17 research outputs found
Peculiar velocity measurement in a clumpy universe
In this work we address the issue of peculiar velocity measurement in a
perturbed Friedmann universe using the deviations from measured luminosity
distances of standard candles from background FRW universe. We want to show and
quantify the statement that in intermediate redshifts (),
deviations from the background FRW model are not uniquely governed by peculiar
velocities. Luminosity distances are modified by gravitational lensing. We also
want to indicate the importance of relativistic calculations for peculiar
velocity measurement at all redshifts.
For this task we discuss the relativistic correction on luminosity distance
and redshift measurement and show the contribution of each of the corrections
as lensing term, peculiar velocity of the source and Sachs-Wolfe effect. Then
we use the SNe Ia sample of Union 2, to investigate the relativistic effects we
consider.
We show that, using the conventional peculiar velocity method, that ignores
the lensing effect, will result in an overestimate of the measured peculiar
velocities at intermediate redshifts. Here we quantify this effect. We show
that at low redshifts the lensing effect is negligible compare to the effect of
peculiar velocity. From the observational point of view, we show that the
uncertainties on luminosity of the present SNe Ia data prevent us from precise
measuring the peculiar velocities even at low redshifts ().Comment: 15 pages, 5 figures, Int. J. Mod. Phys. D 27, 1850019 (2018
The challenge of large and empty voids in the SDSS DR7 redshift survey
We present catalogues of voids for the SDSS DR7 redshift survey and for
Millennium I simulation mock data. We aim to compare the observations with
simulations based on a CDM model and a semi-analytic galaxy formation
model. We use the void statistics as a test for these models. We assembled a
mock catalogue that closely resembles the SDSS DR7 catalogue and carried out a
parallel statistical analysis of the observed and simulated catalogue. We find
that in the observation and the simulation, voids tend to be equally spherical.
The total volume occupied by the voids and their total number are slightly
larger in the simulation than in the observation. We find that large voids are
less abundant in the simulation and the total luminosity of the galaxies
contained in a void with a given radius is higher on average than observed by
SDSS DR7 survey. We expect these discrepancies to be even more significant in
reality than found here since the present value of given by WMAP7 is
lower than the value of 0.9 used in the Millennium I simulation. The reason why
the simulation fails to produce enough large and dark voids might be the
failure of certain semi-analytic galaxy formation models to reduce the
small-scale power of CDM and to produce sufficient power on large
scales.Comment: Kolmogrov-Smirnov test added. version to appear in A&
Unraveling the nature of Gravity through our clumpy Universe
We propose a new probe to test the nature of gravity at various redshifts
through large-scale cosmological observations. We use our void catalog,
extracted from the Sloan Digital Sky Survey (SDSS, DR10), to trace the
distribution of matter along the lines of sight to SNe Ia that are selected
from the Union 2 catalog. We study the relation between SNe Ia luminosities and
convergence and also the peculiar velocities of the sources. We show that the
effects, on SNe Ia luminosities, of convergence and of peculiar velocities
predicted by the theory of general relativity and theories of modified
gravities are different and hence provide a new probe of gravity at various
redshifts. We show that the present sparse large-scale data does not allow us
to determine any statistically- significant deviation from the theory of
general relativity but future more comprehensive surveys should provide us with
means for such an exploration.Comment: Essay received honorable mention in the Gravity Research Foundation
2014 essay competition. To appear in IJMPD. 10 pages, 2 figure
A statistical study of the luminosity gap in galaxy groups
The luminosity gap between the two brightest members of galaxy groups and
clusters is thought to offer a strong test for the models of galaxy formation
and evolution. This study focuses on the statistics of the luminosity gap in
galaxy groups, in particular fossil groups, e.g. large luminosity gap, in an
analogy with the same in a cosmological simulation. We use spectroscopic legacy
data of seventh data release (DR7) of SDSS, to extract a volume limited sample
of galaxy groups utilizing modified friends-of-friends (mFoF) algorithm.
Attention is paid to galaxy groups with the brightest group galaxy (BGG) more
luminous than \Mr = -22. An initial sample of 620 groups in which 109 optical
fossil groups, where the luminosity gap exceeds 2 magnitude, were identified.
We compare the statistics of the luminosity gap in galaxy groups at low mass
range from the SDSS with the same in the Millennium simulations where galaxies
are modeled semi-analytically. We show that the BGGs residing in galaxy groups
with large luminosity gap, i.e. fossil groups, are on average brighter and live
in lower mass halos with respect to their counter parts in non-fossil systems.
Although low mass galaxy groups are thought to have recently formed, we show
that in galaxy groups with 15 galaxies brighter than ,
evolutionary process are most likely to be responsible for the large luminosity
gap. We also examine a new probe of finding fossil group. In addition we extend
the recently introduced observational probe based on the luminosity gap, the
butterfly diagram, to galaxy groups and study the probe as a function of halo
mass. This probe can, in conjunction with the luminosity function, help to fine
tune the semi-analytic models of galaxies employed in the cosmological
simulations.Comment: 11 pages, 11 figures, accepted to PASP journa
The Effects of Blade Configurations on Performance of a Tidal Vertical Axis Turbine
Hydrokinetic energy contains the major uncontrolled source of renewable marine energy. The highest level of converter technology readiness offered in the last three decades is TRL8â9, which is related to the first-generation horizontal axis converters. In low-depth calm waters, one of the best options to harvest tidal energy is vertical axis turbines. About 16% of the conceptual designs presented in the last 30Â years apply this type of converter, which does not have a high level of technological readiness. In this study, a laboratory-designed vertical axis turbine has been introduced in which the effects of the number of blades, the blade profile, and attack angle on the performance of the turbine were analyzed. A 3D incompressible viscous turbulent computational finite volume approach is applied, with the spatial second-order and temporal first-order accuracies. The turbulent model k-Ï SST was used to obtain the flow inside the turbine. Rotors include two, three, and six blades with three different profiles, including NACA2421, NACA16021, and NACA0020. Computational results reveal that the turbine with three blades and an angle of attack of +8 using the NACA2421 profile has a maximum generation capacity of about 4Â kW, with a strength factor of 0.4 and a power factor of about 20%. The capacity, however, was lower for a higher number of blades
The Spatial Distribution Function of Galaxies at High Redshift
This is the first exploration of the galaxy distribution function at
redshifts greater than about 0.1. Redshifts are based on the North and South
GOODS Catalogs. In each catalog we examine clustering in the two redshift bands
0.47 < z < 0.8 and 0.9 < z < 1.5. The mean redshifts of the samples in these
bands are about 0.6 and 1.1. Our main result is that at these redshifts the
galaxy spatial distribution function f_V(N) has the form predicted by
gravitational quasi-equilibrium dynamics for cosmological many-body systems.
This constrains related processes such as galaxy merging and the role of dark
matter in the range of these redshifts.Comment: Accepted ApJ, 6 pages, 5 figure