Tidal Encounters Research Articles

The Effects of Starburst Activity on Low Surface Brightness Disk Galaxies

Although numerous simulations have been done to understand the effects of intense bursts of star formation on high surface brightness galaxies, few attempts have been made to understand how localized starbursts would affect both the color and surface brightness of low surface brightness (LSB) galaxies. To remedy this, we have run 53 simulations involving bursts of star formation activity on LSB galaxies, varying both the underlying galaxy properties and the parameters describing the starbursts. We discovered that although changing the total color of a galaxy was fairly straightforward, it was virtually impossible to alter a galaxy's central surface brightness and thereby remove it from the LSB galaxy classification without placing a high (and fairly artificial) threshold for the underlying gas density. The primary effect of large amounts of induced star formation was to produce a centralized core (bulge) component, which is generally not observed in LSB galaxies. The noisy morphological appearance of LSB galaxies, as well as their noisy surface brightness profiles, can be reproduced by considering small bursts of star formation that are localized within the disk. The trigger mechanism for such bursts is likely distant/weak tidal encounters. The stability of disk central surface brightness to these periods of star formation argues that the large space density of LSB galaxies at z = 0 should hold to substantially higher redshifts.

On the survival and destruction of spiral galaxies in clusters

We follow the evolution of disk galaxies within a cluster that forms hierarchically in a cold dark matter N-body simulation. At a redshift z=0.5 we select several dark matter halos that have quiet merger histories and are about to enter the newly forming cluster environment. The halos are replaced with equilibrium high resolution model spirals that are constructed to represent examples of low surface brightness (LSB) and high surface brightness (HSB) galaxies. Varying the disk and halo structural parameters reveals that the response of a spiral galaxy to tidal encounters depends primarily on the potential depth of the mass distribution and the disk scale length. LSB galaxies, characterised by slowly rising rotation curves and large scale lengths, evolve dramatically under the influence of rapid encounters with substructure and strong tidal shocks from the global cluster potential --- galaxy harassment. We find that up to 90% of their stars are tidally stripped and congregate in large diffuse tails that trace the orbital path of the galaxy and form the diffuse intra-cluster light. The bound stellar remnants closely resemble the dwarf spheroidals (dE's) that populate nearby clusters. HSB galaxies are stable to the chaos of cluster formation and tidal encounters. These disks lie well within the tidally limited dark matter halos and their potentials are more concentrated. Although very few stars are stripped, the scale height of the disks increases substantially and no spiral features remain, therefore we speculate that these galaxies would be identified as S0 galaxies in present day clusters.

Dynamical Properties of Tidally Induced Galactic Bars

We have simulated a series of N-body models of tidal encounters between a disk galaxy and a perturbing galaxy to investigate the dynamical properties of tidally-induced galactic bars (Noguchi 1987, MNRAS, 228, 635). We have also calculated N-body models on isolated galaxies with bar-unstable disks (Ostriker & Peebles 1973, ApJ, 186, 467) to make a comparison between galactic bars of different origins (Miwa & Noguchi 1997, submitted).

Tidal Dwarf Galaxies

The life and evolution of galaxies are dramatically affected by environmental effects. Interactions with the intergalactic medium and collisions with companions cause major perturbations in the morphology and contents of galaxies: in particular stars and gas clouds may be gravitationally pulled out from their parent galaxies during tidal encounters, forming rings, tails and bridges. This debris of collisions lies at the origin of a new generation of small galaxies, the so-called “tidal dwarf galaxies” (hereafter TDGs). Such an exotic way of forming galaxies was put forward by Schweizer (1978) and by Mirabel et al. (1992), who clearly observed the genesis of a star-forming object, out of material tidally expelled from the interacting system NGC 4038/39 (“The Antennae”). Recent studies, based on optical and HI observations, have shown that TDGs actually form a class of “recycled” objects with some properties similar to the more classical dwarf irregulars (dIrr) and blue compact dwarf galaxies (BCDs).

