448 research outputs found
The non-Gaussianity of the cosmic shear likelihood - or: How odd is the Chandra Deep Field South?
(abridged) We study the validity of the approximation of a Gaussian cosmic
shear likelihood. We estimate the true likelihood for a fiducial cosmological
model from a large set of ray-tracing simulations and investigate the impact of
non-Gaussianity on cosmological parameter estimation. We investigate how odd
the recently reported very low value of really is as derived from
the \textit{Chandra} Deep Field South (CDFS) using cosmic shear by taking the
non-Gaussianity of the likelihood into account as well as the possibility of
biases coming from the way the CDFS was selected.
We find that the cosmic shear likelihood is significantly non-Gaussian. This
leads to both a shift of the maximum of the posterior distribution and a
significantly smaller credible region compared to the Gaussian case. We
re-analyse the CDFS cosmic shear data using the non-Gaussian likelihood.
Assuming that the CDFS is a random pointing, we find
for fixed . In a
WMAP5-like cosmology, a value equal to or lower than this would be expected in
of the times. Taking biases into account arising from the way the
CDFS was selected, which we model as being dependent on the number of haloes in
the CDFS, we obtain . Combining the CDFS data
with the parameter constraints from WMAP5 yields and for a flat
universe.Comment: 18 pages, 16 figures, accepted for publication in A&A; New Bayesian
treatment of field selection bia
Calibration of colour gradient bias in shear measurement using HST/CANDELS data
Accurate shape measurements are essential to infer cosmological parameters from large area weak gravitational lensing studies. The compact diffraction-limited point-spread function (PSF) in space-based observations is greatly beneficial, but its chromaticity for a broad band observation can lead to new subtle effects that could hitherto be ignored: the PSF of a galaxy is no longer uniquely defined and spatial variations in the colours of galaxies result in biases in the inferred lensing signal. Taking Euclid as a reference, we show that this colourgradient bias (CG bias) can be quantified with high accuracy using available multi-colour Hubble Space Telescope (HST) data. In particular we study how noise in the HST observations might impact such measurements and find this to be negligible. We determine the CG bias using HST observations in the F606W and F814W filters and observe a correlation with the colour, in line with expectations, whereas the dependence with redshift is weak. The biases for individual galaxies are generally well below 1%, which may be reduced further using morphological information from the Euclid data. Our results demonstrate that CG bias should not be ignored, but it is possible to determine its amplitude with sufficient precision, so that it will not significantly bias the weak lensing measurements using Euclid data
Cosmic shear analysis of archival HST/ACS data: I. Comparison of early ACS pure parallel data to the HST/GEMS Survey
This is the first paper of a series describing our measurement of weak
lensing by large-scale structure using archival observations from the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).
In this work we present results from a pilot study testing the capabilities
of the ACS for cosmic shear measurements with early parallel observations and
presenting a re-analysis of HST/ACS data from the GEMS survey and the GOODS
observations of the Chandra Deep Field South (CDFS). We describe our new
correction scheme for the time-dependent ACS PSF based on observations of
stellar fields. This is currently the only technique which takes the full time
variation of the PSF between individual ACS exposures into account. We estimate
that our PSF correction scheme reduces the systematic contribution to the shear
correlation functions due to PSF distortions to < 2*10^{-6} for galaxy fields
containing at least 10 stars. We perform a number of diagnostic tests
indicating that the remaining level of systematics is consistent with zero for
the GEMS and GOODS data confirming the success of our PSF correction scheme.
For the parallel data we detect a low level of remaining systematics which we
interpret to be caused by a lack of sufficient dithering of the data.
