2,141 research outputs found
Probing Brownstein-Moffat Gravity via Numerical Simulations
In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e.,
a spiral galaxy) is well described by a disk and a bulge embedded in a halo
mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim
that late-type galaxy systems would not need to have halos, avoiding as a
result the dark matter problem, i.e., a modified gravity (non-Newtonian) would
account for the galactic structure with no need of dark matter. In the present
paper, we probe this claim via numerical simulations. Instead of using a
"static galaxy," where the centrifugal equilibrium is usually adopted, we probe
the Brownstein-Moffat gravity dynamically via numerical -body simulations.Comment: 33 pages and 14 figures - To appear in The Astrophysical Journa
Tomography of Collisionless Stellar Systems
In this paper the concept of tomography of a collisionless stellar system of
general shape is introduced, and a generalization of the Projected Virial
Theorem is obtained. Applying the tomographic procedure we then derive a new
family of virial equations which coincides with the already known ones for
spherically symmetric systems. This result is obtained without any use of
explicit expressions for the line-of-sight velocity dispersion, or spherical
coordinate system.Comment: BAP-06-1994-016-OAB. 7 pages, postscript file. In press on Celestial
Mechanic
Evidence of a Metal Rich Galactic Bar from the Vertex Deviation of the Velocity Ellipsoid
We combine radial velocities, proper motions, and low resolution abundances
for a sample of 315 K and M giants in the Baade's Window (l,b)=(0.9,-4)
Galactic bulge field. The velocity ellipsoid of stars with [Fe/H]>-0.5 dex
shows a vertex deviation in the plot of radial versus transverse velocity,
consistent with that expected from a population with orbits supporting a bar.
We demonstrate that the significance of this vertex deviation using
non-parametric rank correlation statistic is >99%. The velocity ellipsoid for
the metal poor ([FeH]<-0.5) part of the population shows no vertex deviation
and is consistent with an isotropic, oblate rotating population. We find no
evidence for kinematic subgroups, but there is a mild tendency for the vertical
velocity dispersion sigma_b to decrease with increasing metallicity.Comment: 4 pages, ApJ Letters, submitte
The Ellipticity of the Disks of Spiral Galaxies
The disks of spiral galaxies are generally elliptical rather than circular.
The distribution of ellipticities can be fit with a log-normal distribution.
For a sample of 12,764 galaxies from the Sloan Digital Sky Survey Data Release
1 (SDSS DR1), the distribution of apparent axis ratios in the i band is best
fit by a log-normal distribution of intrinsic ellipticities with ln epsilon =
-1.85 +/- 0.89. For a sample of nearly face-on spiral galaxies, analyzed by
Andersen and Bershady using both photometric and spectroscopic data, the best
fitting distribution of ellipticities has ln epsilon = -2.29 +/- 1.04. Given
the small size of the Andersen-Bershady sample, the two distribution are not
necessarily inconsistent. If the ellipticity of the potential were equal to
that of the light distribution of the SDSS DR1 galaxies, it would produce 1.0
magnitudes of scatter in the Tully-Fisher relation, greater than is observed.
The Andersen-Bershady results, however, are consistent with a scatter as small
as 0.25 magnitudes in the Tully-Fisher relation.Comment: 19 pages, 5 figures; ApJ, accepte
Probing a regular orbit with spectral dynamics
We have extended the spectral dynamics formalism introduced by Binney &
Spergel, and have implemented a semi-analytic method to represent regular
orbits in any potential, making full use of their regularity. We use the
spectral analysis code of Carpintero & Aguilar to determine the nature of an
orbit (irregular, regular, resonant, periodic) from a short-time numerical
integration. If the orbit is regular, we approximate it by a truncated Fourier
time series of a few tens of terms per coordinate. Switching to a description
in action-angle variables, this corresponds to a reconstruction of the
underlying invariant torus. We then relate the uniform distribution of a
regular orbit on its torus to the non-uniform distribution in the space of
observables by a simple Jacobian transformation between the two sets of
coordinates. This allows us to compute, in a cell-independent way, all the
physical quantities needed in the study of the orbit, including the density and
in the line-of-sight velocity distribution, with much increased accuracy. The
resulting flexibility in the determination of the orbital properties, and the
drastic reduction of storage space for the orbit library, provide a significant
improvement in the practical application of Schwarzschild's orbit superposition
method for constructing galaxy models. We test and apply our method to
two-dimensional orbits in elongated discs, and to the meridional motion in
axisymmetric potentials, and show that for a given accuracy, the spectral
dynamics formalism requires an order of magnitude fewer computations than the
more traditional approaches.Comment: 13 pages, 18 eps figures, submitted to MNRA
Majority-vote model on (3,4,6,4) and (3^4,6) Archimedean lattices
On Archimedean lattices, the Ising model exhibits spontaneous ordering. Two
examples of these lattices of the majority-vote model with noise are considered
and studied through extensive Monte Carlo simulations. The order/disorder phase
transition is observed in this system. The calculated values of the critical
noise parameter are q_c=0.091(2) and q_c=0.134(3) for (3,4,6,4) and (3^4,6)
Archimedean lattices, respectively. The critical exponents beta/nu, gamma/nu
and 1/nu for this model are 0.103(6), 1.596(54), 0.872(85) for (3,4,6,4) and
0.114(3), 1.632(35), 0.978(104) for (3^4,6) Archimedean lattices. These results
differs from the usual Ising model results and the majority-vote model on
so-far studied regular lattices or complex networks. The effective
dimensionality of the system [D_{eff}(3,4,6,4)=1.802(55) and
D_{eff}(3^4,6)=1.860(34)] for these networks are reasonably close to the
embedding dimension two.Comment: 6 pages, 7 figures in 12 eps files, RevTex
Intrinsic Axis Ratio Distribution of Early-type Galaxies From Sloan Digital Sky Survey
Using Sloan Digital Sky Survey Data Release 5, we have investigated the
intrinsic axis ratio distribution (ARD) for early-type galaxies. We have
constructed a volume-limited sample of 3,922 visually-inspected early-type
galaxies at carefully considering sampling biases
caused by the galaxy isophotal size and luminosity. We attempt to de-project
the observed ARD into three-dimensional types (oblate, prolate, and triaxial),
which are classified in terms of triaxiality. We confirm that no linear
combination of -distributed axis ratios of the three types can
reproduce the observed ARD. However, using Gaussian intrinsic distributions, we
have found reasonable fits to the data with preferred mean axis ratios for
oblate, prolate, and triaxial (triaxials in two axis ratios), where the fractions of
oblate, prolate and triaxial types are
\textrm{O:P:T}=0.29^{\pm0.09}:0.26^{\pm0.11}:0.45^{\pm0.13}-23.3 < M_r \leq -21.2-21.2 < M_r <-19.3$) sample does. Oblate is
relatively more abundant among the less luminous galaxies. Interestingly, the
preferences of axis ratios for triaxial types in the two luminosity classes are
remarkably similar. We have not found any significant influence of the local
galaxy number density on ARD. We show that the results can be seriously
affected by the details in the data selection and type classification scheme.
