216 research outputs found
Improved correction of VIPERS angular selection effects in clustering measurements
AbstractClustering estimates in galaxy redshift surveys need to account and correct for the way targets are selected from the general population, as to avoid biasing the measured values of cosmological parameters. The VIMOS Public Extragalactic Redshift Survey (VIPERS) is no exception to this, involving slit collisions and masking effects. Pushed by the increasing precision of the measurements, e.g. of the growth rate f, we have been re-assessing these effects in detail. We present here an improved correction for the two-point correlation function, capable to recover the amplitude of the monopole of the two-point correlation function ξ(r) above 1 h-1 Mpc to better than 2
Augmenting photometric redshift estimates using spectroscopic nearest neighbours
As a consequence of galaxy clustering, close galaxies observed on the plane
of the sky should be spatially correlated with a probability that is inversely
proportional to their angular separation. In principle, this information can be
used to improve photometric redshift estimates when spectroscopic redshifts are
available for some of the neighbouring objects. Depending on the depth of the
survey, however, this angular correlation is reduced by chance projections. In
this work, we implement a deep-learning model to distinguish between apparent
and real angular neighbours by solving a classification task. We adopted a
graph neural network architecture to tie together photometry, spectroscopy, and
the spatial information between neighbouring galaxies. We trained and validated
the algorithm on the data of the VIPERS galaxy survey, for which photometric
redshifts based on spectral energy distribution are also available. The model
yields a confidence level for a pair of galaxies to be real angular neighbours,
enabling us to disentangle chance superpositions in a probabilistic way. When
objects for which no physical companion can be identified are excluded, all
photometric redshift quality metrics improve significantly, confirming that
their estimates were of lower quality. For our typical test configuration, the
algorithm identifies a subset containing ~75% high-quality photometric
redshifts, for which the dispersion is reduced by as much as 50% (from 0.08 to
0.04), while the fraction of outliers reduces from 3% to 0.8%. Moreover, we
show that the spectroscopic redshift of the angular neighbour with the highest
detection probability provides an excellent estimate of the redshift of the
target galaxy, comparable to or even better than the corresponding
template-fitting estimate.Comment: 9 pages, 12 figures, matching the accepted version. NezNet is
available at https://github.com/tos-1/NezNe
Attractancy of Racemic Disparlure and Certain Analogues to Male Gypsy Moths and the Effect of Trap Placement
Traps hung on small trees of 3-8 cm diam and baited with racemic epoxides, hydrocarbons and other analogues related to racemic cis-7,8-epoxy-2-methyloctadecane (disparlure) resulted in male Lymantria dispar L. (gypsy moth) catches statistically indistinguishable from those of unbaited traps. Only (±)-disparlure yielded trap catches statistically above the level of unbaited traps. However, trap placement on trees of ca. 0.5 m diam produced appreciable trap catches, even in unbaited trap
Average luminosity distance in inhomogeneous universes
The paper studies the correction to the distance modulus induced by
inhomogeneities and averaged over all directions from a given observer. The
inhomogeneities are modeled as mass-compensated voids in random or regular
lattices within Swiss-cheese universes. Void radii below 300 Mpc are
considered, which are supported by current redshift surveys and limited by the
recently observed imprint such voids leave on CMB. The averaging over all
directions, performed by numerical ray tracing, is non-perturbative and
includes the supernovas inside the voids. Voids aligning along a certain
direction produce a cumulative gravitational lensing correction that increases
with their number. Such corrections are destroyed by the averaging over all
directions, even in non-randomized simple cubic void lattices. At low
redshifts, the average correction is not zero but decays with the peculiar
velocities and redshift. Its upper bound is provided by the maximal average
correction which assumes no random cancelations between different voids. It is
described well by a linear perturbation formula and, for the voids considered,
is 20% of the correction corresponding to the maximal peculiar velocity. The
average correction calculated in random and simple cubic void lattices is
severely damped below the predicted maximal one after a single void diameter.
That is traced to cancellations between the corrections from the fronts and
backs of different voids. All that implies that voids cannot imitate the effect
of dark energy unless they have radii and peculiar velocities much larger than
the currently observed. The results obtained allow one to readily predict the
redshift above which the direction-averaged fluctuation in the Hubble diagram
falls below a required precision and suggest a method to extract the background
Hubble constant from low redshift data without the need to correct for peculiar
velocities.Comment: 34 pages, 21 figures, matches the version accepted in JCA
The VIMOS Public Extragalactic Redshift Survey (VIPERS). Never mind the gaps: comparing techniques to restore homogeneous sky coverage
[Abridged] Non-uniform sampling and gaps in sky coverage are common in galaxy
redshift surveys, but these effects can degrade galaxy counts-in-cells and
density estimates. We carry out a comparison of methods that aim to fill the
gaps to correct for the systematic effects. Our study is motivated by the
analysis of the VIMOS Extragalactic Redshift Survey (VIPERS), a flux-limited
survey (i<22.5) based on one-pass observations with VIMOS, with gaps covering
25% of the surveyed area and a mean sampling rate of 35%. Our findings are
applicable to other surveys with similar observing strategies. We compare 1)
two algorithms based on photometric redshift, that assign redshifts to galaxies
based on the spectroscopic redshifts of the nearest neighbours, 2) two Bayesian
methods, the Wiener filter and the Poisson-Lognormal filter. Using galaxy mock
catalogues we quantify the accuracy of the counts-in-cells measurements on
scales of R=5 and 8 Mpc/h after applying each of these methods. We also study
how they perform to account for spectroscopic redshift error and inhomogeneous
and sparse sampling rate. We find that in VIPERS the errors in counts-in-cells
measurements on R<10 Mpc/h scales are dominated by the sparseness of the
sample. All methods underpredict by 20-35% the counts at high densities. This
systematic bias is of the same order as random errors. No method outperforms
the others. Random and systematic errors decrease for larger cells. We show
that it is possible to separate the lowest and highest densities on scales of 5
Mpc/h at redshifts 0.5<z<1.1, over a large volume such as in VIPERS survey.
