Flat Galaxies Research Articles

Flat Galaxies of the RFGC Catalog Detected in the HIPASS Survey

Data from the H I Parkes All-Sky Survey (HIPASS) of the southern sky in the neutral hydrogen line are used to determine the radial velocities and widths of the H I line for flat spiral galaxies of the Revised Flat-Galaxy Catalog (RFGC) seen edge-on. The sample of 103 flat galaxies detected in HIPASS is characterized by a median radial velocity of +2037 km/sec and a median width of the H I line at the level of 50% of maximum of 242 km/sec. For RFGC galaxies the 50% detection level in HIPASS corresponds to an apparent magnitude B t = 14 m .5 or an angular diameter a = 2′.9. The relative number of detected galaxies increases from 2% for the morphological types Sbc and Sc to 41% for the type Sm. The median value of the ratio of hydrogen mass to total mass for RFGC galaxies is 0.079. With allowance for the average internal extinction for edge-on galaxies, <ΔB t< = 0 m .75, the median ratio of hydrogen mass to luminosity, M H I/L B = 0.74 M ⊙/L ⊙, is typical for late-type spirals. Because of its small depth, HIPASS reveals only a few RFGC galaxies with previously unknown velocities and line widths.

The bulk motion of flat galaxies on scales of 100 Mpc in the quadrupole and octupole approximations

The peculiarities of non-Hubble bulk motions of galaxies are studied by analyzing a sample of 1271 thin edge-on spirals with distances determined using a multiparametric Tully-Fisher relation that includes the amplitude of the galaxy rotation, the blue and red diameters, surface brightness, and morphological type. In the purely dipole approximation, the bulk motion of galaxies relative to the cosmic microwave background frame can be described by the velocity of 336±96 km s−1 in the direction l=321°, b=−1° within radius Rmax=10000 km s−1. An analysis of more complex velocity field models shows that the anisotropy of the Hubble expansion described by the quadrupole term is equal to ∼5% on scale lengths Rmax=6000–10000 km s−1. The amplitude within the Local Supercluster (Rmax=3000 km s−1) is as high as ∼20%. The inclusion of the octupole component reduces the dipole amplitude to 134±111 km s−1 on scale lengths of ∼8000 km s−1. The most remarkable feature of the galaxy velocity field within Rmax=8000 km s−1 is the zone of minimum centered on l=80°, b=0° (the constellation of Cygnus) whose amplitude reaches 18% of the mean Hubble velocity.

Supercomputer simulations of disk galaxies

The time evolution of models for an isolated disk of highly flattened galaxies of stars is investigated by direct integration of the Newtonian equations of motion of N=30,000 identical stars over a time span of many galactic rotations. Certain astronomical implications of the simulations to actual disk-shaped (i.e. rapidly rotating) galaxies are explored as well.

Mass profiles and anisotropies of early-type galaxies

We discuss the problem of using stellar kinematics of early-type galaxies to constrain the orbital anisotropies and radial mass profiles of galaxies. We demonstrate that compressing the light distribution of a galaxy along the line of sight produces approximately the same signature in the line-of-sight velocity profiles as radial anisotropy. In particular, fitting spherically symmetric dynamical models to apparently round, isotropic face-on flattened galaxies leads to a spurious bias towards radial orbits in the models, especially if the galaxy has a weak face-on stellar disc. Such face-on stellar discs could plausibly be the cause of the radial anisotropy found in spherical models of intermediate luminosity ellipticals such as NGC 2434, 3379 and 6703. In the light of this result, we use simple dynamical models to constrain the outer mass profiles of a sample of 18 round, early-type galaxies. The galaxies follow a Tully—Fisher relation parallel to that for spiral galaxies, but fainter by at least 0.8 mag (I-band) for a given mass. The most luminous galaxies show clear evidence for the presence of a massive dark halo, but the case for dark haloes in fainter galaxies is more ambiguous. We discuss the observations that would be required to resolve this ambiguity.

