ABSTRACT

The infrared luminous galaxy MK 231 appears to exhibit characteristics similar to those of active galactic nuclei — it has been classified as a Seyfert 1 system. However, it has been shown to contain 3 ×x 1010 M⊙ of gas via CO observations. If the CO is confined to the region shielded by dust as in galactic molecular clouds, the molecular gas occupies a much smaller volume than previously thought. The ultimate questions are which characteristic is primarily responsible for most of the luminosity and whether AGN and starbursts are interdependent or coincidental.

THE SUPERGIANT MOLECULAR CLOUD

Scoville et al. (1989) and Radford et al. (1991) have discussed the physical conditions implied by the molecular observations of MK 231. It is inferred from CO observations, that 3 × 1010 M⊙ of molecular gas (H2) resides within a volume of radius RCo < 3 kpc. However, recent infrared observations reveal that the emission from dust at λ ≃ 10 μm arises within a volume of size RIR < 400 pc (Keto et al. 1992). This is an important complementary result to the CO observations because studies within our galaxy show clearly that the molecular gas is largely confined to the region shielded by dust extinction (Young et al. 1982).

ABSTRACT

It has been claimed that spherically symmetric, isothermal (ρ ∼ r-2) halos of invisible material surround certain galaxies and extend to between ≈ 40 kpc (Rubin et al. 1985) and ≈ 1 Mpc (Charlton and Salpeter 1991) from their centers. In this work in progress we consider the tidal effects due to the presence of such halos in binary galaxies. Having investigated the predicted frequency of tidal distortions we then compare with binary galaxy surveys to search for evidence of these effects.

TIDAL RADIUS

A tidal radius was calculated based upon a simplified model of a binary galaxy system. Each member of the system is identical and in a circular orbit about the system's center of mass. The dark matter halos extend to the point where they just begin to overlap and the visible disk of each galaxy, despite rotating on its axis, is taken to be approximately spherically symmetric. Furthermore, the rotational angular momentum vector of the material contained in each visible disk is taken to be parallel to the orbital angular momentum vector for the entire system. This information was used to determine the radial equation of motion for a test particle located at the outer edge of either visible disk and from this equation the tidal radius is obtained (≈ 40 kpc). The tidal radius is the separation between members of the binary system at which the test particle just begins to leave the visible disk.

In previous chapters, we have studied the behavior of charged particlesi using various approximations, in given electric and magnetic field configurations. For instance, we have studied waves in a medium that is assumed to be homogeneous and static. In reality, an astrophysical plasma (such as the solar wind) is neither homogeneous nor static. Furthermore, we have only incomplete information about the structure and time-evolution of the medium. Another important class of problems involves systems that are mildly unstable, in which perturbations grow from some initial small level to a finite but significant level. An example of such a situation is provided by the study of the growth of plasma oscillations in the quasilinear approximation (see, for instance, Drummond and Pines, 1962, or Nicholson, 1983).

In problems such as these, we must develop procedures for representing and perhaps calculating the fluctuations that arise in a plasma. In many cases, we can conveniently represent these fluctuations as a superposition of waves (normal modes) in the plasma. However, the individual particles in such a plasma may behave in a manner that differs in important respects from the behavior of particles in the simpler situations that we have studied so far.

In discussing collision theory, we took account of the fact that a plasma is highly random at the microscopic level that is relevant for the discussion of binary collisions, etc. We now introduce the concept that the plasma may also be random on a macroscopic level, and we study the implications of such randomness.

ABSTRACT

I discuss the various processes which may affect the transfer of angular momentum in gas within the inner parsec surrounding a supermassive black hole. Even after gas has been brought into the gravitational sphere of influence of the black hole, it still has 100–1000 times too much angular momentum to reach the event horizon. Angular momentum loss cannot be accommodated in a scaled-up standard thin accretion disk model, because local self-gravitational instabilities will lead to fragmentation of the disk, which will decrease the efficiency of angular momentum transport. Instead, some nonlocal mechanism or “external” trigger for angular momentum transfer is needed, such as a large-scale magnetized wind, stirring by winds from massive stars and supernovae, or global gravitational instabilities.

