375 research outputs found
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&
COSMOSOMAS Observations of the CMB and Galactic Foregrounds at 11 GHz: Evidence for anomalous microwave emission at high Galactic Latitude
We present observations with the new 11 GHz radiometer of the COSMOSOMAS
experiment at the Teide Observatory (Tenerife). The sky region between 0 deg <=
RA <= 360 deg and 26 deg <= DEC 49 deg (ca. 6500 square degrees) was observed
with an angular resolution of 0.9 deg. Two orthogonal independent channels in
the receiving system measured total power signals from linear polarizations
with a 2 GHz bandwidth. Maps with an average sensitivity of 50 microK per beam
have been obtained for each channel. At high Galactic latitude (|b|>30deg) the
11 GHz data are found to contain the expected cosmic microwave background as
well as extragalactic radiosources, galactic synchrotron and free-free
emission, and a dust-correlated component which is very likely of galactic
origin. At the angular scales allowed by the window function of the experiment,
the dust-correlated component presents an amplitude \Delta T aprox. 9-13 microK
while the CMB signal is of order 27 microK. The spectral behaviour of the
dust-correlated signal is examined in the light of previous COSMOSOMAS data at
13-17 GHz and WMAP data at 22-94 GHz in the same sky region. We detect a
flattening in the spectral index of this signal below 20 GHz which rules out
synchrotron radiation as being responsible for the emission. This anomalous
dust emission can be described by a combination of free-free emission and
spinning dust models with a flux density peaking around 20 GHz.Comment: 17 pages, 10 tables, 20 figures. Details on the COSMOSOMAS experiment
can be found at http://www.iac.es/project/cmb/cosmosomas
Evolution of low-frequency features in the CMB spectrum due to stimulated Compton scattering and Doppler-broadening
We discuss a new solution of the Kompaneets-equation for physical situations
in which low frequency photons, forming relatively narrow spectral details, are
Compton scattered in an isotropic, infinite medium with an intense ambient
blackbody field that is very close to full thermodynamic equilibrium with the
free electrons. In this situation the background-induced stimulated Compton
scattering slows down the motion of photons toward higher frequencies by a
factor of 3 in comparison with the solution that only takes into account
Doppler-broadening and boosting. This new solution is important for detailed
computations of cosmic microwave background spectral distortions arising due to
uncompensated atomic transitions of hydrogen and helium in the early Universe.
In addition we derive another analytic solution that only includes the
background-induced stimulated Compton scattering and is valid for power-law
ambient radiation fields. This solution might have interesting applications for
radio lines arising inside of bright extra-galactic radio source, where
according to our estimates line shifts because of background-induced stimulated
scattering could be amplified and even exceed the line broadening due to the
Doppler-effect.Comment: 5 pages, 2 figures, submitted to A&
Biases in galaxy cluster velocity dispersion and mass estimates in the small number of galaxies regime
We present a study of the statistical properties of three velocity dispersion
and mass estimators, namely biweight, gapper and standard deviation, in the
small number of galaxies regime ().
Using a set of 73 numerically simulated galaxy clusters, we characterise the
statistical bias and the variance for the three estimators, both in the
determination of the velocity dispersion and the dynamical mass of the clusters
via the relation. The results are used to define a new set of
unbiased estimators, that are able to correct for those statistical biases with
a minimal increase of the associated variance. The numerical simulations are
also used to characterise the impact of velocity segregation in the selection
of cluster members, and the impact of using cluster members within different
physical radii from the cluster centre.
The standard deviation is found to be the lowest variance estimator. The
selection of galaxies within the sub-sample of the most massive galaxies in the
cluster introduces a \% bias in the velocity dispersion estimate when
calculated using a quarter of the most massive cluster members. We also find a
dependence of the velocity dispersion estimate on the aperture radius as a
fraction of , consistent with previous results.
The proposed set of unbiased estimators effectively provides a correction of
the velocity dispersion and mass estimates from all those effects in the small
number of cluster members regime. This is tested by applying the new estimators
to a subset of simulated observations. Although for a single galaxy cluster the
statistical and physical effects discussed here are comparable or slightly
smaller than the bias introduced by interlopers, they will be of relevance when
dealing with ensemble properties and scaling relations for large cluster
samples (Abridged).Comment: accepted for publication in A&
Time-Dependent Corrections to the Ly-alpha Escape Probability During Cosmological Hydrogen Recombination
We consider the effects connected with the detailed radiative transfer during
the epoch of cosmological recombination on the ionization history of our
Universe. We focus on the escape of photons from the hydrogen Lyman-alpha
resonance at redshifts 600<~ z <~ 2000, one of two key mechanisms defining the
rate of cosmological recombination. We approach this problem within the
standard formulation, and corrections due to two-photon interactions are
deferred to another paper. As a main result we show here that within a
non-stationary approach to the escape problem, the resulting correction in the
free electron fraction, N_e, is about ~1.6-1.8% in the redshift range
800<~z<~1200. Therefore the discussed process results in one of the largest
modifications to the ionization history close to the maximum of
Thomson-visibility function at z~1100 considered so far. We prove our results
both numerically and analytically, deriving the escape probability, and
considering both Lyman-alpha line emission and line absorption in a way
different from the Sobolev approximation. In particular, we give a detailed
derivation of the Sobolev escape probability during hydrogen recombination, and
explain the underlying assumptions. We then discuss the escape of photons for
the case of coherent scattering in the lab frame, solving this problem
analytically in the quasi-stationary approximation and also in the
time-dependent case. We show here that during hydrogen recombination the
Sobolev approximation for the escape probability is not valid at the level of
DP/P~5-10%. This is because during recombination the ionization degree changes
significantly over a characteristic time Dz/z~10%, so that at percent level
accuracy the photon distribution is not evolving (abridged)Comment: 18 pages, 12 figures, accepted versio
Ly alpha escape during cosmological hydrogen recombination: the 3d-1s and 3s-1s two-photon processes
We give a formulation of the radiative transfer equation for Lyman alpha
photons which allows us to include the two-photon corrections for the 3s-1s and
3d-1s decay channels during cosmological hydrogen recombination. We use this
equation to compute the corrections to the Sobolev escape probability for Lyman
alpha photons during hydrogen recombination, which then allow us to calculate
the changes in the free electron fraction and CMB temperature and polarization
power spectra. We show that the effective escape probability changes by DP/P ~+
11% at z~1400 in comparison with the one obtained using the Sobolev
approximation. This speeds up of hydrogen recombination by DN_e/N_e ~- 1.6% at
z~1190, implying |DC_l/C_l| ~1%-3% at l >~ 1500 with shifts in the positions of
the maxima and minima in the CMB power spectra. These corrections will be
important for the analysis of future CMB data.
