5,090 research outputs found
A study of purely astrometric selection of extragalactic point sources with Gaia
Selection of extragalactic point sources, e.g. QSOs, is often hampered by
significant selection effects causing existing samples to have rather complex
selection functions. We explore whether a purely astrometric selection of
extragalactic point sources, e.g. QSOs, is feasible with the ongoing Gaia
mission. Such a selection would be interesting as it would be unbiased in terms
of colours of the targets and hence would allow selection also with colours in
the stellar sequence. We have analyzed a total of 18 representative regions of
the sky by using GUMS, the simulator prepared for ESAs Gaia mission, both in
the range of mag and mag. For each region we
determine the density of apparently stationary stellar sources, i.e. sources
for which Gaia cannot measure a significant proper motion. The density is
contrasted with the density of extragalactic point sources, e.g. QSOs, in order
to establish in which celestial directions a pure astrometric selection is
feasible. When targeting regions at galactic latitude
the ratio of QSOs to apparently stationary stars is above 50\% and when
observing towards the poles the fraction of QSOs goes up to about \%.
We show that the proper motions from the proposed Gaia successor mission in
about 20 years would dramatically improve these results at all latitudes.
Detection of QSOs solely from zero proper motion, unbiased by any assumptions
on spectra, might lead to the discovery of new types of QSOs or new classes of
extragalactic point sources.Comment: 4 pages, 4 figures, sent in and accepted for publishing to A&
The high A_V Quasar Survey: A z=2.027 metal-rich damped Lyman-alpha absorber towards a red quasar at z=3.21
To fully exploit the potential of quasars as probes of cosmic chemical
evolution and the internal gas dynamics of galaxies it is important to
understand the selection effects behind the quasar samples and in particular if
the selection criteria exclude foreground galaxies with certain properties
(most importantly a high dust content). Here we present spectroscopic follow-up
from the 10.4-m GTC telescope of a dust-reddened quasar, eHAQ0111+0641, from
the extended High A_V Quasar (HAQ) survey. We find that the z=3.21 quasar has a
foreground Damped Lyman-alpha Absorber (DLA) at z=2.027 along the line of
sight. The DLA has very strong metal lines due to a moderately high metallicity
(with an inferred lower limit of 25% of the solar metallicity), but a very
large gas column density along the line-of-sight in its host galaxy. This
discovery is further evidence that there is a dust bias affecting the census of
metals, caused by the combined effect of dust obscuration and reddening, in
existing samples of z>2 DLAs. The case of eHAQ0111+0641 illustrates that dust
bias is not only caused by dust obscuration, but also dust reddening.Comment: 7 pages, 4 figures. Accepted for publication in A&A. A few typos have
been correcte
Serendipitous discovery of a projected pair of QSOs separated by 4.5 arcsec on the sky
We present the serendipitous discovery of a projected pair of quasi-stellar
objects (QSOs) with an angular separation of arcsec. The
redshifts of the two QSOs are widely different: one, our programme target, is a
QSO with a spectrum consistent with being a narrow line Seyfert 1 AGN at
. For this target we detect Lyman-, \ion{C}{4}, and
\ion{C}{3]}. The other QSO, which by chance was included on the spectroscopic
slit, is a Type 1 QSO at a redshift of , for which we detect
\ion{C}{4}, \ion{C}{3]} and \ion{Mg}{2}. We compare this system to previously
detected projected QSO pairs and find that only about a dozen previously known
pairs have smaller angular separation.Comment: 4 pages, 3 figures. Accepted for publication in A
Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths
The sub-population of quasars reddened by intrinsic or intervening clouds of
dust are known to be underrepresented in optical quasar surveys. By defining a
complete parent sample of the brightest and spatially unresolved quasars in the
COSMOS field, we quantify to which extent this sub-population is fundamental to
our understanding of the true population of quasars. By using the available
multiwavelength data of various surveys in the COSMOS field, we built a parent
sample of 33 quasars brighter than mag, identified by reliable X-ray to
radio wavelength selection techniques. Spectroscopic follow-up with the
NOT/ALFOSC was carried out for four candidate quasars that had not been
targeted previously to obtain a 100\% redshift completeness of the sample. The
population of high quasars (HAQs), a specific sub-population of quasars
selected from optical/near-infrared photometry, is found to contribute
of the parent sample. The full population of bright spatially
unresolved quasars represented by our parent sample consists of
reddened quasars defined by having , and
of the sample having assuming the extinction
curve of the Small Magellanic Cloud. We show that the HAQ selection works well
for selecting reddened quasars, but some are missed because their optical
spectra are too blue to pass the color cut in the HAQ selection. This is
either due to a low degree of dust reddening or anomalous spectra. We find that
the fraction of quasars with contributing light from the host galaxy is most
dominant at . At higher redshifts the population of spatially
unresolved quasars selected by our parent sample is found to be representative
of the full population at mag. This work quantifies the bias against
reddened quasars in studies that are based solely on optical surveys.Comment: 22 pages, 10 figures, accepted for publication in A&A. The ArXiv
abstract has been shortened for it to be printabl
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Electroweak measurements from the Tevatron
The two detectors at the Fermilab Tevatron, D0 and CDF, have collected large samples of W and Z decays. With these data measurements of the properties of the W boson have been performed. From the ratio of the W and Z production cross sections a measurement of the W boson width of 2.062 {+-} 0.059 GeV has been obtained. The W boson mass has been measured to be 80.34 {+-} 0.15 GeV. Both detectors have observed diboson production and measured the triple gauge boson couplings. No deviations from Standard Model predictions have been observed
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