ABSTRACT

Results of 3D numerical simulations of a two-component self-gravitating galactic disk embedded in a ‘live’ halo are presented. The pure stellar disk is chosen to be globally unstable and forms a stellar bar. A ‘seed’ black hole (BH) of 5 x 107 M⊙ with an ‘accretion’ radius of 20 pc is placed at the center. The details on the numerical method are given in Shlosman and Heller (these proceedings and references therein). Here we study the effects of star formation (SF) which is introduced when (1) the gas is Jeans unstable; and (2) the gas density exceeds ∼ 100 M⊙ pc-3. The formed massive stars account for the gas heating by means of line-driven winds and by supernovae, assuming an efficiency of kinetic-to-thermal energy conversion of a few percent. Models with and without SF are compared and only the robust features are emphasized.

We find that the SF has induced angular momentum loss by the gas and has increased the radial inflow by a factor < 3. We also find that (1) SF is concentrated at the apocenters of the gaseous circulation in the bar and in the nuclear region; (2) the nuclear starburst phase appears to be very luminous (quasar-like) and episodic; and (3) the nuclear starburst phase correlates with the catastrophic growth of the BH. Without the pre-existing BH, the gas at the center (few x 100 pc) becomes dynamically unstable and forms a gaseous bar which drives a further inflow. The gaseous bar may fission into a massive cloud binary system.

ABSTRACT

We have performed broad-band CCD imaging of a sample of 24 low luminosity radio galaxies containing radio jets. Their optical properties, photometric parameters, and their local environments have been studied and compared with other samples of radio galaxies.

Radio galaxies can be classified in two groups, powerful radio galaxies (PRG, with total power P≥ 1025 W Hz-1 at 1.4 GHz) and low luminosity radio galaxies (LLRG, with total power P≤ 1025 W Hz-1 at 1.4 GHz), both from their radio as well as their optical properties. Among the main works that allowed to establish the differences in line emission, photometric structure, and environment between the two classes we should mention those by Hine and Longair (1979), Heckman et al. (1985, 1986), Lilly and Prestage (1987), Owen and Laing (1989), and Smith and Heckman (1989a, b). PRG usually show Fanaroff-Riley type II radio morphology (Fanaroff and Riley 1974), strong optical emission lines, and photometric structure similar to normal ellipticals. LLRG generally show FR I morphology, weak emission lines or pure absorption line spectra, and optical luminosity and light profiles similar to bright cluster galaxies.

One feature that powerful and low luminosity radio galaxies have in common is the morphological evidence of ongoing or past interaction/merging processes in these systems (Heckman et al. 1986; Lilly and Prestage 1987; Smith and Heckman 1989a, b; Colina and Pérez-Fournon 1990a, b; González-Serrano and Pérez-Fournon 1992 and references therein). However the indications of interaction of each class appear in different form.

ABSTRACT

We describe a program of N-body simulations of clusters of galaxies. Results concerning merging histories and the kinematic properties of multiple nuclei are presented.

THE MODELS

We have completed nine N-body evolutions of models containing 50 galaxies using a total of N = 4 × 104 particles. The models are fully self-consistent in that each galaxy is represented as an extended structure containing many particles and the total gravitational potential arises from the particles alone. The evolutions are carried out with a direct N-body code based on the Barnes and Hut (1986) TREE algorithm for computing the gravitational potential; it is the code developed by Lars Hernquist (1987, 1990) with some modifications (Bode, Cohn, and Lugger 1993).

For all the models, 10% of the particles are ‘luminous’ and the rest represent ‘dark’ matter; the former are given a smaller softening length than the latter. The dark matter is apportioned between galaxy halos and a smoothly distributed cluster background. The percentage of mass initially in this intra-cluster background, β, is varied from 50% to 90%.

The initial mass distribution of the galaxies follows a Schechter function. For β = 50%, the smallest galaxy contains 125 particles and the largest 3500 particles. Galaxies are given a core-halo structure by identifying the most bound particles in each galaxy as luminous. Since the total amount of mass in the cluster is the same for all models, increasing β has the effect of removing mass from the galaxy halos and distributing it through the cluster.

