408 research outputs found
Phase-space shapes of clusters and rich groups of galaxies
Clusters and groups of galaxies are highly aspherical, with shapes
approximated by nearly prolate ellipsoids of revolution. An equally fundamental
property is the shape of these objects in velocity space which is the
anisotropy of the global velocity dispersion tensor. Here we make use of
kinematical data comprising around 600 nearby clusters and rich groups of
galaxies from the SDSS to place constraints on the phase-space shapes of these
objects, i.e. their shapes in both position and velocity space. We show that
the line of sight velocity dispersion normalised by a mass dependent velocity
scale correlates with the apparent elongation, with circular (elongated)
clusters exhibiting an excessive (decremental) normalised velocity dispersion.
This correlation holds for dynamically young or old clusters and, therefore, it
originates from projecting their intrinsic phase-space shapes rather than from
dynamical evolution. It signifies that clusters are preferentially prolate not
only in position space, but also in velocity space. The distribution of the
axial ratios in position space is found to be well approximated by a Gaussian
with a mean 0.66+/-0.01 and a dispersion 0.07+/-0.008. The velocity ellipsoids
representing the shapes in velocity space are more spherical, with a mean axial
ratio of 0.78+/-0.03. This finding has important implications for mass
measurements based on the line of sight velocity dispersion profiles in
individual clusters. For typical axial ratios of the velocity ellipsoids in the
analysed cluster sample, systematic errors on the mass estimates inferred from
the line of sight velocity dispersions become comparable to statistical
uncertainties for galaxy clusters with as few as 40 spectroscopic redshifts.Comment: 9 pages, 7 figures; published in A&A; typo in eq. 5 correcte
Redshift remapping and cosmic acceleration in dark-matter-dominated cosmological models
The standard relation between the cosmological redshift and cosmic scale
factor underlies cosmological inference from virtually all kinds of
cosmological observations, leading to the emergence of the LambdaCDM
cosmological model. This relation is not a fundamental theory and thus
observational determination of this function (redshift remapping) should be
regarded as an insightful alternative to holding its standard form in analyses
of cosmological data. Here we present non-parametric reconstructions of
redshift remapping in dark-matter-dominated models and constraints on
cosmological parameters from a joint analysis of all primary cosmological
probes including the local measurement of the Hubble constant, Type Ia
supernovae, baryonic acoustic oscillations (BAO), Planck observations of the
cosmic microwave background (CMB) radiation (temperature power spectrum) and
cosmic chronometers. The reconstructed redshift remapping points to an
additional boost of redshift operating in late epoch of cosmic evolution, but
affecting both low-redshift observations and the CMB. The model predicts a
significant difference between the actual Hubble constant, h=0.48+/-0.02, and
its local determination, h_obs=0.73+/-0.02. The ratio of these two values
coincides closely with the maximum expansion rate inside voids formed in the
corresponding open cosmological model with Omega_m=0.87+/-0.03, whereas the
actual value of the Hubble constant implies the age of the Universe that is
compatible with the Planck LambdaCDM cosmology. The new dark-matter-dominated
model with redshift remapping provides excellent fits to all data and
eliminates recently reported tensions between the Planck LambdaCDM cosmology,
the local determination of the Hubble constant and the BAO measurements from
the Ly-alpha forest of high-redshift quasars.Comment: 21 pages, 11 figures, 4 tables; accepted for publication in MNRA
Testing the mapping between redshift and cosmic scale factor
The canonical redshift-scale factor relation, 1/a=1+z, is a key element in
the standard LambdaCDM model of the big bang cosmology. Despite its fundamental
role, this relation has not yet undergone any observational tests since
Lemaitre and Hubble established the expansion of the Universe. It is strictly
based on the assumption of the Friedmann-Lemaitre-Robertson-Walker metric
describing a locally homogeneous and isotropic universe and that photons move
on null geodesics of the metric. Thus any violation of this assumption, within
general relativity or modified gravity, can yield a different mapping between
the model redshift z=1/a-1 and the actually observed redshift z_obs, i.e. z_obs
neq z. Here we perform a simple test of consistency for the standard
redshift-scale factor relation by determining simultaneous observational
constraints on the concordance LambdaCDM cosmological parameters and a
generalized redshift mapping z=f(z_obs). Using current baryon acoustic
oscillations (BAO) and Type Ia supernova (SN) data we demonstrate that the
generalized redshift mapping is strongly degenerated with dark energy.
