30 research outputs found
Exploring star formation using the filaments in the Sloan Digital Sky Survey Data Release Five (SDSS DR5)
We have quantified the average filamentarity of the galaxy distribution in
seven nearly two dimensional strips from the SDSS DR5 using a volume limited
sample in the absolute magnitude range -21 < M_r < -20. The average
filamentarity of star forming (SF) galaxies, which are predominantly blue, is
found to be more than that of other galaxies which are predominantly red. This
difference is possibly an outcome of the fact that blue galaxies have a more
filamentary distribution. Comparing the SF galaxies with only the blue other
galaxies, we find that the two show nearly equal filamentarity. Separately
analyzing the galaxies with high star formation rates (SFR) and low SFR, we
find that the latter has a more filamentary distribution. We interpret this in
terms of two effects (1.) A correlation between the SFR and individual galaxy
properties like luminosity with the high SFR galaxies being more luminous (2.)
A relation between the SFR and environmental effects like the density with the
high SFR galaxies preferentially occurring in high density regions. These two
effects are possibly not independent and are operating simultaneously. We do
not find any difference in the filamentarity of SF galaxies and AGNs.Comment: 6 pages, 3 figures, Final accepted version in MNRAS, in pres
The Caltech Faint Galaxy Redshift Survey XII: Clustering of Galaxies
A clustering analysis is performed on two samples of faint
galaxies each, in two widely separated regions of the sky, including the Hubble
Deep Field. One of the survey regions is configured so that some galaxy pairs
span angular separations of up to 1 deg. The median redshift is . Strong clustering is obvious, with every pencil-beam field containing a
handful of narrow redshift-space features, corresponding to galaxy structures
with sizes of 5 to 20 Mpc. The structures are not obviously organized on
planes, though one prominent, colinear triplet of structures is observed,
spanning Mpc. This may be evidence of a filament. A galaxy--galaxy
correlation function calculation is performed. No significant evolution of
clustering (relative to stable clustering) is found in the redshift range
0.3<z<1.0. This is not surprising, since uncertainties in the correlation
amplitude estimated from surveys like these are large; field-to-field
variations and covariances between data points are both shown to be
significant. Consistent with other studies in this redshift range, the
galaxy--galaxy correlation length is found to be somewhat smaller than that
predicted from local measurements and an assumption of no evolution. Galaxies
with absorption-line-dominated spectra show much stronger clustering at
distances of <2 Mpc than typical field galaxies. There is some evidence for
weaker clustering at intermediate redshift than at low redshift, when the
results presented here are compared with surveys of the local Universe. In
subsets of the data, the measured pairwise velocity dispersion of galaxies
ranges from 200 to , depending on the properties of the dominant
redshift structures in each subset.Comment: accepted for publication in the Ap
On the kinematics of the Local cosmic void
We collected the existing data on the distances and radial velocities of
galaxies around the Local Void in the Aquila/Hercules to examine the peculiar
velocity field induced by its underdensity. A sample of 1056 galaxies with
distances measured from the Tip of the Red Giant Branch, the Cepheid
luminosity, the SNIa luminosity, the surface brightness fluctuation method, and
the Tully-Fisher relation has been used for this purpose. The amplitude of
outflow is found to be ~300 km/s. The galaxies located within the void produce
the mean intra-void number density about 1/5 of the mean external number
density of galaxies. The void's population has a lower luminosity and a later
morphological type with the medians: M_B = -15.7^m and T = 8 (Sdm),
respectively.Comment: Version 1. 14 pages, 8 figures, 2 tables. Accepted to Astrophysics,
Volume 54, Issue
A filament of dark matter between two clusters of galaxies
It is a firm prediction of the concordance Cold Dark Matter (CDM)
cosmological model that galaxy clusters live at the intersection of large-scale
structure filaments. The thread-like structure of this "cosmic web" has been
traced by galaxy redshift surveys for decades. More recently the Warm-Hot
Intergalactic Medium (WHIM) residing in low redshift filaments has been
observed in emission and absorption. However, a reliable direct detection of
the underlying Dark Matter skeleton, which should contain more than half of all
matter, remained elusive, as earlier candidates for such detections were either
