816 research outputs found
Alignment of galaxies relative to their local environment in SDSS-DR8
We study the alignment of galaxies relative to their local environment in
SDSS-DR8 and, using these data, we discuss evolution scenarios for different
types of galaxies. We defined a vector field of the direction of anisotropy of
the local environment of galaxies. We summed the unit direction vectors of all
close neighbours of a given galaxy in a particular way to estimate this field.
We found the alignment angles between the spin axes of disc galaxies, or the
minor axes of elliptical galaxies, and the direction of anisotropy. The
distributions of cosines of these angles are compared to the random
distributions to analyse the alignment of galaxies. Sab galaxies show
perpendicular alignment relative to the direction of anisotropy in a sparse
environment, for single galaxies and galaxies of low luminosity. Most of the
parallel alignment of Scd galaxies comes from dense regions, from 2...3 member
groups and from galaxies with low luminosity. The perpendicular alignment of S0
galaxies does not depend strongly on environmental density nor luminosity; it
is detected for single and 2...3 member group galaxies, and for main galaxies
of 4...10 member groups. The perpendicular alignment of elliptical galaxies is
clearly detected for single galaxies and for members of < 11 member groups; the
alignment increases with environmental density and luminosity. We confirm the
existence of fossil tidally induced alignment of Sab galaxies at low z. The
alignment of Scd galaxies can be explained via the infall of matter to
filaments. S0 galaxies may have encountered relatively massive mergers along
the direction of anisotropy. Major mergers along this direction can explain the
alignment of elliptical galaxies. Less massive, but repeated mergers are
possibly responsible for the formation of elliptical galaxies in sparser areas
and for less luminous elliptical galaxies.Comment: 15 pages, 15 figures, accepted for publication in A&
Galaxy filaments as pearl necklaces
Context. Galaxies in the Universe form chains (filaments) that connect groups
and clusters of galaxies. The filamentary network includes nearly half of the
galaxies and is visually the most striking feature in cosmological maps.
Aims. We study the distribution of galaxies along the filamentary network,
trying to find specific patterns and regularities.
Methods. Galaxy filaments are defined by the Bisous model, a marked point
process with interactions. We use the two-point correlation function and the
Rayleigh Z-squared statistic to study how galaxies and galaxy groups are
distributed along the filaments.
Results. We show that galaxies and groups are not uniformly distributed along
filaments, but tend to form a regular pattern. The characteristic length of the
pattern is around 7 Mpc/h. A slightly smaller characteristic length 4 Mpc/h can
also be found, using the Z-squared statistic.
Conclusions. We find that galaxy filaments in the Universe are like pearl
necklaces, where the pearls are galaxy groups distributed more or less
regularly along the filaments. We propose that this well defined characteristic
scale could be used to test various cosmological models and to probe
environmental effects on the formation and evolution of galaxies.Comment: 8 pages, 9 figures, 1 table, accepted for publication in A&
Flux- and volume-limited groups/clusters for the SDSS galaxies: catalogues and mass estimation
We provide flux-limited and volume-limited galaxy group and cluster
catalogues, based on the spectroscopic sample of the SDSS data release 10
galaxies. We used a modified friends-of-friends (FoF) method with a variable
linking length in the transverse and radial directions to identify as many
realistic groups as possible. The flux-limited catalogue incorporates galaxies
down to m_r = 17.77 mag. It includes 588193 galaxies and 82458 groups. The
volume-limited catalogues are complete for absolute magnitudes down to M_r =
-18.0, -18.5, -19.0, -19.5, -20.0, -20.5, and -21.0; the completeness is
achieved within different spatial volumes, respectively. Our analysis shows
that flux-limited and volume-limited group samples are well compatible to each
other, especially for the larger groups/clusters. Dynamical mass estimates,
based on radial velocity dispersions and group extent in the sky, are added to
the extracted groups. The catalogues can be accessed via http://cosmodb.to.ee
and the Strasbourg Astronomical Data Center (CDS).Comment: 16 pages, 18 figures, 2 tables, accepted for publication in A&
Wavelet analysis of the formation of the cosmic web
According to the modern cosmological paradigm galaxies and galaxy systems
form from tiny density perturbations generated during the very early phase of
the evolution of the Universe. Using numerical simulations we study the
evolution of phases of density perturbations of different scales to understand
the formation and evolution of the cosmic web. We apply the wavelet analysis to
follow the evolution of high-density regions (clusters and superclusters) of
the cosmic web. We show that the positions of maxima and minima of density
waves (their spatial phases) almost do not change during the evolution of the
structure. Positions of extrema of density perturbations are the more stable,
the larger is the wavelength of perturbations. Combining observational and
simulation data we conclude that the skeleton of the cosmic web was present
already in an early stage of structure evolution.Comment: 12 pages, 8 figures, revised versio
Discovery of a massive supercluster system at
Superclusters are the largest relatively isolated systems in the cosmic web.
