251 research outputs found
The Caustic Ring Model of the Milky Way Halo
We present a proposal for the full phase space distribution of the Milky Way
halo. The model is axially and reflection symmetric and its time evolution is
self-similar. It describes the halo as a set of discrete dark matter flows with
stated densities and velocity vectors everywhere. We first discuss the general
conditions under which the time evolution of a cold collisionless
self-gravitating fluid is self-similar, and show that symmetry is not necessary
for self-similarity. When spherical symmetry is imposed, the model is the same
as described by Fillmore and Goldreich, and by Bertschinger, twenty-three years
ago. The spherically symmetric model depends on one dimensionless parameter
and two dimensionful parameters. We set = 0.3, a value
consistent with the slope of the power spectrum of density perturbations on
galactic scales. The dimensionful parameters are determined by the Galactic
rotation velocity (220 km/s) at the position of the Sun and by the age of the
Galaxy (13.7 Gyr). The properties of the outer caustics are derived in the
spherically symmetric model. The structure of the inner halo depends on the
angular momentum distribution of the dark matter particles. We assume that
distribution to be axial and reflection symmetric, and dominated by net overall
rotation. The inner caustics are rings whose radii are determined in terms of a
single additional parameter . We summarize the observational
evidence in support of the model. The evidence is consistent with
= 0.18 in Concordance Cosmology, equivalent to = 0.26 in
Einstein - de Sitter cosmology. We give formulas to estimate the flow densities
and velocity vectors anywhere in the Milky Way halo. The properties of the
first forty flows at the location of the Earth are listed.Comment: 35 pages, 6 figure
The Distribution of Dark Matter in a Ringed Galaxy
Outer rings are located at the greatest distance from the galaxy center of
any feature resonant with a bar. Because of their large scale, their morphology
is sensitive to the distribution of the dark matter in the galaxy. We introduce
here how study of these rings can constrain the mass-to-light ratio of the bar,
and so the percentage of dark matter in the center of these galaxies. We
compare periodic orbits integrated in the ringed galaxy NGC 6782 near the outer
Lindblad resonance to the shape of the outer ring. The non-axisymmetric
component of the potential resulting from the bar is derived from a
near-infrared image of the galaxy. The axisymmetric component is derived
assuming a flat rotation curve. We find that the pinched non-self-intersecting
periodic orbits are more elongated for higher bar mass-to-light ratios and
faster bars. The inferred mass-to-light ratio of the bar depends on the assumed
inclination of the galaxy. With an assumed galaxy inclination of i=41 degrees,
for the orbits to be consistent with the observed ring morphology the
mass-to-light ratio of the bar must be high, greater than 70% of a maximal disk
value. For i=45 degrees, the mass-to-light ratio of the bar is of
the maximal disk value. Since the velocity field of these rings can be used to
constrain the galaxy inclination as well as which periodic orbit is represented
in the ring, further study will yield tighter constraints on the mass-to-light
ratio of the bar. If a near maximal disk value for the bar is required, then
either there would be little dark matter within the bar, or the dark matter
contained in the disk of the galaxy would be non-axisymmetric and would rotate
with the bar.Comment: AAS Latex + jpg Figures, Accepted for publication in Ap
Accurate Determination of the Mass Distribution in Spiral Galaxies: II. Testing the Shape of Dark Halos
New high resolution CFHT Fabry-Perot data, combined with published VLA 21 cm
observations are used to determine the mass distribution of NGC 3109 and IC
2574. The multi-wavelength rotation curves allow to test with confidence
different dark halo functional forms from the pseudo-isothermal sphere to some
popular halo distributions motivated by N-body simulations. It appears that
density distribution with an inner logarithmic slope <= -1 are very hard to
reconcile with rotation curves of late type spirals. Modified Newtonian
Dynamics (MOND) is also considered as a potential solution to missing mass and
tested the same way. The new higher resolution data show that MOND can
reproduce in details the rotation curve of IC 2574 but confirm its difficulty
to fit the kinematics of NGC 3109.Comment: 28 pages, accepted by AJ. New HI profile increases the compatibility
of NGC 3109 rotation curve with MON
The Bar Pattern Speed of NGC 1433 Estimated Via Sticky-Particle Simulations
We present detailed numerical simulations of NGC 1433, an intermediate-type
barred spiral showing strong morphological features including a secondary bar,
nuclear ring, inner ring, outer pseudoring, and two striking, detached spiral
arcs known as ``plumes.'' This galaxy is an ideal candidate for recreating the
observed morphology through dynamical models and determining the pattern speed.
