ABSTRACT

N-body simulations of M51-system suggest that the companion is currently moving in a highly inclined (75°) but bound orbit (e = 0.25) with respect to the main system. In this model the inner spiral structure as well as the extended outer tail follow from excitation during an earlier disk plane crossing about 400 million years ago, while the most recent crossing occurred less than 100 million years ago.

INTRODUCTION

The M51 system (NGC 5194/5) is an expectionally well observed spiral galaxy with a clear grand design pattern. Several attempts have been made to explain its structure with N-body modelling, starting with the classic test-particle simulations of Toomre and Toomre (1972). Although these and the later self-gravitating models of Hernquist (1990) have been quite successful in reproducing the tidal bridge and tail structures, they usually predict such a short interval since the principal perturbation at disk plane crossing, that it is hard to explain the existence of the strong inner spiral pattern. Moreover, the arm kinks going from the inner structure to the bridge and tail arms strongly suggest a more complicated process. Finally, the recently observed (Rots et al. 1990) large extent hydrogen “far tail” requires serious revisions in existing simulation models. Howard and Byrd (1990) first suggested that all these features could be simultaneously accounted for by assuming a bound companion orbit, with several strong perturbation events at successive disk plane crossing.

N–BODY MODEL

In the present study the multiple encounter model is refined and studied with a fully 3-dimensional self-gravitating code, similar to that used in modeling of Arp 86 pair (Salo and Laurikainen 1993).

ABSTRACT

The interstellar gas plays a fundamental role in the interacting galaxy systems: it is dissipative and thus responds irreversibly to strong dynamical perturbations; it is also the active component which fuels starbursts and may fuel AGNs in the central regions of the galaxies. In this article we review new high resolution aperture synthesis observations of the atomic and molecular gas in luminous interacting galaxies.

Due to the extended nature of the atomic gas in disk galaxies, the HI observations bear most critically on the long range tidal perburbations and therefore serve to elucidate the interaction history. Here we summarize the HI observations of six systems: the M81/M82 group and five interacting pairs with long tidal tails. In all of these systems the impact of geometry is evident from the spatial and kinematic structure of the atomic gas within the extended tidal features. The six systems may be crudely placed in a temporal sequence for galactic merging — in the early stages large amounts of HI still exist within the galactic disks and the star formation is spread throughout the disks as evidenced by Hα emission while in the final stages the HI is almost entirely contained within the tidal features beyond the merger body and the molecular gas is mostly found in the central remnant.

Molecular line observations have largely concentrated on the intermediate evolutionary phase in which the galaxies become strongly emitting in the far infrared. Molecular line surveys of luminous infrared bright galaxies sampled by the IRAS survey have shown these galaxies to be extraordinarily rich in molecular gas as evidenced by the CO emission.

ABSTRACT

Analysis of quasar broad emission lines suggests that the emitting gas is substantially enriched, often well above solar at high redshifts. The abundances are like those expected in the cores of massive galaxies early in their evolution, suggesting that observable quasars occur near the end of the epoch when rapid star formation, dominated by high mass stars, has created an enriched interstellar medium. An increase in the derived metallicities with both redshift and luminosity suggests that there is a mass-metallicity relation among quasars analogous (or identical) to the mass-metallicity relation in elliptical galaxies. This relation is consistent with the most massive quasars and/or host galaxies forming only at high redshifts.

INTRODUCTION

Observations of strong metallic emission lines in quasars out to redshift ∼4.9 imply some enrichment at times when the Universe was less than 10% of its present age. If quasars reside in the cores of massive galaxies, their gaseous environments could easily have larger than solar abundances, even at the highest redshifts. Observations and models of giant elliptical galaxies show that metallicities of at least several Z⊙ are attained in less than 1 Gyr (cf. Arimoto and Yoshii 1987). In the bulge of our own galaxy the stellar metallicities also reach at least a few Z⊙ (Rich 1988) and the enrichment is again believed to occur in ≲ Gyr (Köppen and Arimoto 1990). Since the gas in any evolving star cluster is as chemically enriched as the most recently formed stars, and thus more enriched than the bulk of the stellar population, metallicities above solar may be typical of the gas in massive galactic nuclei.

ABSTRACT

We present preliminar results of radio observations of 78 southern rich clusters, whose brightest member is a dumbbell galaxy or a multiple nucleus. We identified 41 radio sources with the cluster brightest member: 23 of the 44 observed have a multiple nucleus, and 18 of the 34 mapped have a dumbbell galaxy.

