Dynamical friction on a satellite of a disk galaxy: The circular orbit

In a previous paper, we have studied dynamical friction during a parabolic passage of a companion galaxy past a disk galaxy. This paper continues with the study of satellites in circular orbits around the disk galaxy. Simulations of orbit decay in a self gravitating disk are compared with estimates based on two-body scattering theories; the theories are found to give a satisfactory explanation of the orbital changes. The disk friction is strongly dependent on the sense of rotation of the companion relative to the rotation of the disk galaxy as well as on the amount of mass in a spherical halo. The greatest amount of dynamical friction occurs in direct motion if no spherical halo is present. Then the infall time from the edge of the disk is about one half of the orbital period of the disk edge. A halo twice as massive as the disk increases the infall time four fold. The results of Quinn and Goodman, obtained with a non-self-gravitating method, agree well with our experiments with massive halos (Q ₀ 1.5), but are not usable in a more general case. We give analytic expressions for calculating the disk friction in galaxies of different disk/halo mass ratios.     

The formation of faint ring structures

In a previous paper (Chatterjee, 1984), we studied the formation of ring galaxies due to head-on collisions between disk and spherical galaxies. The rings so formed are quite prominent and well defined. However, in the domain of rings, there are many faint rings which are ill defined and are so faint that they lie on the borderline of detectability. In this paper the formation of such rings is investigated.Using the impulsive approximation technique, in this paper we study an off-center collision between an exponential model disk galaxy and a polytropic (n=4) model spherical galaxy, in which the spherical galaxy moves normal to the plane of the disk galaxy and grazes its periphery. The radii of the two galaxies are taken to be equal, but due to the high central concentration of the spherical galaxy its radius is effectively one-third that of the disk galaxy. It is found that, as a result of the collision, the disk galaxy expands and attains an oblate shape. If the fractional change in binding energy of the disk galaxy, U/|U|, lies between 0.001 to 0.01, then a faint ring structure is formed which does not close on itself. If U/|U|0.01, then in addition to the faint ring a bridge and tail are formed. The ability of such low-energy collisions to affect even minor structural changes in the disk is due to the preferentially higher energy changes occurring in the side nearer to the perturbing galaxy than in the opposite side. Consequently, the structural changes produced are asymmetric.Such a faint ring may elude detection on photographs, but it will be revealed by sensitive photometric observations. There are several examples of such faint ring structures embedded in disks, which have been detected by observation.     

Some peculiarities of four host galaxies,containing Seyfert nuclei

Star-like objects are found in Seyfert galaxies Markarian 290, Markarian 298, NGC 1275, and NGC 7469, being connected with the structure peculiarities of the galaxies. The absolute magnitudes of these objects are –16 ^m M–19 ^m . It has been supposed that these star formations must stimulate the instability in the disk of the galaxy followed by the matter fall toward the centre of the galaxy. The gas inflow toward the centre will allow the recent star formations and Seyfert nuclei generation.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Evolutionary models of nucleosynthesis in the galaxy

A model of the galaxy is constructed and evolved in which the integrated influence of stellar and supernova nucleosynthesis on the composition of the interstellar gas is traced numerically. Our detailed assumptions concerning the character of the matter released from evolving stars and supernovae are guided by the results of recent stellar evolutionary calculations and hydrodynamic studies of supernova events. Stars of main sequence mass in the range 4M8M are assumed to give rise to supernova events, leaving remnants we identify with neutron stars and pulsars and forming both the carbon-to-iron nuclei and ther-process heavy elements in the explosive ejection of the core material. For more massive stars, we assume the core implosion will result in the formation of a Schwarzschild singularity, that is, a black hole or collapsar. The straightforward assumptions (1) that the gas content of the galaxy decreases exponentially with time to its present level of 5% and (2) that the luminosity function characteristic of young clusters and the solar neighborhood is appropriate throughout galactic history, lead to the prediction that 20% of the unevolved stars of approximately one solar mass (M ) in the galaxy today should have metal compositionsZ0.1Z . As Schmidt has argued from similar reasoning, this is quite inconsistent with current observations; an early generation dominated by more massive stars—which would by now have evolved—is suggested by this difficulty. Many of these massive stars, according to our assumptions, will end their lives as collapsed black hole remnants. It is difficult to visualize an epoch of massive star formation in the collapsing gas cloud which formed our galaxy which would enrich the gas rapidly enough to account for the level of heavy element abundances in halo population stars; we have therefore proposed a stage of star formation which is entirely pregalactic in character. We suggest that the Jeans' length-sized initial condensations in the expanding universe discussed by Peebles and Dicke may provide the appropriate setting for this first generation of stars. Guided by these considerations, and by the need for a substantial quantity of unseen mass to bind our local group of galaxies, we have constructed a model of the galaxy in which this violent early phase of massive star formation produces both (1) approximately 25% of the level of heavy elements observed in the solar system and (2) an enormous unseen mass in the form of black holes. The implications of our model for other features of the galaxy, including supernova nucleosynthesis, the cosmic ray production of the light elements, and cosmochronology, are discussed in detail.     