We derived a gravitational potential from an -band image of the galaxy and
simulated the behavior of a two-dimensional disk of 100,000 inelastically
colliding gas particles. We find that the closest matching morphology between a
-band image and a simulation occurs with a pattern speed of 0.89 km s
arcsec 5-10%. We also determine that the ratio of corotation
radius to the average published bar radius is 1.7 0.3, with the ambiguity
in the bar radius being the largest contributor to the error.Comment: Accepted for publication by The Astronomical Journal. 34 pages, 13
figures, 2 table
Searching for Dark Matter with Future Cosmic Positron Experiments
Dark matter particles annihilating in the Galactic halo can provide a flux of
positrons potentially observable in upcoming experiments, such as PAMELA and
AMS-02. We discuss the spectral features which may be associated with dark
matter annihilation in the positron spectrum and assess the prospects for
observing such features in future experiments. Although we focus on some
specific dark matter candidates, neutralinos and Kaluza-Klein states, we carry
out our study in a model independent fashion. We also revisit the positron
spectrum observed by HEAT.Comment: 19 pages, 33 figure
Evidence for universal structure in galactic halos
The late infall of dark matter onto a galaxy produces structure (such as
caustics) in the distribution of dark matter in the halo. We argue that such
structure is likely to occur generically on length scales proportional to , where is the age of the universe and is the
rotation velocity of the galaxy. A set of 32 extended galactic rotation curves
is analyzed. For each curve, the radial coordinate is rescaled according to
, where we choose . A
linear fit to each rescaled rotation curve is subtracted, and the residuals are
binned and averaged. The sample shows significant features near and . This is consistent with the predictions of the
self-similar caustic ring model of galactic halos.Comment: 4 pages, LaTeX, 1 epsf figur
Open Questions in Classical Gravity
We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.Comment: UCONN-93-1, plain TeX format, 22 pages (plus 7 figures - send
requests to [email protected]). To appear in a special issue of
Foundations of Physics honoring Professor Fritz Rohrlich on the occasion of
his retirement, L. P. Horwitz and A. van der Merwe Editors, Plenum Publishing
Company, N.Y., Fall 199
Mass Density Profiles of LSB Galaxies
We derive the mass density profiles of dark matter halos that are implied by
high spatial resolution rotation curves of low surface brightness galaxies. We
find that at small radii, the mass density distribution is dominated by a
nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a,
the distribution of inner slopes a is strongly peaked around a = -0.2. This is
significantly shallower than the cuspy a < -1 halos found in CDM simulations.
While the observed distribution of alpha does have a tail towards such extreme
values, the derived value of alpha is found to depend on the spatial resolution
of the rotation curves: a ~ -1 is found only for the least well resolved
galaxies. Even for these galaxies, our data are also consistent with constant
density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the
illusion of steep cusps when insufficiently resolved. Consequently, there is no
clear evidence for a cuspy halo in any of the low surface brightness galaxies
observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and
emulateapj5.sty Typo in Eq 1 fixe
Limits on Supersymmetric Dark Matter From EGRET Observations of the Galactic Center Region
In most supersymmetic models, neutralino dark matter particles are predicted
to accumulate in the Galactic center and annihilate generating, among other
products, gamma rays. The EGRET experiment has made observations in this
region, and is sensitive to gamma rays from 30 MeV to 30 GeV. We have
used an improved point source analysis including an energy dependent point
spread function and an unbinned maximum likelihood technique, which has allowed
us to significantly lower the limits on gamma ray flux from the Galactic
center. We find that the present EGRET data can limit many supersymmetric
models if the density of the Galactic dark matter halo is cuspy or spiked
toward the Galactic center. We also discuss the ability of GLAST to test these
models.Comment: 4 pages, 3 figure
Stellar and Gas properties of High HI Mass-to-Light Ratio Galaxies in the Local Universe
We present a multi-wavelength study (BVRI band photometry and HI line
interferometry) of nine late-type galaxies selected from the HIPASS Bright
Galaxy Catalog on the basis of apparently high HI mass-to-light ratios (3
M_sun/L_sun < M_HI/L_B < 27 M_sun/L_sun). We found that most of the original
estimates for M_HI/L_B based on available photographic magnitudes in the
literature were too high, and conclude that genuine high HI mass-to-light ratio
(>5 M_sun/L_sun) galaxies are rare in the Local Universe. Extreme high M_HI/L_B
galaxies like ESO215-G?009 appear to have formed only the minimum number of
stars necessary to maintain the stability of their HI disks, and could possibly
be used to constrain galaxy formation models. They may to have been forming
stars at a low, constant rate over their lifetimes. The best examples all have
highly extended HI disks, are spatially isolated, and have normal baryonic
content for their total masses but are deficent in stars. This suggests that
high M_HI/L_B galaxies are not lacking the baryons to create stars, but are
underluminous as they lack either the internal or external stimulation for more
extensive star formation.Comment: 29 Pages, 59 Figures. Accepted for publication in AJ (to be published
~April 2006
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