ABSTRACT

Using the VLA in A-configuration we have obtained λ21cm absorption spectra of 12 Seyfert galaxies with bright, extended continuum radio emission. Currently, we have completed the data reduction for three galaxies: NGC 1068, Markarian 3, and NGC 3079. No absorption is detected in Mrk 3, but multiple and broad absorption lines were detected in NGC 1068 and NGC 3079. Here we present the preliminary analysis for these galaxies.

NGC 1068

The radio continuum emission from NGC 1068 is dominated by a 13″ radio triple extending SW–NE (e.g., Wilson and Ulvestad 1987). We have detected HI absorption over the entire SW radio lobe and the southern half of the central source. Three kinematically distinct regions are apparent: the radio nucleus, the linear radio structure 1–2″ SW of the nucleus (the SW “jet”), and the SW radio lobe. Using the technique of Dickey, Brinks, and Puche (1992), we extracted optical depth spectra for each of these regions (Fig. 1). Broad (FWHM = 130 ± 25 km s-1), double absorption lines are present in the nuclear region. Adopting usys(HI)= 1137 km s-1 (de Vaucouleurs et al. 1991, RC3), the lines are offset by +55 ± 11 and -283 ±11 km s-1. We suspect that these lines may arise in a region of rapid rotation and streaming near the active nucleus. In the SW “jet” multiple absorption lines centered at usys are apparent. Since this region lies along the 2.2 μm stellar bar (Thronson et al. 1989), these multiple absorption lines may be due to gas streaming in the bar potential.

ABSTRACT

We present a new model for the dynamics of molecular clouds in the innermost ≈ 200 pc from the Galactic Center. Our analysis allows us to determine the characteristic parameters of the accretion disk as well as the positions of individual clouds with respect to the Galactic Center. Finally, we show that the results are in good agreement with independent determinations of the same parameters and discuss a physical mechanism that allows for the theoretical understanding of these parameters.

OVERVIEW

We assume that the dynamics of accretion disks in the innermost ≈ 200 pc from the Galactic Center (GC) can be modelled in the framework of an accretion disk description. The two parameters that mainly determine the structure of the disk are the radial mass flow rate through the disk (Ṁ) and the dynamical viscosity (v). For a pair of these two parameters, one can calculate the radial velocities (us) in the disk with respect to the GC and, consequently, the observable quantity ux, the radial velocity with respect to the observer (of course, after applying all the relevant corrections). In an iterative process, with this method, we determine the pair (Ṁ, v) that gives the best agreement between the observed and modelled values of vx. Having found such a solution, we also determined the location of the individual clouds with respect to the GC. Thus, this technique allows us also to construct a map of the distribution of molecular clouds close to the GC. In the next section, we summarize our ansatz for describing the accretion disk.

ABSTRACT

We calculated the orbital evolution of stars due to interaction with an accretion disk around a massive black hole in a galactic nucleus. After circularization the radius of a stellar orbit with initial inclination i to the disk shrinks by a factor 4/(1 + cos i)2 before it settles in the plane of the accretion disk. Next, we calculate the rate at which stars from the star cluster around the hole are captured by a standard Shakura-Sunyaev disk. We find that the majority of captured stars are on retrograde orbits. These stars may reach a small separation from the hole before settling in the disk. AGN with M ∼ 106 M⊙ and Ṁ = MEdd are likely to have stars on inclined orbits with small separation from the hole, i.e. just outside the tidal disruption radius. Observational effects will be most conspicuous in low luminosity AGN.

EVOLUTION OF STAR ORBIT

The pre- and post-impact velocities v and v1 can be calculated from conservation of momentum: m*v + Δmw = (m*, + Δm)v1, where Δm is the mass swept up by the star and w is the velocity of the accretion disk at the impact point. These three equations together with the requirement that the pre– and post–impact positions are the same, provide a set of four relations between the old and new orbital parameters. It is possible to find explicit expressions for the changes in the orbital parameters by linearizing these equations (Roos and Kaastra 1993, in preparation).