Combining the shear estimate of the GEMS and GOODS observations using 96
galaxies arcmin^{-2} with the photometric redshift catalogue of the GOODS-MUSIC
sample, we determine a local single field estimate for the mass power spectrum
normalisation sigma_{8,CDFS}=0.52^{+0.11}_{-0.15} (stat) +/- 0.07 (sys) (68%
confidence assuming Gaussian cosmic variance) at fixed Omega_m=0.3 for a
LambdaCDM cosmology. We interpret this exceptionally low estimate to be due to
a local under-density of the foreground structures in the CDFS.Comment: Version accepted for publication in Astronomy & Astrophysics with 28
pages, 25 figures. A version with full resolution figures can be downloaded
from http://www.astro.uni-bonn.de/~schrabba/papers/cosmic_shear_acs1_v2.pd
GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South
We present first results of our search for high-redshift galaxies in deep CCD
mosaic images. As a pilot study for a larger survey, very deep images of the
Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select
large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break
method. The data of these Lyman-break galaxies are made public as an electronic
table. These objects are good candidates for galaxies at z~3 and z~4 which is
supported by their photometric redshifts. The distributions of apparent
magnitudes and the clustering properties of the two populations are analysed,
and they show good agreement to earlier studies. We see no evolution in the
comoving clustering scale length from z~3 to z~4. The techniques presented here
will be applied to a much larger sample of U-dropouts from the whole survey in
near future.Comment: 11 pages, 11 figures, replaced with version accepted by A&A. Minor
changes and tabular appendix with LBG catalogues. Version with full
resolution figures available at
http://www.astro.uni-bonn.de/~hendrik/2544.pd
The impact of galaxy colour gradients on cosmic shear measurement
Cosmic shear has been identified as the method with the most potential to constrain dark energy. To capitalize on this potential, it is necessary to measure galaxy shapes with great accuracy, which in turn requires a detailed model for the image blurring by the telescope and atmosphere, the point spread function (PSF). In general, the PSF varies with wavelength and therefore the PSF integrated over an observing filter depends on the spectrum of the object. For a typical galaxy the spectrum varies across the galaxy image, thus the PSF depends on the position within the image. We estimate the bias on the shear due to such colour gradients by modelling galaxies using two co-centred, co-elliptical Sérsic profiles, each with a different spectrum. We estimate the effect of ignoring colour gradients and find the shear bias from a single galaxy can be very large depending on the properties of the galaxy. We find that halving the filter width reduces the shear bias by a factor of about 5. We show that, to the first order, tomographic cosmic shear two point statistics depend on the mean shear bias over the galaxy population at a given redshift. For a single broad filter, and averaging over a small galaxy catalogue from Simard et al., we find a mean shear bias which is subdominant to the predicted statistical errors for future cosmic shear surveys. However, the true mean shear bias may exceed the statistical errors, depending on how accurately the catalogue represents the observed distribution of galaxies in the cosmic shear survey. We then investigate the bias on the shear for two-filter imaging and find that the bias is reduced by at least an order of magnitude. Lastly, we find that it is possible to calibrate galaxies for which colour gradients were ignored using two-filter imaging of a fair sample of noisy galaxies, if the galaxy model is known. For a signal-to-noise ratio of 25 the number of galaxies required in each tomographic redshift bin is of the order of 10
Measuring the Stellar Masses of z~7 Galaxies with Spitzer Ultrafaint Survey Program (SURFS UP)
We present Spitzer/IRAC observations of nine -band dropouts highly
magnified (2<mu<12) by the Bullet Cluster. We combine archival imaging with our
Exploratory program (SURFS UP), which results in a total integration time of
~30 hr per IRAC band. We detect (>3sigma) in both IRAC bands the brightest of
these high-redshift galaxies, with [3.6]=23.80+-0.28 mag, [4.5]=23.78+-0.25
mag, and (H-[3.6])=1.17+-0.32 mag. The remaining eight galaxies are undetected
to [3.6]~26.4 mag and [4.5]~26.0 mag with stellar masses of ~5x10^7 M_sol. The
detected galaxy has an estimated magnification of mu=12+-4, which implies this
galaxy has an ultraviolet luminosity of L_1500~0.3 L*_{z=7} --- the lowest
luminosity individual source detected in IRAC at z>7. By modeling the broadband
photometry, we estimate the galaxy has an intrinsic star-formation rate of
SFR~1.3 M_sol/yr and stellar mass of M~2x10^9 M_sol, which gives a specific
star-formation rate of sSFR~0.7 Gyr^-1. If this galaxy had sustained this
star-formation rate since z~20, it could have formed the observed stellar mass
(to within a factor of ~2), we also discuss alternate star-formation histories
and argue the exponentially-increasing model is unlikely. Finally, based on the
intrinsic star-formation rate, we estimate this galaxy has a likely [C II] flux
of = 10^{-17} erg/s/cm2.Comment: Accepted to ApJL. 6 pages, 3 figures, 2 table
Dependence of cosmic shear covariances on cosmology - Impact on parameter estimation
In cosmic shear likelihood analyses the covariance is most commonly assumed
to be constant in parameter space. Therefore, when calculating the covariance
matrix (analytically or from simulations), its underlying cosmology should not
influence the likelihood contours. We examine whether the aforementioned
assumption holds and quantify how strong cosmic shear covariances vary within a
reasonable parameter range. Furthermore, we examine the impact on likelihood
contours when assuming different cosmologies in the covariance. We find that
covariances vary significantly within the considered parameter range
(Omega_m=[0.2;0.4], sigma_8=[0.6;1.0]) and that this has a non-negligible
impact on the size of likelihood contours. This impact increases with
increasing survey size, increasing number density of source galaxies,
decreasing ellipticity noise, and when using non-Gaussian covariances. To
improve on the assumption of a constant covariance we present two methods. The
adaptive covariance is the most accurate method, but it is computationally
expensive. To reduce the computational costs we give a scaling relation for
covariances. As a second method we outline the concept of an iterative
likelihood analysis. Here, we additionally account for non-Gaussianity using a
ray-tracing covariance derived from the Millennium simulation.Comment: 11 pages, 8 figure
3D Cosmic Shear: Cosmology from CFHTLenS
This paper presents the first application of 3D cosmic shear to a wide-field
weak lensing survey. 3D cosmic shear is a technique that analyses weak lensing
in three dimensions using a spherical harmonic approach, and does not bin data
in the redshift direction. This is applied to CFHTLenS, a 154 square degree
imaging survey with a median redshift of 0.7 and an effective number density of
11 galaxies per square arcminute usable for weak lensing. To account for survey
masks we apply a 3D pseudo-Cl approach on weak lensing data, and to avoid
uncertainties in the highly non-linear regime, we separately analyse radial
wave numbers k<=1.5h/Mpc and k<=5.0h/Mpc, and angular wavenumbers l~400-5000.