Marginalization over a class of monotonic functions f(z_obs) changes
substantially degeneracy between matter and dark energy density: the density
parameters become anti correlated with nearly vertical axis of degeneracy.
Furthermore, we show that current SN and BAO data, analysed in a framework with
the generalized redshift mapping, do not constrain dark energy unless the BAO
data include the measurements from the Ly-alpha forest of high-redshift
quasars.Comment: 11 pages, 5 figures, 3 tables; accepted for publication in MNRA
Voids in cosmological simulations over cosmic time
We study evolution of voids in cosmological simulations using a new method
for tracing voids over cosmic time. The method is based on tracking watershed
basins (contiguous regions around density minima) of well developed voids at
low redshift, on a regular grid of density field. It enables us to construct a
robust and continuous mapping between voids at different redshifts, from
initial conditions to the present time. We discuss how the new approach
eliminates strong spurious effects of numerical origin when voids evolution is
traced by matching voids between successive snapshots (by analogy to halo
merger trees). We apply the new method to a cosmological simulation of a
standard LambdaCDM cosmological model and study evolution of basic properties
of typical voids (with effective radii between 6Mpc/h and 20Mpc/h at redshift
z=0) such as volumes, shapes, matter density distributions and relative
alignments. The final voids at low redshifts appear to retain a significant
part of the configuration acquired in initial conditions. Shapes of voids
evolve in a collective way which barely modifies the overall distribution of
the axial ratios. The evolution appears to have a weak impact on mutual
alignments of voids implying that the present state is in large part set up by
the primordial density field. We present evolution of dark matter density
profiles computed on iso-density surfaces which comply with the actual shapes
of voids. Unlike spherical density profiles, this approach enables us to
demonstrate development of theoretically predicted bucket-like shape of the
final density profiles indicating a wide flat core and a sharp transition to
high-density void walls.Comment: 13 pages, 13 figures; accepted for publication in MNRA
Cosmological Parameters From Supernovae Associated With Gamma-ray Bursts
We report estimates of the cosmological parameters and
obtained using supernovae (SNe) associated with gamma-ray
bursts (GRBs) at redshifts up to 0.606. Eight high-fidelity GRB-SNe with
well-sampled light curves across the peak are used. We correct their peak
magnitudes for a luminosity-decline rate relation to turn them into accurate
standard candles with dispersion mag. We also estimate the
peculiar velocity of the low-redshift host galaxy of SN 1998bw, using
constrained cosmological simulations. In a flat universe, the resulting Hubble
diagram leads to best-fit cosmological parameters of . This
exploratory study suggests that GRB-SNe can potentially be used as
standardizable candles to high redshifts to measure distances in the universe
and constrain cosmological parameters.Comment: 7 pages, 3 figures, Accepted for publication in ApJ
Low-redshift measurement of the sound horizon through gravitational time-delays
The matter sound horizon can be inferred from the cosmic microwave background
within the Standard Model. Independent direct measurements of the sound horizon
are then a probe of possible deviations from the Standard Model. We aim at
measuring the sound horizon from low-redshift indicators, which are
completely independent of CMB inference. We used the measured product
from baryon acoustic oscillations (BAO) together with supernovae~\textsc{I}a to
constrain and time-delay lenses analysed by the H0LiCOW
collaboration to anchor cosmological distances ().
