25 research outputs found
The properties of extragalactic radio sources selected at 20 GHz
We present some first results on the variability, polarization and general
properties of radio sources selected in a blind survey at 20 GHz, the highest
frequency at which a sensitive radio survey has been carried out over a large
area of sky. Sources with flux densities above 100 mJy in the AT20G Pilot
Survey at declination -60 to -70 were observed at up to three epochs during
2002-4, including near-simultaneous measurements at 5, 8 and 18 GHz in 2003. Of
the 173 sources detected, 65% are candidate QSOs, BL Lac objects or blazars,
20% galaxies and 15% faint (b > 22 mag) optical objects or blank fields.
On a 1-2 year timescale, the general level of variability at 20 GHz appears
to be low. For the 108 sources with good-quality measurements in both 2003 and
2004, the median variability index at 20 GHz was 6.9% and only five sources
varied by more than 30% in flux density.
Most sources in our sample show low levels of linear polarization (typically
1-5%), with a median fractional polarization of 2.3% at 20 GHz. There is a
trend for fainter sources to show higher fractional polarization.
At least 40% of sources selected at 20GHz have strong spectral curvature over
the frequency range 1-20 GHz. We use a radio `two-colour diagram' to
characterize the radio spectra of our sample, and confirm that the radio-source
population at 20 GHz (which is also the foreground point-source population for
CMB anisotropy experiments like WMAP and Planck) cannot be reliably predicted
by extrapolating the results of surveys at lower frequencies. As a result,
direct selection at 20 GHz appears to be a more efficient way of identifying 90
GHz phase calibrators for ALMA than the currently-proposed technique of
extrapolation from all-sky surveys at 1-5 GHz.Comment: 14-page paper plus 5-page data table. Replaced with published versio
A nearby GRB host prototype for z~7 Lyman-break galaxies: Spitzer-IRS and X-shooter spectroscopy of the host galaxy of GRB031203
Gamma-ray burst (GRB) host galaxies have been studied extensively in optical
photometry and spectroscopy. Here we present the first mid-infrared spectrum of
a GRB host, HG031203. It is one of the nearest GRB hosts at z=0.1055, allowing
both low and high-resolution spectroscopy with Spitzer-IRS. Medium resolution
UV-to-K-band spectroscopy with the X-shooter spectrograph on the VLT is also
presented, along with Spitzer IRAC and MIPS photometry, as well as radio and
sub-mm observations. These data allow us to construct a UV-to-radio spectral
energy distribution with almost complete spectroscopic coverage from 0.3-35
micron of a GRB host galaxy for the first time, potentially valuable as a
template for future model comparisons. The IRS spectra show strong,
high-ionisation fine structure line emission indicative of a hard radiation
field in the galaxy, suggestive of strong ongoing star-formation and a very
young stellar population. The selection of HG031203 via the presence of a GRB
suggests that it might be a useful analogue of very young star-forming galaxies
in the early universe, and hints that local BCDs may be used as more reliable
analogues of star-formation in the early universe than typical local
starbursts. We look at the current debate on the ages of the dominant stellar
populations in z~7 and z~8 galaxies in this context. The nebular line emission
is so strong in HG031203, that at z~7, it can reproduce the spectral energy
distributions of z-band dropout galaxies with elevated IRAC 3.6 and 4.5 micron
fluxes without the need to invoke a 4000A break.Comment: Published in ApJ. 9 pages, 6 figures, emulateapj styl
The optically unbiased GRB host (TOUGH) survey. VI. Radio observations at z<1 and consistency with typical star-forming galaxies
The objective of this paper is to determine the level of obscured star
formation activity and dust attenuation in a sample of gamma-ray burst (GRB)
hosts; and to test the hypothesis that GRB hosts have properties consistent
with those of the general star-forming galaxy populations. We present a radio
continuum survey of all z<1 GRB hosts in The Optically Unbiased GRB Host
(TOUGH) sample supplemented with radio data for all (mostly pre-Swift) GRB-SN
hosts discovered before October 2006. We present new radio data for 22 objects
and have obtained a detection for three of them (GRB 980425, 021211, 031203;
none in the TOUGH sample), increasing the number of radio-detected GRB hosts
from two to five. The star formation rate (SFR) for the GRB 021211 host of ~825
Mo yr^-1, the highest ever reported for a GRB host, places it in the category
of ultraluminous infrared galaxies. We found that at least 63% of GRB hosts
have SFR 500 Mo yr^-1. For the
undetected hosts the mean radio flux (<35 uJy 3sigma) corresponds to an average
SFR < 15 Mo yr^-1. Moreover, ~88% of the z<1 GRB hosts have ultraviolet dust
attenuation A_UV < 6.7 mag (visual attenuation A_V < 3 mag). Hence we did not
find evidence for large dust obscuration in a majority of GRB hosts. Finally,
we found that the distributions of SFRs and A_UV of GRB hosts are consistent
with those of Lyman break galaxies, Halpha emitters at similar redshifts and of
galaxies from cosmological simulations. The similarity of the GRB population
with other star-forming galaxies is consistent with the hypothesis that GRBs, a
least at z<1, trace a large fraction of all star formation, and are therefore
less biased indicators than once thought.Comment: Accepted by ApJ. 13 pages, 4 figures, 2 tables. V3: minor changes to
match the published versio
Formeln und Modelle - Wissensrepraesentation mit Logik
Available from TIB Hannover: RO 8599(9) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
An introduction to non-monotonic reasoning
SIGLETIB: RN 7281 (136) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
KL-DNMS Ein System zur Verwaltung nicht-monotoner Abhaengigkeitsnetze
TIB: RN 7281 (141) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
KL-DNMS Ein System zur Verwaltung nicht-monotoner Abhaengigkeitsnetze
TIB: RN 7281 (141) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Wissensrepraesentation und Schlussfolgerungsverfahren fuer physikalisch-technische Systeme als einheitliche Grundlage technischer Expertensysteme Abschlussbericht
SIGLETIB Hannover: D.Dt.F. AC 1000(32,2) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
Recommender Systems for Configuration Knowledge Engineering ∗
The knowledge engineering bottleneck is still a major challenge in configurator projects. In this paper we show how recommender systems can support knowledge base development and maintenance processes. We discuss a couple of scenarios for the application of recommender systems in knowledge engineering and report the results of empirical studies which show the importance of user-centered configuration knowledge organization.