soft and probably not dominated by young accreting neutron stars. Accreting black holes of stellar masses, hot gas and SNRs may account for the integrated X-ray energy distribution.

Introduction

Although the strength of most emission lines observed in HII galaxies are in excellent agreement with those expected from the ionization by clusters of massive O stars, the presence of narrow, presumably nebular HeII λ4684 emission with an intensity of ≈ 1 % of that of Hβ in some HII galaxies is not yet well understood (see e.g. Campbell 1988; Conti, 1991). There are strong indications that the gas is photoionized and that shock excitation, if present at all, is not responsible for the emission of this high-ionization line. The origin of the luminous He+ Lyman continuum is currently a matter of debate since normal O stars have generally been thought to emit only negligible amounts of He+ ionizing photons.

I discuss some of the predictions of the Starburst model for AGNs, in particular the relation between observational parameters like the average blue luminosity, the amplitude of variability, the frequency of slow peaks in the light curve, and the time-averaged equivalent width of Hβ.

The number of slow peaks or SN events in the light curve of low-luminosity AGNs is uniquely related to the nuclear luminosity. An AGN with MB(min) ∼ −21.5 produces 1 slow peak (or SN) per year. This result is independent of the initial mass function, age and/or total mass of the cluster.

The time-averaged equivalent width of Hβ is related to the total energy of the SN, almost independently of the initial mass function, age and/or total mass of the cluster and of the assumed cosmology; the observed constancy of the value of the equivalent width of Hβ in AGNs is a direct consequence of the universal value of the energy released in a SN explosion.

The long term variability of AGNs as a function of their luminosity has a peak at a luminosity similar to the maximum luminosity of cSNR (i.e. MB ∼ −20). AGNs with both larger and smaller luminosity than MB ∼ −20 should be less variable than those with MB ∼ −20.

Where the phenomenon of wavelength-dependent kinematics has been observed in galaxies, it has been well modelled as a dust extinction effect. Therefore “compact mass features” in rotation curves close to the nuclei of dusty inclined galaxies may not be caused by dynamical effects, but by differential extinction by circumnuclear dust. NGC 7331 exhibits this phenomenology.

Introduction

The kinematics of the zones around the nuclei of galaxies are of exceptional interest, especially in the context of tests for very compact central objects (black holes). A number of nearby galaxies show striking kinematic features around their nuclei and have been well modelled by adding a central point mass to an otherwise smoothly varying bulge distribution, yielding the steep velocity gradients, dispersions and “shoulders” in their rotation curves (Bower et al. 1993). Given the exceptional interest in the presence of supermassive compact objects, however, it may not be surprising that, in at least some cases, remarkable circumnuclear kinematical effects may have been overinterpreted. In previous spectroscopy of dusty, highly inclined galaxies we have found systematic steepening of velocity curves from the near-UV to the near-IR (McKeith et al. 1993), which is convincingly modelled via dust extinction (Prada et al. 1994).

Data and results

To see how the observable rotation curve of the dusty spiral inclined galaxy NGC 7331 varies with wavelength we took long-slit spectra with the ISIS spectrograph on the 4.2-m WHT (La Palma), with the slit along the major axis centred on the optical nucleus.

We present here preliminary results of a study of the stellar population in AGN. Our aim is to quantify the stellar population within the nuclear regions by means of spectroscopic observations and to determine whether the central activity influences the stellar population or vice versa. The results will have general relevance to understanding the evolution of galaxies and the energy generation within the nucleus.

Introduction

We have observed 30 galaxies, of different levels of activity, using long-slit spectroscopy at the CFHT, in the range 5000–10000 Å (including MgI, NaI, TiO, CN and CallT).

In order to determine the composite stellar population of a galaxy, it is necessary to obtain spectroscopy of several different wavelength regions, including a number of different absorption lines. Without such information it is not possible to disentangle the effects of abundance variation, luminosity class and stellar type.

We are able to detect radial gradients in the stellar distribution (if any), as well as the possible dilution in the nucleus of the stellar component by a featureless component. We shall then establish at what wavelengths and to what degree the stellar population is responsible for the observed activity.

Results and Conclusions

Preliminary results for NGC 3516 (type 1 Seyfert, SB0/a), Mkn 620 (type 2 Seyfert, S(B)a) and NGC 3379 (non-active galaxy, E0) are presented.

In mergers of gas-rich spirals powerful starbursts are triggered, in the course of which a secondary population of globular clusters (GCs) may be formed. We present results from our chemical and spectrophotometric evolutionary models and show that even in the case of an old merger remnant like NGC 7252 the star formation (SF) history can be determined quite exactly, if only enough observational data are available. About a Gyr ago, NGC 7252 went through a starburst that, over (1–5)108 yr, increased its stellar mass by 20–50% and created a number of new GCs detected with HST. Young GCs may serve as a tracer for star formation efficiency (SFE). Our models predict metallicities for a secondary population of GCs which should allow to identify Sp-Sp merger remnants among ellipticals. In the case of NGC 7252, follow-up spectroscopy of the two brightest young GCs confirmed our metallicity prediction. We show that once the metallicity is known, very exact age dating of these GCs becomes possible.

