252 research outputs found
The dependence of intrinsic alignment of galaxies on wavelength using KiDS and GAMA
The outer regions of galaxies are more susceptible to the tidal interactions
that lead to intrinsic alignments of galaxies. The resulting alignment signal
may therefore depend on the passband if the colours of galaxies vary spatially.
To quantify this, we measured the shapes of galaxies with spectroscopic
redshifts from the GAMA survey using deep gri imaging data from the KiloDegree
Survey. The performance of the moment-based shape measurement algorithm DEIMOS
was assessed using dedicated image simulations, which showed that the
ellipticities could be determined with an accuracy better than 1% in all bands.
Additional tests for potential systematic errors did not reveal any issues. We
measure a significant difference of the alignment signal between the g,r and
i-band observations. This difference exceeds the amplitude of the linear
alignment model on scales below 2 Mpc/h. Separating the sample into
central/satellite and red/blue galaxies, we find that that the difference is
dominated by red satellite galaxies.Comment: 16 pages, 13 figures, accepted, to appear in A&
Weak gravitational lensing with the Square Kilometre Array
We investigate the capabilities of various stages of the SKA to perform
world-leading weak gravitational lensing surveys. We outline a way forward to
develop the tools needed for pursuing weak lensing in the radio band. We
identify the key analysis challenges and the key pathfinder experiments that
will allow us to address them in the run up to the SKA. We identify and
summarize the unique and potentially very powerful aspects of radio weak
lensing surveys, facilitated by the SKA, that can solve major challenges in the
field of weak lensing. These include the use of polarization and rotational
velocity information to control intrinsic alignments, and the new area of weak
lensing using intensity mapping experiments. We show how the SKA lensing
surveys will both complement and enhance corresponding efforts in the optical
wavebands through cross-correlation techniques and by way of extending the
reach of weak lensing to high redshift.Comment: 19 pages, 6 figures. Cosmology Chapter, Advancing Astrophysics with
the SKA (AASKA14) Conference, Giardini Naxos (Italy), June 9th-13th 201
Testing KiDS cross-correlation redshifts with simulations
Measuring cosmic shear in wide-field imaging surveys requires accurate knowledge of the redshift distribution of all sources. The clustering-redshift technique exploits the angular cross-correlation of a target galaxy sample with unknown redshifts and a reference sample with known redshifts. It represents an attractive alternative to colour-based methods of redshift calibration. Here we test the performance of such clustering redshift measurements using mock catalogues that resemble the Kilo-Degree Survey (KiDS). These mocks are created from the MICE simulation and closely mimic the properties of the KiDS source sample and the overlapping spectroscopic reference samples. We quantify the performance of the clustering redshifts by comparing the cross-correlation results with the true redshift distributions in each of the five KiDS photometric redshift bins. Such a comparison to an informative model is necessary due to the incompleteness of the reference samples at high redshifts. Clustering mean redshifts are unbiased at |Δz|< 0.006 under these conditions. The redshift evolution of the galaxy bias of the reference and target samples represents one of the most important systematic errors when estimating clustering redshifts. It can be reliably mitigated at this level of precision using auto-correlation measurements and self-consistency relations, and will not become a dominant source of systematic error until the arrival of Stage-IV cosmic shear surveys. Using redshift distributions from a direct colour-based estimate instead of the true redshift distributions as a model for comparison with the clustering redshifts increases the biases in the mean to up to |Δz|∼0.04. This indicates that the interpretation of clustering redshifts in real-world applications will require more sophisticated (parameterised) models of the redshift distribution in the future. If such better models are available, the clustering-redshift technique promises to be a highly complementary alternative to other methods of redshift calibration
KiDS-i-800: Comparing weak gravitational lensing measurements in same-sky surveys
We present a weak gravitational lensing analysis of 815 square degree of
-band imaging from the Kilo-Degree Survey (KiDS--800). In contrast to the
deep -band observations, which take priority during excellent seeing
conditions and form the primary KiDS dataset (KiDS--450), the complementary
yet shallower KiDS--800 spans a wide range of observing conditions. The
overlapping KiDS--800 and KiDS--450 imaging therefore provides a unique
opportunity to assess the robustness of weak lensing measurements. In our
analysis, we introduce two new `null' tests. The `nulled' two-point shear
correlation function uses a matched catalogue to show that the calibrated
KiDS--800 and KiDS--450 shear measurements agree at the level of \%. We use five galaxy lens samples to determine a `nulled' galaxy-galaxy
lensing signal from the full KiDS--800 and KiDS--450 surveys and find
that the measurements agree to \% when the KiDS--800 source
redshift distribution is calibrated using either spectroscopic redshifts, or
the 30-band photometric redshifts from the COSMOS survey.Comment: 24 pages, 20 figures. Submitted to MNRAS. Comments welcom
Testing KiDS cross-correlation redshifts with simulations
Measuring cosmic shear in wide-field imaging surveys requires accurate
knowledge of the redshift distribution of all sources. The clustering-redshift
technique exploits the angular cross-correlation of a target galaxy sample with
unknown redshifts and a reference sample with known redshifts, and is an
attractive alternative to colour-based methods of redshift calibration. We test
the performance of such clustering redshift measurements using mock catalogues
that resemble the Kilo-Degree Survey (KiDS). These mocks are created from the
MICE simulation and closely mimic the properties of the KiDS source sample and
the overlapping spectroscopic reference samples. We quantify the performance of
the clustering redshifts by comparing the cross-correlation results with the
true redshift distributions in each of the five KiDS photometric redshift bins.
