scholarly journals Evolution of the vertical structure of galactic disks

1985 ◽  
Vol 106 ◽  
pp. 491-492 ◽  
Author(s):  
Jens Verner Villumsen
Keyword(s):  
Vertical Structure ◽  
Physical Mechanism ◽  
Equations Of Motion ◽  
Giant Molecular Clouds ◽  
Galactic Disks ◽  
Model Galaxy ◽  
Stellar Component ◽  
Adiabatic Cooling ◽  
Soft Particles ◽  
Exponential Disk

Numerical simulations of the evolution of the vertical structure of galactic disks have been performed. The physical mechanism for the evolution is the scattering of stars off Giant Molecular Clouds (GMCs) as proposed by Spitzer and Schwarzschild (1951). A model galaxy consists of a fixed, nearly isothermal halo plus an axisymmetric, thin exponential disk consisting of 1000 stars. A population of GMCs is embedded in the disk. The stars interact with each other via a self-consistent axisymmetric field determined from an expansion in spherical harmonics to twelfth order. The stars scatter off the GMCs that are modelled as soft particles. The equations of motion of the stars and the GMCs are integrated directly to high accuracy. Adiabatic cooling is therefore included implicitly. In order to avoid axisymmetric instabilities, the stellar component is initially relatively hot in the plane of the disk. Nineteen simulations were performed with varying parameters to check the consistency of the results.

Experimental Analysis of a Planar Reconfigurable Mechanism With a Variable Joint

2016 ◽  
Author(s):  
Peter W. Malak ◽  
Anthony J. Buchta ◽  
Philip A. Voglewede
Keyword(s):  
Optimal Control ◽  
Transition Point ◽  
Physical Mechanism ◽  
Equations Of Motion ◽  
Angular Position ◽  
The Other ◽  
Simplified Model ◽  
Physical Prototype ◽  
Kinetics Of ◽  
Kinematics And Kinetics

Previously a specific planar reconfigurable mechanism with a variable joint (RRRR1 -RRRP2 Mechanism) was dynamically modeled. The RRRR-RRRP Mechanism functions as a RRRR mechanism in one configuration and as a in RRRP mechanism the other. The kinematics and kinetics of the RRRP and RRRR configurations were previously analyzed with a Lagrangian approach. The developed equations of motion will be validated with a physical prototype in this paper. In addition, a simplified model of the RRRR-RRRP Mechanism is also developed and compared to the experimental results. The experimental angular position of each joint on the RRRR-RRRP Mechanism will be compared to the model position analysis. Particular attention will be given to the transition point when the physical mechanism changes from an RRRR mechanism to RRRP mechanism and vice versa as it is vital to knowing this point for optimal control of the mechanism.

scholarly journals Starbursts Triggered by Cloud Compression in Interacting Galaxies

1999 ◽  
Vol 186 ◽  
pp. 235-242
Author(s):  
Chanda J. Jog
Keyword(s):  
High Pressure ◽  
Star Formation ◽  
Shock Compression ◽  
Physical Mechanism ◽  
Hydrogen Gas ◽  
Interacting Galaxies ◽  
Giant Molecular Clouds ◽  
Main Mode ◽  
Radiative Shock ◽  
Intercloud Medium

We propose a physical mechanism for the triggering of starbursts in interacting spiral galaxies by shock compression of the pre-existing disk giant molecular clouds (GMCs). We show that as a disk GMC tumbles into the central region of a galaxy following a galactic tidal encounter, it undergoes a radiative shock compression by the pre-existing high pressure of the central molecular intercloud medium. The shocked outer shell of a GMC becomes gravitationally unstable, which results in a burst of star formation in the initially stable GMC. In the case of colliding galaxies with physical overlap such as Arp 244, the cloud compression is shown to occur due to the hot, high-pressure remnant gas resulting from the collisions of atomic hydrogen gas clouds from the two galaxies. The resulting values of infrared luminosity agree with observations. The main mode of triggered star formation is via clusters of stars, thus we can naturally explain the formation of young, luminous star clusters observed in starburst galaxies.

scholarly journals Is NGC 300 a pure exponential disk galaxy?

