678 research outputs found
The QSO evolution derived from the HBQS and other complete QSO surveys
An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has
been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2
with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor
2.2 higher than what measured by the PG survey, corresponding to a surface
density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an
overdensity of a factor 2.5 is observed, corresponding to a density of
0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function
(LF) we used Monte Carlo simulations that take into account of the selection
criteria, photometric errors and QSO spectral slope distribution. The LF can be
represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two
power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the
previous Boyle's (1992) estimations of k=3.45. A flatter slope beta=-3.72 of
the bright part of the LF is also required. The observed overdensity of bright
QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the
luminosity function is flatter than observed at higher redshifts. In this
redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from
our best-fit PLE model. This feature requires a luminosity dependent luminosity
evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2
interval.Comment: Invited talk in "Wide Field Spectroscopy" (20-24 May 1996, Athens),
eds. M. Kontizas et al. 6 pages and 3 eps figures, LaTex file, uses epfs.sty
and crckapb.sty (included
Onbeperkt Houdbaar: Advies voor een Nieuw Natuurbeleid in Nederland.
In mei 2011 ontving Ilkka Hanski uit handen van de Zweedse koning Karel Gustaaf\ud
de prestigieuze Crafoord prize. Hij kreeg de prijs voor zijn bijdrage aan de wiskundige theorie die de effecten van oppervlakte en versnippering van natuur op de populaties van wilde planten en dieren beschrijft. Ongeveer gelijktijdig doemden in Nederland de contouren op van een drastische koerswijziging van de overheid, die een einde zou maken aan het Nederlandse natuurbeleid dat sinds 1990 op deze theorie was gebaseerd. Er werden ingrijpende bezuinigingen aangekondigd, van meer dan 70%. Nooit eerder was de kloof tussen regeringsbeleid en wetenschappelijk inzicht zo diep. Deze koerswijziging heeft de aanzet gegeven tot een politieke en maatschappelijke herbezinning op nut en noodzaak van het natuurbelei
HLA-F*01:01 presents peptides with N-terminal flexibility and a preferred length of 16 residues.
HLA-F belongs to the non-classical HLA-Ib molecules with a marginal polymorphic nature and tissue-restricted distribution. HLA-F is a ligand of the NK cell receptor KIR3DS1, whose activation initiates an antiviral downstream immune response and lead to delayed disease progression of HIV-1. During the time course of HIV infection, the expression of HLA-F is upregulated while its interaction with KIR3DS1 is diminished. Understanding HLA-F peptide selection and presentation is essential to a comprehensive understanding of this dynamic immune response and the molecules function. In this study, we were able to recover stable pHLA-F*01:01 complexes and analyze the characteristics of peptides naturally presented by HLA-F. These HLA-F-restricted peptides exhibit a non-canonical length without a defined N-terminal anchor. The peptide characteristics lead to a unique presentation profile and influence the stability of the protein. Furthermore, we demonstrate that almost all source proteins of HLA-F-restricted peptides are described to interact with HIV proteins. Understanding the balance switch between HLA-Ia and HLA-F expression and peptide selection will support to understand the role of HLA-F in viral pathogenesis
RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1500 Bright X-ray Sources
We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA
Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray
sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that
are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic
sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS
sources is low enough that they can be reliably identified with RBSC sources
having average rms positional uncertainties = 10 arcsec. We used the more
accurate radio positions to make reliable X-ray/radio/optical identifications
down to the POSS plate limits. We obtained optical spectra for many of the
bright identifications lacking published redshifts. The resulting X-ray/radio
sample is unique in its size (N ~ 1500 objects), composition (a mixture of
nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low
average redshift ( ~ 0.1).Comment: 35 LaTeX pages including 6 eps figures + 40 LaTeX page table2
(landscape) w/ AASTeX 5.0; accepted to ApJ
Divergent platforms