Caveats and implications on galaxy formation are discussed.Comment: 9 pages, 11 figures, Accepted for publication in Ap
Reshuffling spins with short range interactions: When sociophysics produces physical results
Galam reshuffling introduced in opinion dynamics models is investigated under
the nearest neighbor Ising model on a square lattice using Monte Carlo
simulations. While the corresponding Galam analytical critical temperature T_C
\approx 3.09 [J/k_B] is recovered almost exactly, it is proved to be different
from both values, not reshuffled (T_C=2/arcsinh(1) \approx 2.27 [J/k_B]) and
mean-field (T_C=4 [J/k_B]). On this basis, gradual reshuffling is studied as
function of 0 \leq p \leq 1 where p measures the probability of spin
reshuffling after each Monte Carlo step. The variation of T_C as function of p
is obtained and exhibits a non-linear behavior. The simplest Solomon network
realization is noted to reproduce Galam p=1 result. Similarly to the critical
temperature, critical exponents are found to differ from both, the classical
Ising case and the mean-field values.Comment: 11 pages, 5 figures in 6 eps files, to appear in IJMP
The Hot Interstellar Medium in Normal Elliptical Galaxies III: The Thermal Structure of the Gas
This is the third paper in a series analyzing X-ray emission from the hot
interstellar medium in a sample of 54 normal elliptical galaxies observed by
Chandra, focusing on 36 galaxies with sufficient signal to compute radial
temperature profiles. We distinguish four qualitatively different types of
profile: positive gradient (outwardly rising), negative gradients (falling),
quasi-isothermal (flat) and hybrid (falling at small radii, then rising). We
measure the mean logarithmic temperature gradients in two radial regions: from
0--2 -band effective radii (excluding the central point source), and
from 2--. We find the outer gradient to be uncorrelated with intrinsic
host galaxy properties, but strongly influenced by the environment: galaxies in
low-density environments tend to show negative outer gradients, while those in
high-density environments show positive outer gradients, suggesting influence
of circumgalactic hot gas. The inner temperature gradient is unaffected by the
environment but strongly correlated with intrinsic host galaxy characteristics:
negative inner gradients are more common for smaller, optically faint, low
radio-luminosity galaxies, whereas positive gradients are found in bright
galaxies with stronger radio sources. There is no evidence for bimodality in
the distribution of inner or outer gradients. We propose three scenarios to
explain the inner temperature gradients: (1) Weak AGN heat the ISM locally,
higher-luminosity AGN heat the system globally through jets inflating cavities
at larger radii; (2) The onset of negative inner gradients indicates a
declining importance of AGN heating relative to other sources, such as
compressional heating or supernovae; (3) The variety of temperature profiles
are snapshots of different stages of a time-dependent flow.Comment: 18 pages, emulateapj, 55 figures (36 online-only figures included in
astro-ph version), submitted to Ap
Models of rotating coronae
Fitting equilibrium dynamical models to observational data is an essential
step in understanding the structure of the gaseous hot haloes that surround our
own and other galaxies. However, the two main categories of models that are
used in the literature are poorly suited for this task: (i) simple barotropic
models are analytic and can therefore be adjusted to match the observations,
but are clearly unrealistic because the rotational velocity does
not depend on the distance from the galactic plane, while (ii) models
obtained as a result of cosmological galaxy formation simulations are more
realistic, but are impractical to fit to observations due to high computational
cost. Here we bridge this gap by presenting a general method to construct
axisymmetric baroclinic equilibrium models of rotating galactic coronae in
arbitrary external potentials. We consider in particular a family of models
whose equipressure surfaces in the plane are ellipses of varying axis
ratio. These models are defined by two one-dimensional functions, the axial
ratio of pressure and the value of the pressure along the galaxy's symmetry axis. These models can have a rotation
speed that realistically decreases as one moves away from the
galactic plane, and can reproduce the angular momentum distribution found in
cosmological simulations. The models are computationally cheap to construct and
can thus be used in fitting algorithms. We provide a python code that given
, and returns ,
, , . We show a few examples of these models using
the Milky Way as a case study.Comment: Accepted for publication in MNRA
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