696 research outputs found
Catastrophic photometric redshift errors: weak lensing survey requirements
We study the sensitivity of weak lensing surveys to the effects of
catastrophic redshift errors - cases where the true redshift is misestimated by
a significant amount. To compute the biases in cosmological parameters, we
adopt an efficient linearized analysis where the redshift errors are directly
related to shifts in the weak lensing convergence power spectra. We estimate
the number Nspec of unbiased spectroscopic redshifts needed to determine the
catastrophic error rate well enough that biases in cosmological parameters are
below statistical errors of weak lensing tomography. While the straightforward
estimate of Nspec is ~10^6 we find that using only the photometric redshifts
with z<=2.5 leads to a drastic reduction in Nspec to ~30,000 while negligibly
increasing statistical errors in dark energy parameters. Therefore, the size of
spectroscopic survey needed to control catastrophic errors is similar to that
previously deemed necessary to constrain the core of the z_s-z_p distribution.
We also study the efficacy of the recent proposal to measure redshift errors by
cross-correlation between the photo-z and spectroscopic samples. We find that
this method requires ~10% a priori knowledge of the bias and stochasticity of
the outlier population, and is also easily confounded by lensing magnification
bias. The cross-correlation method is therefore unlikely to supplant the need
for a complete spectroscopic redshift survey of the source population.Comment: 14 pages, 3 figure
Multifaceted contributions : health workers and smallpox eradication in India
Smallpox eradication in South Asia was a result of the efforts of many grades of health-workers. Working from within the confines of international organisations and government structures, the role of the field officials, who were of various nationalities and also drawn from the cities and rural enclaves of the countries in these regions, was crucial to the development and deployment of policies. However, the role of these personnel is often downplayed in official histories and academic histories, which highlight instead the roles played by a handful of senior officials within the World Health Organization and the federal governments in the sub-continent. This article attempts to provide a more rounded assessment of the complex situation in the field. In this regard, an effort is made to underline the great usefulness of the operational flexibility displayed by field officers, wherein lessons learnt in the field were made an integral part of deploying local campaigns; careful engagement with the communities being targeted, as well as the employment of short term workers from amongst them, was an important feature of this work
Photometric Redshifts with Surface Brightness Priors
We use galaxy surface brightness as prior information to improve photometric
redshift (photo-z) estimation. We apply our template-based photo-z method to
imaging data from the ground-based VVDS survey and the space-based GOODS field
from HST, and use spectroscopic redshifts to test our photometric redshifts for
different galaxy types and redshifts. We find that the surface brightness prior
eliminates a large fraction of outliers by lifting the degeneracy between the
Lyman and 4000 Angstrom breaks. Bias and scatter are improved by about a factor
of 2 with the prior for both the ground and space data. Ongoing and planned
surveys from the ground and space will benefit, provided that care is taken in
measurements of galaxy sizes and in the application of the prior. We discuss
the image quality and signal-to-noise requirements that enable the surface
brightness prior to be successfully applied.Comment: 15 pages, 13 figures, matches published versio
Reconstructing galaxy fundamental distributions and scaling relations from photometric redshift surveys. Applications to the SDSS early-type sample
Noisy distance estimates associated with photometric rather than
spectroscopic redshifts lead to a mis-estimate of the luminosities, and produce
a correlated mis-estimate of the sizes. We consider a sample of early-type
galaxies from the SDSS DR6 for which both spectroscopic and photometric
information is available, and apply the generalization of the V_max method to
correct for these biases. We show that our technique recovers the true
redshift, magnitude and size distributions, as well as the true size-luminosity
relation. We find that using only 10% of the spectroscopic information randomly
spaced in our catalog is sufficient for the reconstructions to be accurate
within about 3%, when the photometric redshift error is dz = 0.038. We then
address the problem of extending our method to deep redshift catalogs, where
only photometric information is available. In addition to the specific
applications outlined here, our technique impacts a broader range of studies,
when at least one distance-dependent quantity is involved. It is particularly
relevant for the next generation of surveys, some of which will only have
photometric information.Comment: 14 pages, 12 figures, 1 table, new section 3.1 and appendix added,
MNRAS in pres
A Substantial Population of Low Mass Stars in Luminous Elliptical Galaxies
The stellar initial mass function (IMF) describes the mass distribution of
stars at the time of their formation and is of fundamental importance for many
areas of astrophysics. The IMF is reasonably well constrained in the disk of
the Milky Way but we have very little direct information on the form of the IMF
in other galaxies and at earlier cosmic epochs. Here we investigate the stellar
mass function in elliptical galaxies by measuring the strength of the Na I
doublet and the Wing-Ford molecular FeH band in their spectra. These lines are
strong in stars with masses <0.3 Msun and weak or absent in all other types of
stars. We unambiguously detect both signatures, consistent with previous
studies that were based on data of lower signal-to-noise ratio. The direct
detection of the light of low mass stars implies that they are very abundant in
elliptical galaxies, making up >80% of the total number of stars and
contributing >60% of the total stellar mass. We infer that the IMF in massive
star-forming galaxies in the early Universe produced many more low mass stars
than the IMF in the Milky Way disk, and was probably slightly steeper than the
Salpeter form in the mass range 0.1 - 1 Msun.Comment: To appear in Natur
Object class recognition using combination of colour dense SIFT and texture descriptors
Object class recognition has recently become one of the most popular research fields. This is due to its importance in many applications such as image classification, retrieval, indexing, and searching. The main aim of object class recognition is determining how to make computers understand and identify automatically which object or scene is being displayed on the image. Despite a lot of efforts that have been made, it still considered as one of the most challenging tasks, mainly due to inter-class variations and intra-class variations like occlusion, background clutter, viewpoint changes, pose, scale and illumination. Feature extraction is one of the important steps in any object class recognition system. Different image features are proposed in the literature review to increase categorisation accuracy such as appearance, texture, shape descriptors. In this paper, we propose to combine different descriptors which are dense colour scale-invariant feature transform (dense colour SIFT) as appearance descriptors with different texture descriptors. The colour completed local binary pattern (CCLBP) and completed local ternary pattern (CLTP) are integrated with dense colour SIFT due to the importance of the texture information in the image. Using different pattern sizes to extract the CLTP and CCLBP texture descriptors will help to find dense texture information from the image. Bag of features is also used in the proposed system with each descriptor while the late fusion strategy is used in the classification stage. The proposed system achieved high recognition accuracy rate when applied in some datasets, namely SUN-397, OT4N, OT8, and Event sport datasets, which accomplished 38.9%, 95.9%, 89.02%, and 88.167%, respectively
Estimating the Redshift Distribution of Faint Galaxy Samples
We present an empirical method for estimating the underlying redshift
distribution N(z) of galaxy photometric samples from photometric observables.
