4,272 research outputs found
Measuring Fundamental Galactic Parameters with Stellar Tidal Streams and SIM PlanetQuest
Extended halo tidal streams from disrupting Milky Way satellites offer new
opportunities for gauging fundamental Galactic parameters without challenging
observations of the Galactic center. In the roughly spherical Galactic
potential tidal debris from a satellite system is largely confined to a single
plane containing the Galactic center, so accurate distances to stars in the
tidal stream can be used to gauge the Galactic center distance, R_0, given
reasonable projection of the stream orbital pole on the X_GC axis.
Alternatively, a tidal stream with orbital pole near the Y_GC axis, like the
Sagittarius stream, can be used to derive the speed of the Local Standard of
Rest (\Theta_LSR). Modest improvements in current astrometric catalogues might
allow this measurement to be made, but NASA's Space Interferometry Mission (SIM
PlanetQuest) can definitively obtain both R_0 and \Theta_LSR using tidal
streams.Comment: 8 pages, 4 figures, accepted for publication in ApJ Letters (minor
text revisions). Version with high resolution figures available at
http://www.astro.caltech.edu/~drlaw/Papers/GalaxyParameters.pd
Low frequency m=1 normal mode oscillations of a self-gravitating disc
A continuous system such as a galactic disc is shown to be well approximated
by an N-ring differentially rotating self-gravitating system. Lowest order
(m=1) non-axisymmetric features such as lopsidedness and warps are global in
nature and quite common in the discs of spiral galaxies. Apparently these two
features of the galactic discs have been treated like two completely disjoint
phenomena. The present analysis based on an eigenvalue approach brings out
clearly that these two features are fundamentally similar in nature and they
are shown to be very Low frequency Normal Mode (LNM) oscillations manifested in
different symmetry planes of the galactic disc. Our analysis also show that
these features are actually long-lived oscillating pattern of the N-ring
self-gravitating system.Comment: 5 figures. Accepted for publication in MNRAS Letter
String Theory in the Penrose Limit of AdS_2 x S^2
The string theory in the Penrose limit of AdS_2 x S^2 is investigated. The
specific Penrose limit is the background known as the Nappi-Witten spacetime,
which is a plane-wave background with an axion field. The string theory on it
is given as the Wess-Zumino-Novikov-Witten (WZNW) model on non-semi-simple
group H_4. It is found that, in the past literature, an important type of
irreducible representations of the corresponding algebra, h_4, were missed. We
present this "new" representations, which have the type of continuous series
representations. All the three types of representations of the previous
literature can be obtained from the "new" representations by setting the
momenta in the theory to special values. Then we realized the affine currents
of the WZNW model in terms of four bosonic free fields and constructed the
spectrum of the theory by acting the negative frequency modes of free fields on
the ground level states in the h_4 continuous series representation. The
spectrum is shown to be free of ghosts, after the Virasoro constraints are
satisfied. In particular we argued that there is no need for constraining one
of the longitudinal momenta to have unitarity. The tachyon vertex operator,
that correspond to a particular state in the ground level of the string
spectrum, is constructed. The operator products of the vertex operator with the
currents and the energy-momentum tensor are shown to have the correct forms,
with the correct conformal weight of the vertex operator.Comment: 30 pages, Latex, no figure
Light scattering study of the “pseudo-layer” compression elastic constant in a twist-bend nematic liquid crystal
The nematic twist-bend (TB) phase, exhibited by certain achiral thermotropic liquid crystalline (LC) dimers, features a nanometer-scale, heliconical rotation of the average molecular long axis (director) with equally probable left- and right-handed domains. On meso to macroscopic scales, the TB phase may be considered as a stack of equivalent slabs or “pseudo-layers”, each one helical pitch in thickness. The long wavelength fluctuation modes should then be analogous to those of a smectic-A phase, and in particular the hydrodynamic mode combining “layer” compression and bending ought to be characterized by an effective layer compression elastic constant Beff and average director splay constant Keff1. The magnitude of Keff1 is expected to be similar to the splay constant of an ordinary nematic LC, but due to the absence of a true mass density wave, Beff could differ substantially from the typical value of ∼10⁶ Pa in a conventional smectic-A. Here we report the results of a dynamic light scattering study, which confirms the “pseudo-layer” structure of the TB phase with Beff in the range 10³–10⁴ Pa. We show additionally that the temperature dependence of Beff at the TB to nematic transition is accurately described by a coarse-grained free energy density, which is based on a Landau-deGennes expansion in terms of a heli-polar order parameter that characterizes the TB state and is linearly coupled to bend distortion of the director
Properties of Active Galaxies Deduced from H I Observations
We completed a new survey for H I emission for a large, well-defined sample
of 154 nearby (z < 0.1) galaxies with type 1 AGNs. We make use of the extensive
database presented in a companion paper to perform a comprehensive appraisal of
the cold gas content in active galaxies and to seek new strategies to
investigate the global properties of the host galaxies and their relationship
to their central black holes (BHs). We show that the BH mass obeys a strong,
roughly linear relation with the host galaxy's dynamical mass. BH mass follows
a looser, though still highly significant, correlation with the maximum
rotation velocity of the galaxy, as expected from the known scaling between
rotation velocity and central velocity dispersion. Neither of these H I-based
correlations is as tight as the more familiar relations between BH mass and
bulge luminosity or velocity dispersion, but they offer the advantage of being
insensitive to the glare of the nucleus and therefore are promising new tools
for probing the host galaxies of both nearby and distant AGNs. We present
evidence for substantial ongoing BH growth in the most actively accreting AGNs.