ABSTRACT

Galaxy bars can be important triggering agents for star formation, radial gas flows, and nuclear activity. This paper reviews the observational evidence for bar-induced star formation and gas redistribution in spiral galaxies. Specific topics include the global star formation rates in barred vs normal galaxies, the spatial distribution and abundances of star forming regions in barred systems, and circumnuclear hotspots.

INTRODUCTION

Barred galaxies present one of the clearest cases of mass-transfer induced activity, and as such are valuable laboratories for understanding the triggering of starbursts and nuclear activity in a broader context. As reviewed by Athanassoula elsewhere in this volume, hydrodynamic simulations suggest that bars can trigger a wide range of phenomena, including large-scale gas compression, star formation, and radial transport of gas into the nuclear region (also see Sellwood and Wilkinson 1993).

This paper reviews the observational evidence for bar-induced star formation and circumnuclear activity. I begin by discussing the integrated properties of barred vs normal spirals, based on surveys in Hα, radio continuum, and the infrared (section 2). Section 3 summarizes the star formation properties of individual barred systems, with emphasis on the bars themselves and their surrounding disks. In section 4 I discuss the circumnuclear “hotspot” star formation regions, which are probably the most distinctive signatures of bar-induced activity. I conclude with a summary of outstanding questions and important areas for future work.

INTEGRATED PROPERTIES OF BARRED VS NORMAL SPIRALS

The first systematic comparisons of the global properties of barred and normal galaxies were based on radio continuum surveys (Cameron 1971; Dressel and Condon 1978; Dressel 1979; Heckman 1980).

ABSTRACT

The idea that Seyfert nuclear activity might be fueled by material inflow induced by a perturbing companion has been fashionable for over a decade. However, the recent literature on the prevalence of Seyfert galaxy companions, is confusing and somewhat contradictory. To clarify this we have constructed a statistical profile of Seyfert galaxy companions in the CfA ZCAT. We use a random sampling technique to estimate the statistical significance of the observed excess of Seyfert companions relative to the rate of pairing found in non-active galaxies. We find the excess of close pairs for Seyfert galaxies to be highly significant.

RESULTS

More than a decade ago, the morphological similarities between patterns seen in Seyfert galaxies and those produced by gravitational forcing led to the suggestion that Seyfert activity might be fueled by material inflow induced by either a central bar or a perturbing companion (Simkin, Su, and Schwarz 1980). The recent literature on the prevalence of Seyfert galaxy companions, however, is somewhat confusing and, at initial glance, contradictory (cf. the extensive review in Osterbrock 1991). Most authors have attributed their disparate conclusions to observational selection effects, (op. cit.). The problems arise because to obtain a valid statistical profile of the non-AGN “control” galaxies requires either an enormous observational effort or a series of ad hoc assumptions which differ with each study.

An alternative approach to this statistical problem is to draw the Seyfert galaxy sample from a larger catalog of galaxies all subject to the same measurement errors and selection effects. With this approach, the non-Seyfert galaxies provide the statistical profile necessary for interpretation.

ABSTRACT

I describe studies of the incidence of nuclear activity and star-formation rates for galaxies in two paired samples, as functions of the encounter direction and kinematic properties of the disturbed disk. One sample, designed to test star-formation diagnostics, is a geometrically derived subset of the Karachentsev catalog. A separate sample, of paired Seyferts, is used to search for common kinematic characteristics among interactiontriggered AGN. Both starbursts and Seyfert nuclei occur with about equal frequency in direct and retrograde encounters. Nuclear and disk star formation are correlated with the form of the velocity curve, and with the normalized amplitude of velocity disturbance in the disk. Seyfert nuclei in pairs show a high fraction of galaxies with large solid-body regions in the rotation curves. Such kinematic properties are associated with higher then normal star-formation rates in the K-pair spirals (but not the highest). For both AGN and starformation processes, the theoretical scheme most nearly accounting for the observations gives a prominent role to a Toomre-style disk instability on large scales, perhaps driving more local processes such as cloud collisions or pressure-induced cloud collapse. Including kinematic information offers a more refined way to identify externally-triggered phenomena than do disturbed morphology or presence of companions alone.