We show how one can recover 2D and tomographic power spectra from the full 3D
cosmic shear power spectra and present a measurement of the 2D cosmic shear
power spectrum, and measurements of a set of 2-bin and 6-bin cosmic shear
tomographic power spectra; in doing so we find that using the 3D power in the
calculation of such 2D and tomographic power spectra from data naturally
accounts for a minimum scale in the matter power spectrum. We use 3D cosmic
shear to constrain cosmologies with parameters OmegaM, OmegaB, sigma8, h, ns,
w0, wa. For a non-evolving dark energy equation of state, and assuming a flat
cosmology, lensing combined with WMAP7 results in h=0.78+/-0.12,
OmegaM=0.252+/-0.079, sigma8=0.88+/-0.23 and w=-1.16+/-0.38 using only scales
k<=1.5h/Mpc. We also present results of lensing combined with first year Planck
results, where we find no tension with the results from this analysis, but we
also find no significant improvement over the Planck results alone. We find
evidence of a suppression of power compared to LCDM on small scales 1.5 < k <
5.0 h/Mpc in the lensing data, which is consistent with predictions of the
effect of baryonic feedback on the matter power spectrum.Comment: Full journal article here
http://mnras.oxfordjournals.org/content/442/2/1326.full.pdf+htm
Spatial matter density mapping of the STAGES Abell A901/2 supercluster field with 3D lensing
We present weak lensing data from the Hubble Space Telescope(HST)/Space Telescope A901/902 Galaxy Evolution Survey (STAGES) survey to study the three-dimensional spatial distribution of matter and galaxies in the Abell 901/902 supercluster complex. Our method improves over the existing 3D lensing mapping techniques by calibrating and removing redshift bias and accounting for the effects of the radial elongation of 3D structures. We also include the first detailed noise analysis of a 3D lensing map, showing that even with deep HST-quality data, only the most massive structures, for example M200≳ 1015M⊙h-1 at z∼ 0.8, can be resolved in 3D with any reasonable redshift accuracy (Δz≈ 0.15). We compare the lensing map to the stellar mass distribution and find luminous counterparts for all mass peaks detected with a peak significance >3σ. We see structures in and behind the z= 0.165 foreground supercluster, finding structure directly behind the A901b cluster at z∼ 0.6 and also behind the south-west (SW) group at z∼ 0.7. This 3D structure viewed in projection has no significant impact on recent mass estimates of A901b or the SW group components SWa and SWb. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Measuring cosmic shear with the ring statistics
Commonly used methods to decompose E- and B-modes in cosmic shear, namely the
aperture mass dispersion and the E/B-mode shear correlation function, suffer
from incomplete knowledge of the two-point correlation function (2PCF) on very
small and/or very large scales. The ring statistics, the most recently
developed cosmic shear measure, improves on this issue and is able to decompose
E- and B-modes using a 2PCF measured on a finite interval. First, we improve on
the ring statistics' filter function with respect to the signal-to-noise ratio.
Second, we examine the ability of the ring statistics to constrain cosmology
and compare the results to cosmological constraints obtained with the aperture
mass dispersion. Third, we use the ring statistics to measure a cosmic shear
signal from CFHTLS (Canada-France-Hawaii Telescope Legacy Survey) data. We
consider a scale-dependent filter function for the ring statistics which
improves its signal-to-noise ratio. In addition, we show that there exist
filter functions which decompose E- and B-modes using a finite range of 2PCFs
(EB-statistics) and have higher S/N ratio than the ring statistics. However, we
find that data points of the latter are significantly less correlated than data
points of the aperture mass dispersion and the EB-statistics. As a consequence
the ring statistics is an ideal tool to identify remaining systematics
accurately as a function of angular scale. We use the 2PCF of the latest CFHTLS
analysis and therefrom calculate the ring statistics and its error bars.Comment: 10 pages, 5 figures, submitted to Astronomy and Astrophysic
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