NGC 2442: Tidal Encounters and the Evolution of Spiral Galaxies

Using imaging Fabry-Perot data, we study the star-forming properties and kinematics of the nearby barred spiral galaxy NGC 2442. The Hα emission is very localized along the strong spiral arms of the galaxy, and shows a marked asymmetry between the sharp, well-defined northern tidal arm and the weaker southern arm. The Hα velocity field appears highly distorted, with a rapidly rotating nuclear component. We find evidence for strong noncircular motions along the northern arm, coincident with the pronounced dust lane and regions of intense star formation. The strong asymmetries, the disturbed velocity field, and the presence of a perturbed companion suggest that we are witnessing a strong kinematic response to a close interaction, which has redistributed the star formation activity throughout the disk of NGC 2442. Dynamical modeling of the NGC 2442 system supports this hypothesis and suggests that the regions of strongest star formation are coincident with strong shocks occurring along the tidally perturbed northern arm. Despite this strong redistribution of the gas on small scales, this galaxy does not show a significant departure from the Tully-Fisher relation, nor does it appear to be experiencing any strong starburst. Moreover, our models predict that in a few times 108 yr, NGC 2442 will have recovered from this first tidal encounter and will experience another passage—and ultimately a merger—in a few gigayears. This merger may provoke stronger, permanent changes in the structural properties of the galaxy, depending on the detailed response of the disk. Given the environment of many disk galaxies, this tidal encounter cycle seems likely to be a normal phase of disk galaxy evolution.

An Ultraviolet and Near‐Infrared View of NGC 4214: A Starbursting Core Embedded in a Low Surface Brightness Disk

During the Astro-2 Spacelab mission in 1995 March, the Ultraviolet Imaging Telescope (UIT) obtained far-UV (λ = 1500 A) imagery of the nearby Sm/Im galaxy NGC 4214. The UIT images have a spatial resolution of ~3'' and a limiting surface brightness, μ1500 > 25 mag arcsec-2, permitting detailed investigation of the intensity and spatial distribution of the young, high-mass stellar component. These data provide the first far-UV imagery covering the full spatial extent of NGC 4214. Comparison with a corresponding I-band image reveals the presence of a starbursting core embedded in an extensive low surface brightness disk. In the far-UV (FUV), NGC 4214 is resolved into several components: a luminous, central knot; an inner region (r 2.5 kpc) with ~15 resolved sources embedded in bright, diffuse emission; and a population of fainter knots extending to the edge of the optically defined disk (r ≈ 5 kpc). The FUV light, which traces recent massive star formation, is observed to be more centrally concentrated than the I-band light, which traces the global stellar population. The FUV radial light profile is remarkably well represented by an R1/4 law, providing evidence that the centrally concentrated massive star formation in NGC 4214 is the result of an interaction, possibly a tidal encounter, with a dwarf companion(s). The brightest FUV source produces ~8% of the global FUV luminosity. This unresolved source, corresponding to the Wolf-Rayet knot described by Sargent & Filippenko, is located at the center of the FUV light distribution, giving NGC 4214 an active galactic nucleus-like morphology. Another strong source is present in the I band, located 19'' west, 10'' north of the central starburst knot, with no FUV counterpart. The I-band source may be the previously unrecognized nucleus of NGC 4214 or an evolved star cluster with an age greater than ~200 Myr. The global star formation rate derived from the total FUV flux is consistent with rates derived using data at other wavelengths and lends support to the scenario of roughly constant star formation during the last few hundred million years at a level significantly enhanced relative to the lifetime averaged star formation rate. The hybrid disk/starburst-irregular morphology evident in NGC 4214 emphasizes the danger of classifying galaxies based on their high surface brightness components at any particular wavelength.