Introduction

A most violent mode of SF is observed in mergers of massive gas-rich spirals. These starbursts have been considered as the near-by analogues of violent SF during the initial collapse at galaxy formation. Luminosities of 1012−1014L⊙ are observed, predominantly emitted in the IR. All the IR-UL galaxies and many of the luminous IRAS galaxies have by now been shown to be in an advanced stage of merging.

The quasi-static gravitational collapse of clouds should lead to fragmentation into small Jeans-unstable cloudlets with a column density similar to that of the clouds as a whole. This criterion defines the mass and size of newly-formed fragments as the collapse proceeds. One could in principle expect that all collapsing matter should end up in a massive singularity at the centre of the collapsing configurations. However, the onset of star formation is shown here to stop the collapse. This should happen if the newly-formed stars produce winds while ramming through the left-over cloud and in this way cause the stirring required to stabilize the collapse. An inmediate consequence of this is the velocity dispersion generated in the star-forming region, which is supersonic in the case of massive clusters and detectable upon the appearance of massive stars in giant HII regions. The stirring cause by the supersonically moving wind-driven sources is also shown to cause a distinct cloud structure, or filling factor, in excellent agreement with recent observations of regions of violent star formation. The two effects, i.e. the acquired velocity dispersion and the filling factor caused in the parent cloud, allow us to differentiate between high- and low-mass clusters. In both cases, however, the disruptive energy from massive stars ends up erasing the clues stored in the gas during cluster formation.

Introduction

There are several trivial but definitive conclusions regarding the history of spheroidal stellar systems that one can infer from the present agglomeration of stars in globular clusters and galactic bulges.

The results of 3-D numerical simulations for superbubbles expanding in a sheared and cloudy gaseous disk are presented. Assuming a disk in rotational equilibrium, the effects due to the gravitational force perpedicular to the Galactic plane, differential rotation, cloud evaporation, and radiative cooling of the gas inside the hot cavity have been included. We consider a set of different stratifications for the cloud component and discuss their influence on the final bubble properties.

Introduction

Massive stars are born in groups and their collective energy input (via UV radiation, stellar winds, and supernova explosions) causes the agglomeration of large masses of gas in cold expanding supershells (see Tenorio-Tagle & Bodenheimer 1988 and references therein). These expanding supershells are common interstellar features in our Galaxy (Heiles 1979; Lozinskaya & Sitnik 1988) and in other nearby galaxies (see Brinks 1994 and references therein). In this contribution we continue the study of expanding multi-supernova shells with 3-D numerical simulations (Palouŝ 1990; Bisnovatyi-Kogan & Silich 1991; Palouŝ 1992; Silich 1992; Silich et al. 1994a) as an attempt to understand their link with the general structure of the ISM and with the star formation process.

Equations and model parameters

The expansion of large multi-supernova shells is described using the thin layer approximation. This approximation corresponds to the late stages of supernova remnant evolution and has been applied to a wide variety of astrophysical problems with both analytical and numerical methods (Bisnovatyi-Kogan & Blinnikov 1982; Tenorio-Tagle & Palouŝ 1987; Mac Low & McCray 1988; see recent reviews by Ostriker & McKee 1988; Franco et al. 1992; Tomisaka 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).

Long-slit spectroscopic observations have been made for two starburst galaxies, Henize 2-10 and Markarian 52, in order to discuss physical conditions of the starburst regions in the galactic nuclei. We obtained the spatial variations of physical conditions of the ionized gas, such as Hα surface brightness, electron density, and ionization/excitation conditions. In particular we found an anti-correlation in the spatial variation between the [OIII]/Hβ ratio and the [SII]/Hα and [OI]/Hα ratios in each galaxy. This anti-correlation suggests that the nuclear starburst region consists of a central massive-star cluster and a single envelope of ionized gas.

Which structure do starburst regions have?

One of the most important issues on starburst galaxies has been the typical size of the distribution of ionizing stars. Although it is well known that ionized gas in starburst galaxies extends over a few hundred pc to one kpc, the distribution of ionizing stars themselves is still a controversial issue (e.g. Sugai & Taniguchi 1992; Puxley et al. 1990). There are two models for the structure of starburst regions. In one model, ionizing stars exist in a central cluster with its surrounding ionized nebula (Figure la). This structure is just like that of one giant/supergiant HII region, such as NGC 604 in M33 and 30 Doradus in LMC. In the other model, on the other hand, ionizing stars are distributed over the whole starburst region, with individual ionized regions (Figure 1b).