2,157 research outputs found
The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter
A detailed analysis of the November 15, 2006 data release (Clowe et al.,
2006) X-ray surface density Sigma-map and the strong and weak gravitational
lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed
and the results are compared with the predictions of a modified gravity (MOG)
and dark matter. Our surface density Sigma-model is computed using a King
beta-model density, and a mass profile of the main cluster and an isothermal
temperature profile are determined by the MOG. We find that the main cluster
thermal profile is nearly isothermal. The MOG prediction of the isothermal
temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with
the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the
two-dimensional convergence kappa-map data are obtained without non-baryonic
dark matter, accounting for the 8-sigma spatial offset between the Sigma-map
and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the
kappa-map results in two baryonic components distributed across the Bullet
Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM)
gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on
average 76% dark matter (neglecting the indeterminant contribution due to the
galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a
statistically significant result when compared to the predicted Lambda-CDM
cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al.,
2006).Comment: Accepted for publication in Mon. Not. Roy. Astron. Soc. -- July 26,
2007. In press. 28 pages, 15 figures, 5 table
Quantifying the Biases of Spectroscopically Selected Gravitational Lenses
Spectroscopic selection has been the most productive technique for the
selection of galaxy-scale strong gravitational lens systems with known
redshifts. Statistically significant samples of strong lenses provide a
powerful method for measuring the mass-density parameters of the lensing
population, but results can only be generalized to the parent population if the
lensing selection biases are sufficiently understood. We perform controlled
Monte Carlo simulations of spectroscopic lens surveys in order to quantify the
bias of lenses relative to parent galaxies in velocity dispersion, mass axis
ratio, and mass density profile. For parameters typical of the SLACS and BELLS
surveys, we find: (1) no significant mass axis ratio detection bias of lenses
relative to parent galaxies; (2) a very small detection bias toward shallow
mass density profiles, which is likely negligible compared to other sources of
uncertainty in this parameter; (3) a detection bias towards smaller Einstein
radius for systems drawn from parent populations with group- and cluster-scale
lensing masses; and (4) a lens-modeling bias towards larger velocity
dispersions for systems drawn from parent samples with sub-arcsecond mean
Einstein radii. This last finding indicates that the incorporation of
velocity-dispersion upper limits of \textit{non-lenses} is an important
ingredient for unbiased analyses of spectroscopically selected lens samples. In
general we find that the completeness of spectroscopic lens surveys in the
plane of Einstein radius and mass-density profile power-law index is quite
uniform, up to a sharp drop in the region of large Einstein radius and steep
mass density profile, and hence that such surveys are ideally suited to the
study of massive field galaxies.Comment: Accepted for publication in Astrophys. J., June 7, 2012. In press. 9
pages, 5 figures, 1 tabl
Modified Gravity and the Phantom of Dark Matter
Astrophysical data analysis of the weak-field predictions support the claim
that modified gravity (MOG) theories provide a self-consistent,
scale-invariant, universal description of galaxy rotation curves, without the
need of non-baryonic dark matter. Comparison to the predictions of Milgrom's
modified dynamics (MOND) provide a best-fit and experimentally determined
universal value of the MOND acceleration parameter. The predictions of the
modified gravity theories are compared to the predictions of cold non-baryonic
dark matter (CDM), including a constant density core-modified fitting formula,
which produces excellent fits to galaxy rotation curves including the low
surface brightness and dwarf galaxies.
Upon analysing the mass profiles of clusters of galaxies inferred from X-ray
luminosity measurements, from the smallest nearby clusters to the largest of
the clusters of galaxies, it is shown that while MOG provides consistent fits,
MOND does not fit the observed shape of cluster mass profiles for any value of
the MOND acceleration parameter. Comparison to the predictions of CDM confirm
that whereas the Navarro-Frenk-White (NFW) fitting formula does not fit the
observed shape of galaxy cluster mass profiles, the core-modified dark matter
fitting formula provides excellent best-fits, supporting the hypothesis that
baryons are dynamically important in the distribution of dark matter halos.Comment: Ph.D. Thesis. 251 pages, 22 figures, 17 table
Why Same-Sex Spouses Should Be Granted Preferential Immigration Status: Reevaluating Adams v. Howerton
Galaxy Cluster Masses Without Non-Baryonic Dark Matter
We apply the modified acceleration law obtained from Einstein gravity coupled
to a massive skew symmetric field, F_{\mu\nu\lambda}, to the problem of
explaining X-ray galaxy cluster masses without exotic dark matter. Utilizing
X-ray observations to fit the gas mass profile and temperature profile of the
hot intracluster medium (ICM) with King beta-models, we show that the dynamical
masses of the galaxy clusters resulting from our modified acceleration law fit
the cluster gas masses for our sample of 106 clusters without the need of
introducing a non-baryonic dark matter component. We are further able to show
for our sample of 106 clusters that the distribution of gas in the ICM as a
function of radial distance is well fit by the dynamical mass distribution
arising from our modified acceleration law without any additional dark matter
component. In previous work, we applied this theory to galaxy rotation curves
and demonstrated good fits to our sample of 101 LSB, HSB and dwarf galaxies
including 58 galaxies that were fit photometrically with the single parameter
(M/L)_{stars}. The results there were qualitatively similar to those obtained
using Milgrom's phenomenological MOND model, although the determined galaxy
masses were quantitatively different and MOND does not show a return to
Keplerian behavior at extragalactic distances. The results here are compared to
those obtained using Milgrom's phenomenological MOND model which does not fit
the X-ray galaxy cluster masses unless an auxiliary dark matter component is
included.Comment: Submitted to MNRAS, July 8, 2005. 16 pages, 2 figures, 1 table, 106
galaxy cluster
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