falsified or suffered from low signal-to-noise ratios and unphysical
misalignements of dark and luminous matter. Here we report the detection of a
dark matter filament connecting the two main components of the Abell 222/223
supercluster system from its weak gravitational lensing signal, both in a
non-parametric mass reconstruction and in parametric model fits. This filament
is coincident with an overdensity of galaxies and diffuse, soft X-ray emission
and contributes mass comparable to that of an additional galaxy cluster to the
total mass of the supercluster. Combined with X-ray observations, we place an
upper limit of 0.09 on the hot gas fraction, the mass of X-ray emitting gas
divided by the total mass, in the filament.Comment: Nature, in pres
Minivoids in the Local Volume
We consider a sphere of 7.5 Mpc radius, which contains 355 galaxies with
accurately measured distances, to detect the nearest empty volumes. Using a
simple void detection algorithm, we found six large (mini)voids in Aquila,
Eridanus, Leo, Vela, Cepheus and Octans, each of more than 30 Mpc^3. Besides
them, 24 middle-size "bubbles" of more than 5 Mpc^3 volume are detected, as
well as 52 small "pores". The six largest minivoids occupy 58% of the
considered volume. Addition of the bubbles and pores to them increases the
total empty volume up to 75% and 81%, respectively. The detected local voids
look like oblong potatoes with typical axial ratios b/a = 0.75 and c/a = 0.62
(in the triaxial ellipsoide approximation). Being arranged by the size of their
volume, local voids follow power law of volumes-rankes dependence. A
correlation Gamma-function of the Local Volume galaxies follows a power low
with a formally calculated fractal dimension D = 1.5. We found that galaxies
surrounding the local minivoids do not differ significantly from other nearby
galaxies on their luminosity, but have appreciably higher hydrogen
mass-to-luminosity ratio and also higher star formation rate. We recognize an
effect of local expansion of typical minivoid to be \Delta H/H_0~(25+-15)%.Comment: 23 pages, 18 figures. Astrophysical Journal, accepte
The clustering of massive galaxies at z~0.5 from the first semester of BOSS data
We calculate the real- and redshift-space clustering of massive galaxies at
z~0.5 using the first semester of data by the Baryon Oscillation Spectroscopic
Survey (BOSS). We study the correlation functions of a sample of 44,000 massive
galaxies in the redshift range 0.4<z<0.7. We present a halo-occupation
distribution modeling of the clustering results and discuss the implications
for the manner in which massive galaxies at z~0.5 occupy dark matter halos. The
majority of our galaxies are central galaxies living in halos of mass
10^{13}Msun/h, but 10% are satellites living in halos 10 times more massive.
These results are broadly in agreement with earlier investigations of massive
galaxies at z~0.5. The inferred large-scale bias (b~2) and relatively high
number density (nbar=3e-4 h^3 Mpc^{-3}) imply that BOSS galaxies are excellent
tracers of large-scale structure, suggesting BOSS will enable a wide range of
investigations on the distance scale, the growth of large-scale structure,
massive galaxy evolution and other topics.Comment: 11 pages, 12 figures, matches version accepted by Ap
The Sloan Great Wall. Morphology and galaxy content
We present the results of the study of the morphology and galaxy content of
the Sloan Great Wall (SGW). We use the luminosity density field to determine
superclusters in the SGW, and the fourth Minkowski functional V_3 and the
morphological signature (the K_1-K_2 shapefinders curve) to show the different
morphologies of the SGW, from a single filament to a multibranching, clumpy
planar system. The richest supercluster in the SGW, SCl~126 and especially its
core resemble a very rich filament, while another rich supercluster in the SGW,
SCl~111, resembles a "multispider" - an assembly of high density regions
connected by chains of galaxies. Using Minkowski functionals we study the
substructure of individual galaxy populations determined by their color in
these superclusters. We assess the statistical significance of the results with
the halo model and smoothed bootstrap. We study the galaxy content and the
properties of groups of galaxies in two richest superclusters of the SGW,
paying special attention to bright red galaxies (BRGs) and to the first ranked
galaxies in SGW groups. About 1/3 of BRGs are spirals. The scatter of colors of
elliptical BRGs is smaller than that of spiral BRGs. About half of BRGs and of
first ranked galaxies in groups have large peculiar velocities. Groups with
elliptical BRGs as their first ranked galaxies populate superclusters more
uniformly than the groups, which have a spiral BRG as its first ranked galaxy.