Intergalactic stars in the Fornax cluster

We have identified ten candidate intergalactic planetary nebulae in the Fornax galaxy cluster. These objects were found during observations in 1992 and 1993 in three fields chosen well away from any Fornax galaxy at 15 arcmin, 30 arcmin, and 45 arcmin from the centre of Fornax. We used the usual method of blinking images taken in a narrow OIII filter, with images taken in an adjacent broad filter. The measured fluxes in the narrow, broad, and I bands are consistent with these unresolved objects being planetary nebulae immersed in an intergalactic population of stars. Such a population is expected to arise as a consequence of tidal encounters between galaxies, and our findings strengthen the case for the existence of such tidal debris. The confirmation of some or all of these ten candidates as planetary nebulae would imply that intergalactic stars constitute a substantial fraction of all the stars in Fornax, up to an estimated 40 per cent. Intergalactic planetary nebulae could prove useful in probing the underlying cluster potential, since they would be far more abundant than galaxies. We discuss possible contamination of the sample by emission-line galaxies, but conclude that planetary nebulae is the most likely identification for the detected objects. keywords: intergalactic medium-- galaxies: interactions -- planetary nebulae: general -- clusters: individual: Fornax

Shaping of Earth-Crossing Asteroids by Tidal Forces

Near-Earth asteroids (NEAs) that have been imaged by Earth-based radar have conspicuously elliptical or potato shapes. If they are {open_quote}{open_quote}rubble piles{close_quote}{close_quote} rather than single chunks of competent material, their shapes may be fossil remnants of past close gravitational tidal encounters with Earth and other planets. The NEAs are removed from the steady-state population by hitting Earth or being pumped by close encounters with it into orbits that are strongly perturbed by Jupiter. Cross sections for tidal encounters exceed those for direct collision with Earth, so a sizable fraction of the NEOs have passed within the tidal radius of Earth. We model the asteroids as agglomerations of spherical rocks, bound together only by gravity (rubble pile). We used an {ital N}-body code to determine the amount of elongation that they acquire in a tidal encounter as a function of impact parameter and velocity with respect to Earth. The model enjoys a similarity relation that allows detailed geometric and energetic features of the tidal distortion to be scaled from one asteroid size to another. The maximum elongation of tidally distorted asteroids exceeds that of any observed NEA including Geographos, so tidal encounters appear to adequately explain the high degree of elongation shownmore » by these objects. {copyright} {ital 1996 American Astronomical Society.}« less

Seyfert Galaxies With Companions: Orbital and Kinematic Clues to AGN Triggering

view Abstract Citations (90) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Seyfert Galaxies With Companions: Orbital and Kinematic Clues to AGN Triggering Keel, William C. Abstract This paper presents imaging and optical spectroscopy of paired Seyfert galaxies and their companions. The aim is to seek common properties of Seyfert galaxies in interacting systems, which might provide evidence of AGN triggering in a way independent of the usual two-sample statistics which have proven ambiguous on this issue. Three kinds of comparison have been made-the kinds of interactions involving Seyfert galaxies, the relative luminosities of the Seyferts and their companions, and the level of kinematic disturbance as measured from rotation curves. (1) Dynamics and tidal features have been used to determine (or at least limit) the sense of orbital motion (direct/retrograde/polar with respect to the Seyfert galaxy's disk) for many of these pairs. There is no obviously preferred kind of interaction-direct, polar, and retrograde encounters are all well represented, despite the gross differences in dynamical response of a disk to these various kinds of encounter. To the extent that triggering of Seyfert nuclei occurs due to tidal encounters, the existence of a perturbation seems more important than its exact duration or detailed effects on the disk. However, the ratio of merging to paired Seyferts is higher than for disk galaxies in general, consistent with more effective triggering of AGN in this specific phase; the implied time scale for enhanced occurrence during mergers is the same as the timescape for merger remnants to appear as such, a few disk-edge crossing times (typically several times 10^8^ yr). (2) Seyfert nuclei occur preferentially in the brighter members of galaxy pairs, by a median of 0.93 mag after making the maximal correction for contaminating nonstellar light in the nuclei. Only about 1/3 of this effect can be accounted for by the known tendency of Seyfert nuclei to occur in more luminous galaxies. Enhancement of AGN by interactions is evidently more effective for more luminous galaxies (though this will also be the case if both star formation and AGN occurrence are enhanced in the same galaxies). (3) The rotation curves of the paired Seyferts show systematically small regions of rising or solid-body rotation compared to the disk radius, as a group comparable to Sa but very different from Sb or Sc galaxies (even for Seyfert galaxies with Hubble type later than Sa). There is weak evidence that this difference is also present with respect to more isolated Seyfert galaxies. Despite the obvious utility of a dynamically disturbed disk for transport of angular momentum and "feeding the monster," Seyfert galaxies in pairs actually have smaller kinematic disturbances (measured by the maximum departure from a symmetric rotation curve, normalized to the full rotation amplitude) than found in a complete sample of non-Seyfert spirals in pairs. Publication: The Astronomical Journal Pub Date: February 1996 DOI: 10.1086/117816 Bibcode: 1996AJ....111..696K Keywords: GALAXIES: SEYFERT; GALAXIES: INTERACTIONS; GALAXIES: ACTIVE full text sources ADS | data products NED (115) SIMBAD (91) CDS (1)