A Structural and Dynamical Study of Late-Type, Edge-on Galaxies. I. Sample Selection and Imaging Data

We present optical (B and R) and infrared (Ks) images and photometry for a sample of 49 extremely late-type, edge-on galaxies selected from the Flat Galaxy Catalog of Karenchentsev et al. Our sample was selected to include galaxies with particularly large axial ratios, increasing the likelihood that the galaxies in the sample are truly edge-on. We have also concentrated the sample on galaxies with low apparent surface brightness in order to increase the representation of intrinsically low surface brightness galaxies. Finally, the sample was chosen to have no apparent bulges or optical warps so that the galaxies represent undisturbed, pure disk systems. The resulting sample forms the basis for a much larger spectroscopic study designed to place constraints on the physical quantities and processes that shape galaxies. The imaging data presented in this paper have been painstakingly reduced and calibrated to allow accurate surface photometry of features as faint as 30 mag arcsec-2 in B and 29 mag arcsec-2 in R on scales larger than 10''. Because of limitations in sky subtraction and flat-fielding, the infrared data can reach only to 22.5 mag arcsec-2 in Ks on comparable scales. As part of this work, we have developed a new method for quantifying the reliability of surface photometry, which provides useful diagnostics for the presence of scattered light, optical emission from infrared cirrus, and other sources of nonuniform sky backgrounds.

Landau excitation of spiral density waves in an inhomogeneous disk of stars

Drift-resonant Landau excitation of spiral density waves in a stellar disk of flat galaxies is proposed. This excitation of waves is suggested as a mechanism for the formation of structural features such as spiral arms and the slow dynamical relaxation of galaxies in a regime of hydrodynamical Jeans-type stability.

An H I survey of highly flattened, edge-on, pure disk galaxies

We have undertaken an 21-cm line survey of 472 late-type, edge-on spiral galaxies using the NancayRadio Telescope and the Green Bank 140-Foot Telescope. Our targets consisted of highly inclined spirals with large disk axial ratios and little or no bulge component–i.e., highly flattened, pure disk galaxies. These objects were primarily selected from the Flat Galaxy Catalogue (FGC) of Karachentsev et al. (1993). Most were classified as Hubble types Scd or later, with an emphasis on the largest angular size and lowest optical surface brightness objects in the FGC. Our detection rate was %, underscoring that relatively small, gas-rich, moderate-to-low surface brightness pure disk spirals are an extremely common constituent of the nearby universe. Our data also suggest that the most highly flattened spirals are primarily isolated systems. The majority of our detected galaxies had no previously published redshifts or data. Here we present parameters for the 239 detected sources and 45 marginal detections, together with search limits for the undetected galaxies.

Linear kinetic theory of instabilities of a gravitatingstellar disk

Linear kinetic theory is developed to describe collective oscillations (and their instabilities) propagating in a rapidly rotating disk of stars, representing a highly flattened galaxy. The analysis is carried out for the special case of a self-gravitating, infinitesimally thin, and spatially inhomogeneous system, taking into account the effects both of thermal movements of stars and of gravitational encounters between stars and giant molecular clouds of an interstellar medium. The star–cloud encounters are described with the use of the Landau collision integral. The dynamics of gravity perturbations with rare interparticle encounters is considered. Such a disk is treated by employing the well elaborated mathematical formalisms from plasma perturbation theory using normal-mode analysis. In particular, the method of solving the Boltzmann equation is applied by integration along paths, neglecting the influence of star–cloud encounters on the distribution of stars in the zeroth-order approximation. We are especially interested in important kinetic effects due to wave–star resonances, which we have little knowledge about. The kinetic effects are introduced via a minor drift motion of stars which is computed from the equations of stellar motion in an unperturbed central force field of a galaxy. The dispersion laws for two main branches of disk's oscillations, that is the classical Jeans branch and an additional gradient branch, are deduced. The resonant Landau-type instabilities of hydrodynamically stable Jeans and gradient gravity perturbations is considered to be a long-term generating mechanism for propagating density waves, thereby leading to spiral-like and/or ring-like patterns in the flat galaxies.

Central NGC 2146: A Firehose‐Type Bending Instability in the Disk of Newly Formed Stars?

