ABSTRACT

Galaxy-galaxy collisions induce nuclear and extranuclear starbursts. The sudden reduction of angular momentum of the interstellar medium due to the gravitational impact of the encounter leads to the subsequent infall to the central regions of a large fraction of the overall interstellar gas. Starburst galaxies with bolometric luminosities ≥ 1011 L⊙ have converted most of the H I into H2 reaching extreme nuclear densities of molecular gas. We also discuss extranuclear starbursts in relation to the formation of dwarf galaxies in mergers. As a consequence of tidal interactions a fraction of the less gravitationally bound atomic hydrogen that populates the outskirsts of the pre-encounter disk galaxies may escape into intergalactic space. We find that the ejected gas may assemble again and collapse, leading to the formation of intergalactic starbursts, namely, tidal dwarf galaxies.

“STARBURST GALAXIES”

“Starburst” denotes star formation at higher rates than in normally, self-regulated processes. They are non-equilibrium episodes that last only a small fraction of the total life-time of the host stellar systems. “Starburst galaxies” are stellar systems where the overall energy output is dominated by recently formed stars. In the context of this definition we must distinguish the “extragalactic H II regions” (Searle and Sargent, 1972) from the “nuclear starburst galaxies” (Weedman et al. 1981). The first are small, irregular, and dust-poor galaxies where the starburst is encompassing most of the visible galaxy; the second are massive luminous galaxies where the most violent starburst takes place embedded in dust in the central regions.

RESULTS AND DISCUSSION

Mkn 298 is known as a morphologically peculiar system (Fig. 1), showing a chain (c, d, e) of small compact blue emitting regions (Stockton 1972) aligned on its eastern side up to ∼ 80 arcsec from the main body of the galaxy. These regions are aligned, with a tail (b) resembling a spiral arm located on the eastern side of the galaxy (a). On the western side a very faint trace of spiral arm is also visible, d and e are characterized by an emission line spectrum, while spectra of c do not show any trace of either emission or absorption (Stockton 1972; Metik and Pronik 1982).

The contour maps of the extended emission lines Hα, Hβ and [OIII]λλ4959, 5007 have been used to isolate six different emitting regions in a and b. Each strip on the 2D-spectrum has then been mashed into a ID-spectrum and the diagnostic line ratios proposed by Veilleux and Osterbrock (1987) have been plotted in the diagnostic diagrams giving [OIII]λ5007/Hβ versus [SII]λλ6716+6731/Hα, [N II]λ6583/Hα, [OI]λ6300/Hα. The effect of reddening has been evaluated from the Hα/Hβ ratio using the Whitford (1958) reddening curve as parameterized by Miller and Mat hews (1972) and adopting the intrinsic ratio 2.81.

ABSTRACT

Here we summarize the main results derived from optical fiber observations (bidimensional spectroscopy) related to the kinematics of the circumnuclear region of NGC 5728. We present additional arguments supporting that the ‘true’ nucleus of this galaxy is displaced in velocity and space from the optical nucleus. In view of this result, the region of double-peaked emission line profiles is connected to the nucleus, and the red component probably represents outflowing gas.

INTRODUCTION

NGC 5728 is a spiral barred galaxy classified as Seyfert 2 (Véron-Cetty et al. 1982; Phillips et al. 1983), which shows some peculiar features in its circumnuclear environment. Here we can point out the presence of a region showing double-peaked emission line profiles in the vicinity of its nucleus, the asymmetric position (with respect to the optical nucleus) of a ring of blue stars and ionized gas (Rubin 1981; Schommer et al. 1988; Wagner and Appenzseller 1988), and the possible presence of a weak BLR misaligned with respect to the NEL maximum (Pecontal et al. 1990).