Self-Gravitating Gas Dynamics in a Nuclear Region of a Weak Barred Galaxy

Starburst regions are frequently located in galactic central regions and CO observations indicate that these regions contain a large amount of molecular gas (e.g. Lo et al. 1987). However, the triggering mechanism for starbursts and the mechanism of the high mass supply rate of gas into a galactic center are still unclear.It is suggested that tidal encounters of galaxies remove angular momentum of gas and trigger rapid gas accretion and starburst. Noguchi(1988) has shown by computer simulations that galaxy-galaxy interactions induce a stellar bar, and gas loses its angular momentum and accumulates to a galactic center. In his numerical simulation, non-axisymmetric potential of a stellar bar plays an important role in the accretion of gas. However, it is not obvious whether or not gas accretes into a nuclear region within a few hundred pc only by the effects of stellar bar.Wada and Habe (1992) investigated the dynamics of self-gravitating gas in a barred potential by 2-D numerical simulation, and show that even if the bar is weak, for the initial gas mass ratio to stellar mass grater than 10%, a central elongated gas ring formed at near ILRs becomes unstable and collapses. As a result, a large amount of gas can be supplied to galactic center. They conclude that both the effect of the self-gravity of the gas and the existence of ILRs are necessary to the rapid gas fueling in a weak barred potential.

A Simulation Atlas of Tidal Features in Galaxies

Detailed simulations of tidally induced structure in disk galaxies have either concentrated on specific systems or consisted of a few encounters with relatively small numbers of particles and no self-gravity. Observers need a 'dictionary' of simulations that covers many encounter parameters with fine morphological resolution and includes effects of self-gravitation. Observers can then search the dictionary for the parameters that best match a particular observed morphology. Alternatively, the dictionary can be used with observational samples for statistical studies of system parameters. To fill this need, we present a survey of model tidal encounters using a self-gravitating, 180,000 particle, two-component ('stars' and 'gas') disk. A wide variety of fascinating morphologies results. There are 86 different encounters that vary orbit tilt, perigalacticon distance, galaxy to companion mass ratio, and the amount of halo dark matter relative to the disk. For morphological comparisons, over 1700 images of the entire survey are available in video form. While there is a rich variety of tidal structure covering much of this parameter space, some general patterns may be remarked. There is a strong orbital inclination dependence of the symmetry of tidal patterns, most symmetric for planar orbits and nearly one-sided for polar encounters. Retrograde encounters produce only broad fanlike global patterns, but rich small-scale internal structure. In both kinds of encounter, our numerical resolution allows us to track internal spiral structure driven by the outer material arms, especially in the lighter halo simulations. We note also that polar encounters generate series of expanding, essentially non-rotating loops resembling shell structures in some respects.

