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).

We present the application of the multiphase model to the Galactic bulge to assure that this model may be used in all regions. Results show a star formation rate that is higher in the bulge. The logical consequence is a higher metallicity due to the burst of star formation at early phases of the evolution. But when a comparison is made between models applied to an elliptical galaxy and a bulge, a different chemical evolution results: the relation [O/Fe] versus [Fe/H] is not the same for elliptical galaxies and bulges.

The multiphase model

The bulge is the central region of the Galaxy (Frogel, 1988) with a radius of 1–2 kpc and a total mass of ∼1010 M⊙. In the outside region, the bulge connects with the spheroidal halo and with the adjacent disc. Characteristics for the stellar population of this bulge are obtained from observations: stars are old (Terndrup 1988; Lee 1992), metal rich (Gratton & Ortolani 1986; Rich 1988) and spatially distributed in two components (Rich, 1990; IRAS results). There is also a spheroidal metal-poor component without rotation and a more centrally concentrated and more metal-rich component with rotation.

The observed metallicity distribution is well reproduced by a simple “closed box” model. It implies no slow “infall” of gas in this region. Other models for the bulge are those of Arimoto & Yoshii (1987), based on their elliptical galaxy models, and Matteucci & Brocato (1990).

The irregular magellanic galaxy NGC 2366 is usually assumed to belong to the M 81/ NGC 2403 group. (B − V)-V and (V − R)-V photometric diagrams of its stellar content are presented. Using its brightest blue and red stars, its distance is estimated to be about 3.0 Mpc.

Introduction

For some time now, a considerable effort has been devoted to the analysis of the stellar content of nearby galaxies through the photometry of their resolved stars. These data, combined with spectroscopic and radio observations, provide interesting information about the history of the star formation and about the star formation processes in galaxies. The understanding of these processes is relevant, since they define the path followed by the galaxy through its evolution.

Our project is devoted to the analysis of the stellar content in nearby galaxies and is included in the GEFE project.

We present the first results of our photometric analysis of NGC 2366. This galaxy is usually assumed to belong to the M 81–NGC 2403 group. Table 1, summarizes the global parameters of NGC 2366.

Data and results

Observations of the stellar content of NGC 2366 were carried out in 1992 using the 1150 × 1250 EEV5 chip at the prime focus of the 2.5-m Isaac Newton Telescope of the Observatory of Roque de los Muchachos in La Palma (Canary Islands, Spain). The limiting magnitudes, are about B = 22.1, V = 23.2 and R = 22.9 (defined as those for which 50% of the stars are lost).

Variability is one of the most conspicuous properties of AGN. The starburst model postulates that the variability observed in radio-quiet sources is produced by the supernova (SN) and compact supernova remnant (cSNR) activity resulting from the evolution of a metal-rich massive stellar cluster, product of a starburst in the nucleus of an early-type galaxy. In this context, the optical light curves of AGN are reproduced by a random sequence of SN events. The parameters that describe a given light curve are the overall rate of explosions (νSN), the energy released in each cSNR (∊51), and the density of the circumstellar medium in which the remnants evolve (n7). In the case of low-luminosity AGN (MB ≳ −22 mag) these three parameters are well constrained by the observations: νSN by the minimum of luminosity and/or by the number of peaks of the light curves; ∊51 by the amplitude and duration of typical oscillations in the light curve and/or by the equivalent width of recombination lines, such as Hβ; and n7 by the decay rate of well-isolated peaks (Aretxaga & Terlevich 1993). The physics involved in the parameters above provides two independent constraints on the distance to a low-luminosity AGN.

Firts distance indicator: SN rate versus stellar luminosity

The B-band luminosity arising from a coeval cluster at its SN II explosion phase, 10 to 60 Myr, is mainly due to Main Sequence stars and cSNR.

The evolution of fast, radiative shocks in a high-density medium is discussed. Approximate broad-band light curves of the shocked gas are calculated, and the emitted spectra are used as the input spectra for photoionization models. The results are in good agreement with parameters characteristic of Active Galactic Nuclei.

Introduction

Over a decade ago Chevalier & Imamura (1982) showed that radiative, steady shocks are subject to an oscillatory instability. This result was confirmed on the basis of non-linear hydrodynamical analysis for nonstationary shocks (Gaetz, Edgar & Chevalier 1988) as well as for steady radiative shocks (Innes, Giddings & Falle 1987). Both groups found unstable behavior of shocks faster than ∼ 130 km s−1.