ABSTRACT

Preliminary results of a search for gas transfer between the galaxies in E+S pairs are presented. In the pair considered K542, ionized gas is detected in the nucleus of the elliptical but no gas in the process of being transferred between the two components has been found.

INTRODUCTION

In the past ten years observations of the ionized (Phillips et al. 1986) and HI (Knapp et al. 1985) gas content of early type galaxies have shown that 50-60% of these galaxies contain a modest amount of interstellar gas. On the basis of the H I data, Knapp and coworkers have suggested that the interstellar gas in ellipticals may be of external origin. At least two external sources for the gas have been suggested: the capture of small gas-rich companions and accretion from nearby gas-rich galaxies. Mixed morphology E+S pairs make an ideal laboratory to test the second hypothesis, since the presumed external source for the gas is relatively unambigous. Two possible strategies exist to test the hypothesis, 1) define a sample of E+S pairs and a control sample of isolated ellipticals and compare the global gas content of the two samples, or 2) examine a sample of E+S pairs that are likely candidates to be currently crossfueling and attempt to catch them in the act. We have chosen the second strategy, since optical spectra had already been obtained for several E+S pairs.

ABSTRACT

I briefly discuss several specific issues regarding the possible inter-relationships between starbursts, quasars, and their extranuclear environments. First, I will argue that the case for fueling starbursts from the extranuclear environment is very strong. The luminosities of extreme starbursts are so large that they essentially require the complete conversion of a galaxy's interstellar medium into massive stars within a single dynamical time. Such starbursts should make good local laboratories for studying the processes involved in galaxy (spheroid) formation. Next, I will discuss the recent proposal by Terlevich and Boyle that the population of high-redshift radio-quiet quasars can be understood as the post-starburst-cores of young/proto elliptical galaxies (with no supermassive black holes required), and will argue that it has a serious energetics problem. Finally, I will describe the effects that the mechanical energy released by starbursts (and possibly quasars) has on their gaseous environments.

INTRODUCTION

The subject I have been asked to review — the complex inter-connections between starbursts, quasars, and their surrounding environment — is far too broad in scope to adequately summarize here in its entirety. I will therefore restrict my review to discussing some specific topics, all pertaining in some way to two general issues.

The first general issue concerns the connection between starbursts and quasars. How can we tell starbursts and quasars apart? Is there any causal or evolutionary connection between them? These issues have been recently reviewed by several different authors (Blandford 1992; Filippenko 1993; Heckman 1987, 1991). The second general issue concerns the two-way communication between starbursts or quasars and their environments.

ABSTRACT

The evolution of the gas distribution in a globally unstable galactic disk embedded in a ‘live’ halo is studied numerically on scales ∼ 100 pc –10 kpc. The gas and stars are evolved using a 3D hybrid SPH/N-body code and gravitational interactions are fully accounted for. The gas is assumed to obey the isothermal equation of state with T – 104 K. The effect of a massive object at the disk center is simulated by placing a ‘seed’ black hole (BH) of 5 x 107 M⊙ with an ‘accretion’ radius of 20 pc. Modifications introduced by star formation in the disk are discussed elsewhere (Heller and Shlosman, these proceedings).

We find that the global stability of a stellar disk can be heavily affected by the gas, given that the gas mass fraction fg is high enough and the gas is dissipative. We also find that the rate of radial inflow in disk galaxies is a robust function of global parameters: the inflow is bar-driven for small fg and dynamical friction-driven for largefg. Without star formation the radial inflows lead to (1) domination of the central kpc by a few massive clouds that evolve into a single object via a cloud binary system; and (2) sporadic accretion onto the BH.

ABSTRACT

We have measured the line-of-sight velocity profiles of M32. The major axis velocity profiles are asymmetric, with opposite asymmetry on opposite sides of the nucleus. Existing models for M32 cannot account for these asymmetries. We present new models which assume the distribution function to be of the form f = f(E, Lz). Such models require a central black hole of ∼ 1.8 × 106 M⊙ to fit the observed rotation velocities and velocity dispersions. Without invoking any further free parameters, these models provide a good fit to the observed velocity profile asymmetries.