We have observed the circumnuclear region of NGC 5728 with a new observational arrangement based on the use of optical fibers (Arribas et al. 1991), which is similar to the one presented by Shapovalova (see Afanasiev and Shapovalova, these proceedings). The observations were performed on May 8, 1989, using the 4.2m WHT sited at the ORM. The detailed analysis and results on NGC 5728 can be found in Arribas and Mediavilla (1993).

ABSTRACT

The circularization and accretion of the debris created by tidally disrupting a star passing near a supermassive black hole depends on the transverse structure of the debris stream. The transverse structure is modified by crossing points in the stream where the orbits are focussed across the stream center or through the orbital plane, the velocity shear across the stream, self-gravity, recombination, shocks, and shear viscosity. Stream-stream collisions may have a weak effect on the orbits because of Lense-Thirring precession, mismatched geometric cross sections, and the kinematics of collisions.

INTRODUCTION

Tidal disruption of stars passing near supermassive black holes (M ∼ 106 M⊙) provides a mechanism for fueling low-luminosity active galactic nuclei (AGN). It is ineffective for more massive AGN (M ≳ 108 M⊙) because the tidal gravity of the black hole is too small to destroy a star before it passes through the event horizon. Other processes in dense central star clusters such as star-star collisions may provide a steady accretion rate (Hills 1975, 1978; Frank 1978; Young, Shields, and Wheeler 1977), but the disruption of a star may lead to an observable flare in the luminosity of the AGN. Lacy, Townes, and Hollenbach (1982) first understood the kinematics of disruption, and the current picture of tidal disruption is reviewed in Rees (1988) and Phinney (1989). For a star of mass M* and radius R* passing at pericentric distance Rp from a black hole of mass M, the strength of the encounter can be parametrized by the square root of the ratio of the surface gravity to the tidal gravity η = (M*R3p/MR3*)½ (Press and Teukolsky 1977).

This book is based on a series of lectures that has been given at Stanford University, for longer than I care to remember, to graduate students from several departments: Aeronautics and Astronautics, Applied Physics, Electrical Engineering, Mechanical Engineering, and Physics. The course has also formed part of the Astronomy Course Program and of the Space Science Program.

The course has changed over the years, beginning as a three-quarter sequence emphasizing laboratory and geophysical plasmas, and evolving into a two-quarter sequence emphasizing solar and other astrophysical applications. Selected material has also been offered as a one-quarter course. The course has been much improved by input from many students (in fact, the first set of lecture notes was produced by students in the class) and from a sequence of dedicated teaching assistants, notably, in recent years, Dr Anton Bergmann, Ms Lisa Porter and Dr Yuri Taranenko.

For invaluable assistance in the preparation of this text, I am indebted to Mrs Louise Meyers-Norney, who entered the text, to Dr James and Mrs Maria Klimchuk, who entered the equations, and to Dr Taeil Bai and Mr David Faust, who helped prepare the figures. Thanks are due also to Dr George Field, Dr Robert Helliwell, Dr Eric Priest and Dr Gerard Van Hoven, who kindly reviewed some of the chapters, and to Dr Simon Mitton and Ms Fiona Thomson of Cambridge University Press for their generous support.

ABSTRACT

We present numerical N-body simulations of galactic interactions in which a compact body or galaxy penetrates a disklike galaxy. The results are ringlike structures having morphologies in good agreement with what one observes as ring galaxies. The code used is the TREE-code by L. Hernquist.

NUMERICAL FEATURES

The disk used as target is imbedded in a massive halo and the whole system is in dynamical equilibrium. It models fairly well the kinematical behaviour of spiral galaxies as far as the velocity distribution is concerned. We performed numerical simulations of collisions between stellar disks (embedded in static halos) and suitable intruders.

The disks have been settled down by solving numerically the Laplace equation in cylindrical coordinates and then immersed in a massive halo structure (King model). The system is evolved several rotation periods to test out the stability. The halo has an important heating effect on the disk during the assessment.

The companions used as intruders are massive points or small king spheres, having different masses and different radii. A series of collisions have been performed varying the direction of the companion's velocity.

