50,041 research outputs found
Relation Between the Thickness of Stellar Disks and the Relative Mass of Dark Halo in Galaxies
We consider a thickness of stellar disks of late-type galaxies by analyzing
the R and K_s band photometric profiles for two independent samples of edge-on
galaxies. The main goal is to verify a hypotesis that a thickness of old
stellar disks is related to the relative masses of the spherical and disk
components of galaxies. We confirm that the radial-to-vertical scale length
ratio for galactic disks increases (the disks become thinner) with the
increasing of total mass-to-light ratio of the galaxies, which characterize the
contribution of dark halo to the total mass, and with the decreasing of central
deprojected disk brightness (surface density). Our results are in good
agreement with numerical models of collisionless disks evolved from subcritical
velocity dispersion state to a marginally stable equilibrium state. This
suggests that in most galaxies the vertical stellar velocity dispersion, which
determine the equilibrium disk thickness, is close to the minimum value, that
ensures disk stability. The thinnest edge-on disks appear to be low brightness
galaxies (after deprojection) in which a dark halo mass far exceeds a mass of
the stellar disk.Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002
Convective magneto-rotational instabilities in accretion disks
We present a study of instabilities occuring in thick magnetized accretion
disks. We calculate the growth rates of these instabilities and characterise
precisely the contribution of the magneto-rotational and the convective
mechanism. All our calculations are performed in radially stratified disks in
the cylindrical limit. The numerical calculations are performed using the
appropriate local dispersion equation solver discussed in Blokland et al.
(2005). A comparison with recent results by Narayan et al. (2002) shows
excellent agreement with their approximate growth rates only if the disks are
weakly magnetized. However, for disks close to equipartition, the dispersion
equation from Narayan et al. (2002) loses its validity. Our calculations allow
for a quantitative determination of the increase of the growth rate due to the
magneto-rotational mechanism. We find that the increase of the growth rate for
long wavelength convective modes caused by this mechanism is almost neglible.
On the other hand, the growth rate of short wavelength instabilities can be
significantly increased by this mechanism, reaching values up to 60%.Comment: 10 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Dynamical evolution of thin dispersion-dominated planetesimal disks
We study the dynamics of a vertically thin, dispersion-dominated disk of
planetesimals with eccentricities and inclinations (normalized in Hill
units) satisfying , . This situation may be typical
for e.g. a population of protoplanetary cores in the end of the oligarchic
phase of planet formation. In this regime of orbital parameters planetesimal
scattering has an anisotropic character and strongly differs from scattering in
thick () disks. We derive analytical expressions for the planetesimal
scattering coefficients and compare them with numerical calculations. We find
significant discrepancies in the inclination scattering coefficients obtained
by the two approaches and ascribe this difference to the effects not accounted
for in the analytical calculation: multiple scattering events (temporary
captures, which may be relevant for the production of distant planetary
satellites outside the Hill sphere) and distant interaction of planetesimals
prior to their close encounter. Our calculations show that the inclination of a
thin, dispersion-dominated planetesimal disk grows exponentially on a very
short time scale implying that (1) such disks must be very short-lived and (2)
planetesimal accretion in this dynamical phase is insignificant. Our results
are also applicable to the dynamics of shear-dominated disks switching to the
dispersion-dominated regime.Comment: 16 pages, 12 figures, submitted to A
Nonlinear Single-Armed Spiral Density Waves in Nearly Keplerian Disks
Single-armed, stationary density waves can propagate in very weakly
self-gravitating gas disks dominated by a central mass. Examples include
circumstellar disks of protostars and molecular disks in galactic nuclei. We
explore the linear and nonlinear dynamics of such waves. Variational methods
yield nonlinear versions of the dispersion relation, angular momentum flux, and
propagation velocity in the tight-winding limit. The pitch angle increases with
amplitude until the tight-winding approximation breaks down. We also find a
series of nonlinear logarithmic spirals which is exact in the limit of small
disk mass and which extends to large pitch angle.Comment: 16 pages, 3 figures. Uses mn.sty and mncite.sty. Accepted by MNRA
Gravitational stability and dynamical overheating of stellar disks of galaxies
We use the marginal stability condition for galactic disks and the stellar
velocity dispersion data published by different authors to place upper limits
on the disk local surface density at two radial scalelengths .
Extrapolating these estimates, we constrain the total mass of the disks and
compare these estimates to those based on the photometry and color of stellar
populations. The comparison reveals that the stellar disks of most of spiral
galaxies in our sample cannot be substantially overheated and are therefore
unlikely to have experienced a significant merging event in their history. The
same conclusion applies to some, but not all of the S0 galaxies we consider.
However, a substantial part of the early type galaxies do show the stellar
velocity dispersion well in excess of the gravitational stability threshold
suggesting a major merger event in the past. We find dynamically overheated
disks among both seemingly isolated galaxies and those forming pairs. The ratio
of the marginal stability disk mass estimate to the total galaxy mass within
four radial scalelengths remains within a range of 0.4---0.8. We see no
evidence for a noticeable running of this ratio with either the morphological
type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter
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