The total correction is the result of the superposition of three independent
processes, related to (i) time-dependent aspects of the problem, (ii)
corrections due to quantum mechanical deviations in the shape of the emission
and absorption profiles in the vicinity of the Lyman alpha line from the normal
Lorentzian, and (iii) a thermodynamic correction factor, which occurs to be
very important. All these corrections are neglected in the
Sobolev-approximation, but they are important in the context of future CMB
observations. All three can be naturally obtained in the two-photon formulation
of the Lyman alpha absorption process. However, the corrections (i) and (iii)
can also be deduced in the normal '1+1' photon language, without necessarily
going to the two-photon picture. Therefore only (ii) is really related to the
quantum mechanical aspects of the two-photon process (abridged)Comment: 30 pages, 21 figures, submitted to A&
Triaxial stellar systems following the r1/n luminosity law: an analytical mass-density expression, gravitational torques and the bulge/disc interplay
We have investigated the structural and dynamical properties of triaxial stellar systems whose surface brightness profiles follow the r1/n luminosity law - extending the analysis by Ciotti, who explored the properties of spherical r1/n systems. A new analytical expression that accurately reproduces the spatial (i.e., deprojected) luminosity density profiles (error less than 0.1 per cent) is presented for detailed modelling of the Sérsic family of luminosity profiles. We evaluate both the symmetric and the non-axisymmetric components of the gravitational potential and force, and compute the torques as a function of position. For a given triaxiality, stellar systems with smaller values of n have a greater non-axisymmetric gravitational field component. We also explore the strength of the non-axisymmetric forces produced by bulges with differing n and triaxiality on systems having a range of bulge-to-disc ratios. The increasing disc-to-bulge ratio with increasing galaxy type (decreasing n) is found to greatly reduce the amplitude of the non-axisymmetric terms, and therefore reduce the possibility that triaxial bulges in late-type systems may be the mechanism or perturbation for non-symmetric structures in the disc. Using seeing-convolved r1/n-bulge plus exponential-disc fits to the K-band data from a sample of 80 nearby disc galaxies, we probe the relations between galaxy type, Sérsic index n and the bulge-to-disc luminosity ratio. These relations are shown to be primarily a consequence of the relation between n and the total bulge luminosity. In the K band, the trend of decreasing bulge-to-disc luminosity ratio along the spiral Hubble sequence is predominantly, though not entirely, a consequence of the change in the total bulge luminosity; the trend between the total disc luminosity and Hubble type is much weake
Cosmological Hydrogen Recombination: influence of resonance and electron scattering
In this paper we consider the effects of resonance and electron scattering on
the escape of Lyman alpha photons during cosmological hydrogen recombination.
We pay particular attention to the influence of atomic recoil, Doppler boosting
and Doppler broadening using a Fokker-Planck approximation of the
redistribution function describing the scattering of photons on the Lyman alpha
resonance of moving hydrogen atoms. We extend the computations of our recent
paper on the influence of the 3d/3s-1s two-photon channels on the dynamics of
hydrogen recombination, simultaneously including the full time-dependence of
the problem, the thermodynamic corrections factor, leading to a
frequency-dependent asymmetry between the emission and absorption profile, and
the quantum-mechanical corrections related to the two-photon nature of the
3d/3s-1s emission and absorption process on the exact shape of the Lyman alpha
emission profile. We show here that due to the redistribution of photons over
frequency hydrogen recombination is sped up by DN_e/N_e~-0.6% at z~900. For the
CMB temperature and polarization power spectra this results in
|DC_l/C_l|~0.5%-1% at l >~ 1500, and therefore will be important for the
analysis of future CMB data in the context of the PLANCK Surveyor, SPT and ACT.
The main contribution to this correction is coming from the atomic recoil
effect (DN_e/N_e~-1.2% at z~900), while Doppler boosting and Doppler broadening
partially cancel this correction, again slowing hydrogen recombination down by
DN_e/N_e~0.6% at z~900. The influence of electron scattering close to the
maximum of the Thomson visibility function at z~1100 can be neglected.
(abridged)Comment: 11 pages, 13 figures, submitted to A&
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