16,561 research outputs found
Deriving the Dyer-Roeder Equation from the Geodesic Deviation Equation via the Newman-Penrose Null Tetrad
In this paper we examine the geodesic deviation equation using the Newman-Penrose (N-P) formalism for a flat Friedmann-Lemaitre-Robertson-Walker(FLRW) metric [Carroll,S. (2004), Ryden, B. (2003), Newman Penrose(1962)]. We solved the geodesic deviation equation for angular diameter distance, using the relevant N-P components, and the resulting expression was the Dyer-Roeder equation of cosmology [Ryden, B. (2003)] [Schneider et al.(1992)]. This leads us to believe that we can apply the N-P formalism to a perturbed FLRW metric and find a solvable equation for angular diameter distance [Kling Campbell(2008)]. The perturbed FLRW metric incorporates clumps of matter into a metric that is on average homogeneous and isotropic. Deriving a solvable equation for angular diameter distance, in a perturbed FLRW metric, could prove useful to astronomers. By including clumps of matter along the line-of-sight into the math, we can calculate distances to light emitting objects that are obstructed by weak gravitational fields. This paper should serve as a test of both our tetrad and our methodology, by showing that they work in the flat unperturbed metric
The Size and Shape of Voids in Three-Dimensional Galaxy Surveys
The sizes and shapes of voids in a galaxy survey depend not only on the
physics of structure formation, but also on the sampling density of the survey
and on the algorithm used to define voids. Using an N-body simulation with a
CDM power spectrum, we study the properties of voids in samples with different
number densities of galaxies, both in redshift space and in real space. When
voids are defined as regions totally empty of galaxies, their characteristic
volume is strongly dependent on sampling density; when they are defined as
regions whose density is 0.2 times the mean galaxy density, the dependence is
less strong. We compare two void-finding algorithms, one in which voids are
nonoverlapping spheres, and one, based on the algorithm of Aikio and Mahonen,
which does not predefine the shape of a void. Regardless of the algorithm
chosen, the characteristic void size is larger in redshift space than in real
space, and is larger for low sampling densities than for high sampling
densities. We define an elongation statistic Q which measures the tendency of
voids to be stretched or squashed along the line of sight. Using this
statistic, we find that at sufficiently high sampling densities (comparable to
the number densities of galaxies brighter than L_*), large voids tend to be
slightly elongated along the line of sight in redshift space.Comment: LaTex, 21 pages (including 7 figures), ApJ, submitte
A Comparison of the Intrinsic Shapes of Two Different Types of Dwarf Galaxies: Blues Compact Dwarfs and Dwarf Ellipticals
We measure the apparent shapes for a sample of 62 blue compact dwarf galaxies
(BCDs), and compare them with the apparent shapes for a sample of 80 dwarf
elliptical galaxies (dEs). The BCDs are flatter, on average, than the dEs, but
the difference is only marginally significant. We then use both non-parametric
and parametric techniques to determine possible distributions of intrinsic
shapes for the BCDs. The hypothesis that BCDs are oblate spheroids can be ruled
out with a high confidence level (), but the hypothesis that they are
prolate spheroids cannot be excluded. The apparent shapes of BCDs are totally
consistent with the hypothesis that they are triaxial ellipsoids. If the
intrinsic axis ratios, and , are distributed according to a
Gaussian with means and and standard deviation ,
we find the best-fitting distribution for BCDs has , while that for dEs has . Our results are consistent with the hypothesis that BCDs
have a close evolutionary relation with dEs.Comment: total 23 pages, 9 figures, and 1 Table, submitted to ApJ on Sep 19
1997. Email addresses: [email protected],
[email protected], [email protected],
[email protected], [email protected]
The Dependence of Galaxy Shape on Luminosity and Surface Brightness Profile
For a sample of 96,951 galaxies from the Sloan Digital Sky Survey Data
Release 3, we study the distribution of apparent axis ratios as a function of
r-band absolute magnitude and surface brightness profile type. We use the
parameter fracDeV to quantify the profile type (fracDeV = 1 for a de
Vaucouleurs profile; fracDeV = 0 for an exponential profile). When the apparent
axis ratio q_{am} is estimated from the moments of the light distribution, the
roundest galaxies are very bright (M_r \sim -23) de Vaucouleurs galaxies and
the flattest are modestly bright (M_r \sim -18) exponential galaxies. When the
apparent axis ratio q_{25} is estimated from the axis ratio of the 25
mag/arcsec^2 isophote, we find that de Vaucouleurs galaxies are flatter than
exponential galaxies of the same absolute magnitude. For a given surface
brightness profile type, very bright galaxies are rounder, on average, than
fainter galaxies. We deconvolve the distributions of apparent axis ratios to
find the distribution of the intrinsic short-to-long axis ratio gamma, assuming
constant triaxiality T. For all profile types and luminosities, the
distribution of apparent axis ratios is inconsistent with a population of
oblate spheroids, but is usually consistent with a population of prolate
spheroids. Bright galaxies with a de Vaucouleurs profile (M_r < -21.84, fracDeV
> 0.9) have a distribution of q_{am} that is consistent with triaxiality in the
range 0.4 < T < 0.8, with mean intrinsic axis ratio 0.66 < gamma < 0.69. The
fainter de Vaucouleurs galaxies are best fit with prolate spheroids (T = 1)
with mean axis ratio gamma = 0.51.Comment: 32 pages, 12 figures, to appear in Ap
The intrinsic shape of galaxy bulges
The knowledge of the intrinsic three-dimensional (3D) structure of galaxy
components provides crucial information about the physical processes driving
their formation and evolution. In this paper I discuss the main developments
