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.

ABSTRACT

We have studied the effects on line emission due to the reprocessing of AGN continuum by cool (T ≲ 104 K) stellar winds from stars, such as those from red giants, expected to be present in the region (Fabian 1979; Penston, 1988; Norman and Scoville 1988; Kazanas 1989). Using model stellar phase space distribution functions appropriate for stellar clusters with black holes, we have calculated the expected covering factors, line profiles, and two dimensional line transfer functions of these systems. Our stellar models contain between 107 and 108 stars within a parsec from the central source, of which we assume a fraction are able to reprocess continuum radiation. The line profiles of our models have FWHM up to 5,000 km sec-1 for black hole masses between 107 and 108 solar masses and covering factors up to approximately 10-1. The line transfer functions we obtain peak at lags from 5 to 200 days for optically thick clouds. Forbidden line emission has also been studied for this effect. For some of our models, the wind line emission has been found to be quite significant.

INTRODUCTION

Active galactic nuclei (AGN) have very high stellar densities. That the effects of stars in the study of AGN has been largely ignored is somewhat surprising. In addition to the stellar densities, the X-ray continuum radiation is also quite high in AGN. We have calculated the magnitude and observational characteristics of line emission from reprocessing of the central continuum radiation of stellar winds of stars in AGN.

ABSTRACT

The fact of starburst activity in a non-negligible fraction of galaxies implies a refuelling mechanism, since the star formation rates observed are sufficient to exhaust the gas in situ over relatively short timescales. A number of “gas bars” – elongated structures in which molecular gas is observed to flow in markedly non-circular orbits – has been observed in galaxies with circumnuclear star-forming activity. In this paper I discuss whether the observed properties of these flows fit the dynamical models which have been proposed for refuelling starbursts, concluding that they may well do so, but not according to some rather over-simplified previously postulated scenarios.

INTRODUCTION

Observations during the past five years of molecular gas in external galaxies on scales of order 100 pc, made possible by the commissioning of millimetre wave interferometers, have virtually put within our research the solution to the problem of how starbursts are refuelled. The need for refuelling is clear since in the majority of cases the observed star-formation rates, of order 0.1 to 1 M⊙ per year, would exhaust the molecular gas observed within 100 pc of the nucleus in times between 108 and 107 years. On the other hand the observed association of starbursts and also Seyferts with the presence of bars in spirals, implies a causal link which would be satisfied naturally by the property of a bar to brake the rotational motion of interstellar gas, and thus allow it to flow down the gravitational potential gradient towards the nucleus. These considerations have led the molecular observers to search for evidence of the phenomenology of molecular inflow, with some success as I describe below.

ABSTRACT

The kinematics of ionized gas in active (and normal) galaxies is reviewed. For clarity, discussion is divided first by emission region size, and then by galaxy type. Although a wide range of velocity fields are encountered on all scales, a number of recent developments are stressed : large scale outflows in Seyfert and Starburst galaxies (1–20 kpc); gravitationally dominated motion on intermediate scales in all galaxies (few × 102 pc); radial flows of uncertain direction on small scales in Seyferts (3 – 100 pc); and possible continuity of velocity field in Seyferts down to very small scales (≲0.1 pc, BLR).

INTRODUCTION

Ionized gas can be found on many scales in both active and normal galaxies — on large scales ∼ 1 – 20 kpc in the body and near environment of the host galaxy; on intermediate scales ∼ 0.1 – 1 kpc in the bulge dominated regions; on small scales ∼ 3 – 100 pc in the inner bulge cores; and on very small scales ∼ 0.01 – 1 pc within the Broad Line Region (BLR). My intention is to review observations which shed light on the kinematics of this ionized gas, recognizing that the velocity fields on one scale may be quite unrelated to those on another.

Studies of ionized gas kinematics focus, of course, on emission lines and their Doppler profiles. First order information comes from the profile center and width, while higher order information comes from profile asymmetry, kurtosis, and substructure, as well as the comparison of these for different emission lines.

ABSTRACT

Strong central starbursts occur most frequently in early-type disk galaxies, especially barred galaxies. We have taken U, B, R, I, and Ha images and long slit nuclear spectra of eight galaxies of type SO/SBO to Sa/SBa with bright central starbursts, selected on the basis of IRAS colors and fluxes. Four of the galaxies have circumnuclear ring-like distributions of hot stars and gas. Some or all of these four galaxies may be cases of barred galaxies with inner Lindblad resonances (ILRs), in which gas has been channelled into a ring at an ILR and compressed into star formation. Another galaxy is clearly interacting with a companion Seyfert galaxy. The strong interaction has apparently lead to the producion of the bright nuclear and near-nuclear emitting regions and the complex of fainter star-forming clumps and filaments surrounding them. Two of the remaining galaxies have compact nuclear starbursts. We conjecture that these galaxies may not have ILRs, so that gas perturbed by a bar potential or by a companion has accumulated at the nucleus instead of in a ring. The final galaxy appears to have a very bright off-center clump of star formation, conspicuous at all wavelengths. Although the possibility of a centered but dust-obscured burst is discussed, the apparent asymmetry may be real and in need of explanation.

INTRODUCTION

Strong central starbursts are occurring in a significant minority of S0 and early-type spiral galaxies. The IRAS survey data can be used reliably to find complete samples of these galaxies, since on-going bursts produce far infrared radiation that is warmer than that produced by disks.