The time scale of evolution of the system is around 0.5 Gyr and is consistent with theory (Theys and Spiegel 1977)

EVOLUTION OF THE SYSTEM

The passage of the massive point through the disk generates a transient ringshaped mass distribution. The ring is produced by a single density wave propagating through the disk. The wave has a damped oscillatory behaviour, since after an initial outward propagation, goes backward with decreasing amplitude toward the center of the structure. Correspondingly to the inward propagation, the ring disappears.

ABSTRACT

We model the kinematics of the molecular gas in the nearly edge-on disk in M82, by considering velocity and surface density perturbations caused by a possible rotating kpc long bar consistent with the angle of the bar observed from K (2.2 μm) isophotes. A model with a bar that has an Inner Linblad Resonance (ILR) at r ∼ 10″ ∼ 150 pc fits the molecular observations of the inner torus. The clouds have a cloud-cloud velocity dispersion of less than ∼ 30 km s-1.

The nearby “starburst” galaxy M82 is one of the most powerful infrared sources which is the result of a high star formation (SF) rate. There is a high concentration of molecular gas (∼ 5.5 × 107 M⊙) in the vicinity of the central starbursting region. The double lobed molecular structure observed is thought to be in the form of a rotating ring with a radius of approximately 250 pc (e.g. Weliachew et al. 1984). In the near infrared K or 2.2 μm band, there is a plateau of emission which is interpreted as evidence for a kpc long bar (Telesco et al. 1991) which may have caused the molecular gas to sink into the nuclear region. This mechanism for fueling a starburst has been predicted by numerical simulations of galaxy-galaxy collisions which include gas dynamics (e.g. Barnes and Hernquist 1991).

ABSTRACT

We have measured or compiled stellar velocity dispersions, σ*, for a sample of ∼ 80 Seyfert galaxies. The [OIII] λ5007Å emission line width correlates quite strongly with σ*, suggesting ionized gas velocities result principally from motion in the host bulge potential. Here we concentrate on second order effects, looking for parameters which correlate with the scatter on the [OIII] FWHM vs. σ* relation. In decreasing order of clarity, we find that Seyferts with relatively broad emission lines (e.g. Vgas > Vstars) have strong linear radio sources, are disturbed, or have bars. Since these galaxies show no unusual scatter on the Faber-Jackson plot of σ* vs Mbul, we conclude that radio luminous Seyferts and tidally disturbed Seyferts have unusual gas kinematics rather than unusual stellar kinematics.

The profiles of forbidden emission lines characterize the ionized gas kinematics in the Narrow Line Region (NLR) of Seyfert galaxies. What physical processes accelerate this gas, and are they related to the active nucleus or to the host galaxy? To explore the role of the host galaxy, we have measured stellar velocity dispersions, σ*, for 78 objects using the cross-correlation method. We also include published measurements of σ* (e.g. Terlevich, Dias, and Terlevich 1990; Whitmore et al. 1985) and complementary data on NLR and host properites, from Whittle (1992). In particular, we include the Perturbation Class, PC, which rates the degree of galaxy disturbance and/or interaction on a scale of 1 to 6 (based on the scheme of Dahari [1985]).

ABSTRACT

We have obtained 10 μm continuum images of a flux-limited sample of bright infrared galaxies with a spatial resolution of 0.8 arcseconds. All observations were made with UCSD's Mid-Infrared Camera on the Mt. Lemmon 1.5 meter telescope, Tucson, AZ. Most of the galaxies imaged display centrally condensed cores of emission. Two galaxies in our sample, NGC 253 and Markarian 171, are well resolved due to their proximity to the Galaxy and show extended emission. In the case of NGC 253, we have also obtained 20 μm continuum images. In this paper we present some results of our observations of NGC 253.