Such a comparison to an informative model is necessary due to the
incompleteness of the reference samples at high redshifts. Clustering mean
redshifts are unbiased at under these conditions. The
redshift evolution of the galaxy bias can be reliably mitigated at this level
of precision using auto-correlation measurements and self-consistency
relations, and will not become a dominant source of systematic error until the
arrival of Stage-IV cosmic shear surveys. Using redshift distributions from a
direct colour-based estimate instead of the true redshift distributions as a
model for comparison with the clustering redshifts increases the biases in the
mean to up to . This indicates that the interpretation of
clustering redshifts in real-world applications will require more sophisticated
(parameterised) models of the redshift distribution in the future. If such
better models are available, the clustering-redshift technique promises to be a
highly complementary alternative to other methods of redshift calibration.Comment: 21 pages, 18 figures, 10 tables, submitted to A&
Geometry versus growth. Internal consistency of the flat LambdaCDM model with KiDS-1000
We carry out a multi-probe self-consistency test of the flat ΛCDM model with the aim of exploring potential causes of the reported tensions between high- and low-redshift cosmological observations. We divide the model into two theory regimes determined by the smooth background (geometry) and the evolution of matter density fluctuations (growth), each governed by an independent set of Lambda Cold Dark Matter (ΛCDM) cosmological parameters. This extended model is constrained by a combination of weak gravitational lensing measurements from the Kilo-Degree Survey, galaxy clustering signatures extracted from Sloan Digital Sky Survey campaigns and the Six-Degree Field Galaxy Survey, and the angular baryon acoustic scale and the primordial scalar fluctuation power spectrum measured in Planck cosmic microwave background (CMB) data. For both the weak lensing data set individually and the combined probes, we find strong consistency between the geometry and growth parameters, as well as with the posterior of standard ΛCDM analysis. In the non-split analysis, for which one single set of parameters was used, tension in the amplitude of matter density fluctuations as measured by the parameter S 8 persists at around 3σ, with a 1.5 % constraint of S 8 = 0.776+0.016 for −0.008 the combined probes. We also observe a less significant preference (at least 2σ) for higher values of the Hubble constant, H0 = 70.5+0.7 km s−1 Mpc−1 , as well as for lower values of the total matter density parameter Ωm = 0.289+0.007 compared to the full Planck −1.5 −0.005 analysis. Including the subset of the CMB information in the probe combination enhances these differences rather than alleviate them, which we link to the discrepancy between low and high multipoles in Planck data. Our geometry versus growth analysis does not yet yield clear signs regarding whether the origin of the discrepancies lies in ΛCDM structure growth or expansion history but holds promise as an insightful test for forthcoming, more powerful data
Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS
We present a tomographic cosmological weak lensing analysis of the HST COSMOS
Survey. Applying our lensing-optimized data reduction, principal component
interpolation for the ACS PSF, and improved modelling of charge-transfer
inefficiency, we measure a lensing signal which is consistent with pure
gravitational modes and no significant shape systematics. We carefully estimate
the statistical uncertainty from simulated COSMOS-like fields obtained from
ray-tracing through the Millennium Simulation. We test our pipeline on
simulated space-based data, recalibrate non-linear power spectrum corrections
using the ray-tracing, employ photometric redshifts to reduce potential
contamination by intrinsic galaxy alignments, and marginalize over systematic
uncertainties. We find that the lensing signal scales with redshift as expected
from General Relativity for a concordance LCDM cosmology, including the full
cross-correlations between different redshift bins. For a flat LCDM cosmology,
we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect
agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023,
sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness
and using HST Key Project and BBN priors only, we find a negative deceleration
parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing
independent evidence for the accelerated expansion of the Universe. For a flat
wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark
energy constraints are still relatively weak solely due to the limited area of
COSMOS. However, they provide an important demonstration for the usefulness of
tomographic weak lensing measurements from space. (abridged)Comment: 26 pages, 25 figures, matches version accepted for publication by
Astronomy and Astrophysic
Period Change Rates of Large Magellanic Cloud Cepheids using MESA
Pulsating stars, such as Cepheids and RR Lyrae, offer us a window to measure
and study changes due to stellar evolution. In this work, we study the former
by calculating a set of evolutionary tracks of stars with an initial mass of 4
to 7 , varying the initial rotation rate and metallicity, using the
stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA).
