166 research outputs found
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
Properties of galaxy dark matter halos from weak lensing
We present the results of a study of weak lensing by galaxies based on 45.5
deg of band imaging data from the Red-Sequence Cluster Survey (RCS).
We present the first weak lensing detection of the flattening of galaxy dark
matter halos. We use a simple model in which the ellipticity of the halo is
times the observed ellipticity of the lens. We find a best fit value of
, suggesting that the dark matter halos are somewhat
rounder than the light distribution. The fact that we detect a significant
flattening implies that the halos are well aligned with the light distribution.
Given the average ellipticity of the lenses, this implies a halo ellipticity of
, in fair agreement with results from
numerical simulations of CDM. This result provides strong support for the
existence of dark matter, as an isotropic lensing signal is excluded with 99.5%
confidence. We also study the average mass profile around the lenses, using a
maximum likelihood analysis. We consider two models for the halo mass profile:
a truncated isothermal sphere (TIS) and an NFW profile. We adopt
observationally motivated scaling relations between the lens luminosity and the
velocity dispersion and the extent of the halo. The best fit NFW model yields a
mass and a scale
radius kpc. This value for the scale radius is
in excellent agreement with predictions from numerical simulations for a halo
of this mass.Comment: Significantly revised version, accepted for publication in ApJ 11
pages, 6 figure
Can multistate dark matter annihilation explain the high-energy cosmic ray lepton anomalies?
Multistate dark matter (DM) models with small mass splittings and couplings
to light hidden sector bosons have been proposed as an explanation for the
PAMELA/Fermi/H.E.S.S. high-energy lepton excesses. We investigate this proposal
over a wide range of DM density profiles, in the framework of concrete models
with doublet or triplet dark matter and a hidden SU(2) gauge sector that mixes
with standard model hypercharge. The gauge coupling is bounded from below by
the DM relic density, and the Sommerfeld enhancement factor is explicitly
computable for given values of the DM and gauge boson masses M, mu and the
(largest) dark matter mass splitting delta M_{12}. Sommerfeld enhancement is
stronger at the galactic center than near the Sun because of the radial
dependence of the DM velocity profile, which strengthens the inverse Compton
(IC) gamma ray constraints relative to usual assumptions. We find that the
PAMELA/Fermi/H.E.S.S. lepton excesses are marginally compatible with the model
predictions, and with CMB and Fermi gamma ray constraints, for M ~ 800 GeV, mu
~ 200 MeV, and a dark matter profile with noncuspy Einasto parameters alpha >
0.20, r_s ~ 30 kpc. We also find that the annihilating DM must provide only a
subdominant (< 0.4) component of the total DM mass density, since otherwise the
boost factor due to Sommerfeld enhancement is too large.Comment: 20 pages, 12 figures; v2: Corrected branching ratio for ground state
DM annihilations into leptons, leading to boost factors that are larger than
allowed. Added explicit results for doublet DM model. Some conclusions
changed; main conclusion of tension between inverse Compton constraints and
N-body simulations of halo profiles is unchange
HI studies of the Sculptor group galaxies. VIII. The background galaxies: NGC 24 and NGC 45
In order to complete our HI survey of galaxies in the Sculptor group area,
VLA observations of NGC 24 and NGC 45 are presented. These two galaxies of
similar magnitude M_B ~ -17.4 lie in the background of the Sculptor group and
are low surface brightness galaxies, especially NGC 45. The HI distribution and
kinematics are regular for NGC 24 while NGC 45 exhibits a kinematical twist of
its major axis. A tilted-ring model shows that the position angle of the major
axis changes by ~25 degrees. A best-fit model of their mass distribution gives
mass-to-light ratios for the stellar disk of 2.5 and 5.2 for NGC 24 and NGC 45
respectively. These values are higher than the ones expected from stellar
population synthesis models. Despite the large dark matter contribution, the
galaxy mass is still dominated by the stellar component in their very inner
regions. These high mass-to-light ratios are typical of what is seen in low
surface brightness galaxies and may indicate that, in those galaxies, disks are
far from the maximum disk case. The halo parameters derived from the best-fit
models are thus lower limits.Comment: 23 pages, 10 figures, accepted for publication in The Astronomical
Journa
A Magellanic Origin for the Warp of the Galaxy
We show that a Magellanic Cloud origin for the warp of the Milky Way can
explain most quantitative features of the outer HI layer recently identified by
Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg
(1998) that produces distortions in the dark matter halo, and we calculate the
combined effect of these dark-halo distortions and the direct tidal forcing by
the Magellanic Clouds on the disk warp in the linear regime. The interaction of
the dark matter halo with the disk and resonances between the orbit of the
Clouds and the disk account for the large amplitudes observed for the vertical
m=0,1,2 harmonics. The observations lead to six constraints on warp forcing
mechanisms and our model reasonably approximates all six. The disk is shown to
be very dynamic, constantly changing its shape as the Clouds proceed along
their orbit. We discuss the challenges to MOND placed by the observations.Comment: 4 pages, 3 figures, submitted to ApJ Letters. Additional graphics, 3d
visualizations and movies available at
http://www.astro.umass.edu/~weinberg/lm
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
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
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
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
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
- âŠ