Weak Tidal Encounters of a Star with a Massive Black Hole

We have computed encounters of stars with a massive black hole using a three-dimensional Eulerian hydrodynamic code. The stars are assumed to be polytropes with indices n=1.5, 2, and 3, and the adiabatic index γ is taken to be 5/3. In this paper encounters which do not lead to stripping or disruption of the star are considered. Nonlinear effects and the transfer of energy and angular momentum are discussed, and the results are compared with the predictions of the linear theory

MAPPING THE STELLAR BACKBONES OF SPIRAL GALAXIES

ABSTRACT We present optical and IR surface photometry of M51 (NGC 5194) at B V R I J K and CO(2.3mu). These data are used to establish whether K-band (2.2mu) images of spiral galaxies provide reliable maps of stellar surface mass density features such as massive spiral arms or bars. The main distorting agents in the mapping at shorter wavelengths are dust extinction and luminous young stars. From modeling the color chagnes across the main dust lanes in M51, we find the optical depths to be ~0.5 in the K-band. For these optical depths the K-band flux is attenuated by only <~ 10% even in the dust lanes. From monitoring the gravity-sensitive CO (2.3mu) index across the spiral arms we find that young, red supergiants do not distort significantly the K-band image. On this basis, we conclude that K-band images do trace the massive disk star population and allow to map azimuthal variation int he surface mass density of the stellar disk. In M51 we find the surface mass density contrast (arm/inter-arm), Imax/Imin, to ranage from 1.8 to 3, as expected from N-body simulations of the galaxy's tidal encounter with NGC 5195. This density contrast is larger than the light contrast in I band images, where the spiral arm crest is affected by dust extinction. The spiral arm amplitude in M51 clearly shows smooth, strong variations, with a maximum at ~130" and a minima at 45" adn 170". These variations may arise from interference of a pre-existing spiral pattern with the tidally induced spiral arms. An ongoing K-band imaging study of a sample of spiral galaxies will yield a more representative picture of the role of bars and massive spiral arms features.

The dynamic interstellar medium in NGC 4438

We present multiwavelength observations of the interstellar medium in the disturbed galaxy NGC 4438. We find extended emission-line filaments, which are kinematically unrelated to the galaxy disk. For one of these features, an extended loop, we propose a scheme in which the 'bubble' seen at radio wavelengths has swept a clear path for outflowing material from the nucleus to ionize more distant material. Another radial emission-line feature might represent shocks in the disk along a gravitationally-defined caustic produced during a tidal encounter, or a 'searchlight beam' escaping the inner dusty regions of the galaxy. Energy-balance arguments favor the shock picture, but further study will be important in understanding this feature. Emission-line ratios indicate that shocks may be important in the energetics of the interstellar medium (ISM) through much of the disk of NGC 4438; the alternative is star formation in a uniformly very dense ISM. We use K-band data to locate the stellar peak of the galaxy, which is just outside the prominent radio-continuum loop or shell feature. The nucleus itself is at most a very weak radio source. However, the more extended radio structure shows that energetic events have taken place in the innermost few hundred parsees of NGC 4438. The interstellar medium of NGC 4438 may show at once the impact of nuclear energy input (from an AGN or cocooned starburst), perturbation by the companion NGC 4435, and interaction with the dense intergalactic medium in the Virgo core.

Tidally triggered galaxy formation. II - Galaxy number counts

A model is presented for the evolution of the galaxy luminosity function in which the distribution of the galaxy masses, star formation time scales, and star formation turn-on redshifts is derived from the initial power spectrum of density fluctuations. The mass function of galaxy halos is very steep at low masses. Star formation occurs quite late, especially for massive galaxies, because it is assumed to begin only when the group surrounding a galaxy has collapsed and tidal encounters commenced. A good fit of the model to the present luminosity function of bright galaxies and to the relation between half-light radius and luminosity for galaxies fainter than M(B) about -21. The predicted relation between halo circular velocity and blue luminosity for bluer galaxies is consistent with the observed Tully-Fisher relation for spiral galaxies. A reasonable fit to the B- and R-band number counts up to B less than 24 is found. The model's main problems are discussed.

