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.

ABSTRACT

We have found possible evidence that boxy-type elliptical galaxies favor the environment of clusters of galaxies, while disky-type ellipticals prefer the field environment.

MOTIVATION, DATA, AND RESULTS

Recent high-quality imaging and spectroscopic studies have shown that there is a fundamental distinction in elliptical galaxies : the isophotal shapes of elliptical galaxies (cf. Bender et al. 1989). Elliptical galaxies have three types of isophotes: boxy, elliptic, and disky. It should be stressed that these types are more physically related to the dynamical properties of the elliptical galaxies than are the fine morphologically peculiar features such as shells. The boxy elliptical galaxies owe their shape to the anisotropy of velocity dispersion of stars while the disky type are flattened by rotational motion. This dynamical difference between them suggests that the two types of galaxies have different formation histories. It is therefore natural to ask whether the difference in the isophotal shapes of elliptical galaxies may have some relation to their environments.

Our study is based on the data in Bender et al. (1989) who compiled published data as well as their own results of detailed CCD photometry of 109 elliptical galaxies. Among the 109 elliptical galaxies, we assigned the environments for 96 galaxies based on the paper by Faber et al. (1989). The elliptical galaxies in Virgo, Coma, Abell 194, and Abell 1367 clusters of galaxies are classified as cluster ellipticals. The elliptical galaxies to which no group identification has been given are classified as field ellipticals. The remaining ellipticals belong to the group of galaxies. In this way, we obtained a sample of 96 elliptical galaxies.

ABSTRACT

Using deep HST/WFC images, originally taken to study faint radio galaxies, we find 81 clear serendipitous galaxy images, of which 34 are pair members. Based on nearby magnitude-limited samples, this is an excess of more than 4σ above the expected number of pair members. We take this result as strong evidence that the galaxy merger rate was higher in the past, and has declined over time.

INTRODUCTION: GALAXY MERGERS AND EVOLUTION

Galaxy interactions and mergers have been implicated as driving galaxy evolution in several ways:

Triggering starbursts, thus making the star-forming history episodic Driving global winds from starbursts, sweeping merger remnants free of gas and dust Transforming galaxy morphology through mergers and tidal impulses Triggering nuclear activity

Counts of local pairs and mergers, plus N-body modelling of orbital decays, suggest that many (perhaps most) present galaxies underwent mergers during cosmic history. This means that the merger rate was probably higher in the past. We are using galaxy and pair counts from deep HST serendipitous fields to constrain the merger rate.

We cannot uniformly trace mergers themselves to large redshifts, because (1) cosmological (1 + z)4 surface-brightness dimming makes the characteristic tidal features too faint for detection and (2) at large redshifts, the disturbed structures can be too small for detection given surface-brightness constraints. We therefore trace the merger rate by studying the evolution of galaxy pairs some of which are the immediate precursors of mergers.

ABSTRACT

In order to study effects of the starburst activity on far-infrared (FIR) colors, we have constructed starburst models which are able to trace FIR color evolution of starburst activity.

INTRODUCTION

The IRAS mission discovered that FIR emission properties of galaxies are significantly altered by starburst activity. Therefore, the FIR properties of galaxies are used to study the nature of starburst activity, in principle. Since, however, it is difficult to know properties of dust grains unambiguously, little effort has been made to construct starburst models which are able to reproduce FIR properties of galaxies (cf. Rowan-Robinson 1992). We here present a simple starburst model capable for calculating FIR emission of starburst galaxies.

MODEL AND RESULTS

The infrared continuum emission comes from dust grains embedded in ionized gas and from those in molecular clouds. The large grains are in thermal equilibrium with the radiation field and radiate as a blackbody with an emissivity Qλ ∝ λ-1. On the other hand, small grains are heated transiently by single UV photons. In our model, emission at 12 and 25 μm mainly comes from small grains in the molecular clouds and large grains in the ionized gas, while emission at 60 and 100 μm comes from large grains in the molecular clouds. The basic concept of our model was given in Mouri and Taniguchi (1992). Our new model is now able to calculate FIR emission from 12 to 100 μm directly and to trace the full phase of starburst evolution. Detailed description will be given elsewhere.

We saw in the previous chapter that the equation governing the evolution of the particle distribution function has the form of a diffusion equation (18.2.19). When starting from the Fokker–Planck equation, it is not obvious why this should be the case.

In this chapter, we consider a homogeneous plasma and assume that the fluctuating electric and magnetic fields can be expressed as a distribution of normal modes of the plasma. We shall find that this approach leads directly to a diffusion equation. In addition, we shall find that the diffusion coefficient is related to the emission and absorption processes for the waves that comprise the normal modes.

Our analysis follows the semi-classical or quasi-quantum-mechanical approach due originally to Ginzburg (1939) and subsequently applied to plasma physics by Smerd and Westfold (1949), Pines and Schrieffer (1962), Melrose (1968) and Harris (1969). Our analysis will be restricted to the case that the unperturbed system contains no magnetic field. For a recent and more complete exposition of this procedure (that includes a possible magnetic field) see Melrose (1980).