and results in the quest to better understand the 3D shape of galaxy bulges. I
start by establishing the basic geometrical description of the problem. Our
understanding of the intrinsic shape of elliptical galaxies and galaxy discs is
then presented in a historical context, in order to place the role that the 3D
structure of bulges play in the broader picture of galaxy evolution. Our
current view on the 3D shape of the Milky Way bulge and future prospects in the
field are also depicted.Comment: Invited Review to appear in "Galactic Bulges" Editors: Laurikainen
E., Peletier R., Gadotti D. Springer Publishing. 24 pages, 7 figure
Dependence of Galaxy Shape on Environment in the Sloan Digital Sky Survey
Using a sample of galaxies from the Sloan Digital Sky Survey (SDSS) Data
Release 4, we study the trends relating surface brightness profile type and
apparent axis ratio to the local galaxy environment. We use the SDSS parameter
`fracDeV' to quantify the profile type. We find that galaxies with M_r > -18
are mostly described by exponential profiles in all environments. Galaxies with
-21 < M_r < -18 mainly have exponential profiles in low density environments
and de Vaucouleurs profiles in high density environments. The most luminous
galaxies, with M_r < -21, are mostly described by de Vaucouleurs profiles in
all environments. For galaxies with M_r < -19, the fraction of de Vaucouleurs
galaxies is a monotonically increasing function of local density, while the
fraction of exponential galaxies is monotonically decreasing. For a fixed
surface brightness profile type, apparent axis ratio is frequently correlated
with environment. As the local density of galaxies increases, we find that for
-20 < M_r < -18, galaxies of all profile types become slightly rounder, on
average; for -22 < M_r < -20, galaxies with exponential profiles tend to become
flatter, while galaxies with de Vaucouleurs profiles become rounder; for M_r <
-22, galaxies with exponential profiles become flatter, while the de
Vaucouleurs galaxies become rounder in their inner regions, yet exhibit no
change in their outer regions. We comment on how the observed trends relate to
the merger history of galaxies.Comment: 23 pages, 7 figures, accepted by Ap
The ellipticities of Galactic and LMC globular clusters
The globular clusters of the LMC are found to be significantly more
elliptical than Galactic globular clusters, but very similar in virtually all
other respects. The ellipticity of the LMC globular clusters is shown not be
correlated with the age or mass of those clusters. It is proposed that the
ellipticity differences are caused by the different strengths of the tidal
fields in the LMC and the Galaxy. The strong Galactic tidal field erases
initial velocity anisotropies and removes angular momentum from globular
clusters making them more spherical. The tidal field of the LMC is not strong
enough to perform these tasks and its globular clusters remain close to their
initial states.Comment: 3 pages LaTeX file with 3 figures incorporated accepted for
publication in MNRAS. Also available by e-mailing spg, or by ftp from
ftp://star-www.maps.susx.ac.uk/pub/papers/spg/ellip.ps.
Galaxy Clusters: Oblate or Prolate?
It is now well known that a combined analysis of the Sunyaev-Zel'dovich (SZ)
effect and the X-ray emission observations can be used to determine the angular
diameter distance to galaxy clusters, from which the Hubble constant is
derived. Given that the SZ/X-ray Hubble constant is determined through a
geometrical description of clusters, the accuracy to which such distance
measurements can be made depends on how well one can describe intrinsic cluster
shapes. Using the observed X-ray isophotal axial ratio distribution for a
sample of galaxy clusters, we discuss intrinsic cluster shapes and, in
particular, if clusters can be described by axisymmetric models, such as oblate
and prolate ellipsoids. These models are currently favored when determining the
SZ/X-ray Hubble constant. We show that the current observational data on the
asphericity of galaxy clusters suggest that clusters are more consistent with a
prolate rather than an oblate distribution. We address the possibility that
clusters are intrinsically triaxial by viewing triaxial ellipsoids at random
angles with the intrinsic axial ratios following an isotropic Gaussian
distribution. We discuss implications of our results on current attempts at
measuring the Hubble constant using galaxy clusters and suggest that an
unbiased estimate of the Hubble constant, not fundamentally limited by
projection effects, would eventually be possible with the SZ/X-ray method.Comment: 6 pages, 6 figures. MNRAS (in press
Some improvements to the spherical collapse model
I study the joint effect of dynamical friction, tidal torques and
cosmological constant on clusters of galaxies formation I show that within
high-density environments, such as rich clusters of galaxies, both dynamical
friction and tidal torques slows down the collapse of low-? peaks producing an
observable variation in the time of collapse of the perturbation and, as a
consequence, a reduction in the mass bound to the collapsed perturbation
Moreover, the delay of the collapse produces a tendency for less dense regions
to accrete less mass, with respect to a classical spherical model, inducing a
biasing of over-dense regions toward higher mass I show how the threshold of
collapse is modified if dynamical friction, tidal torques and a non-zero
cosmological constant are taken into account and I use the Extended Press
Schecter (EPS) approach to calculate the effects on the mass function Then, I
compare the numerical mass function given in Reed et al (2003) with the
theoretical mass function obtained in the present paper I show that the barrier
obtained in the present paper gives rise to a better description of the mass
function evolution with respect to other previous models (Sheth & Tormen 1999,
MNRAS, 308, 119 (hereafter ST); Sheth & Tormen 2002, MNRAS, 329, 61 (hereafter
ST1)
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