2020 ◽  
Vol 640 ◽  
pp. L19
Author(s):  
In Sung Jang ◽  
Roelof S. de Jong ◽  
Ivan Minchev ◽  
Eric F. Bell ◽  
Antonela Monachesi ◽  
...  
Keyword(s):  
Stellar Populations ◽  
Disk Galaxy ◽  
Type I ◽  
Density Profiles ◽  
Stellar Component ◽  
Stellar Halo ◽  
Cast Doubt ◽  
Low Mass ◽  
Red Giant ◽  
Exponential Disk

NGC 300 is a low-mass disk galaxy in the Sculptor group. In the literature, it has been identified as a pure exponential disk galaxy, as its luminosity profile can be well fit with a single exponential law over many disk scale lengths (Type I). We investigate the stellar luminosity distribution of NGC 300 using Hubble Space Telescope archive data, reaching farther and deeper than any other previous studies. Color-magnitude diagrams show a significant population of old red giant branch (RGB) stars in all fields out to R ∼ 19 kpc (32′), as well as younger populations in the inner regions. We construct the density profiles of the young, intermediate-aged, and old stellar populations, and find two clear breaks in the density profiles of the old RGB and intermediate-aged stars: one down bending (Type II) at R ∼ 5.9 kpc, and another up bending (Type III) at R ∼ 8.3 kpc. Moreover, the old RGB stars exhibit a negative radial color gradient with an upward bend at R ∼ 8 kpc, beyond which the stellar populations are uniformly old (>7 Gyr) and metal poor ([Fe/H] = −1.6−0.4+0.2 dex). The outer stellar component at R ⪆ 8 kpc is therefore well separated from the inner disk in terms of stellar density and stellar population. While our results cast doubt on the currently established wisdom that NGC 300 is a pure exponential disk galaxy, a more detailed survey should be carried out to identify the outskirts as either a disk or a stellar halo.

The centring dynamics of a thin liquid shell in capillary oscillations

1988 ◽  
Vol 188 ◽  
pp. 411-435 ◽  
Author(s):  
Chun P. Lee ◽  
Taylor G. Wang
Keyword(s):  
Physical Mechanism ◽  
Numerical Study ◽  
Equations Of Motion ◽  
Thin Sheet ◽  
Nonlinear Effects ◽  
Experimental Information ◽  
Centre Of Mass ◽  
One Dimensional ◽  
The Core ◽  
Nonlinear Equations Of Motion

The physical mechanism governing the centring of a hollow liquid shell in capillary oscillations, which has been observed in experiments, is investigated theoretically. First, the shell is assumed to be inviscid and to have a thickness that is much less than its spherical radius. A system of one-dimensional nonlinear equations of motion is derived using a thin-sheet model. From a numerical study the nonlinear effects of the wave are found to cause the core to oscillate slowly relative to the shell while the centre of mass of the whole system remains stationary. The effects of small viscosity are then considered in an approximation. Finally the strength of the centring mechanism is compared with that of the decentring effect due to buoyancy. The findings are consistent with the limited experimental information available.

The vertical structure of galactic disks

1987 ◽  
Vol 322 ◽  
pp. 59 ◽  
Author(s):  
R. G. Carlberg
Keyword(s):  
Vertical Structure ◽  
Galactic Disks

scholarly journals On the Generalized Eigenvalue Problem for the Rossby Wave Vertical Velocity in the Presence of Mean Flow and Topography

2012 ◽  
Vol 42 (6) ◽  
pp. 1045-1050 ◽  
Author(s):  
Rémi Tailleux
Keyword(s):  
Eigenvalue Problem ◽  
Rossby Wave ◽  
Vertical Velocity ◽  
Vertical Structure ◽  
Nonlinear Term ◽  
Eigenvalue Problems ◽  
Equations Of Motion ◽  
Generalized Eigenvalue Problem ◽  
Mean Flow ◽  
Generalized Eigenvalue

Abstract In a series of papers, Killworth and Blundell have proposed to study the effects of a background mean flow and topography on Rossby wave propagation by means of a generalized eigenvalue problem formulated in terms of the vertical velocity, obtained from a linearization of the primitive equations of motion. However, it has been known for a number of years that this eigenvalue problem contains an error, which Killworth was prevented from correcting himself by his unfortunate passing and whose correction is therefore taken up in this note. Here, the author shows in the context of quasigeostrophic (QG) theory that the error can ultimately be traced to the fact that the eigenvalue problem for the vertical velocity is fundamentally a nonlinear one (the eigenvalue appears both in the numerator and denominator), unlike that for the pressure. The reason that this nonlinear term is lacking in the Killworth and Blundell theory comes from neglecting the depth dependence of a depth-dependent term. This nonlinear term is shown on idealized examples to alter significantly the Rossby wave dispersion relation in the high-wavenumber regime but is otherwise irrelevant in the long-wave limit, in which case the eigenvalue problems for the vertical velocity and pressure are both linear. In the general dispersive case, however, one should first solve the generalized eigenvalue problem for the pressure vertical structure and, if needed, diagnose the vertical velocity vertical structure from the latter.

scholarly journals Indications of the invalidity of the exponentiality of the disk within bulges of spiral galaxies

2020 ◽  
Vol 640 ◽  
pp. A20 ◽  
Author(s):  
Iris Breda ◽  
Polychronis Papaderos ◽  
Jean-Michel Gomes
Keyword(s):  
Galactic Center ◽  
Current Knowledge ◽  
Spectral Synthesis ◽  
Intensity Profile ◽  
Data Cube ◽  
Significant Fraction ◽  
Galactic Disks ◽  
Underlying Assumption ◽  
Dynamical Interaction ◽  
Exponential Disk