INTRODUCTION

In many galaxy clusters, the first–ranked galaxy is not a single isolated object but has two or more components. Such galaxies show a wide range of morphologies, from dumbbell systems (two galaxies of roughly equal brightness (Δm < 1 – 2) inside a common halo) to galaxies with multiple nuclei (two or more condensations visible within the image of a single galactic spheroid, with each secondary nucleus at least two magnitudes fainter than the main system).

It is likely that in some multiple systems the companions are gravitationally bound to the central galaxy (often a cD) and may eventually be cannibalized; while in others we see unbound galaxies whose eccentric orbits in the cluster potential well bring them close to the cluster center where the cD is located (Tonry 1985).

Since many active galaxies are located in dense environments and show signs of interaction, it has often been suggested that gravitational interactions between galaxies may trigger nuclear activity.

ABSTRACT

We are investigating the properties of gas in the elliptical (E) components of E+S pairs. This is being done both by determining the physical conditions of the gas and by spectral synthesis of the underlying stellar population. A major goal is to determine whether cross fueling occurs in such pairs. We present data for AM 0327-285 which is an E+S pair showing signs of tidal interaction between the E and S components. We present evidence for gas transfer from the spiral to the elliptical component.

INTRODUCTION

Paired galaxies as nonequilibrium systems provide a unique chance to study the physics of interaction, galaxy evolution, and the effects of environment on galaxies. If environment at the formation epoch plays a major role on morphology, components of binary galaxies might be expected to show morphological concordance. More recently the formation of elliptical galaxies has been revisited in the light of new observational evidence for fine structure suggesting a “nurture” origin. Therefore, the existence of a large number of true mixed pairs (E+S) (about 25% of an unbiased binary sample are mixed pairs [Sulentic 1990]) raises interesting questions.

We are studying the properties of mixed pairs using low resolution spectroscopy and stellar population synthesis. Such pairs provide a unique insight because they involve one gas rich galaxy in the presence of a relatively clean perturber.

ABSTRACT

I present results of several simulations of a flat self-gravitating disk composed of gas clouds and stars, to show that cloud collisions cause bulge formation.

THE CODE

I have performed a dozen runs using N-body code with a self-gravitating “zerothickness” disk containing stars and/or gas clouds, and imbedded inside a spherical halo. Clouds could either collide or not. I have developed a new elaborate cloud collision routine, allowing clouds to either coalesce, fragment, or undergo star formation. The type of cloud collisions depends on their relative distance, while the relative speed and total mass of a colliding pair determine the outcome of a collision.

RESULTS

Bulge formation occurs in all runs with a low halo/disk mass ratio (H/D < 3) containing colliding clouds. A low H/D allows clouds to coalesce more often via cloud collisions into bigger clouds. These bigger clouds then act as very effective scattering agents, throwing stars and some other clouds out of the disk plane. The scattered particles make the disk thicker, and most of them end up, under the influence of halo potential, around the nucleus creating a bulge. Longer lasting, higher cloud collision rates, especially those between giant molecular clouds (GMC-GMC collisions), cause creation of bigger bulges. Lower H/Ds and larger initial cloud radii produce higher GMC-GMC collision rates, as seen in Figure 1 (clouds and stars, and bigger clouds).

ABSTRACT

We present r – i colors of GHz–Peaked Spectrum (GPS) Radio Galaxies. We find that most GPS radio galaxies have r – i colors in the range of 0.2 – 0.4, typical of passively evolving elliptical galaxies. However, several have much redder colors in the range 1–2. We suggest several possible explanations for the very red colors, including (1) large amounts of dust in the galaxy, (2) a significant post-starburst population, and (3) a highly reddened active galactic nucleus. The red colors are consistent with a hypothesis in which significant mass transfer has occurred, producing a dense, clumpy, and possibly dusty ISM in the GPS host galaxy.

INTRODUCTION

GPS radio galaxies are characterized by the following properties: (1) a simple, convex, radio spectrum, peaking near 1 GHz, (2) mostly compact (sub kpc) radio structure, (3) low radio and optical polarization, and (4) low variability. We (O'Dea, Baum, and Stanghellini 1991) have suggested that the GPS galaxies have a dense, clumpy ISM (probably acquired externally via cannibalism) that confines and depolarizes the radio source. Our r and i band optical imaging has shown that the GPS galaxies are often in strongly interacting systems (Stanghellini et al. 1993).

THE R – I COLORS

We find that most GPS radio galaxies have integrated r – i colors in the range of 0.2 – 0.4, typical of passively evolving elliptical galaxies. This suggests that most GPS galaxies are not currently undergoing a significant starburst (though they might have in the past), unless the starburst population is highly obscured. However, several galaxies have much redder r – i colors in the range 1–2.