EVOLUTION OF GALAXIES IN COMPACT GROUPS

ABSTRACT We investigate the role of mergers and interactions in the evolution of galaxies by studying galaxies in compact groups. Compact groups of galaxies have high spatial densities and low velocity dispersions making these regions ideal laboratories in which to study the effect of interactions and mergers on galaxies. We present a large multicolor survey of the elliptical galaxies in compact groups and show that several of the elliptical galaxies in these groups have unusually blue colors indicative of recent interactions or mergers. We also find that a small fraction of galaxies in these groups are currently merging, based on their disturbed optical morphologies, warm far-infrared colors, and unusual dynamics. However, unusually blue elliptical galaxies and currently merging systems are relatively rare, indicating that the time scale for a compact group to evolve through merging into a single object is at least 3 Gyr. Although merging is not occurring rapidly, the elliptical galaxies appear to be affected by tidal interactions within the group. We find that the compact group elliptical galaxies are less likely than similar galaxies in other environments to have boxy isophotes and more likely to have irregular isophotal shapes indicative of a recent tidal encounter. The most striking difference between galaxies in compact groups and those in other regions is that the elliptical galaxies in compact groups appear to have systematically low velocity dispersions compared to other elliptical galaxies of the same effective radii, average surface brightness, absolute magnitudes, or colors. Therefore, they do not lie on the fundamental plane defined by elliptical galaxies in other environments.

Evolution of luminous IRAS sources - Radio imaging

view Abstract Citations (57) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Luminous IRAS Sources: Radio Imaging Neff, S. G. ; Hutchings, J. B. Abstract We present and discuss observations of the compact radio morphology of 111 luminous IRAS-selected active galaxies covering a wide range of IR and optical properties. Of these sources, 72 are observed for the first time with the VLA^1^ A configuration. The circumnuclear radio sources are generally small and weak, with 15 nondetections to a limit of ~0.4 mJy. Comparison with the IR and optical properties of the objects indicates that the radio sources turn on within 10^8^ yr of the tidal encounter that is presumed to generate the IR activity. However, the radio sources do not all appear at the same time. The radio observations are consistent with the evolution scenario for luminous IRAS galaxies suggested by Hutchings & Neff [AJ, 101,434(1991)]. Publication: The Astronomical Journal Pub Date: June 1992 DOI: 10.1086/116192 Bibcode: 1992AJ....103.1746N Keywords: Galactic Evolution; Infrared Sources (Astronomy); Luminosity; Radio Sources (Astronomy); Astronomical Spectroscopy; Galactic Nuclei; Infrared Astronomy Satellite; Optical Properties; Very Large Array (Vla); Astrophysics; GALAXIES: ACTIVE; INFRARED: GALAXIES; RADIO: GALAXIES; GALAXIES: EVOLUTION full text sources ADS | data products NED (111) SIMBAD (108)

The leading arm, ringed spiral galaxy NGC 4622 - A diagnostic case of a retrograde tidal encounter

