ABSTRACT

We present a study of the O/H gradients derived from large numbers of HII regions in a sample of barred galaxies. It is shown that the slopes of the O/H gradients in galaxies with a barred structure are related to the bar strength: the stronger is the bar, the flatter the abundance gradient becomes. This result is in agreement with a scenario of mixing of the interstellar gas by radial flows across the disks of barred galaxies. Thus we concluded that a bar acts as a homogenizer on the chemical composition in a spiral galaxy.

INTRODUCTION: RADIAL FLOWS

Observations and numerical simulations have shown that strong radial flows are present in barred spiral galaxies (e.g. Sellwood and Wilkinson 1993). Although the exact origin of such flows is unclear, their amplitudes seem to be related to the bar properties and to the presence of radiative shocks induced when the gas passes through the bar. A possible important consequence of radial flows in a spiral galaxy is a radial redistribution of the chemical composition across the disk. Such mechanism was suggested to explain why the O/H gradients in few barred galaxies (e.g. NGC 1365) are shallower than gradients observed in normal spirals.

Review works by Vila-Costas and Edmunds (1992, VE) and Zaritsky et al. (1993, ZKH) have confirmed this difference. However, their large samples of galaxies included very few barred spirals where the O/H gradient is derived from a large number of HII regions.

ABSTRACT

A nearby interacting galaxy NGC 3627 was observed in the CO (1–0) transition and in H I using aperture synthesis technique. The combined CO and H I data indicated that, the gravitational torque experienced by NGC 3627 during its close encounter with NGC 3628 triggered a sequence of dynamical processes, including the formation of prominent spiral structures, the central concentration of both the stellar and gaseous mass, the formation of two widely separated and outwardly located Inner Lindblad Resonances, and the formation of a gaseous bar inside the inner resonance. These processes in coordination allow the continuous and efficient radial mass accretion across the entire galactic disk. The observational result in the current work provides a detailed picture of a nearby interacting galaxy which is very likely in the process of evolving into a nuclear active galaxy. It also suggests one of the possible mechanisms for the formation of successive instabilities in the post-interacting galaxies, which facilitates the central channeling of interstellar medium to fuel nuclear activities.

INTRODUCTION

In the recent years, it has become increasingly apparent that the interaction among galaxies plays an important role in the evolution of a disk galaxy (Barnes and Hernquist 1992 and the references therein). It is well known that the gravitational tide excited during the galaxy interaction could induce significant inflow of gas to fuel nuclear activities. But since very few interacting galaxies have so far been mapped with high resolution, a detailed picture of the type of instabilities that occur after galaxy interaction, and of the mass accretion process in the inner kiloparsec region of the galaxy is still lacking.

ABSTRACT

Major features of star complexes as basic “building blocks” of disk galaxies are presented and their role in the galactic formation and evolution is discussed.

Star complexes are the largest aggregates of individual stars, stellar associations and clusters, together with interstellar gas clouds. They have been recently recognized as universal and ubiquitous “building blocks” of disk galaxies and the contemporary star formation is concentrated within the complexes (Efremov 1979, 1988, 1989).

A typical complex is a kpc-size system containing about 107 M⊙, mostly in HI and H2 clouds. Young star clusters and OB-associations are strongly confined within the star complex and usually fill in only a small part of its volume. Star formation lasts 50–70 Myr in a common complex, and it is probable that after 100 Myr the complexes are disrupted. Super-associations may be considered as a rare kind of complexes with violent star formation over the whole complex: there are more than 200 star complexes in M31 and only one super-association.

The well known Gould's Belt, the Local system, is the only star complex whose structure and dynamics we are able to study. It contains 8 small clusters, 3 OB associations, at least 3 Cepheides, a dozen other late supergiants, and surely a lot of main sequence stars. The oldest stars and clusters in the Local system complex are about 60 Myr old. Tayler et al. (1987) estimated that the total stellar, HI, and molecular masses of the complex are 0.5, 1.0, and 0.4 × 106 M⊙, respectively. Other complexes in the Galaxy are probably more massive, especially those concentrated within the Car – Sgr arm (Alfaro et al. 1992).

ABSTRACT

We report the results of ROSAT PSPC observations of two merging galaxy systems. The nearest classic example, the Antennae, shows X-ray features alt the two galactic nuclei, together with features interpreted as large H II regions, all embedded in apparently diffuse emission. Another feature with no radio or optical counterparts may be a hot outflow. NGC 2623, a more infrared-luminous system, is at a later merger stage. Although less X-ray lumiiious than the Anteniiae, it has a hotter nuclear feature, and a larger outflow. Thesfc observations are part of a chronological sequence of merging galaxies, carefully chosen in order to study the evolution of their X-ray properties through the merging process. These early results suggest that, as a rrierger progresses, the X-ray emission may evolve more rapidly than the infrared, and that massive hot outflows are generated.