Using the SDSS BOSS survey we search for the largest superclusters in the
redshift range .
We generate a luminosity-density field smoothed over
to detect the large-scale over-density regions. Each individual over-density
region is defined as single supercluster in the survey. We define the
superclusters in the way that they are comparable with the superclusters found
in the SDSS main survey.
We found a system we call the BOSS Great Wall (BGW), which consists of two
walls with diameters 186 and 173 Mpc, and two other major superclusters
with diameters of 64 and 91 Mpc. As a whole, this system consists of
830 galaxies with the mean redshift 0.47. We estimate the total mass to be
approximately . The morphology of the
superclusters in the BGW system is similar to the morphology of the
superclusters in the Sloan Great Wall region.
The BGW is one of the most extended and massive system of superclusters yet
found in the Universe.Comment: 4 pages, accepted as a letter in A&
An integrative approach based on probabilistic modelling and statistical inference for morpho-statistical characterization of astronomical data
This paper describes several applications in astronomy and cosmology that are
addressed using probabilistic modelling and statistical inference
Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum
We present precise stellar radial velocity measurements for the K giant star
Beta Gem spanning over 25 years. These data show that the long period low
amplitude radial velocity variations found by Hatzes & Cochran (1993) are
long-lived and coherent. An examination of the Ca II K emission, spectral line
shapes from high resolution data (R = 210,000), and Hipparcos photometry show
no significant variations of these quantities with the RV period. These data
confirm the planetary companion hypothesis suggested by Hatzes & Cochran
(1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a
period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis,
and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the
seventh intermediate mass star shown to host a sub-stellar companion and
suggests that planet-formation around stars much more massive than the sun may
common.Comment: 10 pages, 9 figures, Astronomy and Astrophysics, in pres
A Spitzer/IRAC Search for Substellar Companions of the Debris Disk Star epsilon Eridani
We have used the InfraRed Array Camera (IRAC) onboard the Spitzer Space
telescope to search for low mass companions of the nearby debris disk star
epsilon Eridani. The star was observed in two epochs 39 days apart, with
different focal plane rotation to allow the subtraction of the instrumental
Point Spread Function, achieving a maximum sensitivity of 0.01 MJy/sr at 3.6
and 4.5 um, and 0.05 MJy/sr at 5.8 and 8.0 um. This sensitivity is not
sufficient to directly detect scattered or thermal radiation from the epsilon
Eridani debris disk. It is however sufficient to allow the detection of Jovian
planets with mass as low as 1 MJ in the IRAC 4.5 um band. In this band, we
detected over 460 sources within the 5.70 arcmin field of view of our images.