Quantum-mechanical description

The relationship between emissivity, absorption and particle diffusion rests upon certain quantum-mechanical relationships. We therefore begin by describing the system in quantum-mechanical terms, taking note of the required relationships, and then making the transition to a classical description.

It is sufficient to consider a simple quantum-mechanical system with discrete energy levels (assumed non-degenerate) of energy E1, E2, …, with particle populations n1, n2,. … (See Fig. 19.1.)

The significance of stellar activity cycles

During the twentieth century, our perception of the fundamental nature of stars and stellar systems has undergone a revolution almost as profound as that initiated by Copernicus in relation to the solar system. In the nineteenth century, a star was regarded as a luminous, spherically symmetric system, for which the only available energy source appeared to be the energy released by gravitational contraction. Unfortunately, simple calculations showed that, on this basis, the Sun's luminous lifetime (the Kelvin-Helmholtz time) was far too short to accommodate the age of geological structures, the development of life, and the evolution of species.

The discovery that nuclear energy could provide the source necessary to prolong the luminous lifetimes of stars by several orders of magnitude was the first significant development in our understanding and provided the background structure for the picture of a star that emerged in the first half of this century: i.e. that of an equilibrium system, in which the internal generation of nuclear energy remained in long-term balance with the radiation emitted at the surface. In this system, it was assumed that hydrostatic pressure balance applied and that the outward temperature gradient was monotonically negative, in conformity with the well-understood principles of thermodynamics.

In February of 1985, an international group of solar astronomers, including both observers and theoreticians, met in Tucson, Arizona, and agreed to plan a series of workshops with the aim of mounting a coordinated study of the new solar cycle, Cycle 22, which was expected to begin in 1986. Meetings were hosted by the California Institute of Technology at the Big Bear Solar Observatory in August of 1986, by Stanford University at Fallen Leaf Lake in May 1987, by the University of Sydney in Sydney, Australia, in January 1989, and by the National Solar Observatory in Sunspot, New Mexico, in October 1991.

This volume does not seek to provide a formal account of the workshop proceedings, which may be found elsewhere. It has, however, been inspired by the intellectual stimulation generated by these meetings and by the many contacts with scientists throughout the world which have followed them.

While making full acknowledgment to the many people whose work and ideas have provided me with excitement and stimulation, I do not wish to imply that this book represents a general consensus. It is not possible to provide a definitive account of the mechanisms underlying cyclic activity at the present time; opinions differ strongly on some aspects, whereas a general bafflement prevails in other areas. It is thus an exciting field, and this volume is intended to set forth a summary of the current state of our understanding of stellar cycles, as interpreted by the author.

The polar fields

The first observations of the Sun's weak polar magnetic fields were obtained in 1915 by Hale at Mount Wilson but, at that time, little attention was paid to their polarities in relation to those of sunspots. In 1957, however, Horace Babcock noted that, at the beginning of Cycle 19, the north polar fields were positive, as were the leader spots of the new cycle in the northern hemisphere. He further noticed that, as the cycle proceeded, the polar fields weakened and, in 1959, the mean magnetic polarity of the north polar field reversed, so that, for a period, the polarity of both polar fields was negative. Eighteen months later, the polarity of the south polar field also changed, so that, at the start of the next cycle (Cycle 20), the polar polarities in each hemisphere again corresponded to those of the leader spots. Similar reversals also occurred shortly after maximum during Cycles 20 and 21. On the basis of these three occurrences, it is now widely assumed that the global polar field of the Sun reverses with a period comparable to that of the sunspot magnetic cycle, but with a phase difference of ∼ 90°.

The polar fields are weak; even at sunspot minimum they are only a few gauss, and their reversals are not well defined. At times during the reversals, the polar regions are covered with weak patches of field of either polarity and the net polarity of the region is uncertain.

Introduction

Although the solar cycle was identified as a sunspot number fluctuation in 1850, and as a magnetic oscillation in 1923, little progress was made in developing theories which might explain the somewhat diverse phenomena associated with it until the latter half of this century. Since 1950, however, several ingenious, and essentially heuristic, models have been proposed, and some have subsequently been supported by more detailed mathematical analyses. Each of these has offered some insight into possible cyclic processes, but none has provided an account consistent with all the available data, or with our current understanding of the physical processes operating within the Sun. Nevertheless, in order to set the stage for later, more mathematical discussions, it may be helpful to review these models briefly in order to see where they have succeeded and to understand how they have failed.

These models may be classified as (i) relaxation models (e.g. Babcock 1961), (ii) forced oscillator models (e.g. Bracewell 1988), or (iii) dynamo wave models (e.g. Parker 1955, Krause and Rädler 1980). Both (i) and (iii) may be regarded as particular examples of the formal mathematical discipline known as dynamo theory, which will be discussed in more detail in Chapter 11. The models to be discussed here have all arisen from attempts to understand the phenomena of the solar cycle and are part of the background against which more recent observational data should be considered.