INTRODUCTION AND OBSERVATIONS

Although merging galaxies have been the subject of intensive multiwavelength studies, particularly over the past decade, studies in the X-ray band have been severely restricted by the limited spatial resolution and sensitivity of instrumentation available. The best study prior to the launch of ROSAT was the Einstein IPC observation of the Antennae, which produced interesting but inconclusive results.

We are studying the X-ray development of merging galaxies at different stages of coalescence. ROSAT PSPC results for two such systems are presented here.

The Antennae, NGC 4038/9 (Arp 244), has much multi-wavelength information available. It is one of the closest merging systems (hence both large and bright) and its classic two-tailed form makes the presence of an interaction unambiguous.

ABSTRACT

A 2D hydrodynamic Eulerian code is applied to study the effect of induced star formation in galactic cooling flows. Our main findings are: 1) Star formation activity is induced by ram pressure when the mass loss is dominated by late type giants. 2) Star formation activity is insensitive to the environment when the mass loss is dominated by early type giants. The present calculation supports the claim that the Butcher-Oemler effect is caused by star formation activity and provides a natural explanation for its source.

INTRODUCTION

The intra-cluster gas (ICG) is a hot rarefied gas that experiences negligible cooling during a Hubble time. This fact changes as the gas flows into the inner parts of elliptical galaxies and encounters the hot inter stellar medium (ISM). The cooling time in the ISM dominated region may become 6 to 7 orders of magnitude smaller then the Hubble time (cf. Portnoy et al. 1993, hereafter PPS). Regions in which cooling times are very short are also found in the centers of many cluster of galaxies (for a review cf. Sarazin 1988 and references therein). These high density and low temperatures regions are strong sources of thermal X-ray radiation, and are known as cooling flows.

The ISM Removal Problem

In contrast to cluster cooling flows that were discovered observationally (cf. Sarazin 1988), galactic cooling flows (in normal ellipticals in contrast to CDs) were discovered numerically in simulations performed by Shaviv and Salpeter (1982, here-after ShS). The main motivation for their numerical simulation was the introduction of the cooling process of the gas into the simulations. As a consequence ShS frequently encountered a cooling instability.

ABSTRACT

During the stage of galaxy formation, or because of subsequent accretion events, cold, rotationally supported matter carrying substantial angular momentum is deposited and organized in disks/rings at the outer parts of many types of galaxies. Examples are warped HI disks in spirals, disks of gas and dust in ellipticals, and polar rings around SOs/ellipticals. The observed nuclear activity in many galaxies has its origin in mass transfer from large radii into the centers of galaxy potential wells. We describe two stages of such a mass transfer process and present related numerical multidimensional hydrodynamical simulations. In the first stage, gas still carrying some angular momentum flows toward the nuclear region as a moderately inclined outer disk is attempting to settle toward an energetically “preferred orientation.” In the second stage, a nuclear accretion disk, influenced by the gravitational potential well of a central massive black hole and possibly by a coherent weak magnetic field, suffers one of several known dynamical nonaxisymmetric instabilities that drives gas deeper into the potential well and regulates its accretion onto the black hole.

INTRODUCTION

Although gas is predominantly found in spiral galaxies distributed in rotationally supported large-scale disks (∼ 10 kpc), recent observations reveal the existence of counter-rotating gaseous disks in the central regions of many elliptical and SO galaxies (Franx and Illingworth 1988; Bertola et al. 1990; Bertola, Buson, and Zeilinger 1992). The accepted interpretation of such observations is that gas in ellipticals and SOs must be of external origin.

ABSTRACT

We have made the first neutral hydrogen maps of the galaxy pair, NGC 5775, and NGC 5774, as well as a number of radio continuum maps from several independent observing runs at 6 and 20 cm. Although the galaxies are not strongly distorted, optically, we have discovered clear evidence for an interaction in the form of two connecting H I bridges through which gas is travelling from NGC 5774 to NGC 5775. Along the southern bridge, we have also detected non-thermal radio continuum emission, suggesting (but not requiring) that star formation may also be occuring between the galaxies. In addition, we have discovered H I arcs and extensions away from the plane of the IR-bright, edge-on galaxy, NGC 5775, and can confirm previous detections of arcs and plumes in the radio continuum. In this respect, NGC 5775 appears to be very similar to NGC 891.

INTRODUCTION

NGC 5775 is an edge-on, spiral galaxy with a face-on companion, NGC 5774, 4.3′ to the NW. It belongs to a small group (Group #148; Geller and Huchra 1983) which, in addition, includes IC 1070, only 3.9′ to the SW of NGC 5775. NGC 5775 is the dominant galaxy in the group, it is an IR bright galaxy (Soifer et al. 1987), and has a nuclear HII region spectrum (Giuricin et al. 1990). NGC 5774 is a barred “low surface brightness” (LSB) galaxy (Romanishin et al. 1983); however, its central surface brightness is more than a factor of 5 brighter than the brightest LSB galaxies studied by G. Bothun (these proceedings). Neither galaxy appears strongly tidally disturbed on the POSS prints.