In these nearby systems, BH growth appears to be delayed with respect to the
assembly of the host galaxy but otherwise has left no detectable perturbation
to its mass-to-light ratio or its global gas content. The host galaxies of type
1 AGNs, including those luminous enough to qualify as quasars, are generally
gas-rich systems, possessing a cold interstellar medium reservoir at least as
abundant as that in inactive galaxies of the same morphological type. This
calls into question current implementations of AGN feedback in models of galaxy
formation that predict strong cold gas depletion in unobscured AGNs. (Abridged)Comment: To appear in ApJ; 14 page
Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC
I construct a near-IR star count map of the LMC and demonstrate, using the
viewing angles derived in Paper I, that the LMC is intrinsically elongated. I
argue that this is due to the tidal force from the Milky Way. The near-IR data
from the 2MASS and DENIS surveys are used to create a star count map of RGB and
AGB stars, which is interpreted through ellipse fitting. The radial number
density profile is approximately exponential with a scale-length 1.3-1.5 kpc.
However, there is an excess density at large radii that may be due to the tidal
effect of the Milky Way. The position angle and ellipticity profile converge to
PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 deg. At
large radii there is a drift of the center of the star count contours towards
the near side of the plane, which can be undrestood as due to viewing
perspective. The fact that PA_maj differes from the line of nodes position
angle Theta = 122.5 +/- 8.3 (cf. Paper I) indicates that the LMC disk is not
circular, but has an intrinsic ellipticity of 0.31. The LMC is elongated in the
general direction of the Galactic center, and is elongated perpendicular to the
Magellanic Stream and the velocity vector of the LMC center of mass. This
suggests that the elongation of the LMC has been induced by the tidal force of
the Milky Way. The position angle of the line of nodes differs from the
position angle Theta_max of the line of maximum line of sight velocity
gradient: Theta_max - Theta = 20-60 degrees. This could be due to: (a)
streaming along non-circular orbits in the elongated disk; (b) uncertainties in
the transverse motion of the LMC center of mass; (c) precession and nutation of
the LMC disk as it orbits the Milky Way (expected on theoretical grounds).
[Abridged]Comment: Astronomical Journal, in press. 34 pages, LaTeX, with 7 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution color figures from
http://www.stsci.edu/~marel/abstracts/abs_R32.htm
Dynamics of a ferromagnetic domain wall: avalanches, depinning transition and the Barkhausen effect
We study the dynamics of a ferromagnetic domain wall driven by an external
magnetic field through a disordered medium. The avalanche-like motion of the
domain walls between pinned configurations produces a noise known as the
Barkhausen effect. We discuss experimental results on soft ferromagnetic
materials, with reference to the domain structure and the sample geometry, and
report Barkhausen noise measurements on FeCoB amorphous
alloy. We construct an equation of motion for a flexible domain wall, which
displays a depinning transition as the field is increased. The long-range
dipolar interactions are shown to set the upper critical dimension to ,
which implies that mean-field exponents (with possible logarithmic correction)
are expected to describe the Barkhausen effect. We introduce a mean-field
infinite-range model and show that it is equivalent to a previously introduced
single-degree-of-freedom model, known to reproduce several experimental
results. We numerically simulate the equation in , confirming the
theoretical predictions. We compute the avalanche distributions as a function
of the field driving rate and the intensity of the demagnetizing field. The
scaling exponents change linearly with the driving rate, while the cutoff of
the distribution is determined by the demagnetizing field, in remarkable
agreement with experiments.Comment: 17 RevTeX pages, 19 embedded ps figures + 1 extra figure, submitted
to Phys. Rev.
Sloshing in High Speed Galaxy Interactions
Observations of lopsided spiral galaxies motivated us to explore whether the
rapid passage of a companion galaxy could cause them. We examine whether the
center of mass of the visible matter becomes displaced from the center of mass
of the dark halo during the intruder's passage, thereby causing an asymmetric
response and asymmetric structure. Two dimensional -body simulations
indicate that this can happen.
We also explore some consequences of this offset. These include the center of
mass of the visible disk following a decaying orbit around the halo center of
mass and the development of transient one-armed spirals that persist for up to
six rotation periods.
We then study the results of a variety of initial conditions based on such
offsets. We report on the results of several runs in which we initially offset
a disk from its halo's center of mass by an amount typical of the above
interaction. In some runs the halo is free to move, while in others it is held
fixed. We used three different mass distributions for the halo in these runs.
We find that the disk's center of mass spiraled inward creating a variety of
observed or observable phenomena including one-armed spirals, massive clumps of
particles, and counter-rotating waves. The systems settle into relatively
axisymmetric configurations. Whether or not the end states included a bar
depended on a variety of initial conditions.Comment: 20 text pages, 3 tables, 24 figures. A gzipped postscripped version
with higher resolution figures can be downloaded from
http://butch.umephy.maine.edu/kickers/Research/Sloshing/ . Accepted for
publication in The Astrophysical Journa
Catalog of Radio Galaxies with z>0.3. I:Construction of the Sample
The procedure of the construction of a sample of distant () radio
galaxies using NED, SDSS, and CATS databases for further application in
statistical tests is described. The sample is assumed to be cleaned from
objects with quasar properties. Primary statistical analysis of the list is
performed and the regression dependence of the spectral index on redshift is
found.Comment: 9 pages, 6 figures, 2 table
Dynamics of a ferromagnetic domain wall and the Barkhausen effect
We derive an equation of motion for the the dynamics of a ferromagnetic
domain wall driven by an external magnetic field through a disordered medium
and we study the associated depinning transition. The long-range dipolar
interactions set the upper critical dimension to be , so we suggest that
mean-field exponents describe the Barkhausen effect for three-dimensional soft
ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a
function of the field driving rate and the intensity of the demagnetizing
field, and find results in quantitative agreement with experiments on
crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde
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