Context. A fundamental subject in extragalactic astronomy concerns the formation and evolution of late-type galaxies (LTGs). The standard scenario envisages a two-phase build-up for these systems, comprising the early assembly of the bulge followed by disk accretion. However, recent observational evidence points to a joint formation and perpetual coevolution of these structural components. Our current knowledge on the properties of the bulge and the disk is, to a large degree, founded on photometric decomposition studies, which sensitively depend on the adopted methodology and enclosed assumptions on the structure of LTGs. A critical assumption whose validity had never been questioned before is that galactic disks conserve their exponential nature up to the galactic center. This, although seemingly plausible, implies that bulge and disk coexist without significant dynamical interaction and mass exchange over nearly the entire Hubble time. Aims. Our goal is to examine the validity of the standard assumption that galactic disks preserve their exponential intensity profile inside the bulge radius (RB) all the way to the galactic center, as is generally assumed in photometric decomposition studies. Methods. We developed a spectrophotometric bulge-disk decomposition technique that provides an estimation for the net (i.e., disk-subtracted) spectrum of the bulge. Starting from an integral field spectroscopy (IFS) data cube, this tool computes the integrated spectrum of the bulge and the disk, scales the latter considering the light fraction estimated from photometric decomposition techniques, and subtract it from the former, thereby allowing for the extraction of the net-bulge spectrum. Considering that the latter depends on the underlying assumption for the disk luminosity profile, checking its physical plausibility (for instance, positiveness and spectral slope) places indirect constraints on the validity of the disk’s assumed profile inside the radius R⋆ < RB. In this pilot study, we tested the following three different disk configurations: the standard exponential disk profile as well as a centrally flattened or down-bending exponential disk profile. Results. A systematic application of our spectrophotometric bulge-disk decomposition tool to a representative sample of 135 local LTGs from the CALIFA survey yields a significant fraction (up to ∼30 (20)%) of unphysical net-bulge spectra when a purely exponential (centrally flattened) intensity profile is assumed for the disk. This never occurs for disks’ profiles involving a centrally decreasing intensity. Conclusions. The obtained results suggest that, for a significant fraction of LTGs, the disk component shows a down-bending beneath the bulge. If proven to be true, this result will call for a substantial revision of structural decomposition studies for LTGs and it will have far-reaching implications in our understanding of the photometric properties of their bulges. Given its major relevance, it appears worthwhile to explore the central stellar surface density of galactic disks further, through an improved version of the spectrophotometric decomposition tool presented here and its application combining deep surface photometry, spatially resolved spectral synthesis, and kinematical analyses.

The motion of a lunar orbiter

1966 ◽  
Vol 25 ◽  
pp. 373
Author(s):  
Y. Kozai
Keyword(s):  
Degrees Of Freedom ◽  
Dimensional Space ◽  
Artificial Satellite ◽  
Equations Of Motion ◽  
Triaxial Ellipsoid ◽  
The Moon ◽  
Secular Motion ◽  
The Earth ◽  
Close Satellite ◽  
The Mean

The motion of an artificial satellite around the Moon is much more complicated than that around the Earth, since the shape of the Moon is a triaxial ellipsoid and the effect of the Earth on the motion is very important even for a very close satellite.The differential equations of motion of the satellite are written in canonical form of three degrees of freedom with time depending Hamiltonian. By eliminating short-periodic terms depending on the mean longitude of the satellite and by assuming that the Earth is moving on the lunar equator, however, the equations are reduced to those of two degrees of freedom with an energy integral.Since the mean motion of the Earth around the Moon is more rapid than the secular motion of the argument of pericentre of the satellite by a factor of one order, the terms depending on the longitude of the Earth can be eliminated, and the degree of freedom is reduced to one.Then the motion can be discussed by drawing equi-energy curves in two-dimensional space. According to these figures satellites with high inclination have large possibilities of falling down to the lunar surface even if the initial eccentricities are very small.The principal properties of the motion are not changed even if plausible values ofJ3andJ4of the Moon are included.This paper has been published in Publ. astr. Soc.Japan15, 301, 1963.

On the motion of comet P/d’Arrest

1974 ◽  
Vol 22 ◽  
pp. 145-148
Author(s):  
W. J. Klepczynski
Keyword(s):  
Equations Of Motion ◽  
Variational Equations ◽  
Partial Derivatives

AbstractThe differences between numerically approximated partial derivatives and partial derivatives obtained by integrating the variational equations are computed for Comet P/d’Arrest. The effect of errors in the IAU adopted system of masses, normally used in the integration of the equations of motion of comets of this type, is investigated. It is concluded that the resulting effects are negligible when compared with the observed discrepancies in the motion of this comet.

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