430 research outputs found
On the Probability Distributions of Ellipticity
In this paper we derive an exact full expression for the 2D probability
distribution of the ellipticity of an object measured from data, only assuming
Gaussian noise in pixel values. This is a generalisation of the probability
distribution for the ratio of single random variables, that is well-known, to
the multivariate case. This expression is derived within the context of the
measurement of weak gravitational lensing from noisy galaxy images. We find
that the third flattening, or epsilon-ellipticity, has a biased maximum
likelihood but an unbiased mean; and that the third eccentricity, or normalised
polarisation chi, has both a biased maximum likelihood and a biased mean. The
very fact that the bias in the ellipticity is itself a function of the
ellipticity requires an accurate knowledge of the intrinsic ellipticity
distribution of the galaxies in order to properly calibrate shear measurements.
We use this expression to explore strategies for calibration of biases caused
by measurement processes in weak gravitational lensing. We find that upcoming
weak lensing surveys like KiDS or DES require calibration fields of order of
several square degrees and 1.2 magnitude deeper than the wide survey in order
to correct for the noise bias. Future surveys like Euclid will require
calibration fields of order 40 square degree and several magnitude deeper than
the wide survey. We also investigate the use of the Stokes parameters to
estimate the shear as an alternative to the ellipticity. We find that they can
provide unbiased shear estimates at the cost of a very large variance in the
measurement. The python code used to compute the distributions presented in the
paper and to perform the numerical calculations are available on request.Comment: 24 pages, 18 figures, 2 Tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Main Journa
Implications for the missing low-mass galaxies (satellites) problem from cosmic shear
The number of observed dwarf galaxies, with dark matter mass M in the Milky Way or the Andromeda galaxy does not agree
with predictions from the successful CDM paradigm. To alleviate this
problem a suppression of dark matter clustering power on very small scales has
been conjectured. However, the abundance of dark matter halos outside our
immediate neighbourhood (the Local Group) seem to agree with the
CDM--expected abundance. Here we connect these problems to
observations of weak lensing cosmic shear, pointing out that cosmic shear can
make significant statements about the missing satellites problem in a
statistical way. As an example and pedagogical application we use recent
constraints on small-scales power suppression from measurements of the CFHTLenS
data. We find that, on average, in a region of Gpc there is no
significant small-scale power suppression. This implies that suppression of
small-scale power is not a viable solution to the `missing satellites problem'
or, alternatively, that on average in this volume there is no `missing
satellites problem' for dark matter masses M.
Further analysis of current and future weak lensing surveys will probe much
smaller scales, Mpc corresponding roughly to masses .Comment: Matches published version in MNRAS Letters; no change
Report on trial of SatScan tray scanner system by SmartDrive Ltd.
Smartdrive Ltd. has developed a prototype imaging system, SatScan, that captures digitised images of large areas while keeping smaller objects in focus at very high resolution. The system was set up in the Sackler Biodiversity Imaging laboratory of Natural History Museum on March 8, 2010 for a one-month trial. A series of projects imaging parts of the entomological, botanical and palaeoentomological collection were conducted to assess the systems utility for museum collection management and biodiversity research. The technical and practical limitations of the system were investigated as part of this process
Can we measure the neutrino mass hierarchy in the sky?
Cosmological probes are steadily reducing the total neutrino mass window,
resulting in constraints on the neutrino-mass degeneracy as the most
significant outcome. In this work we explore the discovery potential of
cosmological probes to constrain the neutrino hierarchy, and point out some
subtleties that could yield spurious claims of detection. This has an important
implication for next generation of double beta decay experiments, that will be
able to achieve a positive signal in the case of degenerate or inverted
hierarchy of Majorana neutrinos. We find that cosmological experiments that
nearly cover the whole sky could in principle distinguish the neutrino
hierarchy by yielding 'substantial' evidence for one scenario over the another,
via precise measurements of the shape of the matter power spectrum from large
scale structure and weak gravitational lensing.Comment: Submitted to JCA
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