1,102 research outputs found
Neural-network selection of high-redshift radio quasars, and the luminosity function at z~4
We obtain a sample of 87 radio-loud QSOs in the redshift range 3.6<z<4.4 by
cross-correlating sources in the FIRST radio survey S{1.4GHz} > 1 mJy with
star-like objects having r <20.2 in SDSS Data Release 7. Of these 87 QSOs, 80
are spectroscopically classified in previous work (mainly SDSS), and form the
training set for a search for additional such sources. We apply our selection
to 2,916 FIRST-DR7 pairs and find 15 likely candidates. Seven of these are
confirmed as high-redshift quasars, bringing the total to 87. The candidates
were selected using a neural-network, which yields 97% completeness (fraction
of actual high-z QSOs selected as such) and an efficiency (fraction of
candidates which are high-z QSOs) in the range of 47 to 60%. We use this sample
to estimate the binned optical luminosity function of radio-loud QSOs at , and also the LF of the total QSO population and its comoving density. Our
results suggest that the radio-loud fraction (RLF) at high z is similar to that
at low-z and that other authors may be underestimating the fraction at high-z.
Finally, we determine the slope of the optical luminosity function and obtain
results consistent with previous studies of radio-loud QSOs and of the whole
population of QSOs. The evolution of the luminosity function with redshift was
for many years interpreted as a flattening of the bright end slope, but has
recently been re-interpreted as strong evolution of the break luminosity for
high-z QSOs, and our results, for the radio-loud population, are consistent
with this.Comment: 20 pages. Accepted for publication in MNRAS on 3 March 201
Use of neural networks for the identification of new z>=3.6 QSOs from FIRST-SDSS DR5
We aim to obtain a complete sample of redshift > 3.6 radio QSOs from FIRST
sources having star-like counterparts in the SDSS DR5 photometric survey
(r<=20.2). We found that simple supervised neural networks, trained on sources
with SDSS spectra, and using optical photometry and radio data, are very
effective for identifying high-z QSOs without spectra. The technique yields a
completeness of 96 per cent and an efficiency of 62 per cent. Applying the
trained networks to 4415 sources without DR5 spectra we found 58 z>=3.6 QSO
candidates. We obtained spectra of 27 of them, and 17 are confirmed as high-z
QSOs. Spectra of 13 additional candidates from the literature and from SDSS DR6
revealed 7 more z>=3.6 QSOs, giving and overall efficiency of 60 per cent. None
of the non-candidates with spectra from NED or DR6 is a z>=3.6 QSO,
consistently with a high completeness. The initial sample of z>=3.6 QSOs is
increased from 52 to 76, i.e. by a factor 1.46. From the new identifications
and candidates we estimate an incompleteness of SDSS for the spectroscopic
classification of FIRST 3.6<=z<=4.6 QSOs of 15 percent for r<=20.2.Comment: 16 pages, 9 figures accepted for publication in MNRA
Tomography of high-redshift clusters with OSIRIS
High-redshift clusters of galaxies are amongst the largest cosmic structures.
Their properties and evolution are key ingredients to our understanding of
cosmology: to study the growth of structure from the inhomogeneities of the
cosmic microwave background; the processes of galaxy formation, evolution, and
differentiation; and to measure the cosmological parameters (through their
interaction with the geometry of the universe, the age estimates of their
component galaxies, or the measurement of the amount of matter locked in their
potential wells). However, not much is yet known about the properties of
clusters at redshifts of cosmological interest. We propose here a radically new
method to study large samples of cluster galaxies using microslits to perform
spectroscopy of huge numbers of objects in single fields in a narrow spectral
range-chosen to fit an emission line at the cluster redshift. Our objective is
to obtain spectroscopy in a very restricted wavelength range (~100 A in width)
of several thousands of objects for each single 8x8 square arcmin field.