This is vital for the characterisation of cosmic variance and rare populations
(e.g, brightest galaxies) in environmental studies at these redshifts.Comment: 17 pages, 13 figures, accepted for publication in A&A (revised
version after minor revision and language editing
Measuring the growth rate of structure around cosmic voids
Using an algorithm based on searching for empty spheres we identified 245 voids in the VIMOS Public Extragalactic Redshift Survey (VIPERS). We show how by modelling the anisotropic void-galaxy cross correlation function we can probe the growth rate of structure
Looking the void in the eyes - the kSZ effect in LTB models
As an alternative explanation of the dimming of distant supernovae it has
recently been advocated that we live in a special place in the Universe near
the centre of a large void described by a Lemaitre-Tolman-Bondi (LTB) metric.
The Universe is no longer homogeneous and isotropic and the apparent late time
acceleration is actually a consequence of spatial gradients in the metric. If
we did not live close to the centre of the void, we would have observed a
Cosmic Microwave Background (CMB) dipole much larger than that allowed by
observations. Hence, until now it has been argued, for the model to be
consistent with observations, that by coincidence we happen to live very close
to the centre of the void or we are moving towards it. However, even if we are
at the centre of the void, we can observe distant galaxy clusters, which are
off-centre. In their frame of reference there should be a large CMB dipole,
which manifests itself observationally for us as a kinematic Sunyaev-Zeldovich
(kSZ) effect. kSZ observations give far stronger constraints on the LTB model
compared to other observational probes such as Type Ia Supernovae, the CMB, and
baryon acoustic oscillations. We show that current observations of only 9
clusters with large error bars already rule out LTB models with void sizes
greater than approximately 1.5 Gpc and a significant underdensity, and that
near future kSZ surveys like the Atacama Cosmology Telescope, South Pole
Telescope, APEX telescope, or the Planck satellite will be able to strongly
rule out or confirm LTB models with giga parsec sized voids. On the other hand,
if the LTB model is confirmed by observations, a kSZ survey gives a unique
possibility of directly reconstructing the expansion rate and underdensity
profile of the void.Comment: 20 pages, 9 figures, submitted to JCA
The VIMOS Public Extragalactic Redshift Survey (VIPERS). Exploring the dependence of the three-point correlation function on stellar mass and luminosity at 0.5<z<1.1
The three-point correlation function (3PCF) is a powerful probe to
investigate the clustering of matter in the Universe in a complementary way
with respect to lower-order statistics, providing additional information with
respect to the two-point correlation function and allowing us to shed light on
biasing, nonlinear processes, and deviations from Gaussian statistics. In this
paper, we analyse the first data release of the VIMOS Public Extragalactic
Redshift Survey (VIPERS), determining the dependence of the three-point
correlation function on luminosity and stellar mass at . We
exploit the VIPERS Public Data Release 1, consisting of more than 50,000
galaxies with B-band magnitudes in the range and stellar masses in the range
. We measure both the
connected 3PCF and the reduced 3PCF in redshift space, probing different
configurations and scales, in the range [Mpc/h]. We find a
significant dependence of the reduced 3PCF on scales and triangle shapes, with
stronger anisotropy at larger scales ( Mpc/h) and an almost flat trend
at smaller scales, Mpc/h. Massive and luminous galaxies present a
larger connected 3PCF, while the reduced 3PCF is remarkably insensitive to
magnitude and stellar masses in the range we explored. These trends, already
observed at low redshifts, are confirmed for the first time to be still valid
up to , providing support to the hierarchical scenario for which massive
and bright systems are expected to be more clustered. The possibility of using
the measured 3PCF to provide independent constraints on the linear galaxy bias
has also been explored, showing promising results in agreement with other
probes.Comment: 12 pages, 11 figures, 2 tables, accepted for publication in A&
The VIMOS Public Extragalactic Redshift Survey (VIPERS): PCA-based automatic cleaning and reconstruction of survey spectra
Identifying spurious reduction artefacts in galaxy spectra is a challenge for
large surveys. We present an algorithm for identifying and repairing residual
spurious features in sky-subtracted galaxy spectra with application to the
VIPERS survey. The algorithm uses principal component analysis (PCA) applied to
the galaxy spectra in the observed frame to identify sky line residuals
imprinted at characteristic wavelengths. We further model the galaxy spectra in
the rest-frame using PCA to estimate the most probable continuum in the
corrupted spectral regions, which are then repaired. We apply the method to
90,000 spectra from the VIPERS survey and compare the results with a subset
where careful editing was performed by hand. We find that the automatic
technique does an extremely good job in reproducing the time-consuming manual
cleaning and does it in a uniform and objective manner across a large data
sample. The mask data products produced in this work are released together with
the VIPERS second public data release (PDR-2).Comment: Find the VIPERS data release at http://vipers.inaf.i
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