INTRODUCTION

Most participants in this conference seem to agree that large-scale, non-axisym-metric gravitational disturbances can be very effective in transferring angular momentum on scales of hundreds of parsecs or larger. These disturbances might be driven by tidal encounters or may be “self-starting”, as in the “bars-in-bars” scenario. What is less certain is whether similar mechanisms can be effective all the way into the nucleus, say, into the inner parsec surrounding a supermassive black hole. Gravitational triggers for inflow usually involve at least a mild form of self-gravitational instability, which requires that the stars plus gas constitute a significant fraction of the total mass enclosed within the region under consideration (see, e.g., Friedli, these proceedings).

ABSTRACT

The apparent low covering factor of the narrow line region (NLR), the nearly “empty” intermediate region between the broad line region (BLR) and the NLR, and the size of the BLR, are all naturally explained if dust is embedded in the narrow line emitting gas. Such dust, together with the observed blue-excess asymmetry of the narrow line profiles, imply that the NLR gas has a net inflow motion.

THE BASIC QUESTIONS

In this paper we address the following three questions: 1) Why does RBLR ∼ 0.1 L½46 pc? This scaling is indicated by the remarkably similar emission spectra of AGNs over a very large luminosity range. 2) What is the actual covering factor (C) of the cold gas in the NLR? Photon counting using the Hβ recombination line indicates C ∼ 2%, assuming optically thick clouds. The 3 – 30 μm IR continuum, if due to dust reprocessing of the UV continuum, indicates C ∼ 20 – 40%. High resolution imagings of the NLR by the HST in nearby AGNs indicate, if the clouds are resolved, C ≥ 20%. 3). Why is there an apparent gap in the gas distribution between the BLR and the NLR? A gap is indicated most clearly in Seyfert 1.5 galaxies and some quasars where the permitted line profiles show distinct narrow and broad components.

ABSTRACT

We used optical spectra to investigate the nuclear regions of the starburst galaxies NGC 2782, NGC 4102 and NGC 6764. In addition to the central starburst, we find evidence for extranuclear shock-ionised gas, with kinematical properties consistent with outflow along the minor axis. The observations are consistent with the presence of dense shockionised shells, formed by the starburst-driven winds. The shells, observed in NGC 2782, NGC 4102 and NGC 6764 respectively, appear to be in a different evolutionary phase, which is explained by differences in age and strength of the central starbursts.

INTRODUCTION

Evolutionary models of starburst nuclei predict the formation of a thin, dense, shock-ionised shell, surrounding a hot cavity (e.g. Tomisaka and Ikeuchi 1988). The shell expands predominantly perpendicular to the galactic plane, gradually elongating until it finally breaks open at the top. The nearby starburst galaxies NGC 253 and M82 are probably examples of the broken-shell phase.

We investigated the starbust galaxies NGC 2782, NGC 4102 and NGC 6764 with the aid of long-slit spectra with high spatial resolution. All three galaxies have been classified previously as starburst galaxies, but the optical line ratios derived from 1-dimensional spectra put the galaxies close to the borderline between starbursts and LINERs in diagnostic diagrams.

RESULTS

From the spatial behaviour of the line ratios in our spectra we find, that the high line ratios are due to the presence of an extranuclear high-ionisation component in addition to the central starburst. The line ratios of the high-ionisation component are in agreement with shock-ionisation; its kinematical properties are consistent with outflow along the minor axis.

ABSTRACT

Theoretical studies of gas dynamics in disk galaxies are reviewed in relation to the fueling of nuclear activity. Importance of self-gravitational effects in the interstellar gas component is emphasized.