INTRODUCTION

Among the class of infrared luminous galaxies established by the Infrared Astronomical Satellite (IRAS), NGC 253 is a modest example of the “starburst” type. Due to its proximity to the Galaxy (∼ 3 Mpc), it is well resolved at many wavelengths. NGC 253 is an SABc galaxy with an inclination of 78.5∘. It displays no peculiarities in morphology. Still, within R < 500 pc, the far-infrared luminosity is ∼ 3 x 1O10 L⊙ (Telesco and Harper 1980).

OBSERVATIONS AND RESULTS

The UCSD mid-IR camera, the “Golden Gopher”, operates in the spectral region from 5 to 27 μm using a 20 x 64 element Si : As Impurity Band Conduction (IBC) device manufactured by GenCorp Aerojet Electronics Systems Division.

ABSTRACT

In this paper I briefly review the flow of gas in and around the bars of early type, strongly barred galaxies. I discuss the formation and location of the shocks near the leading edges of bars and the parameters that influence them. Straight shock loci can also be loci of such high shear that no stars can form there, although they correspond to important density enhancements. The flows found in barred galaxies entail a considerable amount of inflow. If inner Lindblad resonances are absent, or if one or more secondary bars exist within the primary one, then this inflowing gas can come very near to the galactic center.

INTRODUCTION

Modelling the interstellar medium in order to follow the gas flow in and around bars is not a straightforward matter. Two families of approaches have been developed so far for that purpose:

1. Codes treating the cool dense clouds as ballistic particles with a finite cross section, often called “sticky particle” codes. Exactly when two such clouds are considered to collide and what happens in such a case varies from one code to the other (Taff and Savedoff 1972; Larson 1978; Schwarz 1979).

2. Codes considering a collection of these clouds as a fluid with a sound speed of the order of the velocity dispersion of the clouds, i.e. of the order of 5 to 10 km sec-1 (cf. Cowie 1980). This group of codes is composed of at least three subgroups:

1. i) Difference schemes, in which several different ways of solving the hydrodynamic equations are used (see e.g. Prendergast 1983 and references therein).

2. ii) the beam scheme (Sanders and Prendergast 1974).

3. […]

ABSTRACT

Arp 86 is studied in terms of 3-dimensional N-body simulations and compared to CCD-observations. Deep, high resolution BVRI images were obtained in order to determine initial parameters for dynamical modelling, and to study star formation properties of these galaxies. The models suggest that the companion galaxy is moving in a low inclination, low eccentrity orbit thus performing several revolutions around the main galaxy, the obtained colors being in agreement with this interpretation. The orbit geometry favors material transfer between the components, which is proposed to be the cause of the anomalously large activity in the companion, and of the ongoing star formation in the bridge. The prolonged perturbation due to dynamically bound companion explains the grand-design structure of the main galaxy.

OBSERVATIONS AND N-BODY MODEL

Arp 86 (NGC 7753/54) is a spiral pair resembling M51 system. In order to study its star formation properties deep, high resolution BVRI CCD-photometry has been obtained (Laurikainen et al. 1993). Same observations are utilized in determination of initial parameters for dynamical modelling.

Dynamical modelling of the pair is performed with a new fast N-body code (Salo and Laurikainen 1993), capable of following the simultaneous evolution of two or more systems including both stellar and gaseous components. In the code galaxy disks are described in terms of self-gravitating particles, while analytical models are used for the spherical halo components. Potential evaluation is based on multiple, comoving logarithmic spherical potential grids. The code is 3-dimensional and thus allows arbitrary orbital geometry, and retains good spatial resolution simultaneously near the nuclei of both systems as well as in the interaction zone.

ABSTRACT

Two SO galaxies with extended counter-rotating [O III] 5007A gas are presented. The first, NGC 3941, is a barred field SO inclined 50° to the line-of-sight. The second, NGC 7332, is an edge-on field SO with a prominent boxy bulge. Multiple stellar components are not obviously present in either object nor do their structures have major distortions. This suggests that the progenitors of the extended emission were low mass dwarf/satellites or tidally stripped material.