view Abstract Citations (32) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Leading Arm, Ringed Spiral Galaxy NGC 4622: A Diagnostic Case of a Retrogade Tidal Encounter Buta, R. ; Crocker, D. A. ; Byrd, G. G. Abstract The southern galaxy NGC 4622 is a rare example of a spiral with two sets of spiral arms winding in opposite senses: A single inner arm which opens counterclockwise, and two outer arms which open clockwise, The two sets of arms are linked by a nearly circular, off-centered inner ring of high contrast, and the presence of the two opposing senses means that NGC 4622 has at least one leading spiral arm, a phenomenon that up until now has never been firmly established in any galaxy. In this paper, we describe some of the basic morphologic and photometric properties of this galaxy, which has recently been suggested to have experienced a retrograde tidal encounter. Of the three observed arms, the inner arm is the prime suspect for the leading sense, based on published n-body models. We establish with BVI surface photometry that this arm is present in the stellar disk component and is not an artifact of dust absorption. The suspected leading arm begins ~3 kpc from the center and winds more than 360^deg^ to a radius of ~4.4 kpc. The winding is logarithmic in projection with a pitch angle of only 4^deg^. A substantial portion of the inner ring may be made of the leading arm. A large variation in B - I color index around the ring is also found. At the average radius of the ring, the phase of the 1θ Fourier component in B is shifted ~40^deg^ in the direction of rotation relative to that in I. The shift is consistent with the hypothesis that part of the ring is a one-armed density wave turning opposite the disk rotation at ~1.4 times the orbital speed. The galaxy may be the most definite case to date demonstrating this density wave effect because of the large difference between the leading arm pattern rate and the orbital rate. The perturbing companion has not yet been identified. We suspect that a small galaxy l.85' east, of unknown redshift, may be the most promising to explain not only the leading arm, but also the two trailing outer arms. Publication: The Astronomical Journal Pub Date: May 1992 DOI: 10.1086/116165 Bibcode: 1992AJ....103.1526B Keywords: Galactic Evolution; Galactic Structure; Ring Galaxies; Spiral Galaxies; Astronomical Photometry; Galactic Clusters; Luminosity; Southern Sky; Astrophysics; GALAXIES: SPIRAL; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: INDIVIDUAL: NGC 4622 full text sources ADS | data products SIMBAD (5) NED (1)

Three-dimensional hydrodynamical simulations of colliding stars . III - Collisions and tidal captures of unequal-mass main-sequence stars

A 3D smooth-particle hydrodynamic code with 7000 particles is used to simulate collisions between lower main sequence (MS) stars whose masses differ by a factor of five. Collisions between point-mass intruders and MS stars are simulated to study how the physics of the encounters depends on the finite radius of the smaller MS stars. It is found that the two MS stars become gravitationally bound in a physical collision if their relative velocity at infinity is less than a critical velocity Vd. Vd decreases from 1000 km/s in a head-on collision to 150 km/s in a grazing one. If the less massive star is replaced by a point mass, Vd remains the same for grazing collision and tidal encounters but drops to about 600 km/s in head-on collisions. The collision and coalescence of two MS stars resets the nuclear clock of the coalesced star, so it contracts to the MS. Tidal capture of binary stars in globular clusters can only occur in encounters in which the closest approach of the two stars to their center of mass is less than 2.0 times the sum of their radii.

Dynamical segregation of direct and retrograde orbits in binary disk galaxies

view Abstract Citations (26) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical Segregation of Direct and Retrograde Orbits in Binary Disk Galaxies Keel, William C. Abstract A subset of the Karachentsev galaxy-pair catalog has been identified, using geometric and velocity criteria, which must be undergoing either direct or retrograde encounters. Rotation curves have been used to define the sense of disk rotation in each case and thus the orbital sense of each companion. A significant difference in orbital properties appears between direct and retrograde encounters, such that direct companions are moving systematically faster than retrograde ones. This is interpreted as evidence of more circular orbits in retrograde cases, perhaps indicating that such orbits are less strongly modified during tidal encounters than are direct ones. The difference between direct and retrograde companions is likely to reflect differential dynamical evolution, so that the properties of companion orbits now have a nearly bimodal distribution. Any difference in energy transfer between pairs of different orbital senses requires either relatively low halo masses, so that the disk is dynamically significant, or that the halo rotates in the same direction as the disk. Publication: The Astrophysical Journal Pub Date: July 1991 DOI: 10.1086/186075 Bibcode: 1991ApJ...375L...5K Keywords: Disk Galaxies; Interacting Galaxies; Orbital Mechanics; Velocity Distribution; Astronomical Spectroscopy; Galactic Evolution; Astrophysics; GALAXIES: EVOLUTION; GALAXIES: INTERACTIONS full text sources ADS |