2,522 research outputs found
Correlations in the Spatial Power Spectrum Inferred from Angular Clustering: Methods and Application to APM
We reconsider the inference of spatial power spectra from angular clustering
data and show how to include correlations in both the angular correlation
function and the spatial power spectrum. Inclusion of the full covariance
matrices loosens the constraints on large-scale structure inferred from the APM
survey by over a factor of two. We present a new inversion technique based on
singular value decomposition that allows one to propagate the covariance matrix
on the angular correlation function through to that of the spatial power
spectrum and to reconstruct smooth power spectra without underestimating the
errors. Within a parameter space of the CDM shape Gamma and the amplitude
sigma_8, we find that the angular correlations in the APM survey constrain
Gamma to be 0.19-0.37 at 68% confidence when fit to scales larger than k=0.2h
Mpc^-1. A downturn in power at k<0.04h Mpc^-1 is significant at only 1-sigma.
These results are optimistic as we include only Gaussian statistical errors and
neglect any boundary effects.Comment: 37 pages, LaTex, 9 figures. Submitted to Ap
The Galaxy Angular Correlation Functions and Power Spectrum from the Two Micron All Sky Survey
We calculate the angular correlation function of galaxies in the Two Micron
All Sky Survey. We minimize the possible contamination by stars, dust, seeing
and sky brightness by studying their cross correlation with galaxy density, and
limiting the galaxy sample accordingly. We measure the correlation function at
scales between 1-18 arcdegs using a half million galaxies. We find a best fit
power law to the correlation function has a slope of 0.76 and an amplitude of
0.11. However, there are statistically significant oscillations around this
power law. The largest oscillation occurs at about 0.8 degrees, corresponding
to 2.8 h^{-1} Mpc at the median redshift of our survey, as expected in halo
occupation distribution descriptions of galaxy clustering.
We invert the angular correlation function using Singular Value Decomposition
to measure the three-dimensional power spectrum and find that it too is in good
agreement with previous measurements. A dip seen in the power spectrum at small
wavenumber k is statistically consistent with CDM-type power spectra. A fit of
CDM-type power spectra to k < 0.2 h Mpc^{-1} give constraints of
\Gamma_{eff}=0.116 and \sigma_8=0.96. This suggest a K_s-band linear bias of
1.1+/-0.2. This \Gamma_{eff} is different from the WMAP CMB derived value. On
small scales the power-law shape of our power spectrum is shallower than that
derived for the SDSS. These facts together imply a biasing of these different
galaxies that might be nonlinear, that might be either waveband or luminosity
dependent, and that might have a nonlocal origin.Comment: 14 pages, 20 figures, to be published in ApJ January 20th, revision
included two new figures, version with high resolution figures can be found
here http::ww
The Redshift Distribution of FIRST Radio Sources at 1 mJy
We present spectra for a sample of radio sources from the FIRST survey, and
use them to define the form of the redshift distribution of radio sources at
mJy levels.We targeted 365 sources and obtained 46 redshifts (13 per cent of
the sample). We find that our sample is complete in redshift measurement to R
, corresponding to . Early-type galaxies represent the
largest subset (45 per cent) of the sample and have redshifts 0.15\la z \la
0.5 ; late-type galaxies make up 15 per cent of the sample and have redshifts
0.05\la z \la 0.2; starbursting galaxies are a small fraction ( per
cent), and are very nearby (z\la 0.05). Some 9 per cent of the population
have Seyfert1/quasar-type spectra, all at z\ga 0.8, and there are 4 per cent
are Seyfert2 type galaxies at intermediate redshifts (). Using our
measurements and data from the Phoenix survey, we obtain an estimate for
at mJy and compare this with model predictions. At
variance with previous conclusions, we find that the population of starbursting
objects makes up \la 5 per cent of the radio population at S mJy.Comment: 20 pages, sumbitted to MNRA
Catalog of quasars from the Kilo-Degree Survey Data Release 3
We present a catalog of quasars selected from broad-band photometric ugri
data of the Kilo-Degree Survey Data Release 3 (KiDS DR3). The QSOs are
identified by the random forest (RF) supervised machine learning model, trained
on SDSS DR14 spectroscopic data. We first cleaned the input KiDS data from
entries with excessively noisy, missing or otherwise problematic measurements.