In a series of papers Terlevich and collaborators (see e.g. Terlevich et al. 1992) developed the starburst model for Active Galactic Nuclei (AGNs). In this model AGNs are powered by compact, dense supernova remnants (cSNRs), and the bulk of the radiation is emitted by the supernova shock wave evolving in a dense medium (n = 107 cm−3). Terlevich et al. (1992) calculated 1-D and 2-D hydrodynamical models of cSNR evolution and demonstrated that it was possible to recover observed characteristics of the Broad Line Region of AGNs. Using very simple assumptions, Terlevich et al. (1994a) successfully explained observed differences between times of maximum continuum and line emission taking into account the dependence on the ionization parameter.

The highly inclined very late type galaxy NGC 55 is a perfect target to study the effects of massive star formation on the interstellar medium, especially in the disk-halo interface.

We present deep broad- and narrow-band images of a field centered on the two largest HII regions of NGC 55. The color-magnitude diagrams produced from broad band V, R, i, and z images show the effects of a strong burst of recent star formation and imply the presence of a population of blue clusters which show up as clearly exdended sources on our best seeing images.

In the Hα image a spectacular arrangement of filaments, shells and supergiant shells is visible. This ionized gas at distances of up to 1 kpc from the nearest star-forming region makes NGC 55 a new example of a galaxy in ‘chimney mode’.

Supergiant shells in irregular galaxies

HII supergiant shells (SGSs) in irregular galaxies were first detected in the Magellanic Clouds on deep Hα plates taken with the UK Schmidt telescope (Goudis & Meaburn 1978). Thereafter, more and more such structures were detected in nearby irregular galaxies using modern detectors (e.g. Hunter & Gallagher 1990; Bomans & Hopp 1992; Hunter, Hawley & Gallagher 1993). The objects seem to be related to active star formation and HI holes, but the pattern is far from being understood.

Introduction

VV 523 (= NGC 3991 – UGC 6933 – IRAS11549+3237) is a typical clumpy irregular galaxy. Its distance is 42.6 Mpc, its absolute blue magnitude is −20.2 and its U – B index –0.44. On CCD frames taken at the Bernard Lyot 2-m telescope of the Observatoire du Pic-du- Midi VV 523 shows a complex structure with three components of bright clumps scattered in a common envelope. The extension of the envelope is 80″ × 2″ within a surface brightness of 25 mag arcsec−2.

Observations

Three filters (B, R and I) were used for the CCD observations. The seeing was about 1 arcsecond. A wavelet transform with a scale parameter suited to the size of the starforming cells was used to detect the clumps and to provide their size, location and colour indexes. Figure 1 shows the wavelet transform of the B image. The photometric results are in Table 1. The clumps have a mean size of 200 parsecs and the B – I indexes exhibit a sequence in colour which corresponds to their location in the galaxy.

Long-slit spectra with a dispersion of 33 and 260 Å mm−1 were recorded with the Carelec spectrograph at the 1.93-m telescope of the Observatoire de Haute Provence. The spectra show strong emission lines typical of HII regions. Three regions of different intensities are detected and named A, B and C. In Table 1 we give the physical properties of each of the three regions.

We discuss the observational constraints to the chemical evolution of the interstellar medium, ISM, provided by the abundances of HII regions. We present a review of the results derived from these constraints for the solar vicinity and for metal-poor galaxies. It is found that, contrary to previous results, black holes do not play an important role in the chemical evolution of galaxies. Chemical evolution considerations indicate that substellar objects (M ≤ 0.08 M⊙) have a mass density smaller than 0.02 M⊙ pc−3 (2σ) in the solar neighborhood. One or more of the following ingredients are needed to explain the Z versus µ diagram of metal-poor galaxies: a) a heavy-element yield increasing with Z due to a variation of the low-mass end of the IMF, b) outflow of Z-rich material, c) outflow of well-mixed material, and d) dark matter that does not participate in the chemical evolution process.

Introduction

HII regions are excellent probes of the chemical composition of the ISM of the Galaxy and of other galaxies. From the study of bright HII regions it is possible to derive accurate abundances of H, He, C, N, O, Ne, S, and Ar for galaxies that are located at many megaparsecs from us. Two important sources of error in the abundance determinations of bright HII regions are the temperature structure and the fraction of heavy elements that is embedded in dust.

Photometric observations on a group of eleven Haro starburst galaxies were carried out with the 1.0-m JKT over several semesters giving details of galaxy morphology and colour. These photometric images were then used in spectroscopic observations with the 2.5-m INT to enable accurate slit placement across the star forming knots. High and low dispersion spectra of the HII star forming regions were obtained, allowing the calculation of element abundance, abundance gradients, ionised gas dynamics, and estimates on burst age and possible starburst cause.