To test if any of these sources could be a low mass companion to epsilon
Eridani, we have compared their colors and magnitudes with models and
photometry of low mass objects. Of the sources detected in at least two IRAC
bands, none fall into the range of mid-IR color and luminosity expected for
cool, 1 Gyr substellar and planetary mass companions of epsilon Eridani, as
determined by both models and observations of field M, L and T dwarf. We
identify three new sources which have detections at 4.5 um only, the lower
limit placed on their [3.6]-[4.5] color consistent with models of planetary
mass objects. Their nature cannot be established with the currently available
data and a new observation at a later epoch will be needed to measure their
proper motion, in order to determine if they are physically associated to
epsilon Eridani.Comment: 36 pages, to be published on The Astrophysical Journal, vol. 647,
August 200
A Blind Search for Magnetospheric Emissions from Planetary Companions to Nearby Solar-type Stars
This paper reports a blind search for magnetospheric emissions from planets
around nearby stars. Young stars are likely to have much stronger stellar winds
than the Sun, and because planetary magnetospheric emissions are powered by
stellar winds, stronger stellar winds may enhance the radio luminosity of any
orbiting planets. Using various stellar catalogs, we selected nearby stars (<~
30 pc) with relatively young age estimates (< 3 Gyr). We constructed different
samples from the stellar catalogs, finding between 100 and several hundred
stars. We stacked images from the 74-MHz (4-m wavelength) VLA Low-frequency Sky
Survey (VLSS), obtaining 3\sigma limits on planetary emission in the stacked
images of between 10 and 33 mJy. These flux density limits correspond to
average planetary luminosities less than 5--10 x 10^{23} erg/s. Using recent
models for the scaling of stellar wind velocity, density, and magnetic field
with stellar age, we estimate scaling factors for the strength of stellar
winds, relative to the Sun, in our samples. The typical kinetic energy carried
by the stellar winds in our samples is 15--50 times larger than that of the
Sun, and the typical magnetic energy is 5--10 times larger. If we assume that
every star is orbited by a Jupiter-like planet with a luminosity larger than
that of the Jovian decametric radiation by the above factors, our limits on
planetary luminosities from the stacking analysis are likely to be a factor of
10--100 above what would be required to detect the planets in a statistical
sense. Similar statistical analyses with observations by future instruments,
such as the Low Frequency Array (LOFAR) and the Long Wavelength Array (LWA),
offer the promise of improvements by factors of 10--100.Comment: 11 pages; AASTeX; accepted for publication in A
HD 77407 and GJ 577: two new young stellar binaries detected with the Calar Alto Adaptive Optics system ALFA
We present the first results from our search for close stellar and
sub-stellar companions to young nearby stars on the northern sky. Our infrared
imaging observations are obtained with the 3.5 m Calar Alto telescope and the
AO system ALFA. With two epoch observations which were separated by about one
year, we found two co-moving companion candidates, one close to HD 77407 and
one close to GJ 577. For the companion candidate near GJ 577, we obtained an
optical spectrum showing spectral type M4.5; this candidate is a bound low-mass
stellar companion confirmed by both proper motion and spectroscopy. We estimate
the masses for HD 77407 B and GJ 577 B to be ~0.3 to 0.5 Msun and ~0.16 to 0.2
Msun, respectively. Compared to Siess al.(2000) models, each of the two pairs
appears co-eval with HD 77407 A,B being 10 to 40 Myrs old and GJ 577 A,B being
older than 100 Myrs. We also took multi-epoch high-resolution spectra of HD
77407 to search for sub-stellar companions, but did not find any with 3 Mjup as
upper mass (msin(i)) limit (for up to 4 year orbits); however, we detected a
long-term radial velocity trend in HD 77407 A, consistent with a ~ 0.3 Msun
companion at ~ 50 AU separation, i.e. the one detected by the imaging. Hence,
HD 77407 B is confirmed to be a bound companion to HD 77407 A. We also present
limits for undetected, but detectable companions using a deep image of HD 77407
A and B, also observed with the Keck NIRC2 AO system; any brown dwarfs were
detectable outside of 0.5 arcsec (17 AU at HD 77407), giant planets with masses
from ~ 6.5 to 12 Mjup were detectable at > 1.5 arcsec.Comment: in pres
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