Applying a feature importance analysis, we then tune the algorithm and identify
in the KiDS multiband catalog the 17 most useful features for the
classification, namely magnitudes, colors, magnitude ratios, and the stellarity
index. We used the t-SNE algorithm to map the multi-dimensional photometric
data onto 2D planes and compare the coverage of the training and inference
sets. We limited the inference set to r<22 to avoid extrapolation beyond the
feature space covered by training, as the SDSS spectroscopic sample is
considerably shallower than KiDS. This gives 3.4 million objects in the final
inference sample, from which the random forest identified 190,000 quasar
candidates. Accuracy of 97%, purity of 91%, and completeness of 87%, as derived
from a test set extracted from SDSS and not used in the training, are confirmed
by comparison with external spectroscopic and photometric QSO catalogs
overlapping with the KiDS footprint. The robustness of our results is
strengthened by number counts of the quasar candidates in the r band, as well
as by their mid-infrared colors available from WISE. An analysis of parallaxes
and proper motions of our QSO candidates found also in Gaia DR2 suggests that a
probability cut of p(QSO)>0.8 is optimal for purity, whereas p(QSO)>0.7 is
preferable for better completeness. Our study presents the first comprehensive
quasar selection from deep high-quality KiDS data and will serve as the basis
for versatile studies of the QSO population detected by this survey.Comment: Data available from the KiDS website at
http://kids.strw.leidenuniv.nl/DR3/quasarcatalog.php and the source code from
https://github.com/snakoneczny/kids-quasar
Large-scale structure and matter in the universe
This paper summarizes the physical mechanisms that encode the type and
quantity of cosmological matter in the properties of large-scale structure, and
reviews the application of such tests to current datasets. The key lengths of
the horizon size at matter-radiation equality and at last scattering determine
the total matter density and its ratio to the relativistic density; acoustic
oscillations can diagnose whether the matter is collisionless, and small-scale
structure or its absence can limit the mass of any dark-matter relic particle.
The most stringent constraints come from combining data on present-day galaxy
clustering with data on CMB anisotropies. Such an analysis breaks the
degeneracies inherent in either dataset alone, and proves that the universe is
very close to flat. The matter content is accurately consistent with pure Cold
Dark Matter, with about 25% of the critical density, and fluctuations that are
scalar-only, adiabatic and scale-invariant. It is demonstrated that these
conclusions cannot be evaded by adjusting either the equation of state of the
vacuum, or the total relativistic density.Comment: 17 Pages. Review paper from the January 2003 Royal Society Discussion
Meeting, "The search for dark matter and dark energy in the universe
Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio Source Populations
We present the first results from a study of the radio continuum properties
of galaxies in the 2dF Galaxy Redshift Survey, based on thirty 2dF fields
covering a total area of about 100 square degrees. About 1.5% of galaxies with
b(J) < 19.4 mag are detected as radio continuum sources in the NRAO VLA Sky
Survey (NVSS). Of these, roughly 40% are star-forming galaxies and 60% are
active galaxies (mostly low-power radio galaxies and a few Seyferts). The
combination of 2dFGRS and NVSS will eventually yield a homogeneous set of
around 4000 radio-galaxy spectra, which will be a powerful tool for studying
the distriibution and evolution of both AGN and starburst galaxies out to
redshift z=0.3.Comment: 14 pages, 7 figures, accepted for publication in PAS
Cumulant Correlators from the APM
This work presents a set of new statistics, the cumulant correlators, aimed
at high precision analysis of the galaxy distribution. They form a symmetric
matrix, , related to moment correlators the same way as cumulants are
related to the moments of the distribution. They encode more information than
