527 research outputs found
Equilibrium Disk-Bulge-Halo Models for the Milky Way and Andromeda Galaxies
We describe a new set of self-consistent, equilibrium disk galaxy models that
incorporate an exponential disk, a Hernquist model bulge, an NFW halo and a
central supermassive black hole. The models are derived from explicit
distribution functions for each component and the large number of parameters
permit detailed modeling of actual galaxies. We present techniques that use
structural and kinematic data such as radial surface brightness profiles,
rotation curves and bulge velocity dispersion profiles to find the best-fit
models for the Milky Way and M31. Through N-body realizations of these models
we explore their stability against the formation of bars. The models permit the
study of a wide range of dynamical phenomenon with a high degree of realism.Comment: 58 pages, 20 figures, submitted to the Astrophysical Journa
Sterile Neutrinos as Dark Matter
The simplest model that can accomodate a viable nonbaryonic dark matter
candidate is the standard electroweak theory with the addition of right-handed
or sterile neutrinos. We reexamine this model and find that the sterile
neutrinos can be either hot, warm, or cold dark matter. Since their only direct
coupling is to left-handed or active neutrinos, the most efficient production
mechanism is via neutrino oscillations. If the production rate is always less
than the expansion rate, then these neutrinos will never be in thermal
equilibrium. However, enough of them may be produced so that they provide the
missing mass necessary for closure. We consider a single generation of neutrino
fields with a Dirac mass, , and a Majorana
mass for the right-handed components only, . For we show that the
number density of sterile neutrinos is proportional to so that the
energy density today is {\it independent of} . However is crucial in
determining the large scale structure of the Universe. In particular, leads to warm dark matter and a structure formation
scenario that may have some advantages over both the standard hot and cold dark
matter scenarios.Comment: 10 pages (1 figure available upon request) phyzzx,
FERMILAB-Pub-93/057-
The dynamics of curved gravitating walls
We examine the dynamics of a self-gravitating domain wall using the model as a specific example. We find that the Nambu motion of the wall
is quite generic and dominates the wall motion even in the presence of gravity.
We calculate the corrections to this leading order motion, and estimate the
effect of the inclusion of gravity on the dynamics of the wall. We then treat
the case of a spherical gravitating thick wall as a particular example, solving
the field equations and calculating the corrections to the Nambu motion
analytically for this specific case. We find that the presence of gravity
retards collapse in this case.Comment: 19 pages revtex, 3 figures, references added, equations correcte
Chiral fermion mass and dispersion relations at finite temperature in the presence of hypermagnetic fields
We study the modifications to the real part of the thermal self-energy for
chiral fermions in the presence of a constant external hypermagnetic field. We
compute the dispersion relation for fermions occupying a given Landau level to
first order in g'^2, g^2 and g_phi^2 and to all orders in g'B, where g' and g
are the U(1)_Y and SU(2)_L couplings of the standard model, respectively, g_phi
is the fermion Yukawa coupling, and B is the hypermagnetic field strength. We
show that in the limit where the temperature is large compared to sqrt{g'B},
left- and right-handed modes acquire finite and different B-dependent masses
due to the chiral nature of their coupling with the external field. Given the
current bounds on the strength of primordial magnetic fields, we argue that the
above is the relevant scenario to study the effects of magnetic fields on the
propagation of fermions prior and during the electroweak phase transition.Comment: 11 pages 4 figures, published versio
EXCITATION of COUPLED STELLAR MOTIONS in the GALACTIC DISK by ORBITING SATELLITES
We use a set of high-resolution N-body simulations of the Galactic disk to study its interactions with the population of cosmologically predicted satellites. One simulation illustrates that multiple passages of massive satellites with different velocities through the disk generate a wobble, which has the appearance of rings in face-on projections of the stellar disk. They also produce flares in the outer disk parts and gradually heat the disk through bending waves. A different numerical experiment shows that an individual satellite as massive as the Sagittarius dwarf galaxy passing through the disk will drive coupled horizontal and vertical oscillations of stars in underdense regions with small associated heating. This experiment shows that vertical excursions of stars in these low-density regions can exceed 1 kpc in the Solar neighborhood, resembling the recently locally detected coherent vertical oscillations. They can also induce non-zero vertical streaming motions as large as 10-20 km s-1, which is consistent with recent observations in the Galactic disk. This phenomenon appears as a local ring with modest associated disk heating. © 2016. The American Astronomical Society. All rights reserved
Dynamics in the satellite system of Triangulum: Is AndXXII a dwarf satellite of M33?