Introduction

The sample

The galaxies were chosen from Haro's (1956) list of 44 blue galaxies. They are characterised by having an ultra-violet excess spectrum and emission lines from hot gas. Most of the galaxies showed unusual or chaotic morphologies, with the blue emission emanating from compact, usually central, regions. Our more detailed observation revealed a diverse sample of morphological species, including two spirals, five nuclear ellipticals (Loose 1986), two cigar-shaped irregulars and two clumpy irregulars. Table 1 lists the name, type, absolute visual magnitude, heliocentric distance, actual diameter and integrated colour of each galaxy. A Hubble constant of 75 kms−1 Mpc−1 is used throughout.

Observations and data reduction

Photometric observations were carried out on the 1.0-m JKT in February 1985 and January 1988 with 512×320 pixel CCD detectors. The galaxies were observed through B, V and I filters. Contour maps are given in Figure 1.

Observations in the line of Hα and of [OIII] λ5007 of the galaxy NGC 4449 taken at the William Herschel Telescope of the Observatorio del Roque de los Muchachos with TAURUS-2, a Fabry-Perot interferometer, are being used to study the correlation between the diameter or flux and the velocity dispersion of its HII regions. Two different catalogues of HII regions are being compared. In the first we consider each flux relative maximum as a differentiated HII region, while in the second sample we use kinematical criteria to identify the different regions.

Introduction

In 1981, Terlevich and Melnick established a correlation between the diameter or the luminosity of giant HII regions and the turbulent width of their emission lines, which represented a chance to improve present extragalactic distance estimations. To study the possible existence of similar correlations for HII regions in a single galaxy, we have chosen NGC 4449, a giant irregular type I galaxy. It is located 5 Mpc away from the Milky Way and is very rich in HII regions. New criteria to identify HII regions are explored and compared with the samples and measurements already present in the literature. Detailed information will be presented in Fuentes-Masip et al. (1994, in preparation).

Data analysis

The product of the calibrated Fabry-Perot observations was a three dimensional set where the X-Y axes were the spatial coordinates, and the Z axis was wavelength calibrated (the dispersion direction).

We are carrying out a deep study of the CfA Seyfert sample of Seyfert galaxies in broad-band and narrow-band Hα. Our aim is to perform a complete analysis of the morphological properties of the galaxies hosting the Seyfert nuclei. We will also study the location and number of circumnuclear star-forming regions, and the incidence of interactions and galaxy-wide starbusts, emphasizing the similarities and differences between the type 1 and type 2 objects.

Introduction

Previous studies of active galaxies have been mainly concerned with the properties of the active nucleus. In fact, the many varieties of AGN recognised today were separated according to their nuclear properties. Yet the importance of the host galaxy in the understanding of the nuclear activity has been pointed out by several authors. For instance, some authors suggest that the host galaxies of Seyfert nuclei are substantially more luminous than similar field galaxies in the far infrared (Rodríguez Espinosa et al. 1987; Edelson et al. 1987; Rieke 1992), implying a connection between galaxy-wide star formation and Seyfert activity. Other authors find differences between the stellar formation rates in the host galaxies of Seyfert 1 and Seyfert 2 (Heckman et al. 1989).

It has also been known for some time that Seyfert 1's and 2's tend to be hosted by spiral galaxies of different morphological types, Seyfert 2's ocurring more often in latetype spirals than Seyfert 1's.

Observations of faint blue extended objects with the Hubble Space Telescope by Dressier et al. (1993) are discussed. It is argued that the objects may be regions of violent star formation in unseen galaxies at z = 0.4.

Nascent Galaxies?

On a 6-hr Wide Field Camera exposure with the Hubble Space Telescope of the rich cluster CL 0939+4713, Dressier et al. (1993) have found an apparent group of 15–30 faint extended objects with magnitudes 22 < r < 25. The objects are typically 1″ in angular size with bright central regions only a few tenths of an arcsecond in size, and are distributed over a region 40″ × 20″. The size and appearance of individual objects, their blue colors (known from broad-band ground-based imaging), and their clustering suggest, as Dressier et al. (1993) speculate, that they could be associated with each other, and – most importantly – that they are considerably more distant than the cluster CL 0939+4713 at z = 0.40. Furthermore, they assume that these objects could be physically associated with an unresolved, extremely blue object with the spectrum of a QSO at z = 2.055. If so, it means that at least a part of the objects may represent the early stage of formation of galaxies.

Should this association of the objects with QSO be confirmed, it would have lead to the conclusion that the luminous parts of these objects have physical diameters of about 1–3 kpc.