the usual cumulants, 's, and their extraction from data is similar to the
calculation of the two-point correlation function. Perturbation theory (PT),
its generalization, the extended perturbation theory (EPT), and the
hierarchical assumption (HA) have simple predictions for these statistics. As
an example, the factorial moment correlators measured by Szapudi, Dalton,
Efstathiou & Szalay (1996) in the APM catalog are reanalyzed using this
technique. While the previous analysis assumed hierarchical structure
constants, this method can directly investigate the validity of HA, along with
PT, and EPT. The results in agreement with previous findings indicate that, at
the small scales used for this analysis, the APM data supports HA. When all
non-linear corrections are taken into account it is a good approximation at the
20 percent level. It appears that PT, and a natural generalization of EPT for
cumulant correlators does not provide such a good fit for the APM at small
scales. Once the validity the HA is approximately established, cumulant
correlators can separate the amplitudes of different tree-types in the
hierarchy up to fifth order. As an example, the weights for the fourth order
tree topologies are calculated including all non-linear corrections.Comment: 9 pages+.ps figure, final version accepted for publication in Apj
Letter
Correlation Function of Superclusters of Galaxies
We present a study of the two-point correlation function of superclusters of
galaxies. The largest catalogs are used. The results show negligible
correlation less than 0.1-0.2 for separations up to 500-600 h^{-1} Mpc. Small
correlations are obtained using various estimates and samples. Seemingly there
are no structures of superclusters of galaxies.Comment: 19 pages, 3 figures, 4 tables. To appear in 1998 ApJ, 506, No. 2 (Oct
20
Coccidiose hos kvæg: En oversigt over coccidiearter, patogenese, epidemiologi og forebyggelse specielt i økologiske besætninger
Coccidiose hos kalve forårsages af arter af de encellede parasitter, Eimeria, der findes vidt udbredt i alle verdensdele, og som giver anledning til diarré og utrivelighed hos især unge og modtagelige dyr. Hos kvæg findes 12 forskellige Eimeria arter, der adskiller sig fra hinanden med hensyn til morfologi, livscyklus og patogenitet. Infektion med Eimeria kan diagnosticeres ved forskellige la-boratoriemetoder, hvorved bl.a. antallet og arten af coccidier forsøges fastslået. Således skabes grundlag for bestemmelse af hvilke forebyggende tiltag, der eventuelt bør iværksættes. Ved under-søgelse af 258 kalve fra 25 danske økologiske malkekvægsbesætninger fandt vi Eimeria-arter i alle (100%) besætninger og i 88% af kalvene. De fleste kalve udskilte meget lavgradige mængder af coccidier i gødningen, medens nogle havde særdeles høje coccidieudskillelser.
Coccidiose er en multifaktoriel sygdom, og sværhedsgraderne af sygdommen varierer derfor fra subkliniske tilfælde til nogle med løs, pastøs fæces af en enkelt dags varighed eller profus diarré med dysenteri, tenesmus og evt. mors. De kliniske symptomer tillægges traditionelt tilstedeværelsen af coccidiearterne Eimeria zuernii og E. bovis. Kalvens modtagelighed for coccidiose afhænger bl.a. af råmælksforsyning og immunitet, ernæringsstatus, stress, og evt. forekomst af andre sygdomme (f.eks. samtidige helminth- eller virusinfektioner). Endelig har også miljøet betydning for udvikling af coccidiose, herunder opstaldningstæthed, klima og hygiejne. Total udryddelse af coccidier er ikke muligt pga. parasittens ubikvitære forekomst og resistens i miljøet, men kalven kan styrkes ved sikring af tilstrækkelige råmælksmængder og ved opretholdelse af høj hygiejne i kælvningsboksen, samt efterfølgende opstaldningsforhold. Aktiv immunisering (vaccination) har endnu ikke vist sig effektiv i større målestok. Der foreligger ikke yderligere retningslinier for forebyggelse i perspektiv af de økologiske regler. Kalve, der har klinisk coccidiose, bør isoleres fra de øvrige kalve og gives væsketerapi for at opretholde væskebalancen. Den bedste behandling er imidlertid forebyggelse - med fokus på belægningsgrad og høj hygiejne inklusive rigeligt med tør strøelse
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