Models of electoral competition between two opportunistic, office-motivated parties typically predict that both parties become indistinguishable in equilibrium. I show that this strong connection between the office motivation of parties and their equilibrium choice of identical platforms depends on two—possibly false—assumptions: (1) Issue spaces are uni-dimensional and (2) Parties are unitary actors whose preferences can be represented by expected utilities. I provide an example of a two-party model in which parties offer substantially different equilibrium platforms even though no exogenous differences between parties are assumed. In this example, some voters’ preferences over the 2-dimensional issue space exhibit non-convexities and parties evaluate their actions with respect to a set of beliefs on the electorate
A Study of CO Emission in High Redshift QSOs Using the Owens Valley Millimeter Array
Searches for CO emission in high-redshift objects have traditionally suffered
from the accuracy of optically-derived redshifts due to lack of bandwidth in
correlators at radio observatories. This problem has motivated the creation of
the new COBRA continuum correlator, with 4 GHz available bandwidth, at the
Owens Valley Radio Observatory Millimeter Array. Presented here are the first
scientific results from COBRA. We report detections of redshifted CO(J=3-2)
emission in the QSOs SMM J04135+10277 and VCV J140955.5+562827, as well as a
probable detection in RX J0911.4+0551. At redshifts of z=2.846, z=2.585, and
z=2.796, we find integrated CO flux densities of 5.4 Jy km/s, 2.4 Jy km/s, and
2.9 Jy km/s for SMM J04135+10277, VCV J140955.5+562827, and RX J0911.4+0551,
respectively, over linewidths of Delta(V_{FWHM}) ~ 350 km/s. These
measurements, when corrected for gravitational lensing, correspond to molecular
gas masses of order M(H_2) ~ 10^{9.6-11.1} solar masses, and are consistent
with previous CO observations of high-redshift QSOs. We also report 3-sigma
upper limits on CO(3-2) emission in the QSO LBQS 0018-0220 of 1.3 Jy km/s. We
do not detect significant 3mm continuum emission from any of the QSOs, with the
exception of a tentative (3-sigma) detection in RX J0911.4+0551 of S_{3mm}=0.92
mJy/beam.Comment: 18 pages, 5 figures, 2 tables, accepted to ApJ. Changes made for
version 2: citations added, 2 objects added to Table 2 and Figure
Integral field spectroscopy of four lensed quasars: analysis of their neighborhood and evidence for microlensing
CONTEXT: Gravitationally lensed quasars constitute an independent tool to
derive H0 through time-delays; they offer as well the opportunity to study the
mass distribution and interstellar medium of their lensing galaxies and,
through microlensing they also allow one to study details of the emitting
source. AIMS: For such studies, one needs to have an excellent knowledge of the
close environment of the lensed images in order to model the lensing potential:
this means observational data over a large field-of-view and spectroscopy at
high spatial resolution. METHODS: We present VIMOS integral field observations
around four lensed quasars: HE 0230-2130, RX J0911.4+0551, H 1413+117 and B
1359+154. Using the low, medium and high resolution modes, we study the quasar
images and the quasar environments, as well as provide a detailed report of the
data reduction. RESULTS: Comparison between the quasar spectra of the different
images reveals differences for HE 0230-2130, RX J0911.4+0551 and H 1413+117:
flux ratios between the images of the same quasar are different when measured
in the emission lines and in the continuum. We have also measured the redshifts
of galaxies in the neighborhood of HE 0230-2130 and RX J0911.4+0551 which
possibly contribute to the total lensing potential. CONCLUSIONS: A careful
analysis reveals that microlensing is the most natural explanation for the
(de)magnification of the continuum emitting region of the background sources.
In HE 0230-2130, image D is likely to be affected by microlensing
magnification; in RX J0911.4+0551, images A1 and A3 are likely to be modified
by microlensing de-magnification and in H 1413+117, at least image D is
affected by microlensing.Comment: 13 pages, 18 figures. Accepted for publication in A&A: January 7,
200
Two New Gravitationally Lensed Double Quasars from the Sloan Digital Sky Survey
We report the discoveries of the two-image gravitationally lensed quasars,
SDSS J0746+4403 and SDSS J1406+6126, selected from the Sloan Digital Sky Survey
(SDSS). SDSS J0746+4403, which will be included in our lens sample for
statistics and cosmology, has a source redshift of z_s=2.00, an estimated lens
redshift of z_l~0.3, and an image separation of 1.08". SDSS J1406+6126 has a
source redshift of z_s=2.13, a spectroscopically measured lens redshift of
z_l=0.27, and an image separation of 1.98". We find that the two quasar images
of SDSS J1406+6126 have different intervening MgII absorption strengths, which
are suggestive of large variations of absorbers on kpc scales. The positions
and fluxes of both the lensed quasar systems are easily reproduced by simple
mass models with reasonable parameter values. These objects bring to 18 the
number of lensed quasars that have been discovered from the SDSS data.Comment: 25 pages, 6 figures, The Astronomical Journal accepte
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