INTRODUCTION

Early-type galaxies offer a unique laboratory in which to study galaxy interactions and their effects on the stellar and nonstellar components of the participants. The initial low gas and dust content of the host galaxy and the generally smooth morphology of early-type galaxies facilitates the detection of the perturbing influence of interactions and mergers. Whether these signatures of interaction manifest themselves as starbursts, bars, dust lanes, counter-rotating disks, or other phenomena depends on specifics such as the nature of the secondary object, the parameters of the collision, and the resulting evolution of the material.

The origins of the features displayed by the two objects mentioned here are suggestive of galaxy interactions playing an important role in the evolution of some early-type galaxies. These galaxies join the list (see Bertola et al. 1992) of SO galaxies displaying a decoupling of the angular momentum between their gas and stars.

ABSTRACT

We have obtained bi-dimensional spectroscopy of intermediate spectral resolution of the circumnuclear region of NGC 3227. We found two-peaked emission line profiles which evidence the presence of different kinematical components. The classification of the line profiles after attempting their two-components Gaussian fitting is discussed.

INTRODUCTION

The presence of asymmetries in the profiles of the narrow emission lines (NEL) of Seyfert nuclei is a common result (see e.g. Veilleux 1991). In NGC 3227 the asymmetrical profiles of the NEL corresponding to the optical nucleus suggest the existence of a substructure. Observing with intermediate spectral resolution an extended region in the environment of the nucleus of this galaxy, we obtained profiles showing at least two components. We present here this result whose importance for kinematics is obvious.

Our instrumental set-up is based on an optical fiber bundle of 95 fibers disposed in an hexagonal lattice, covering a projected rectangle of 9 arcsec × 12 arcsec. The spatial sampling was ∼ 1 arcsec, and the spectral resolution 2Å. The spectral range covered was (4600–5400Å) which includes the Hβ and [0 III]λλ4959,5007 lines. We used this fiber bundle in combination with the ISIS spectrograph and the 4.2m WHT sited on the island of La Palma. The data were acquired on December 10, 1992. For details about bi-dimensional spectroscopy with optical fibers see Arribas, Mediavilla, and Rasilla (1991), Vanderriest (1993), or Shapovalova (these proceedings).

ABSTRACT

The Hawaii Imaging Fabry-Perot Interferometer (HIFI) was used to produce a large data cube of the edge-on SBc galaxy NGC 3079 covering Hα + [Nil] λλ6548, 6583. The complete two-dimensional coverage of the Fabry-Perot data allowed us to derive the general flow pattern of the nuclear gas making up the superbubble in this object. Comparisons of our results with the well-known outflows in the Seyfert galaxy NGC 1068 and the starburst galaxy M82 indicate that the mass of entrained material is similar in these three galaxies, but that the kinetic energy involved in the outflow of NGC 3079 is at least an order of magnitude larger than in NGC 1068 and M82. The active nucleus in NGC 3079 is probably powering some of the outflow.

INTRODUCTION

Recent observations suggest that a violent outflow is taking place in the core of the edge-on SB(s)c galaxy NGC 3079. The optical line emission in the nucleus is LINER-like (Heckman 1980), and Hα presents faint, broad wings (Stauffer 1982; Keel 1983) reminiscent of low-luminosity Seyfert 1 galaxies. On closer inspection, however, the line emission responsible for the broad wings in the Hα profile is produced by a complex of extranuclear high-velocity clouds (Heckman, Armus, and Miley 1990 [HAM]; Filippenko and Sargent 1992 [FS]) which coincides in position with a looplike structure first discovered in Hα + [NII] images (Ford et al. 1986).