Using Radial Stellar Pulsations (RSP), a recently added functionality of MESA,
we obtained theoretical instability strip (IS) edges and linear periods for the
radial fundamental mode. Period-age, period-age-temperature, period-luminosity,
and period-luminosity-temperature relationships were derived for three rotation
rates and metallicities, showing a dependence on crossing number, position in
the IS, rotation, and metallicity. We calculated period change rates (PCRs)
based on the linear periods from RSP. We compared our models with literature
results using the Geneva code, and found large differences, as expected due to
the different implementations of rotation between codes. In addition, we
compared our theoretical PCRs with those measured in our recent work for Large
Magellanic Cloud Cepheids. We found good overall agreement, even though our
models do not reach the short-period regime exhibited by the empirical data.
Implementations of physical processes not yet included in our models, such as
pulsation-driven mass loss, an improved treatment of convection that may lead
to a better description of the instability strip edges, as well as
consideration of a wider initial mass range, could all help improve the
agreement with the observed PCRs.Comment: 19 pages, 17 figures. Accepted by MNRA
KiDS-1000: Constraints on the intrinsic alignment of luminous red galaxies
We constrain the luminosity and redshift dependence of the intrinsic alignment (IA) of a nearly volume-limited sample of luminous red galaxies selected from the fourth public data release of the Kilo-Degree Survey (KiDS-1000). To measure the shapes of the galaxies, we used two complementary algorithms, finding consistent IA measurements for the overlapping galaxy sample. The global significance of IA detection across our two independent luminous red galaxy samples, with our favoured method of shape estimation, is ∼10.7σ. We find no significant dependence with redshift of the IA signal in the range 0.2 < z < 0.8, nor a dependence with luminosity below Lr ≲ 2.9 × 1010 h−2Lr, ⊙. Above this luminosity, however, we find that the IA signal increases as a power law, although our results are also compatible with linear growth within the current uncertainties. This behaviour motivates the use of a broken power law model when accounting for the luminosity dependence of IA contamination in cosmic shear studies
KiDS-1000: Cross-correlation with Planck cosmic microwave background lensing and intrinsic alignment removal with self-calibration
CONTEXT:
Galaxy shear and cosmic microwave background (CMB) lensing convergence cross-correlations contain additional information on cosmology with respect to auto-correlations. While remaining immune to certain systemic effects, these cross-correlations are nonetheless affected by the galaxy's intrinsic alignments (IA). These effects may, in fact, be responsible for the reported low lensing amplitude of the galaxy shear × CMB convergence cross-correlations, compared to the standard Planck ACDM (cosmological constant and cold dark matter) cosmology predictions.
AIMS:
In this work, we investigate how IA affects the Kilo-Degree Survey (KiDS) galaxy lensing shear and Planck CMB lensing convergence cross-correlation and we compare it to previous treatments, both with and without IA taken into consideration.
METHODS:
We compared the marginalization over IA parameters and the IA self-calibration (SC) method (with additional observables defined only from the source galaxies) to demonstrate that SC can efficiently break the degeneracy between the CMB lensing amplitude, Alens, and the IA amplitude, AIA. We further investigated how different systematics affect the resulting AIA and Alens and we validated our results with the MICE2 simulation.
RESULTS:
We find that by including the SC method to constrain IA, the information loss due to the degeneracy between CMB lensing and IA is strongly reduced. The best-fit values are Alens = 0.84-0.22+0.22 and AIA = 0.60-1.03+1.03, while different angular scale cuts can affect Alens by ~10%. We show that an appropriate treatment of the boost factor, cosmic magnification, and photometric redshift modeling is important for obtaining the correct IA and cosmological results
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