{Additionally, we investigated the influence of adding a sample of quasars with
higher redshift with standardisable UV-Xray luminosity distances. We adopted
polynomial expansions in or in comoving distances} so that our inference
was completely independent of any cosmological model on which the expansion
history might be based. Our measurements are independent of Cepheids and
systematics from peculiar motions {to within percent-level accuracy.} The
inferred sound horizon varies between ~Mpc and ~Mpc across different models. The discrepancy with CMB measurements is
robust against model choice. Statistical uncertainties are comparable to
systematics. The combination of time-delay lenses, supernovae, and BAO yields a
distance ladder that is independent of cosmology (and of Cepheid calibration)
and a measurement of that is independent of the CMB. These cosmographic
measurements are then a competitive test of the Standard Model, regardless of
the hypotheses on which the cosmology is based.Comment: 2019A&A...632A..91A (subm. 28/05/2019), 6 pages, 3 figure
Językowa i stylistyczna wielogłosowość publicystyki okolicznościowej
Artykuł jest pomyślany jako próba charakterystyki konkretnej publikacji prasowej, mającej kształt gatunkowej kolekcji. Jest to bowiem dodatek do „Tygodnika Powszechnego” (nr 29 z 2018 roku) pt. Festiwal Stolica Języka Polskiego. Publikacja ta powiela formę magazynu prasowego, gdyż oprócz programu festiwalu zawiera: editorial, wywiady, recenzje, teksty komentujące i biogramy wybranych uczestników festiwalu. Mamy zatem do czynienia z typowym dla kolekcji współwystępowaniem konkretnych wypowiedzi w określonym miejscu i czasie. Całości przypisać można funkcję powiadamiania o wydarzeniu kulturalnym (festiwalu) oraz funkcję promowania imprezy. Autorka analizuje wspomnianą publikację w perspektywie dyskursywnej, a więc zwraca uwagę na określone, typowe dla współczesnej prasy praktyki komunikacyjne, tworzące skomplikowaną siatkę strategii, celów i form komunikacji, składających się na kształt współczesnej publicystyki. Analitycznym celem artykułu jest pokazanie komunikacyjnej polimorficzności konkretnego zbioru wypowiedzi publicystycznych, a więc interpretujących określoną rzeczywistość i służących przekonaniu czytelników do tej interpretacji. Poszczególne wypowiedzi są traktowane jako głosy, czyli komunikacyjnie (pragmatycznie) i formalnie ukształtowane typy działań językowych.The article has been conceived as an attempt to characterize a particular press publication in the shape of a genre collection. It is an extra to „Tygodnik Powszechny” (No. 29, 2018) under the title “Festiwal Stolica Języka Polskiego” (“Festival the Capital of the Polish Language)”. The publication reproduces a form of press magazine, since apart from the festival program, it includes: an editorial, interviews, reviews, commentaries and silhouettes of selected participants of the festival. Therefore, we deal with a co-occurrence of particular utterances at a specific place and time, typical for a collection. The function of notification about a cultural event (the festival) and the function of promoting the event can be ascribed to the whole issue. The author analyzes the abovementioned publication in the discursive perspective, and so she draws attention to specified communication practices, typical of contemporary press that create a complicated network of strategies, goals and forms of communication, contributing to the shape of contemporary journalism. The analytic purpose of the article is to present a communication polymorphism of a particular set of press utterances that interpret a specified reality and aim at convincing the readers to this interpretation. Particular utterances are treated as voices, i.e. communicatively (pragmatically) and formally shaped types of language activities
Effect of asphericity in caustic mass estimates of galaxy clusters
The caustic technique for measuring mass profiles of galaxy clusters relies
on the assumption of spherical symmetry. When applied to aspherical galaxy
clusters, the method yields mass estimates affected by the cluster orientation.
Here we employ mock redshift catalogues generated from cosmological simulations
to study the effect of clusters intrinsic shape and surrounding filamentary
structures on the caustic mass estimates. To this end, we develop a new method
for removing perturbations from large-scale structures, modelled as the
two-halo term, in a caustic analysis of stacked cluster data.
We find that the cluster masses inferred from kinematical data of ~10^14 Msun
clusters observed along the major axis are larger than masses from those
observed along the minor axis by a factor of 1.7 within the virial radius,
increasing to 1.8 within three virial radii. This discrepancy increases by 20%
for the most massive clusters. In addition a smaller but still significant mass
discrepancy arises when filamentary structures are present near a galaxy
cluster.
We find that the mean cluster mass from random sightlines is unbiased at all
radii and their scatter ranges from 0.14 to 0.17 within one and three virial
radii, with a 40% increase for the most massive clusters. We provide tables
which estimate the caustic mass bias given observational constraints on the
cluster orientation.Comment: 19 pages, 9 figures, 6 tables, accepted for publication in MNRA
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