ABSTRACT

Massive binary black holes are expected to form in the nuclei of galaxies as a result of mergers between galaxies containing massive black holes. Evolutionary schemes (e.g. Roos 1988) where a galaxy merger leads to rapid evolution of a pre-existing wide binary towards a close binary predict that most binary black holes will be either wide (in a more or less undisturbed mode) or narrow (in a rather rapidly evolving stage). A nice example of a massive binary in such a rapidly evolving stage may have been found recently in the quasar 1928+738, where the observed wiggles in radio jets of this superluminal quasar could be interpreted as due to modulation of the jet velocity by the orbital motion of the binary (Roos, Kaastra, and Hummel 1993) We are doing numerical simulations of the (restricted) three body problem in order to study the evolution and accretion rate of massive binary black holes. Some results are presented.

SIMULATIONS

We are performing numerical simulations of the three body interactions between massive binaries with stars drawn randomly from a stellar cusp distribution around the holes. The equations of motion are solved using a pulsating-rotating coordinate system in which the binary is at rest (Szebehely 1967). The changes in orbital parameters of the binary are deduced from the change in energy and angular momentum of the star.

ABSTRACT

Galaxy interactions that agitate the interstellar medium by increasing the gas velocity dispersion and removing peripheral gas in tidal arms can lead to the formation and possible ejection of self-gravitationally bound cloud complexes with masses in excess of 108 M⊙. Some of these complexes may eventually appear as independent dwarf galaxies.

MASSIVE CLOUDS IN IC 2163/NGC 2207

VLA HI observations (Elmegreen et al. 1993a) reveal 10 clouds each with HI mass > 108 M⊙ in the outer parts and in the main disks of the interacting galaxy pair IC 2163/NGC 2207. Our observations apparently catch this pair in the early stages of massive cloud formation. The clouds, which are comparable in mass to dwarf galaxies, are fundamentally clumps in the gas, not clumps in the stellar component. The HI velocity dispersion in the clouds and in much of the main disk of NGC 2207 is, typically, 40 km s-1, a factor of 4 times higher than in normal disk galaxies. We propose that the high velocity dispersion of the gas is the key to why these clouds are at least 10 times more massive than the largest clouds in normal disk galaxies: the Jeans mass scales as the fourth power of the effective velocity dispersion. Such massive clouds can form by common gravitational instabilities where the Jeans mass is high and where the local value of the instability parameter for the gas is below threshold, e.g. in the outer disk and arms. Some of the massive clouds may later become large star formation complexes.

ABSTRACT

We used HST Planetary Camera images of M83 in the Hα, U, V, and I filters to study the ionizing clusters in the nuclear starburst region. Our high resolution images revealed detailed structure, previously not visible in ground-based observations.

INTRODUCTION

M83 is a well-studied barred spiral galaxy due to its proximity and its near face-on orientation (i=24°). The estimated distance to M83 ranges from 3.75 Mpc (de Vaucouleurs 1976) to 7.5 Mpc (Lord 1991). The nuclear region is well resolved in ground-based observations (e.g. Gallais et al. 1991), but with the HST Planetary Camera it is possible to probe the region in much greater detail. Our HST images actually allowed us to resolve individual clusters within the starburst regions.

OBSERVATIONS

We obtained high-resolution images of the nuclear regions with the Planetary Camera (pixel size= 0.″ 0436) on the Hubble Space Telescope (HST) on 14 December 1992. We chose U, V, and I filters which would reveal the ionizing star clusters in the bands. We were also able to obtain Ha images of M83 because its readshift (vhel = 504 km s-1 RC3) places Hα in the [NII] filter, F658N, near its peak sensitivity. Since the images were taken 126 days after the most recent WFPC decontamination, it was necessary to correct for contamination especially in the U-band image.

REDUCTION

The primary processing of the data was done at the Space Telescope Institute, which inclused flat-fielding, bias removal, and “dark” image subtraction. The nuclear region of M83 was on PC6. Processing at Goddard Space Flight Center included absolute flux calibration and the removal of cosmic rays.