We present results from a spectroscopic survey of the dwarf spheroidal And
XXII and the two extended clusters EC1 and EC2. These three objects are
candidate satellites of the Triangulum galaxy, M33, which itself is likely a
satellite of M31. We use the DEep Imaging Multi-Object Spectrograph mounted on
the Keck-II telescope to derive radial velocities for candidate member stars of
these objects and thereby identify the stars that are most likely actual
members. Eleven most probable stellar members (of 13 candidates) are found for
AndXXII. We obtain an upper limit of sigma_v < 6.0 km s-1 for the velocity
dispersion of AndXXII, [Fe/H] ~ -1.6 for its metallicity, and 255pc for the
Plummer radius of its projected density profile. We construct a colour
magnitude diagram for AndXXII and identify both the red giant branch and the
horizontal branch. The position of the latter is used to derive a heliocentric
distance to And XXII of 853 pm 26 kpc. The combination of the radial velocity,
distance, and angular position of AndXXII indicates that it is a strong
candidate for being the first known satellite of M33 and one of the very few
examples of a galactic satellite of a satellite. N-body simulations imply that
this conclusion is unchanged even if M31 and M33 had a strong encounter in the
past few Gyr. We test the hypothesis that the extended clusters highlight
tidally stripped galaxies by searching for an excess cloud of halo-like stars
in their vicinity. We find such a cloud for the case of EC1 but not EC2. The
three objects imply a dynamical mass for M33 that is consistent with previous
estimates.Comment: 14 pages, 14 figures, revised for MNRAS publicatio
Adaptive filtering techniques for gravitational wave interferometric data: Removing long-term sinusoidal disturbances and oscillatory transients
It is known by the experience gained from the gravitational wave detector
proto-types that the interferometric output signal will be corrupted by a
significant amount of non-Gaussian noise, large part of it being essentially
composed of long-term sinusoids with slowly varying envelope (such as violin
resonances in the suspensions, or main power harmonics) and short-term ringdown
noise (which may emanate from servo control systems, electronics in a
non-linear state, etc.). Since non-Gaussian noise components make the detection
and estimation of the gravitational wave signature more difficult, a denoising
algorithm based on adaptive filtering techniques (LMS methods) is proposed to
separate and extract them from the stationary and Gaussian background noise.
The strength of the method is that it does not require any precise model on the
observed data: the signals are distinguished on the basis of their
autocorrelation time. We believe that the robustness and simplicity of this
method make it useful for data preparation and for the understanding of the
first interferometric data. We present the detailed structure of the algorithm
and its application to both simulated data and real data from the LIGO 40meter
proto-type.Comment: 16 pages, 9 figures, submitted to Phys. Rev.
Rat urate oxidase produced by recombinant baculovirus expression: formation of peroxisome crystalloid core-like structures.
Pseudo-Stable Bubbles
The evolution of spherically symmetric unstable scalar field configurations
(``bubbles'') is examined for both symmetric (SDWP) and asymmetric (ADWP)
double-well potentials. Bubbles with initial static energies E_0\la E_{{\rm
crit}}, where is some critical value, shrink in a time scale
determined by their linear dimension, or ``radius''. Bubbles with E_0\ga
E_{{\rm crit}} evolve into time-dependent, localized configurations which are
{\it very} long-lived compared to characteristic time-scales in the models
examined. The stability of these configurations is investigated and possible
applications are briefly discussed.tic time-scales in the models examined. The
stability of these configurations is investigated and possible applications are
briefly discussed.Comment: 10 pages, LaTeX (uses revtex 3.0), 4 figures (postscript files of
figs.1 and 2 appended starting on line 497), report DART-HEP-93/0
The Merger Rate of Extremely Low Mass White Dwarf Binaries: Links to the Formation of AM CVn Stars and Underluminous Supernovae
We study a complete, colour-selected sample of double-degenerate binary
systems containing extremely low mass (ELM) <0.25 Msol white dwarfs (WDs). We
show, for the first time, that Milky Way disk ELM WDs have a merger rate of
approximately 4 x 10^(-5)/yr due to gravitational wave radiation. The merger
end-product depends on the mass ratio of the binary. The ELM WD systems that
undergo stable mass transfer can account for >3% of AM CVn stars. More
importantly, the ELM WD systems that may detonate merge at a rate comparable to
the estimated rate of underluminous SNe, rare explosions estimated to produce
only ~0.2 Msol worth of ejecta. At least 25% of our ELM WD sample belong to the
old thick disk and halo components of the Milky Way. Thus, if merging ELM WD
systems are the progenitors of underluminous SNe, transient surveys must find
them in both elliptical and spiral galaxies.Comment: MNRAS Letters, in pres
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