ABSTRACT

We find that Sy 2 galaxies have compact radio cores much more often than Sy 1 galaxies. This result is inconsistent with the popular unified models which explain the difference between Sy 1 and Sy 2 in terms of orientation. We propose a mechanism which is consistent with the observations while still supporting the unified model.

INTRODUCTION

The orientation unification scheme has enjoyed considerable success in accounting for the difference between Sy 1 and Sy 2 galaxies. This scheme suggests that the apparent differences between Sy 1 and Sy 2 galaxies are due simply to our viewing angle. It invokes a dense dusty torus which obscures our view of the broad-line region (BLR) when our line of sight lies close to the plane of the torus. In this case, only the lines from the more distant narrow-line region (NLR) are visible, and the galaxy then appears as a Sy 2. Light from the BLR is visible only when viewed from within a cone centred on the polar axis of the dust torus, and the galaxy then appears as a Sy 1.

This model has received support from observations such as those of Miller and Goodrich (1990), who found that the polarized emission from some Sy 2 galaxies has broad lines characteristic of a Sy 1 galaxy. They suggest that the light scattered from dust or electrons above and below the torus enable us to see the BLR in scattered light. This scheme predicts that, since the dust in the torus should be optically thin at radio wavelengths, Sy 1 and Sy 2 galaxies should appear identical at radio wavelengths.

ABSTRACT

Southern nearby active and starburst galaxies are being mapped in the 12CO(1-0) and 12CO(2-1) lines with the SEST telescope in order to investigate possible peculiarities in the dynamics, content and distribution of the gas which could lead to one or another type of activity, as was proposed by some recent models. Large-scale stellar bars, rings or closely interacting companions seem to be present in most active galaxies, being obviously related with the mechanism of gas transport from the disc into the nuclear or circumnuclear regions. But given that many of the barred or interacting galaxies do not show enhanced nuclear activity, other relevant parameters must exist, like the total gas content, the strength of the bar, or the impact parameters of the interactions. A signature of these processes would also be nuclear elongated features or “mini-bars” that are being observed in many galaxies, mainly in the infrared. We present CO data obtained for five active interacting and barred galaxies: NGC 134, NGC 986, NGC 4027, IC 1623 and IC 2554.

We mapped the nearby starburst and active galaxies described in Table 1 in the 12CO(1-0) and 12CO(2-1) lines with the 15m SEST telescope, with resolutions of 45″ and 22″ respectively. The galaxies in our sample were selected for their angular size, morphology and significant FIR luminosity, which is an indication of their activity. All the galaxies studied happen to have companions, sometimes clearly interacting.

ABSTRACT

By means of models for supernovae of type II during the photospheric phase and for accretion disks around (super–)massive black holes it is found that the pressures, temperatures and velocities in the spectrum-forming regions are quite comparable so that the energy distributions have to be similar. The observed similarity of moderate resolution optical spectra from supernovae of type II and from active galactic nuclei may therefore be just fortuitously. For the distinction of these classes of objects in particular line profiles can be used in addition to gamma–iay fluxes and polarization data.

INTRODUCTION

Optical spectra of type II supernovae (abbreviated subsequently “SNell”) during the photospheric phase look often very similar to those of active galactic nuclei (AGNs, see e.g. Filippenko 1992). This similarity was used by Terlevitch and Melnick (1985, see also subsequent papers by Terlevitch) to support the hypothesis that the activity of galaxies is caused by starbursts that produce lots of massive stars which subsequently explode as supernovae. On the other hand, there are many convincing, though indirect arguments that the optical/UV luminosity of AGNs is produced by disks around (super-)massive black holes (see e.g. Shields 1978; Rees 1984; Malkan 1991).

Since unique spectral signatures of accretion disks have still to be found this raises the following questions:

1. (i) is the optical light of AGNs indeed produced predominantly by SNell?

2. (ii) what does the similarity tell us, if the AGN spectra are in fact produced by accretion disks?

3. […]

ABSTRACT

The large-scale dynamics of the interstellar medium in spiral galaxies is significantly determined by molecular cloud physics. Inelastic cloud-cloud collisions, coupled with gravitational instabilities generated by the gas cooling, give rise to a net flux of angular momentum which is equivalent to a (gravitational) “viscosity”. By using N–body model calculations of a self-gravitating disk we study the efficiency of collisions coupled with gravitational waves, and its importance for large-scale mass accretion in disk galaxies.