Approximately 100 of them will be identified as cluster emission-line objects
and will yield basic measurements of the dynamics and the star formation in the
cluster (that figure applies to a cluster at z~0.50, and becomes ~40 and ~20
for clusters at z~0.75 and z~1.00 respectively). This is a pioneering approach
that, once proven, will be followed in combination with photometric redshift
techniques and applied to other astrophysical problems.Comment: 4 pages, 3 figures. Proceedings of "Science with the GTC", Granada
(Spain), February 2002, RMxAA in pres
Ground temperatures, landforms and processes in an Atlantic mountain. Cantabrian Mountains (Northern Spain)
This research was supported by the Formación de Profesorado Universitario FPU13/05837 (Ministerio de Educación Cultura y Deporte) program, by the OAPN 053/2010 (Organismo Autónomo Parques Nacionales, MAGRAMA) project, by the I + D + I CGL2015-68144-R (Ministerio de Economia y Competitividad) project, by the Leverhulme Trust International Network Grant IN-2012-140 and the Royal Geographical Society Dudley Stamp Memorial Award.Peer reviewedPostprin
Dissolution experiments of commercial PWR (52 MWd/kgU) and BWR (53 MWd/kgU) spent nuclear fuel cladded segments in bicarbonate water under oxidizing conditions. Experimental determination of matrix and instant release fraction
The denominated instant release fraction (IRF) is considered in performance assessment (PA) exercises to govern the dose that could arise from the repository. A conservative definition of IRF comprises the total inventory of radionuclides located in the gap, fractures, and the grain boundaries and, if present, in the high burn-up structure (HBS). The values calculated from this theoretical approach correspond to an upper limit that likely does not correspond to what it will be expected to be instantaneously released in the real system. Trying to ascertain this IRF from an experimental point of view, static leaching experiments have been carried out with two commercial UO2 spent nuclear fuels (SNF): one from a pressurized water reactor (PWR), labelled PWR, with an average burn-up (BU) of 52 MWd/kgU and fission gas release (FGR) of 23.1%, and one from a boiling water reactor (BWR), labelled BWR, with an average BU of and 53 MWd/kgU and FGR of 3.9%.; One sample of each SNF, consisting of fuel and cladding, has been leached in bicarbonate water during one year under oxidizing conditions at room temperature (25 +/- 5) degrees C. The behaviour of the concentration measured in solution can be divided in two according to the release rate. All radionuclides presented an initial release rate that after some days levels down to a slower second one, which remains constant until the end of the experiment. Cumulative fraction of inventory in aqueous phase (FIAPc) values has been calculated. Results show faster release in the case of the PWR SNF. In both cases Np, Pu, Am, Cm, Y, Tc, La and Nd dissolve congruently with U, while dissolution of Zr, Ru and Rh is slower. Rb, Sr, Cs and Mo, dissolve faster than U. The IRF of Cs at 10 and 200 days has been calculated, being (3.10 +/- 0.62) and (3.66 +/- 0.73) for PWR fuel, and (035 +/- 0.07) and (0.51 +/- 0.10) for BWR fuel. (C) 2015 Elsevier B.V. All rights reserved.Peer ReviewedPostprint (author's final draft
On the origin of the X-ray emission from a narrow-line radioquasar at z>1
We present new XMM-Newton X-ray observations of the z=1.246 narrow-line
radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray
spectrum previously measured by ROSAT and ASCA is shown to be the result of a
steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing
column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident
with the nucleus of the radio source that we have resolved in new sensitive VLA
observations at 3.6 and 6 cm, implying that scattering in the radio lobes is
not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint
patch of X-ray emission is apparent 14'' to the NE of the main X-ray source.
The former is positionally coincident with an apparently extended optical
object with R~21.9, but there is no associated radio emission, thus ruling out
the possibility that this represents a hotspot in a jet emanating from the
primary X-ray source. No reflection features are detected in the X-ray spectrum
of the narrow-line radioquasar, although an Fe line with equivalent width of up
to 600 eV cannot be ruled out.Comment: 7 pages, 6 figures, MNRAS in the pres
Red quasars not so dusty
Webster et al (1995) claimed that up to 80% of QSOs may be obscured by dust.