OBSERVED VELOCITY PROFILES

The presence of a massive black hole has been invoked to match the observed rotation velocities and velocity dispersions at the center of M32 (Tonry 1987; Richstone, Bower and Dressier 1990). Previous studies have assumed the line-of-sight velocity distributions of the stars, henceforth referred to as the velocity profiles, to be Gaussian. We have determined the velocity profile shapes of M32 from high S/N spectra taken with the William Herschel Telescope at La Palma (van der Marel et al. 1993), using the techniques of Rix and White (1992) and van der Marel and Franx (1993). The velocity profiles are asymmetric, with the asymmetry changing sign upon going from one side of the nucleus to the other (see Fig. 1). None of the existing models, in which the local (unprojected) velocity distributions of the stars are assumed to be Gaussian, can reproduce the observed asymmetries of the velocity profiles.

ABSTRACT

Radial gas flows can be induced in a galactic disk by a bar potential, and its implied gravitational torques. All gas inside corotation is driven towards the center, and forms a nuclear ring at the inner Lindblad resonance (ILR). When the mass concentration is high enough, there exists two ILRs, and the existence of periodic orbits perpendicular to the bar makes the gas response to shift in phase with respect to the stellar main bar. This produces a strong torque on the gas, and drives a rapid nuclear gas flow inside ILR. With a more viscous gas, however, a second bar of stars and gas can decouple from the primary one, with a higher pattern speed. In this “bar within bar” configuration, the gas is in phase with the stellar component, and the gravity torques are minimised. The gas inside the second corotation flows slowly inwards. The nuclear bar is relatively long- lived, which explains its frequent occurence in observed barred spirals.

NUCLEAR BARS AND THEIR POSSIBLE INTERPRETATIONS

Bars are the way to redistribute angular momentum in a galaxy, and to reshape the mass distribution. The induced gas flow towards the center is the cause of star-bursts, hot spots in nuclear rings, and may be of nuclear activity. A clue to the detailed mechanisms of the central gas flow is the observation of nuclear bars and central isophote twists. This phenomenon in barred spiral galaxies has been observed for a long time. Already de Vaucouleurs (1974) had noticed bars within bars, and Sandage and Brucato (1979) high surface brightness nuclear bars, as independent entities.

ABSTRACT

We present details of galaxy morphology in the compact group Seyfert's Sextet obtained with the HST-WFC.

INTRODUCTION

Seyfert's Sextet (SS, HCG 79) exhibits one of the highest galaxy surface density enhancements (> 103, Sulentic 1987) outside the core of a rich galaxy cluster. There are convincing signs of interaction between the component galaxies, including: 1) optical evidence of bridges, tails and a common low light level envelope (Sulentic and Lorre 1983), and 2) a distorted distribution of neutral hydrogen (Williams et al. 1991).

HST OBSERVATION AND DECONVOLUTION

SS was observed with the WFC on 12 May 1992. Nine 15-minute exposures were taken with the F439W (B) filter and processed with the standard STScI pipeline. The frames were accurately registered (to within 1 pixel), allowing us to combine them into a single averaged picture. Cosmic rays were removed by ignoring pixels which were significantly (3σ) deviant from the corresponding pixels on other frames.

Deconvolution was performed using both the Richardson-Lucy package bundled in IRAF/STSDAS (refer to the STSDAS User Guide), and the σ-CLEAN algorithm (Keel 1991), optimized to change the point-spread function (PSF) according to position on the chip. PSFs were computed using the Tiny Tim (refer to The Tiny Tim User's Manual) package. A 50 iteration deconvolution was carried out with the Lucy package. After 80,000 iterations with CLEAN the model brightness distribution showed little change. Each of these algorithms presents strong and weak points. Lucy handled very well large structures (low spatial frequencies), such as the halo connecting the galaxies. However, the final brightness distribution seemed to be dependent on the number of iterations used.