THE IDEA

The mass of the interstellar medium of galaxies is essentially included in an ensemble of clouds, whose physics on a small-scale dominates the dynamical behaviour. In a non-axisymmetric potential, the gas clouds are submitted to gravitational torques, which transfer angular momentum outwards, inducing an accumulation of gas in the central regions. Combes and Gèrin (1985) developed a collision model for the interstellar medium, including a mass spectrum of molecular clouds. If on the other hand, differential rotation is sufficient, the gas is submitted to viscous torques, which also accumulates gas in the central regions (Lesch et al. 1990). It is the aim of our investigation to connect these different approaches, by identifying the angular momentum transport in a non-axisymmetric potential with a viscosity. A study of the exchange of angular momentum and energy between clouds will provide an understanding of the nature of the gas “viscosity”.

OVERVIEW

One–dimensional numerical simulations of a large–scale flow of interstellar gas in galactic spiral density waves were produced using the approach described by Roberts (1969), Shu et al. (1972, 1973), Woodward (1975), Marochnik et al. (1983), Lubow et al. (1986), and Berman et al. (1990).

We have employed a two–phase model for the initial state of the interstellar medium (ISM). Thermal processes and self–gravitation of the gas were taken into account. Magnetic fields were not included in this version of the mathematical model. A discussion on the role of the magnetic field and status of the two–phase model of ISM versus Supernova–dominated model can be found in Marochnik and Suchkov (1991).

We have used the Schmidt model of the galaxy and the following parameters of its spiral structure: Ωp = 23, 24, and 13.5 km s-1 kpc-1 (the angular velocity of the spiral pattern), F = 10% (the gravitational field of a spiral arm). The right-hand side of the energy equation is L = n(nΛ – Γ), where nΛ(T) and Γ are the standard cooling and heating functions, respectively (Penston 1970).

Numerical simulations of the interstellar gas flow have been produced for a number of galactocentric distances, R = 4, 5, 6, 8, 10 kpc and two different values of the initial density of the interstellar gas, n0 = 0.05 and 0.5 cm-3. Figure 1 demonstrates one of the examples simulated.

ABSTRACT

We have made new N-body simulations of ringed, barred spiral galaxies that extend and improve upon the previous work of M. P. Schwarz. Using a comprehensive database of multicolor images of a large sample of ringed galaxies, we are able to morphologically “match” galaxies to simulation frames that differ in bar pattern speed and time step. From these matches, we can reliably identify both low and high pattern speed galaxies.

INTRODUCTION

It is now well-established that the rings commonly observed in disk galaxies are probably caused by orbital resonances with a bar or bar-like potential in the disk (see Buta and Crocker 1991, A. J., 102, 1715, hereafter BC; 1993, A. J., 105, 1344, for recent discussions). In a pioneering N-body investigation, Schwarz (1981, Ap. J., 247, 77) simulated the behavior of gas clouds in two dimensional rotating bar potentials, and identified the outer Lindblad resonance (OLR), the inner 4:1 resonance (UHR), and the inner Lindblad resonance (ILR) as the resonances responsible for the features known as outer, inner, and nuclear rings, respectively. To better understand ring formation and evolution, we have carried out new simulations at many more bar pattern speeds (Ωp) than Schwarz with more particles and an improved treatment of cloud collisions. We also take advantage of a recently acquired database of BVI CCD images of 150 ringed galaxies for comparison.

243 Seyfert galaxies of type 1, 2, or 3 are listed in the Catalogue of Quasars and AGNs of Veron and Veron (1989) having mv ≤ 15 and vrad ≤ 20,000 km s-1. Eight of these Seyfert galaxies show two nuclei (IR 0248-11, Mkn 739, Mkn 266, Mkn 463, Mkn 673, Arp 220, NGC 6240, NGC 7593). The separations of the nuclei are 3 – 10 arcsec corresponding to 2 – 6 kpc.

To confirm the double nuclear structure for these disturbed galaxies we determined the internal velocity field to be sure that they are the result of two galaxies in the late stages of merging (Kollatschny et al. 1991).

Making the additional assumption that the Seyfert galaxies Mkn 231 (Kollatschny et al. 1992) and Mkn 273 are galaxies in the final stage of merging, having strong tidal arms but unresolved nuclei, one can estimate that 4 percent of all Seyfert galaxies are in the merging process.

The luminosities of the multiple nuclei Seyfert galaxies are extremely high in comparison to morphologically undisturbed Seyfert galaxies. In Table 1, mean values of the visual and blue luminosities and of the far-infrared and radio (6 cm) luminosities as well as the Ha fluxes are listed for both classes. We have separated Seyfert 1 and Seyfert 2 galaxies.

In all cases the luminosities of double nucleus Seyfert galaxies are higher by a factor of more than two with respect to ‘undisturbed’ Seyfert galaxies — except in the soft X-ray range (ROSAT) and the UV.