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.

ABSTRACT

Many elliptical galaxies contain cold gas of external origin, which often forms a warped disk or ring. Recent observations of the atomic and the molecular gas in nearby ellipticals have shed considerable light on the properties of this accreted material. Resulting progress in understanding the details of the settling process are discussed, with attention to questions such as: Are any of the observed gas disks settled? How do the settling times and the observed properties of the gas disks depend on the structure and shape of the galaxy in which it resides? How much mass is delivered to the nucleus?

INTRODUCTION

Many elliptical galaxies contain significant amounts of cold gas. This is often detectable by its Hα emission which sometimes extends to 3–5 kpc from the center and appears as a warped disk (Demoulin–Ulrich, Butcher and Boksenberg 1984; Trinchieri and di Serego Alighieri 1991; Shields 1991; Macchetto and Sparks 1992; Buson et al. 1993). When seen nearly edge–on, such disks are seen as a dust lane across the image of the galaxy (Bertola and Galletta 1978; Hawarden et al. 1981; Ebneter and Balick 1985; Sadler and Gerhard 1985; Bertola 1987). These disks contain 103–105 M⊙ of ionized gas (Phillips etal 1986), 104–106 M⊙ of dust (Forbes 1991), and 106–108 M⊙ of neutral gas (Lees et al. 1991; Bregman, Hogg, and Roberts 1992). Large amounts of neutral hydrogen extending to tens of kpc have been found in only a half–dozen elliptical galaxies (van Gorkom 1992).

ABSTRACT

From the preliminary analysis of a sample of ≃ 600 galaxies with bj ≤ 19.4, and spanning a redshift interval up to z ≃ 0.3, we deduce that star formation per unit luminosity, as indicated by the OII 3727 line equivalent width, is a function of galaxy luminosity (decreasing at increasing luminosities), redshift (increasing at increasing redshift, or, physically, look–back time) and environment.

INTRODUCTION

At the ESO 3.6m telescope at La Silla, we are currently performing a redshift survey of galaxies with bj ≤ 19.4, in a rectangular area ≃ 22° × 1° (plus a nearby area of ≃ 5° × 1°) in a region around the South Galactic Pole. Up to now we have acccumulated spectra for ≃ 2,000 galaxies over ≃ 70% of the area.

The distribution in distance of the survey galaxies exibits significative peaks above the expectation at D ≃ 180h-1 Mpc and D ≃ 300h-1 Mpc. These peaks correspond to large scale structures extending over a significant fraction of the strip.

A large fraction of galaxies (≃ 40%) shows the presence of one or more emission lines (OII λ3727, Hβ, OIII λλ4959,5007). These objects can be either spiral galaxies, where lines originate mostly from HII regions in the disks, or galaxies undergoing a significant burst of star formation. The observed peaks in the galaxy distribution are much less pronounced when only emission line galaxies are considered. This suggests that either spiral galaxies are less frequent in the densest regions, thus confirming a large scale validity of the well known morphology–density relation, or starburst phenomena in galaxies occur preferentially in low density environments, or both.

ABSTRACT

The nuclear star formation rates have been measured for a complete sample of 156 spiral galaxies. The nuclear star formation rates of barred and unbarred galaxies are compared separately for early and late Hubble types. Significant differences are observed and critically evaluated within the context of a possible causal relationship between stellar bars and nuclear starbursts.

INTRODUCTION

Considerable interest surrounds the possible association between nuclear activity and galaxy morphology (Sersic and Pastoriza 1967; Heckman 1978; Simkin, Su, and Schwarz 1980). Indeed, the unambiguous association with a morphological feature, such as a stellar bar for example, may provide an important clue as to the origin of the activity. A number of studies have demonstrated the frequency of nuclear star formation to be enhanced in the nuclei of barred spiral galaxies. Heckman (1980) demonstrated a higher incidence of HII regions in the nuclei of barred spirals and Balzano (1983) noted an excess of barred spirals in an optically selected sample of starburst galaxies. The results discussed in this contribution, however, I believe are the first to demonstrate that the high mass star formation rate is enhanced in the nuclei of barred spirals when compared to unbarred spirals.

THE SAMPLE

A complete sample of 227 nearby (15 ≤ D)[Mpc] ≤ 40), infrared luminous, L(40 – 120 μm) ≥ 3 × 109 L⊙ spiral galaxies were selected upon comparison of the Infrared Astronomical Satellite (IRAS) Point Source Catalog with the Nearby Galaxies Catalog (Tully 1988).

ABSTRACT

Low-ionization broad absorption line quasars, also known as Mg II BAL QSOs, are intriguing objects that might represent an evolutionary link between the infrared-luminous aftermaths of galaxy-galaxy collisions and the normal, unshrouded quasars thought by some to result from such collisions. This contribution briefly summarizes Mg II BAL QSO phenomenology, sketches the results of a more detailed analysis of Mg II BAL QSO spectra, and estimates the prodigious kinetic energy outputs of these objects. It appears likely that the kinetic luminosities of these quasars exceed 10% of their radiative luminosities.

INTRODUCTION TO MG II BAL QSOS

Broad absorption-line quasars (BAL QSOs), comprising ∼ 10% of all opticallyselected radio-quiet quasars, have puzzled astronomers for over two decades. Broad, blueshifted absorption features in UV resonance lines like CIV, N V, Si IV, and Lyα show that these quasars are ejecting 104 K material at velocities of up to 0.1c. In the spirit of this conference, we could call the BAL QSO phenomenon “Activity-Induced Mass Transfer in Galaxies”.

We still do not understand how these quasars accelerate the absorbing material to such high velocities. Even though BAL outflows could be telling us something important about the innards of active galactic nuclei, BAL QSOs have been somewhat neglected in recent years, partly because we lack a basic understanding of the acceleration mechanism. The field has progressed more by ruling out possible acceleration mechanisms than by identifying schemes that might work. Weymann, Turnshek, and Christiansen (1985) and Begelman, de Kool, and Sikora (1991) provide particularly insightful analyses of the problems involved in accelerating BAL material.