FUELING PROBLEM

Recent observations have revealed that some galaxies show an unusual level of activity near the nuclear regions. One type of activity is the enhanced star formation in the central few kpc regions around galaxy nuclei. Another is the non-thermal activity which originates from sub-parsec region, which is called active galactic nuclei (AGN). This type of activity is believed to be powered by mass accretion onto supermassive black holes located in the galactic nuclei (e.g., Begelman et al. 1984). Both types of activity require adequate source of fuel. In typical AGNs, a gas supply rate of ∼ 1 M0 yr-1 is needed to generate the radiation of the observed amount. CO line observations (e.g., Kenney 1990) have detected large accumulation of a potential fuel in the form of molecular gas in the central kpc in many starburst galaxies.

Possible candidate sources of fuel are divided into local and global ones according to their spatial distribution. The difficulties for local sources such as compact star clusters have been pointed out by several works (e.g., Shlosman et al. 1990). In this article we concentrate on global sources, especially the interstellar gas in the galactic disks. Because interstellar gas is distributed on the 10 kpc scale, one problem is how the material can be channelled into the nuclear region with a sufficiently high efficiency to maintain the observed level of activity.

ABSTRACT

Galaxies with elevated metabolic rates get energy from their gaseous food by extracting its nuclear energy (in stars), and its gravitational energy (via accretion onto massive black holes). There is strong evidence that interactions with other galaxies trigger star formation activity, and weaker evidence that it triggers black hole accretion (nuclear activity). We review the processes by which interactions can remove angular momentum from gas, particularly gravitational torques and the m = 2, m = 1, Jeans and fission instabilities that give rise to them. There is ample evidence, both theoretical and observational, that these can remove enough angular momentum to move much of a galaxy's gas from ∼ 3 kpc to ∼ 300 pc. This is still many decades from the ∼ 10-5 pc scales of stars and black hole horizons. We discuss star formation, the interpretation of simulations, and cosmological implications. The evolution of binary supermassive black holes, and the problem of forming a dense (≲ 1 pc) nuclear star cluster are examined.

WHAT IS MASS-TRANSFER INDUCED ACTIVITY IN GALAXIES?

Before this conference, I wasn't sure. After this conference, I am sure I am not sure. Let me nevertheless attempt a definition, starting from the easy end, the back of the phrase. Galaxies are of course the island universes within which reside most of the stars, much of the gas, and a little of the mass in the cosmos. Activity in Galaxies, like that in animals, is defined by the metabolic rate. When this is well above the average ‘resting’ (aka. basal) level, a galaxy or animal is said to be active.

ABSTRACT

We discuss the results from a study in progress of radio–loud IRAS galaxies. We have discovered a class of gas-rich AGN which are characterized by large IR luminosities, and which are intermediate in radio luminosity, IR colour, and optical spectral class between the IRAS ultraluminous galaxies and powerful quasars. These objects form an important sample in studies of the AGN-starburst connection and may be the evolutionary link between the ultraluminous galaxies and quasars.

INTRODUCTION

The relationship between nuclear activity and galaxy evolution is poorly understood. Speculation on the subject ranges between ideas that Seyfert activity is due to nuclear starbursts, to the theory that the ultraluminous (LFIR ζ 1012 L⊙) far-infrared galaxies like Arp 220 are dust enshrouded, young quasars (Sanders et al. 1988). Unfortunately there exists no sample of active galaxies that can be used to test these various hypotheses since most samples used in these studies are selected either on the basis of optical emission-line equivalent width, or IRAS colour selection criteria, IRAS luminosity, etc. There is no ideal sample, but a good way of studying possible nuclear activity/evolution relationships is to select gas-rich galaxies with nuclear activity. We have selected sources from the IRAS survey (which is biased toward dusty, gas-rich objects) by using a completely independent indicator of galaxy activity: strong non-thermal radio emission. This sample of gas-rich radio galaxies also allows us to investigate the complex relationships between the ambient interstellar medium, the radio source, and the active nucleus.

ABSTRACT

We have studied the luminosity function (LF) of H II regions in the disk of the Seyfert 1 galaxy NGC 6814. We find that the LF is very similar to LFs of other late-type, not necessarily active, spiral galaxies. Although the Seyfert nucleus shows its character by emitting strongly in Hα, the disk H II regions seem not to be influenced by the active nucleus.

