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.

ABSTRACT

The nearby elliptical galaxy NGC 4278 (D = 8.2 h-1 Mpc) has long been known to harbor an active LINER, radio nucleus as well as an extended gas disk. Observations of NGC 4278 in the 21 cm HI line using the VLA and recent deep, long-slit optical spectroscopy of the emission line gas are discussed. The atomic gas disk shows regular rotation and extends to over eleven times the half-light radius Re. Noncircular motions in the central regions can be satisfactorily fit by a triaxial model for the galaxy density distribution in which the gas moves on increasingly elliptic orbits towards the nucleus. These elliptic orbits may help feed gas into the active nuclear source in NGC 4278. The high gas rotational velocities in the outer parts give evidence for a massive, dark halo surrounding this active elliptical galaxy.

INTRODUCTION: WHAT CAN WE LEARN FROM GAS KINEMATICS IN ELLIPTICAL GALAXIES?

Contrary to the early definitions of elliptical galaxies as old stellar systems lacking gas and dust, many early-type systems are now routinely detected in all tracers of the cool interstellar medium. About 60% of ellipticals have optical emission lines from ionized gas (Phillips et al. 1986), 45% show thermal dust emission at 60–100 μ (Knapp et al. 1989), 40% show optical absorption by dust patches (Sadler and Gerhard 1985), and about 20% are detected in HI or CO emission (Knapp 1987; Lees et al. 1991).

ABSTRACT

Near–IR photometry within the nuclei of 3 barred spiral galaxies reveals 63–100° isophote twists between the innermost regions and the PA's of the major bars. New N–body simulations indicate that such twists can be best described as the response of the stars to the influence on the potential of the gas within circumnuclear rings.

INTRODUCTION AND OBSERVATIONS

The clearest consequence of barred potentials is manifest in those galaxies undergoing enhanced nuclear star formation, particularly those possessing circumnuclear rings (CNRs). These rings probably form in the region of the inner Lindblad resonances (ILRs), the accumulation of molecular gas acting to enhance star formation. Isophote twists have been noted previously in the centres of barred potentials, both in the stars (Jarvis et al. 1988) and the gas (Devereux et al. 1992), and a link has been suggested between these twists and CNR's.

We have obtained near–IR (1.2–2.2 μm) photometry within the central regions of the barred spirals NGC 1097, NGC 4736 and NGC 5728. Twists of 63°–100° are observed between the innermost isophotes and the known position angles of the bar/lens components, corresponding to the most unambiguous and extreme such detections in any barred spiral later than S0/a. In NGC 1097 and NGC 5728 such twists are immediately interior to the radii of known CNRs. We have considered three possibilities by which such twists could arise.

In weak–intermediate strength bars, there exist two ILRs. The (stable) x2 orbital family dominate between the ILRs and are orthogonal to the main bar.

ABSTRACT

We have studied the bulge formation process by starbursts in young disk galaxies whose disks and halos are gas-rich. If such galaxies tidally encounter another galaxies, large starbursts are easily induced and create galactic superwinds. We study the interaction between the superwind and the halo gas by using a similarity solution and show that a massive, radiativelly-cooled, gaseous shell is formed and becomes gravitationally unstable. In this way, we expect that shells of stars are formed. In order to study further evolution of these shells and their interaction with the disk, we model both the shell and the disk by using an N-body code. Our numerical results show that a large bulge with de Vaucouleurs' density profile is formed from the shell. We also show that the disk is thickened due to the interaction with the shell. The large bulges and thick disks are very similar to these found in S0 galaxies.

INTRODUCTION

Starburst galaxies release huge energy by frequent supernovae. In some starburst galaxies, hot gas and molecular outflows are observed. These outflows are called superwinds (Heckman et al. 1990; Tomisakak and Ikeuchi 1988; Mac Low and McCray 1989). Since some starburst galaxies are interacting galaxies, it was proposed that the starbursts occurs due to gravitational interaction between galaxies (e.g. Noguchi 1988). Such interactions induce bar formation in a galaxy, and the subsequent gas inflow towards the galactic center, as a result of gas-bar interaction and due to the self-gravity in the gas (e.g. Wada and Habe 1990).

ABSTRACT

We present high resolution spectroscopic measurements of the 2.3 μm CO band of the luminous IRAS galaxies NGC 6240 and Arp 220. A convolution analysis yields velocity dispersions σ (FWHM/2.354) of 355 and 150 km s-1 for NGC 6240 and Arp 220, respectively. The velocity dispersion found for NGC 6240 is amongst the highest ever found in a galaxy and it is probably due to violent relaxation associated with the merging of two galaxies. The stellar velocity dispersion of Arp 220 is much smaller than that inferred from the Brα measurement of DePoy et al (1987). Our result implies that there is no dynamical evidence for an AGN in this galaxy. From the dynamical mass derived from our measurements, we infer an infrared mass-to-light ratio M/LK of 0.3 and 0.1 M⊙/L⊙ for NGC 6240 and Arp 220, respectively. For comparison, the ratio typically found in bulges of normal spiral galaxies is 0.6. These observations suggest that the bulk of the 2.2 μim luminosity emitted from the nucleus of NGC 6240 is associated mostly with normal giant stars while the 2.2 μm continuum of Arp 220 seem to have a significant contribution from young red supergiants.

INTRODUCTION

The velocity dispersion is a fundamental quantity for studying the kinematics and stellar populations of galaxies. In normal galaxies, the velocity dispersion is easily obtained by measuring optical emission or absorption lines. Unfortunately, the situation is more complicated for luminous IRAS galaxies.