462 research outputs found
Dynamically-Driven Star Formation In Models Of NGC 7252
We present new dynamical models of the merger remnant NGC 7252 which include
star formation simulated according to various phenomenological rules. By using
interactive software to match our model with the observed morphology and gas
velocity field, we obtain a consistent dynamical model for NGC 7252. In our
models, this proto-elliptical galaxy formed by the merger of two similar
gas-rich disk galaxies which fell together with an initial pericentric
separation of ~2 disk scale lengths approximately 620 Myr ago. Results from two
different star formation rules--- density-dependent and shock-induced--- show
significant differences in star formation during and after the first passage.
Shock-induced star formation yields a prompt and wide-spread starburst at the
time of first passage, while density-dependent star formation predicts a more
slowly rising and centrally concentrated starburst. A comparison of the
distributions and ages of observed clusters with results of our simulations
favors shock-induced mechanism of star formation in NGC 7252. We also present
simulated color images of our model of NGC 7252, constructed by incorporating
population synthesis with radiative transfer and dust attenuation. Overall the
predicted magnitudes and colors of the models are consistent with observations,
although the simulated tails are fainter and redder than observed. We suggest
that a lack of star formation in the tails, reflected by the redder colors, is
due to an incomplete description of star formation in our models rather than
insufficient gas in the tails.Comment: 11 pages, 9 figures, to be published in MNRA
Structure and Evolution of Giant Cells in Global Models of Solar Convection
The global scales of solar convection are studied through three-dimensional
simulations of compressible convection carried out in spherical shells of
rotating fluid which extend from the base of the convection zone to within 15
Mm of the photosphere. Such modelling at the highest spatial resolution to date
allows study of distinctly turbulent convection, revealing that coherent
downflow structures associated with giant cells continue to play a significant
role in maintaining the strong differential rotation that is achieved. These
giant cells at lower latitudes exhibit prograde propagation relative to the
mean zonal flow, or differential rotation, that they establish, and retrograde
propagation of more isotropic structures with vortical character at mid and
high latitudes. The interstices of the downflow networks often possess strong
and compact cyclonic flows. The evolving giant-cell downflow systems can be
partly masked by the intense smaller scales of convection driven closer to the
surface, yet they are likely to be detectable with the helioseismic probing
that is now becoming available. Indeed, the meandering streams and varying
cellular subsurface flows revealed by helioseismology must be sampling
contributions from the giant cells, yet it is difficult to separate out these
signals from those attributed to the faster horizontal flows of
supergranulation. To aid in such detection, we use our simulations to describe
how the properties of giant cells may be expected to vary with depth, how their
patterns evolve in time, and analyze the statistical features of correlations
within these complex flow fields.Comment: 22 pages, 16 figures (color figures are low res), uses emulateapj.cls
Latex class file, Results shown during a Press release at the AAS meeting in
June 2007. Submitted to Ap
Leading Wave as a Component of the Spiral Pattern of the Galaxy
The spiral pattern of the Galaxy identified by analyzing the kinematics of
young stars within 3 kpc of the Sun is Fourier decomposed into spiral
harmonics. The spiral pattern of the Galaxy is shown to be representable as a
superposition of trailing and leading waves with interarm distances of
1.8(+/-0.4) kpc and 4(+/-2) kpc, respectively. Shock waves are probably present
only in the portions of the trailing spiral pattern where it crosses the crest
of the leading wave. The small interarm distance of the trailing spiral wave
(1.8 kpc) can be explained by its evolution - by the decrease in the interarm
distance as the wave is displaced toward the inner Lindblad resonance. The
Carina arm may be part of this resonance ring.Comment: 17 pages, 4 figures, to be published in Astronomy Letters, 200
Solar Multi-Scale Convection and Rotation Gradients Studied in Shallow Spherical Shells
The differential rotation of the sun, as deduced from helioseismology,
exhibits a prominent radial shear layer near the top of the convection zone
wherein negative radial gradients of angular velocity are evident in the low-
and mid-latitude regions spanning the outer 5% of the solar radius.
Supergranulation and related scales of turbulent convection are likely to play
a significant role in the maintenance of such radial gradients, and may
influence dynamics on a global scale in ways that are not yet understood. To
investigate such dynamics, we have constructed a series of three-dimensional
numerical simulations of turbulent compressible convection within spherical
shells, dealing with shallow domains to make such modeling computationally
tractable. These simulations are the first models of solar convection in a
spherical geometry that can explicitly resolve both the largest dynamical
scales of the system (of order the solar radius) as well as smaller-scale
convective overturning motions comparable in size to solar supergranulation
(20--40 Mm). We find that convection within these simulations spans a large
range of horizontal scales, and that the radial angular velocity gradient in
these models is typically negative, especially in low- and mid-latitude
regions. Analyses of the angular momentum transport indicates that such
gradients are maintained by Reynolds stresses associated with the convection,
transporting angular momentum inward to balance the outward transport achieved
by viscous diffusion and large-scale flows in the meridional plane. We suggest
that similar mechanisms associated with smaller-scale convection in the sun may
contribute to the maintenance of the observed radial shear layer located
immediately below the solar photosphere.Comment: 45 pages, 17 figures, ApJ in press. A preprint of paper with hi-res
figures can be found at
http://www-lcd.colorado.edu/~derosa/modelling/modelling.htm
The Interaction Of Multiple Convection Zones In A-type Stars
A-type stars have a complex internal structure with the possibility of
multiple convection zones. If not sufficiently separated, such zones will
interact through the convectively stable regions that lie between them. It is
therefore of interest to ask whether the typical conditions that exist within
such stars are such that these convections zones can ever be considered as
disjoint.