NGC 6814 is an Sbc galaxy with well-defined spiral arms, of type Seyfert 1. Because of the strong X-ray emission, it is considered a key object for understanding nuclear activity.

We have obtained new Hα observations with the 4.2m William Herschel Telescope (WHT) on La Palma, using the TAURUS instrument in imaging mode. The final continuum subtracted Hα image has high sensitivity (H II region detection limit is L = 1036.9 erg s-1) and resolution (0.8 arcsec, or about 100 pc at the distance of NGC 6814). From the image, we have measured positions, diameters and fluxes of a total number of 735 H II regions (Knapen et al. 1993). We found that the nucleus is a strong Hα emitter, of luminosity L = 1039.9 erg s-1.

From our catalog of H II regions we constructed a luminosity function (LF), for all the H II regions in the disk of the galaxy, and for arm and interarm H II regions separately. Figure 1 shows the total LF. The slope of the LF is a = –2.37 ± 0.09, well within the range of slopes measured in the literature for galaxies of similar morphological type. The arm and interarm LF slopes are equal within the fitting uncertainties.

ABSTRACT Ring galaxies are commonly known as objects where a burst of star formation was triggered by a close encounter with an intruder. Here we present a self-consistent approach to reproduce the observed morphological and photometric behaviour of a sample of ring galaxies using updated N-body simulations and evolutionary synthesis model from UV to far-IR wavelengths.

INTRODUCTION

Galaxies were selected from the list of Appleton and Struck-Marcell (1987). BVRI CCD observations of seven ring galaxies were carried out at Padova-Asiago Observatory using a GEC CCD with a pixel size of 22 μm, corresponding to 0.28 arcsec. Fluxes have been translated to the standard BVRI Johnson's system. In the following we briefly outline fundamentals of our models and summarize our results in section 2. Far-IR (FIR) data come from IRAS catalogue (Version 2).

N-Body Simulations

We performed numerical simulations of collisions between stellar disks (embedded in static massive halos) and suitable intruders. The code used is the Hernquist's (1987) TREECODE. 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. These numerical experiments represent something new with respect to the previous work on the formation of ring structures (Appleton and James 1990) because of the careful and realistic production of the disk target (for details see Curir et al., these proceedings). The passage of the intruder through the disk generates a transient ring-shaped mass distribution. The ring is produced by a single density wave propagating through the disk.

ABSTRACT

Very dense clusters or massive black holes (MBH) located in the nuclei of disc galaxies generate inner Lindblad resonances (ILR) and chaos which then strongly modify the global dynamics. MBH with mass Mbh ≲ Mbhlim ≃ 0.02Md, where MD is the total stellar disc mass, round the central isodensity contours of barred galaxies, and the bar strength decreases with increasing Mbh. Near the horizontal ILR, gas can form a nuclear ring where star formation takes place and reduces the bar-driven gas inflow. Initial MBH with Mbh ≳ Mbhlim are sufficient to prevent an otherwise forming bar. If they slowly increase or are added in barred potentials, they dissolve the bar as soon as Mbh ≳ Mbhlim, stopping the large scale gas fueling. MBH candidates for the upper end of the black hole mass function must preferably be searched in SO, Sa galaxies, some of them being turned-off quasars.

INTRODUCTION

Observations of nearby galaxies indicate non negligible (dark) mass in their nuclei (see e.g. Kormendy 1992) interpreted either as very dense clusters or MBH. The latter hypothesis is sustained by the widespread idea that MBH can be the engine powering AGN. However, there are less AGN at present time than at high redshifts z and it remains to be understood why some AGN are now turned-off. Whereas the influence of MBH on the local dynamics is indisputable, their effects on the global dynamics of multi-component systems remain to be explored in a self-consistent way.