The formation of a ring galaxy during a head-on collision between a disk and a spherical galaxy

The tidal force effects of a spherical galaxy passing head-on through a disk galaxy have been studied at various regions of the disk galaxy and for various orientations of the disk galaxy with respect to the direction of relative motion of the two galaxies. The density distribution of the disk galaxy is taken to be, (r)=_ce^–4r/R , where _c is the central density andR is the radius of the disk. The density distribution of the spherical galaxy is taken to be that of a oolytrope of indexn=4. It is found that as a result of the collision, through the central parts and the outer parts of the disk galaxy remain intact, the region in between these two regions disrupts. Thus a ring galaxy with a nucleus embedded in the ring-i.e., a ring galaxy of the RN-type, is formed.     

An application of sphere of activity in three-body model of interacting galaxies

The sphere of activity which be considered in the test stars of both central galaxies and satellite galaxies is used in three-body model to study the orbital decay of interacting galaxies. It can take account of both semi-restrictedN-body program (Lin and Tremaine, 1983) and multiple three-body algorithm (Borne, 1984) at the same time. The merger time is calculated accurately. The orbital decay of satellite galaxy is characterized. Energy and angular momentum which are carried away by escaping star are computed, too.     

Probing the Baryon Density of the Universe using Light Echoes of High Redshift Radio Sources

It is shown that the high redshift intergalactic gas that probably contains most of the baryonic density b = 0.05(Ho = 75kms-1 Mpc-1) in the standard Cold Dark Matter model can be detected through "true" intergalactic Thompson-scattered halos or light echoes around isotropic radio-loud quasars and radio galaxies. These cosmological halos, which form in the uniform ionized medium around "old" non-evolving sources, have a very large degree of polarization, up to pmax 44 percent at a projected distance half the light-age radius, and a plateau in the annular polarized flux density within 1/3 of the light-age radius, which is about 4 arc min for sources older than 107 years at z 4. The optimal wavelength range for halo polarimetric observations is 20 - 30 cm, depending on the Galactic rotation measure in the direction of the source, the spectral index of the source, and the specific maximized parameter of the halo. If observed with large single-dish radiotelescopes, the 21 cm polarized brightness temperatures of some inner halos, for example the halo around the quasar OQ 172 (S1400 = 2.5 Jy,z = 3.53), are of the same order of magnitude as the current total-intensity limits for cosmic background fluctuations on arc min scales. The halo test can be extended to larger volumes of space by concentric co-adding of source-normalized halos around the much more numerous isotropic radio galaxies. The expected ensemble-averaged profile of the polarized flux density around the symmetry center is calculated by integrating over the halo ages, assuming a uniform source-age distribution. In addition to the electron density or a lower limit for b based on the halo brightness, the characteristic life time of the radio emission can be derived based on its angular size. If there is a moderate deviation of the density evolution of the intergalactic gas from the conservative cubic law due to galaxy formation, high redshift radio halos may be on the rising branch of their brightness beyond its minimum, similar to the well known nonmonotonic behaviour of angular sizes.     

Stability of a Riemann S Ellipsoid with a Spheroidal Halo

The stability of Riemann S ellipsoids inside an oblate halo with respect to the second form of oscillations is investigated. It is shown that some ellipsoids with reverse internal circulation of matter, which are stable inside a spherical halo or in its absence, become unstable with respect to second odd forms of oscillation inside an oblate halo. Here there is asymmetry between conjugate ellipsoids from the standpoint of their stability. Only those conjugate ellipsoids that correspond to higher frequencies of reverse circulation of matter than their corresponding angular rotation rates are unstable. The domains of instability of light and heavy conjugate embedded ellipsoids are obtained as a function of the oblateness measure and relative density of the halo.     

Tidal disruption of a disk galaxy

The tidal force effects of a spherical galaxy passing head-on through a disk galaxy have been studied for various orientations of the disk galaxy with respect to the direction of relative motion of the two galaxies. The density distribution of the spherical galaxy is taken to be that of a polytrope of indexn=4 and that of the disk galaxy is taken to be, (r)=_ce^–4r/R, where _c is the central density andR the radius of the disk. It is found that the disruptive effects due to the tidal force are minimum when the plane of the disk lies along the direction of relative motion, but are maximum when the plane of the disk is slightly inclined to this direction (about 15°). The tidal force effects at the median radius have also been computed. The tadal force effects are much higher in the interior region of the disk.     