As the observations of highly flattened galaxies, including the Milky Way, and gravitational N-body simulations show, the central parts of these systems at distances of, say, r < 0.5-0.7 kpc from the center rotate slowly, and their local circular velocities of regular rotation become less than (or comparable to) the residual (random) velocities. In such a thin, practically nonrotating, pressure-supported central disk, a typical star moves along the bending, perpendicular to the equatorial plane layer, under the action of two forces acting in opposite directions: the destabilizing centrifugal force, c, and the restoring gravitational attraction, g. Obviously, fierce instabilities of the buckling kind developing perpendicular to the plane may not be avoided if Fc > Fg. The latter condition is none other than the condition of the so-called firehose electromagnetic instability in collisionless plasmas. The source of free energy in the instability is the intrinsic anisotropy of a velocity dispersion ("temperature"). It seems reasonable that this is a natural mechanism for building a snake-shaped radio structure, which has recently been observed by Zhao and coworkers in the central region of the spiral starburst galaxy NGC 2146 with the Very Large Array at an angular resolution of 2''. In the current paper, in order to investigate the dynamics of the central region, N-body simulations of the firehose-type bending instability are presented for this galaxy. Use of concurrent computers has enabled us to make long simulation runs using a sufficiently large number of particles in the direct summation code, N ~ 20,000. In contrast to all previous N-body simulations of bending instabilities, we show how bending structures may be longer lived in real starburst galaxies than in the computer models. The simulations clearly confirm the qualitative picture and, moreover, are in fair quantitative agreement with the theory. A theoretical prediction is confirmed that the instability is driven by an excess of plane kinetic energy of random motions of stars, when the ratio of the dispersion of radial velocities of stars in the plane cr to the velocity dispersion in the perpendicular direction cz is large enough, cr ≳ 0.6cz, in other words, if the thickness of the stellar disk h ∝ cz is small enough. The extent to which our results on the stability of the disk can have a bearing on observable spiral galaxies with a high star formation rate in the central parts is discussed.

Low-luminosity BL Lac Objects and Flat-spectrum Radio Galaxies

AbstractWe present first results from a new sample of low radio luminosity flat spectrum radio galaxies.

6.13. Central NGC 2146 – a bending instability in the disk of newly formed stars?

As the observations of highly flattened galaxies including the Milky Way and many-body (N-body) simulations show, the central parts of these systems, say, at distances r &lt; 0.5–0.7 kpc from the center, rotate slowly and their local circular velocities of regular rotation become less than (or comparable to) the residual (random) velocities. In such a thin, practically nonrotating circumnuclear disk, a typical star moves along the bending, perpendicular to the equatorial plane layer under the action of two forces which act in opposite directions: the destabilizing centrifugal force Fc and the restoring gravitational attraction Fg. Obviously, fierce instabilities of the buckling kind developing perpendicular to the plane may not be avoided if Fc &gt; Fg. The latter condition is none other than the condition of “firehose” electromagnetic instability in collisionless plasmas which is driven by the particle “pressure” anisotropy.

Spectroscopy of Edge-On Spirals

We present HI line observations of 744 edge-on spiral galaxies, extracted from the Flat Galaxy Catalog of Karachentsev et al. (1993). Fluxes, systemic velocities and line widths are given for 587 detected galaxies, as well as search parameters for 157 undetected systems. Widths are corrected for instrumental broadening, smoothing, signal-to--noise and profile shape, and an estimate of the error on the width is given. When corrected for turbulent broadening and inclination angle of the disks, the velocity widths presented here can provide the appropriate line width parameter needed to derive distances via the Tully-Fisher relation.

Dynamics of the Interstellar Gas in the Vicinity of a Bar

A numerical simulation of the disk dynamics of a flat stellar-gaseous galaxy is presented. This simulation is based on N-body modelling for a stellar disk, together with integration of the two-dimensional hydrodynamic equations for a gaseous disk. The existence of a quasi-periodic regime of disk evolution found earlier in a purely hydrodynamic simulation is confirmed. Intense gas flows in the central area of the disk due to the saddle point of the bar potential can support the active galactic nuclei in Seyfert galaxies and form double nuclei.

A gaseous disk in external barred potential: Angular momentum and mass transfer

Abstract The evolution of the gaseous disk of a flat galaxy in the field of external gravitational potential of stellar disk, spheroidal component and rotating bar-like perturbation is simulated. The bar-like perturbation amplitude is growing from zero up to some cut-off value and does not change thereafter. It is shown that with rapidly growing bar such disk passes through two stages of evolution. In the first one the mass and angular momentum redistribution (following the density spiral wave generation by bar potential) leads to dynamic separation of the central and peripheral regions of the disk. In the second stage a quasi-periodic process sets up. During every quasi-period there occur consequently: a superposition of two-armed spiral wave and perturbation ring moving outwards; a distortion of the spiral perturbation into a ring; the stop of the resulting ring-like perturbation and the development of the inward motion which reflect at the outer Lindblad resonance. The angular momentum and mass transport and physical mechanisms governing quasi-periodic process are analyzed in detail. A possible application of the results obtained to the interpretation of the dynamics of SB galaxies is discussed.