ABSTRACT

We report on an analysis of a sample of objects observed to have a correlation between FWHM and critical density for de-excitation of forbidden emission lines. Included in the sample is a QSO of MV ≈ – 26 which is the most luminous object to date known to show such a relationship. In addition, this QSO shows that the semi-forbidden lines which arise in the Broad Line Region (BLR) follow the same trend seen in the more distant Narrow Line Region (NLR), suggesting that a smooth velocity field joins both zones. A similar relationship between linewidth and critical density holds for all objects in the sample despite a range of ≈ 5000:1 in luminosity.

INTRODUCTION

Previous studies of the velocity field in NLRs of low redshift AGNs suggested that the physical processes are at work in all objects (Fillipenko 1985). We have extended the early work by obtaining a sample of objects from the literature which show a correlation between linewidth and critical density and supplemented these with data for a QSO recently discovered to show the first direct evidence for such a relationship extending into the BLR itself.

ABSTRACT

Possible consequences of the dynamics of interstellar gas in merging galaxies are discussed and illustrated with numerical simulations which incorporate both collisionless and hydrbdynamical evolution.

INTRODUCTION

Over the past 20 years, considerable observational evidence has accumulated implying that “major” mergers of comparable–mass spirals may play an important role in the evolution of galaxies. Based on statistics on well–known merger candidates, Toomre and Toomre (1972) and later Toomre (1977) argued that major mergers are the dominant process by which early–type galaxies form. More recently, infrared and radio surveys have bolstered long–standing suspicions that nuclear starbursts in some peculiar objects may have been triggered by mergers of gas–rich progenitors [e.g. Sanders et al. 1988a, b; Sanders 1992). Only slightly less compelling are observations implicating galaxy collisions to the onset of activity in bright radio galaxies (e.g. Heckman et al. 1986) and quasars (e.g. Stockton 1990). (For a review, see Barnes and Hernquist 1992a.)

By now, the stellar–dynamics of major mergers have been explored in some detail using N-body simulation (e.g. Barnes 1988, 1992; Hernquist 1992, 1993a); however studies of the hydrodynamical evolution of interstellar gas during these events are less fully developed. Seminal works include simplified calculations, which ignore self–consistency, showing that bars in disks can drive nuclear inflows of gas (Simkin et al. 1980), low–resolution models investigating the fate of a dissipative component during major mergers (Negroponte and White 1983), simulations of gas inflow during transient collisions between galaxies (e.g. Noguchi 1988, 1991; Combes et al. 1990), and possible consequences of star formation and feedback (e.g. Mihos et al. 1991, 1992).

The galaxy NGC 5506 shows features intermediate between Sy 1 and Sy 2. We observed NGC 5506 with ISIS, at the 4.2m William Herschel Telescope. We obtained spectra at three slit inclinations (0° P.A., parallel to the disk minor axis, 68° and 93° P.A.), in the spectral ranges 4,675–5,475Å and 6,300–7,125Å. The relative proximity of this galaxy (z=0.006) allowed us to resolve the Narrow Line Region along the slit.

The Hα, [NII]6,584 and [0 III]5,007 lines show double peaks at distances larger than 300 pc from the center along 0° P.A. The velocity shift vs. position of the two peaks is plotted in Figure 1. We fitted the velocity curve of these peaks with a model similar to that by Wilson, Baldwin, and Ulvestad (1985) where the double peaked lines are emitted by gas flowing through two aligned and opposite cones; we obtain a flow velocity of 400 km s-1, a cone aperture angle of 80° and an inclination angle of the cone axis with respect to the perpendicular to the line of sight of 13°, very close to galaxy inclination.

The relative intensity of the Hα double peaks and their [NII]/Hα ratio (Fig. 2) can be well explained assuming that the emitting clouds within the cones are outflowing and optically thick to Hα, so that the redshifted clouds (NR and SR), presenting us the illuminated side, show line ratios typical of the high ionization region, while the blueshifted clouds (NB and SB), presenting us the dark side, show line ratios typical of the transition (partially ionized) zone.