Infrared emission of dust grains in the clusters of galaxies

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 10⁷–10⁸ yr at a gas concentrationn _g 10^–3 cm^–3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300 are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near =100 and the same for the Perseus cluster near 50–70. The total fluxes of IR emission from these clusters are about 10⁵–10⁶ Jy and can be detected by modern observational methods.     

The chemical composition of matter in the early universe

The results of the computations of the chemical evolution for a galaxy cluster are presented. The matter exchange between galaxies and intergalactic medium is taken into account. Two dependences of star formation rate on time are considered: (i) monotonously decreasing dependence characteristic of elliptical galaxies, (ii) dependence having two peaks associated with creation of spiral galaxy subsystems, with suppression of star formation at the period between maxima. It is assumed that galactic ejection is due to explosions of II-type supernova with massesm5M , and that the accretion on to a galaxy depends but weakly on the time. By comparing the obtained results with total combination of available observations, it is established that the rate of gaseous exchange between a galaxy and intergalactic medium should be rather large: 0.03M _gal Gyr^–1. Besides, the activity of each type of galaxy leads to an approximately equal enrichment of intergalactic gas by new elements synthesized in the stars. The existence of a large accretion on to the Galaxy leads to the decrease of primordial deuterium abundance by a factor of no more than 2 during the galaxy evolution time. It enables us to assume that the standard Big Bang model with baryon density parameter _b 0.1 may be considered as true.     

The maximum and minimum masses of galaxies

Galaxies may have formed by fragmentation in a collapsing cloud of very large mass. The most massive galaxies were formed from fragments which were nearly but not quite opaque: the least massive galaxies were formed from fragments about as large as the Jeans mass. If the maximum mass of galaxies is 10¹³ M , then the minimum mass should be 10⁶ M .     

cD galaxies of apparent supergiant sizes due to the curvature of space

By combining two two-dimensional subspaces, closed into themselves due to curvature, it is possible to create a model of three-dimensional space of the same properties. If the Universe is a space of this type, its effect is that of a monstrous lens. Close objects are observed to diminish according to the normal law of perspective; however, the remote galaxies are seen to be very highly magnified.The apparent angular size² of a galaxy is more than the size¹ in flat space according to relation:² =¹ cosec , where is the angular distance from the observer to the galaxy. The diameter² d of a galaxy in curved space must be in the same relation to a diameter¹ d with no curvature of space:² d=¹ d cosec . The apparent angular size² and diameter² d are distorted shapes in consequence of an optical illusion caused by the spatial curvature.It is necessary to distribute the multitude of galaxies into two parts in accordance with their location on the close or reverse hemihypersphere of the Universe. The minimum of apparent angular size² of a galaxy of diameter¹ d is at the equatorial zone.The most likely candidates for location in the reverse hemi-hypersphere are cD's of apparent supergiant sizes due, probably, to the curvature of space. The existence of supergiant sizes of galaxies is the second indirect proof, besides superluminal velocities, that the Universe is closed into itself through curvature. The third indirect evidence, i.e., inductive confirmation of the same fact, is the superposition of galaxies which need not inevitably be a new alternative to the present theories of collisions, cannibalism, merger, etc.The fourth indirect proof of the positive curvature of the Universe is the occurrence of background radiation, because that must vanish in hyperbolic space irrespective of its origin. The gravitational lens effect acquires another theoretical form, as usual, in the case of remote galaxies, because it is impossible to distinguish between gravitator and lensing image.     

Cooling flows in clusters of galaxies

Summary X-ray images and spectra of clusters of galaxies show strong evidence for cooling flows. In many clusters, the hot gas in the core is cooling at rates of 100Myr^–1 and greater. Few traces of the cooled gas have been observed, but it probably forms into low-mass stars (perhaps brown dwarf or even Jupiter-mass objects). X-ray surface-brightness profiles show that the cooling gas is highly inhomogeneous. Overdense gas cools rapidly to form cooled clumps distributed throughout the flow, with little of the gas ever reaching the cluster centre. Cooled and cooling clumps are disrupted because of their motion relative to the remainder of the gas, tending to produce small cooled fragments and, ultimately, low-mass stars. Large molecular clouds, which are the sites of massive star formation in our galaxy, do not occur in the outer parts of cooling flows. There is evidence of larger gas clumps and the formation of more massive stars in the central few kpc of some cooling flows. It is argued that cooling flows efficiently form dark matter. This has wider implications for the formation of dark matter in massive galaxies.     