Stability of the Stellar Disks of Flat Galaxies. I. A Collisionless, Homogeneous System

view Abstract Citations (26) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stability of the Stellar Disks of Flat Galaxies. I. A Collisionless, Homogeneous System Griv, Evgeny ; Peter, William Abstract A differentially rotating stellar disk, representing highly flattened galaxies, is studied by employing the linear kinetic theory to determine its stability against small-amplitude gravity perturbations. The analysis is carried out for the special case of an infinitely thin, spatially homogeneous system, taKing into account the effects of random (residual) movements of stars, but neglecting collisions. The influence of drift motion on the stability of disk oscillations is investigated. This minor motion of a star, whose value may be determined from the higher order approximation of Lindblad's epicyclic theory, is analogous to the grad B drift of an electrically charged particle in a plasma and is related to the differential rotation of the system. In classical stellar dynamics, this drift has been known as the asymmetric drift. Within the local approximation of the WKB method, it is shown that a disk rotating differentially could be unstable due to either a gravitational Jeans-type instability or an oscillating kinetic-type instability (overstability). To suppress Jeans instabilities, a generalized local stability criterion must be satisfied, which indicates that in a differentially rotating disk, nonaxisymmetric perturbations are more unstable than axisymmetric ones. A Jeans-stable disk is still unstable to oscillatory growing perturbations. The etiology of this oscillating instability is the resonant interaction of drifting stars with Jeans-stable waves at the corotation resonance: nonaxisymmetric Jeans-stable modes of the differentially rotating disk are modified by drifts and can become unstable. It is similar to the instability of the Cherenkov type (the inverse Landau damping effect) in plasmas, and it is due to the nature of the differential rotation of the galactic system. The growth rates of the amplitudes of the excited waves are exponentially small, excepting the corotation resonant regions of the system. As drift motion always exists in the differentially rotating disk, the oscillating instability can be considered to be a long-term generating mechanism for propagating density waves, thereby leading to spiral-like patterns in the flat galaxies. It is suggested also that because of the resonant interaction of stars with the gravitational field of the oscillatory growing waves, the rate of relaxation of a disk toward equilibrium can be enhanced. This collisionless collective relaxation will be accompanied by a secular increase in the stellar velocity dispersion and a redistribution of the surface mass density to a more peaked distribution. The latter may assist in the formation of a condensed nucleus of a galaxy. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177761 Bibcode: 1996ApJ...469...84G Keywords: GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: STRUCTURE; INSTABILITIES full text sources ADS | data products SIMBAD (4) Related Materials (2) Part 2: 1996ApJ...469...99G Part 3: 1996ApJ...469..103G

Stability of the Stellar Disks of Flat Galaxies. II. The Effect of Spatial Inhomogeneity

view Abstract Citations (9) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stability of the Stellar Disks of Flat Galaxies. II. The Effect of Spatial Inhomogeneity Griv, Evgeny ; Peter, William Abstract The investigation of the first paper of the present series is extended to include the effect of spatial inhomogeneity, i.e., the surface density, angular velocity, and velocity dispersion gradients of a stellar disk. Similar to the investigation of the first paper, the well- elaborated mathematical methods of plasma kinetic theory are used. The influence of disk inhomogeneity on the oscillation spectrum of different small-amplitude oscillations in a stellar disk of flat galaxies is studied. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177762 Bibcode: 1996ApJ...469...99G Keywords: GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: STRUCTURE; INSTABILITIES; GALAXIES: INTERACTIONS full text sources ADS | Related Materials (2) Part 1: 1996ApJ...469...84G Part 3: 1996ApJ...469..103G

