1,645 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
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
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
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: mock galaxy catalogues for the low-redshift sample
We present one thousand mock galaxy catalogues for the analysis of the Low
Redshift Sample (LOWZ, effective redshift z ~ 10.32) of the Baryon Oscillation
Spectroscopic Survey Data Releases 10 and 11. These mocks have been created
following the PTHalos method of Manera13 et al. (2013) revised to include new
developments. The main improvement is the introduction of a redshift dependence
in the Halo Occupation Distribution in order to account for the change of the
galaxy number density with redshift. These mock catalogues are used in the
analyses of the LOWZ galaxy clustering by the BOSS collaboration.Comment: 10 pages, 8 figure
Stellar population analysis of MaNGA early-type galaxies: IMF dependence and systematic effects
We study systematics associated with estimating simple stellar population
(SSP) parameters -- age, metallicity [M/H], -enhancement [/Fe]
and IMF shape -- and associated gradients, of elliptical slow rotators
(E-SRs), fast rotators (E-FRs) and S0s from stacked spectra of galaxies in the
MaNGA survey. These systematics arise from (i) how one normalizes the spectra
when stacking; (ii) having to subtract emission before estimating absorption
line strengths; (iii) the decision to fit the whole spectrum or just a few
absorption lines; (iv) SSP model differences (e.g. isochrones, enrichment,
IMF). The MILES+Padova SSP models, fit to the H, Fe,
TiO and [MgFe] Lick indices in the stacks, indicate that out to
the half-light radius : (a) ages are younger and [/Fe] values are
lower in the central regions but the opposite is true of [M/H]; (b) the IMF is
more bottom-heavy in the center, but is close to Kroupa beyond about ;
(c) this makes about larger in the central regions than
beyond . While the models of Conroy et al. (2018) return similar [M/H]
and [/Fe] profiles, the age and (hence) profiles can differ
significantly even for solar abundances and a Kroupa IMF; different responses
to non-solar abundances and IMF parametrization further compound these
differences. There are clear (model independent) differences between E-SRs,
E-FRs and S0s: younger ages and less enhanced [/Fe] values suggest that
E-FRs and S0s are not SSPs, but relaxing this assumption is unlikely to change
their inferred gradients significantly.Comment: 22 pages, 23 figures, accepted for publication in MNRA
Fractal Spacetime Structure in Asymptotically Safe Gravity
Four-dimensional Quantum Einstein Gravity (QEG) is likely to be an
asymptotically safe theory which is applicable at arbitrarily small distance
scales. On sub-Planckian distances it predicts that spacetime is a fractal with
an effective dimensionality of 2. The original argument leading to this result
was based upon the anomalous dimension of Newton's constant. In the present
paper we demonstrate that also the spectral dimension equals 2 microscopically,
while it is equal to 4 on macroscopic scales. This result is an exact
consequence of asymptotic safety and does not rely on any truncation. Contact
is made with recent Monte Carlo simulations.Comment: 20 pages, late
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