The galaxy and group content of the core of the supercluster SCl~126 shows
several differences in comparison with the outskirts of this supercluster and
with the supercluster SCl~111. Our results suggest that the formation history
and evolution of individual neighbour superclusters in the SGW has been
different.Comment: Comments: 26 pages, 20 figures, accepted for publication in Ap
The Hierarchical Structure and Dynamics of Voids
Contrary to the common view voids have very complex internal structure and
dynamics. Here we show how the hierarchy of structures in the density field
inside voids is reflected by a similar hierarchy of structures in the velocity
field. Voids defined by dense filaments and clusters can de described as simple
expanding domains with coherent flows everywhere except at their boundaries. At
scales smaller that the void radius the velocity field breaks into expanding
sub-domains corresponding to sub- voids. These sub-domains break into even
smaller sub-sub domains at smaller scales resulting in a nesting hierarchy of
locally expanding domains. The ratio between the magnitude of the velocity
field responsible for the expansion of the void and the velocity field defining
the sub voids is approximately one order of magnitude. The small-scale
components of the velocity field play a minor role in the shaping of the voids
but they define the local dynamics directly affecting the processes of galaxy
formation and evolution. The super-Hubble expansion inside voids makes them
cosmic magnifiers by stretching their internal primordial density fluctuations
allowing us to probe the small scales in the primordial density field. Voids
also act like time machines by "freezing" the development of the medium-scale
density fluctuations responsible for the formation of the tenuous web of
structures seen connecting proto galaxies in computer simulations. As a result
of this freezing haloes in voids can remain "connected" to this tenuous web
until the present time. This may have an important effect in the formation and
evolution of galaxies in voids by providing an efficient gas accretion
mechanism via coherent low-velocity streams that can keep a steady inflow of
matter for extended periods of time.Comment: High-res version are related media here:
http://skysrv.pha.jhu.edu/~miguel/Papers/Hierarchy_voids/index.htm
SDSS DR7 superclusters. Morphology
We study the morphology of a set of superclusters drawn from the SDSS DR7. We
calculate the luminosity density field to determine superclusters from a flux-
limited sample of galaxies from SDSS DR7, and select superclusters with 300 and
more galaxies for our study. The morphology of superclusters is described with
the fourth Minkowski functional V3, the morphological signature (the curve in
the shapefinder's K1-K2 plane) and the shape parameter (the ratio of the
shapefinders K1/K2). We investigate the supercluster sample using
multidimensional normal mixture modelling, and use Abell clusters to identify
our superclusters with known superclusters and to study the large-scale
distribution of superclusters. The superclusters in our sample form three
chains of superclusters; one of them is the Sloan Great Wall. Most
superclusters have filament-like overall shapes. Superclusters can be divided
into two sets; more elongated superclusters are more luminous, richer, have
larger diameters, and a more complex fine structure than less elongated
superclusters. The fine structure of superclusters can be divided into four
main morphological types: spiders, multispiders, filaments, and multibranching
filaments. We present the 2D and 3D distribution of galaxies and rich groups,
the fourth Minkowski functional, and the morphological signature for all
superclusters. Widely different morphologies of superclusters show that their
evolution has been dissimilar. A study of a larger sample of superclusters from
observations and simulations is needed to understand the morphological variety
of superclusters and the possible connection between the morphology of
superclusters and their large-scale environment.Comment: Comments: 20 pages, 18 figures, accepted for publication in Astronomy
and Astrophysic
The Beginning and Evolution of the Universe
We review the current standard model for the evolution of the Universe from
an early inflationary epoch to the complex hierarchy of structure seen today.
We summarize and provide key references for the following topics: observations
of the expanding Universe; the hot early Universe and nucleosynthesis; theory
and observations of the cosmic microwave background; Big Bang cosmology;
inflation; dark matter and dark energy; theory of structure formation; the cold
dark matter model; galaxy formation; cosmological simulations; observations of
galaxies, clusters, and quasars; statistical measures of large-scale structure;
and measurement of cosmological parameters. We conclude with discussion of some
open questions in cosmology. This review is designed to provide a graduate
student or other new worker in the field an introduction to the cosmological
literature.Comment: 69 pages. Invited review article for Publications of the Astronomical
Society of the Pacific. Supplementary references, tables, and more concise
PDF file at http://www.physics.drexel.edu/univers