Near-Infrared Spectral Mapping of NGC 1614

We present the spatial distribution of the bright near-infrared emission lines, Br, H₂, He I, [Fe II], and the CO band longwards of 2.3 m, for the luminous infrared galaxy NGC 1614. The morphology of the ionised gas is different from that of the stellar light, and possibly forms a circumnuclear ring. Our data imply that the stellar population is older and the extinction is lower in the nucleus relative to the surrounding circumnuclear ring. We suggest that NGC 1614 is a galaxy whose recent interaction triggered massive star formation in the nucleus, which in turn caused a radially outward progression of star formation thereby producing the circumnuclear ring we observe today. There is no evidence for a buried AGN, and it is difficult to reconcile our data with the simple evolutionary model of ultraluminous galaxies proposed by Sanders et al (1988).     

Gravothermal catastrophe in rotating galaxies

Violent relaxation in a rotating galaxy can lead to a core-halo, system. In order to understand this, a very, crude model is studied in which all details are ignored, and only the dimensions of physical quantities are retained. The behaviour depends on the ratio of the potential in the core to the potential in the halo. When >5/2, the core contracts; when >5, the contraction is a runaway process, or gravothermal catastrophe.     

Population synthesis in starburst galaxies: Mkn 710

Evolutionary population synthesis models have been built to study the properties of the starforming processes taking place in starburst regions. BothZ andZ /10 stellar evolutionary tracks have been used to take into account that starburst galaxies are generally metal deficient. A very small time step allows us to follow the evolution of the cluster even during very short phases. Three parameters have been synthesized: the ratio of the Siiv (1400 Å) andCiv (1550 Å) UV absorption lines, that characterize the predominant massive stars in the cluster, the ratio of the WR_bump over the H luminosity, tracer of WR stars and the FIR luminosity, directly related to the extinction due to dust in the cluster. The predictions have been compared with observations of the burst taking place in the galaxy Mkn 710. The burst age is 5–6 Myr. The stars seem to have formed simultaneously with a mass spectrum whose slope is in the range (1,2) for massive stars. The upper mass limit has to be 60M . We have performed a systematic study of a sample of 17 starburst galaxies searching for differences in the star formation process that may be related to different metallicities or morphological types.Based on observations with the International Ultraviolet Explorer collected at the Villafranca Satellite Tracking Station of the European Space Agency.     

Dynamical friction and the escape of a compact supermassive object shot from the center of a galaxy

A numerical study has been made of the motion of a compact object consisting of a supermassive black hole with a dense cluster of stars around through a galaxy which has recoiled from the center of the latter as a result of anisotropic emission of gravitational radiation or asymmetrical plasma emission. We find that the effect of dynamical friction on its motion through the galaxy (mass10¹¹ M ) estimated using the impulsive approximation technique, is minimal for an object mass 10⁹ M and for recoil taking place at a velocity larger than that of escape. A velocity 1.1 times the escape velocity is needed for the object to escape from the galaxy, whereas for velocities of recoil less than this critical velocity, damped oscillatory motion ensures. The energy exchange of the object with the galaxy is not large enough to cause appreciable change in the internal energy of the latter.     

Fragmentation of Cosmic Objects in the Course of Their Evolution and the Possible Role of Hubble Expansion in this Process

A series of phenomenological similarities between activity phenomena in the microscopic world and in the world of galaxies is examined. Proceeding from the high metallicity of quasars, it is shown that the relative amount of light elements, primarily hydrogen, increases during the evolution of the universe. Evidence supporting an analogous enrichment of the world of galaxies by dwarf galaxies is presented. A variant is proposed in which cD galaxies are the generators of the clusters of galaxies in which they are located, while all the galaxies of a given cluster are products of the activity of a central supergiant galaxy. An analogous mechanism is apparently responsible for the formation of systems of globular clusters. A physical connection between activity phenomena and cosmic expansion is sought.     

A cosmic virial theorem

A new type of cosmic virial theorem is derived, representing a relation among the two-point and three-point galaxy correlation functions and the mean square relative peculiar velocity v ²(r) averaged over pairs of galaxies separated by distancer. The two correlation functions seem to be fairly well known, but v ²(r) at best is only very roughly estimated from available data. It might be possible to improve the estimate through a systematic program of redshift measurements in sample areas. If so, the cosmic virial theorem might yield an interesting measure of the mean mass density due to galaxies.Research supported in part by the National Science Foundation.