The method does not rely on photometric redshift (photo-z) estimates for
individual galaxies, which typically suffer from biases. Instead, it assigns
weights to galaxies in a spectroscopic subsample such that the weighted
distributions of photometric observables (e.g., multi-band magnitudes) match
the corresponding distributions for the photometric sample. The weights are
estimated using a nearest-neighbor technique that ensures stability in sparsely
populated regions of color-magnitude space. The derived weights are then summed
in redshift bins to create the redshift distribution. We apply this weighting
technique to data from the Sloan Digital Sky Survey as well as to mock catalogs
for the Dark Energy Survey, and compare the results to those from the
estimation of photo-z's derived by a neural network algorithm. We find that the
weighting method accurately recovers the underlying redshift distribution,
typically better than the photo-z reconstruction, provided the spectroscopic
subsample spans the range of photometric observables covered by the photometric
sample.Comment: 14 pages, 9 figures, submitted to MNRA
Measuring large-scale structure with quasars in narrow-band filter surveys
We show that a large-area imaging survey using narrow-band filters could
detect quasars in sufficiently high number densities, and with more than
sufficient accuracy in their photometric redshifts, to turn them into suitable
tracers of large-scale structure. If a narrow-band optical survey can detect
objects as faint as i=23, it could reach volumetric number densities as high as
10^{-4} h^3 Mpc^{-3} (comoving) at z~1.5 . Such a catalog would lead to
precision measurements of the power spectrum up to z~3-4. We also show that it
is possible to employ quasars to measure baryon acoustic oscillations at high
redshifts, where the uncertainties from redshift distortions and nonlinearities
are much smaller than at z<1. As a concrete example we study the future impact
of J-PAS, which is a narrow-band imaging survey in the optical over 1/5 of the
unobscured sky with 42 filters of ~100 A full-width at half-maximum. We show
that J-PAS will be able to take advantage of the broad emission lines of
quasars to deliver excellent photometric redshifts, \sigma_{z}~0.002(1+z), for
millions of objects.Comment: Matches version published in MNRAS (2012
Enrichment analysis of Alu elements with different spatial chromatin proximity in the human genome
Transposable elements (TEs) have no longer been totally considered as “junk DNA” for quite a time since the continual discoveries of their multifunctional roles in eukaryote genomes. As one of the most important and abundant TEs that still active in human genome, Alu, a SINE family, has demonstrated its indispensable regulatory functions at sequence level, but its spatial roles are still unclear. Technologies based on 3C(chromosomeconformation capture) have revealed the mysterious three-dimensional structure of chromatin, and make it possible to study the distal chromatin interaction in the genome. To find the role TE
playing in distal regulation in human genome, we compiled the new released Hi-C data, TE annotation, histone marker annotations, and the genome-wide methylation data to operate correlation analysis, and found that the density of Alu elements showed a strong positive correlation with the level of chromatin interactions (hESC: r=0.9, P<2.2×1016; IMR90 fibroblasts: r = 0.94, P < 2.2 × 1016) and also have a significant positive correlation withsomeremote functional DNA elements like enhancers and promoters (Enhancer: hESC: r=0.997, P=2.3×10−4; IMR90: r=0.934, P=2×10−2; Promoter: hESC: r = 0.995, P = 3.8 × 10−4; IMR90: r = 0.996, P = 3.2 × 10−4). Further investigation involving GC content and methylation status showed the GC content of Alu covered sequences shared a similar pattern with that of the overall sequence, suggesting that Alu elements also function as the GC nucleotide and CpG site provider. In all, our results suggest that the Alu elements may act as an alternative parameter to evaluate the Hi-C data, which is confirmed by the correlation analysis of Alu elements and histone markers. Moreover, the GC-rich Alu sequence can bring high GC content and methylation flexibility to the regions with more distal chromatin contact, regulating the transcription of tissue-specific genes
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