In this paper we present results from numerical simulations that help in
understanding how increasing the distance between the convectively unstable
regions are likely to interact through the stable region that separates them.
This has profound implications for mixing and transport within these stars.Comment: 9 pages, 15 figures, Preprint accepted for publication in MNRA
Outskirts of Nearby Disk Galaxies: Star Formation and Stellar Populations
The properties and star formation processes in the far-outer disks of nearby
spiral and dwarf irregular galaxies are reviewed. The origin and structure of
the generally exponential profiles in stellar disks is considered to result
from cosmological infall combined with a non-linear star formation law and a
history of stellar migration and scattering from spirals, bars, and random
collisions with interstellar clouds. In both spirals and dwarfs, the far-outer
disks tend to be older, redder and thicker than the inner disks, with the
overall radial profiles suggesting inside-out star formation plus stellar
scattering in spirals, and outside-in star formation with a possible
contribution from scattering in dwarfs. Dwarf irregulars and the far-outer
parts of spirals both tend to be gas dominated, and the gas radial profile is
often non-exponential although still decreasing with radius. The ratio of
H-alpha to far-UV flux tends to decrease with lower surface brightness in these
regions, suggesting either a change in the initial stellar mass function or the
sampling of that function, or a possible loss of H-alpha photons.Comment: 20 pages, 8 figures, Invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure
We present hydrodynamical models of the grand design spiral M51 (NGC 5194),
and its interaction with its companion NGC 5195. Despite the simplicity of our
models, our simulations capture the present day spiral structure of M51
remarkably well, and even reproduce details such as a kink along one spiral
arm, and spiral arm bifurcations. We investigate the offset between the stellar
and gaseous spiral arms, and find at most times (including the present day)
there is no offset between the stars and gas to within our error bars. We also
compare our simulations with recent observational analysis of M51. We compute
the pattern speed versus radius, and like the observations, find no single
global pattern speed. We also show that the spiral arms cannot be fitted well
by logarithmic spirals. We interpret these findings as evidence that M51 does
not exhibit a quasi-steady density wave, as would be predicted by density wave
theory. The internal structure of M51 derives from the complicated and
dynamical interaction with its companion, resulting in spiral arms showing
considerable structure in the form of short-lived kinks and bifurcations.
Rather than trying to model such galaxies in terms of global spiral modes with
fixed pattern speeds, it is more realistic to start from a picture in which the
spiral arms, while not being simple material arms, are the result of tidally
induced kinematic density `waves' or density patterns, which wind up slowly
over time.Comment: 23 pages, 20 figures, accepted for publication in MNRA
Secular Evolution of Galaxy Morphologies
Today we have numerous evidences that spirals evolve dynamically through
various secular or episodic processes, such as bar formation and destruction,
bulge growth and mergers, sometimes over much shorter periods than the standard
galaxy age of 10-15 Gyr. This, coupled to the known properties of the Hubble
sequence, leads to a unique sense of evolution: from Sm to Sa. Linking this to
the known mass components provides new indications on the nature of dark matter
in galaxies. The existence of large amounts of yet undetected dark gas appears
as the most natural option. Bounds on the amount of dark stars can be given
since their formation is mostly irreversible and requires obviously a same
amount of gas.Comment: 8 pages, Latex2e, crckapb.sty macros, 1 Postscript figure, replaced
with TeX source; To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
Age distributions of star clusters in spiral and barred galaxies as a test for theories of spiral structure
We consider models of gas flow in spiral galaxies in which the spiral
structure has been excited by various possible mechanisms: a global steady
density wave, self-gravity of the stellar disc and an external tidal
interaction, as well as the case of a galaxy with a central rotating bar. In
each model we estimate in a simple manner the likely current positions of star
clusters of a variety of ages, ranging from ~ 2 Myr to around 130 Myr,
depending on the model. We find that the spatial distribution of cluster of
different ages varies markedly depending on the model, and propose that
observations of the locations of age-dated stellar clusters is a possible
discriminant between excitation mechanisms for spiral structure in an
individual galaxy.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Mechanisms of the Vertical Secular Heating of a Stellar Disk
We investigate the nonlinear growth stages of bending instability in stellar
disks with exponential radial density profiles.We found that the unstable modes
are global (the wavelengths are larger than the disk scale lengths) and that
the instability saturation level is much higher than that following from a
linear criterion. The instability saturation time scales are of the order of
one billion years or more. For this reason, the bending instability can play an
important role in the secular heating of a stellar disk in the direction.
In an extensive series of numerical -body simulations with a high spatial
resolution, we were able to scan in detail the space of key parameters (the
initial disk thickness , the Toomre parameter , and the ratio of dark
halo mass to disk mass ). We revealed three distinct
mechanisms of disk heating in the direction: bending instability of the
entire disk, bending instability of the bar, and heating on vertical
inhomogeneities in the distribution of stellar matter.Comment: 22 pages including 8 figures. To be published in Astronomy Letters
(v.29, 2003
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