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.

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).