ABSTRACT

A theoretical scheme is developed for computing line profiles from optically thin rotating and expanding disks and systems of emission line clouds moving radially inside cones. The models include turbulent motions, atmospheric seeing and effects induced by the size of the observing aperture. As an example, the asymmetric extranuclear Ha line profiles along the major axis of NGC 7469 are fitted by a rotating disk model.

INTRODUCTION

Disks and cones are basic geometries for systems of emission line clouds in active galaxies. Many lines, as e.g. forbidden emission lines, are formed in optically thin clouds so that the line profiles only depend on the geometrical and kinematical distribution of the clouds and radiative transfer effects can be neglected. Hence, infinitesimally spatially resolved profiles are calculated by integrating along that part of the line-of-sight that intersects the configuration. Those volume elements whose line-of-sight velocity component equals v contribute to the intensity I(v).

For systems following radial power laws for both velocity and emissivity we derived analytical expressions for the profile functions. These results are used to check the pure numerical code whose working method is based on collecting and summing up all emissivities falling in a given velocity bin along the considered line-of-sight. Additional isotropic turbulent motions are taken into account by convolving the profiles with a normalized Gaussian of a given σ. The line-of-sight profiles are stored in a data cube (x, y, v). Spatial convolutions to mimick the effects of atmospheric seeing and finite observing apertur and pixel sizes are simply carried out with the ESO image processing system MIDAS.

INTRODUCTION

It has become evident in recent years that elliptical galaxies harbor many interesting features. Currently in ∼ 40% of all ellipticals dust has been detected (e.g.) Sadler and Gerhard 1985). Several galaxies are found to harbor decoupled cores. Although the best evidence for these entities is provided by kinematical data (i.e. counter-rotating cores), Bender (1988) found in four cases an interesting correspondence between kinematical decoupling and the ellipticity profile within the region where kinematical decoupling takes place.

Since these features (including the gas and/or dust) are often located in or near the nuclei of these galaxies, many of them may still be undetected due to the atmospheric smearing. We therefore undertook a program of high resolution Hubble Space Telescope (HST) imaging of a complete sample of 12 elliptical galaxies in the Virgo cluster. After standard reduction the images were deconvolved by Fourier filtering.

RESULTS

The majority of our galaxies show peculiar near-nuclear morphology (Jaffe et al. 19936): NGC 4261 (3C 270), one of the two active galaxies in our sample, was found to harbor a small, smooth, dusty disk around a point-like nucleus (JafFe et al. 1993a). We found the disk, which we interpret as the outer accretion disk, to be perpendicular to the jet axis. NGC 4476 shows a large circumnuclear ring or disk of dust, whereas the dust is filamentary in NGC 4550 and NGC 4374.

ABSTRACT

We describe the results of an imaging and spectrophotometric investigation of the mixed elliptical-spiral pair Kar 29 (≡ VV 347 ≡ Arp 119). The spiral component (≡ Mrk 984) shows a strong, extended, LINER-like emission line spectrum. Each line is partly resolved into at least four components, redshifted with respect to the underlying galaxy, and covering δvr ∼ 1,300 km s-1. Line ratios indicate that the dominant ionization mechanism is provided by shocks or a mixture of shocks and photoionization by hot stars. One possible interpretation involves a nearly polar crossing of the spiral disk by the elliptical, with the line emitting gas stripped and accelerated toward the elliptical. Some of the data are however better explained if the large δvr between the line components is due to the disk impact of an additional small companion.

INTRODUCTION AND OBSERVATIONS

Interaction between galaxies produces a wide variety of observable phenomena; however, most of them are not yet fully understood in physical terms. One important challenge is to explain the response of the gaseous component of a disk galaxy to strong perturbations.

Long slit CCD spectra of the spiral (classified as Sc pec or Sdm)/elliptical (E5) pair of giant galaxies Kar 29 were obtained at KPNO, San Pedro Martir and ESO. Slits were oriented (1) along the major axis of the pair (P.A. = 8°), (2) along the major axis of the spiral (116°), (3) and at P.A. = 82° (with the slit centered on the knot closest to the nucleus), during several observing runs in 1991/92.

ABSTRACT

Some of ultra-luminous galaxy mergers show a sign of quasar-like activity. We have numerically investigated the dynamical evolution of the interstellar gas in the late phase of disk galaxy mergers in order to clarify how the gas dynamics is related to the triggering of quasar-like activity. It is found that in most cases the efficient gas infall to the central 10 pc of the nucleus is realized only after the coalescence of two galaxy cores. This suggests that the quasar-like activity tends to appear only after the merger has been completed.

MODELS

The cloud particle scheme has been used to follow the gas response to the time dependent gravitational field made by two galaxy cores spiraling into the mass center of the system. We have neglected the influence of galactic disks because the inner region of a late phase merger is likely to be dynamically dominated by spherical components (e.g., bulges and nuclei). The spherical component (hereafter core) of each galaxy is assumed to be rigid. The self-gravity of the gas has been neglected. The dissipational nature of gas has been included by making gas clouds collide inelastically. Gas clouds are initially distributed in a disk around one of two nuclei.

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.