Stability of the Stellar Disks of Flat Galaxies. III. The Effect of Collisions

view Abstract Citations (10) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stability of the Stellar Disks of Flat Galaxies. III. The Effect of Collisions Griv, Evgeny ; Peter, William Abstract According to observations, the occurrence of gravitational collisions in the disk of our own Galaxy is evidenced by the encounters between stars and giant molecular clouds. In the solar vicinity the timescale for encounter between stars and clouds is around t ~ 10^9^ yr. Because this timescale is much smaller than the age of the Galaxy T ~ 10^10^ yr, the theory of stability of the stellar disk has to be extended to include the effects of star-cloud gravitational encounters. In this paper, a method of investigating the small-amplitude oscillations and their stability of a collisional two-dimensional galactic disk is developed, through the studying of dispersion relations. A spatially inhomogeneous, differentially rotating disk is considered, with the property that the epicyclic frequency, as well as the angular velocity of rotation, greatly exceeds the frequency v_c_ ~ 1/t of binary collisions between particles. The resulting kinetic equation for the perturbed distribution function can be solved by successive approximations, neglecting the influence of star-cloud encounters on the equilibrium velocity distribution of stars in the zeroth-order approximation. Kinetic theory with the model Bhatnagar-Gross-Krook collisional integral is used, so that the analysis is extended to regions of wavelengths and eigenfrequencies ω of oscillations inaccessible by the hydrodynamic approach developed by Lynden-Bell & Pringle and others. A general dispersion relation is obtained and analyzed in the different cases of weak, ω^2^ >> v_c_^2^, and strong collisions, ω^2^ << v_c_^2^. Using this dispersion relation, the effects of encounters on the dispersion laws both of Jeans and gradient perturbations are considered. The influence of particle encounters on the growth of the oscillating instability, studied in the first paper of the series, is investigated also. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177763 Bibcode: 1996ApJ...469..103G Keywords: GALAXIES: INTERACTIONS; GALAXIES: KINEMATICS AND DYNAMICS; HYDRODYNAMICS; ISM: CLOUDS full text sources ADS | Related Materials (2) Part 1: 1996ApJ...469...84G Part 2: 1996ApJ...469...99G

Bar Properties as a Function of Hubble Type

AbstractPrevious optical surface photometry of barred spiral galaxies revealed that there are two distinct types of bars: large bars tend to have a nearly constant surface brightness (”flat“ bar), while smaller bars tend to have a decreasing surface brightness with a scale length similar to the disk (”exponential“ bar). Statistically, flat bars tend to occur in early Hubble types and exponential bars in later types. Studies of resonances in spirals indicate that flat bars end inside corotation, while exponential bars end between the inner Lindblad and 4:1 resonances. Near-infrared (JHK) surface photometry of bars is presented in order to compare the stellar distributions and bar potentials in flat and exponential barred galaxies. The presence of isophotal twists in some galaxies provides additional information on resonances. The grand design and fiocculent optical structures in the two types of barred galaxies will be compared and contrasted with their near-infrared light distributions.

Stability of a Collisionless Stellar Disk

The study of stability of the stellar disks of flat galaxies is the first step towards an understanding of the phenomena of spiral structure. To explain the spiral pattern, Lin and Shu (1964) further developed the Lindblad's idea of density waves by considering the spiral structure as the collective effect in a self-gravitating system. As an extension to the original Lin-Shu theory, unstable spiral modes have been obtained from an asymptotic analysis (e.g., Bertin [1980]). In such “modal theory”, the Jeans-type instability (even weak) is important to the maintenance of spiral structure.

Large‐scale streaming of flat galaxies

AbstractWe consider a sample of thin edge‐on galaxies from new a Flat Galaxy Catalogue, FGC (Karachentsev et al. 1993) covering the whole sky. The galaxies have been selected into the Catalogue by their apparent axis ratio a/b &gt; 7 and angular diameter a ≥ 0.6 arcmin. Among 4455 such galaxies 893 have estimates both radial velocity and inner motion amplitude, however most part of them (542) are concetrated in the Arecibo zone. This sample is characterized by the medians: uh = +5300 Km s−1, W50 = 285 km s−1 and Ty = Scd.To determine distances and peculiar velocities of the flat galaxies we use different modifications of the Tully‐Fisher relation between linear diameter and HI linewidth taking into account also their surface brightness. A typical scatter of galaxy distance estimates by this method corresponds to dex (0.09).In a dipole approach we calculated the terms of bulk motion of the galaxies. Relative to the CMB frame the population of FGC galaxies is moving to apex l = 319° ± 10°, b = +28° ± 11° with velocity 260 ± 40 km s−1. Here the formal 1σ errors correspond to a scattering of estimates carried out with different kinds of the TF relation (blue or red diameters, two‐ or three‐parametric regressions).A consecutive removing of nearby galaxies by a condition of V &lt; Vmin leads to a rise of the mean bulk velocity and to a drift of the apex direction: Vbulk = 400 ± 70km s−1, l = 304° ± 6°, b = +23° ± 12° for Vmin = 3500km s−1 and Vbulk = 450 ± 40km s−1, l = 301° ± 7°, b = +23° ± 4° for Vmin = 8500km s−1, what remains not far from the Shapley concentration (l = 311°, b = +30°) of rich clusters.