They inferred the presence of this dust from the remarkably broad range of B-K
optical-infrared colours of a sample of flat-spectrum PKS radio QSOs. If such
dust is typical of QSOs, it will have rendered invisible most of those which
would otherwise been have detected by optical surveys. We used the William
Herschel Telescope on La Palma to obtain K infrared images of 54 B3 radio
quasars selected at low frequency (mainly steep-spectrum), and we find that
although several have very red optical-infrared colours, most of these can be
attributed to an excess of light in K rather than a dust-induced deficit in B.
We present evidence that some of the infrared excess comes from the light of
stars in the host galaxy (some, as previously suggested, comes from synchrotron
radiation associated with flat-spectrum radio sources). The B-K colours of the
B3 QSOs provide no evidence for a large reddened population. Either the Webster
et al QSOs are atypical in having such large extinctions, or their reddening is
not due to dust; either way, the broad range of their B-K colours does not
provide evidence that a large fraction of QSOs has been missed from optical
surveys.Comment: 16 pages TeX file + 2 PostScript figures. Accepted in MNRA
K-band imaging of 52 B3-VLA quasars: Nucleus and host properties
We present K-band imaging and photometry of a sample of 52 radio loud quasars
(RQs) selected from the B3 survey with flux densities above 0.5 Jy at 408 MHz.
The optical completeness of the sample is 90% and the quasars cover the
redshift range 0.4 - 2.3. For ~57% of the sources for which the quality of the
images allowed a detailed morphological study (16/28) resolved extended
emission was detected around the QSO, and its K flux was measured. Interpreting
this ``fuzz'' as starlight emission from the host galaxy, its location on the
K-z plane at z<1 is consistent with radio quasars being hosted by galaxies
similar to radio galaxies (RGs) or giant ellipticals (gEs). At higher redshifts
the detected host galaxies of RQs are more luminous than typical RGs and gEs,
although some weak detections or upper limits are consistent with a similar
fraction of RQs being hosted by galaxies with the expected luminosities for RGs
or gEs. We found a significant correlation between radio power and nuclear
infrared luminosity indicating a direct link between the radio synchrotron
emission and the nuclear emission in K. This correlation is more tight for the
steep-spectrum sources (99.97% significance). In addition, a trend is found
between radio power and infrared luminosity of the host galaxy (or mass), in
the sense that the most powerful quasars inhabit the most luminous galaxies.Comment: tar gzipped file including 1 LaTeX file, 4 latex tables, and 13
PostScript figures. Accepted in AJ (April 1998
The Luminosity Function Evolution of Soft X--ray selected AGN in the RIXOS survey
A sample of 198 soft X--ray selected active galactic nuclei (AGN) from the
ROSAT International X--ray Optical Survey (RIXOS), is used to investigate the
X--ray luminosity function and its evolution. RIXOS, with a flux limit of 3E-14
erg s-1 cm-2 (0.5 to 2.0 keV), samples a broad range in redshift over 20 deg^2
of sky, and is almost completely identified; it is used in combination with the
Einstein Extended Medium Sensitivity Survey (EMSS), to give a total sample of
over 600 AGN. We find the evolution of AGN with redshift to be consistent with
pure luminosity evolution (PLE) models in which the rate of evolution slows
markedly or stops at high redshifts z>1.8. We find that this result is not
affected by the inclusion, or exclusion, of narrow emission line galaxies at
low redshift in the RIXOS and EMSS samples, and is insensitive to uncertainties
in the conversion between flux values measured with ROSAT and Einstein. We
confirm, using a model independent Ve/Va test, that our survey is consistent
with no evolution at high redshifts.Comment: 10 pages, LaTeX file, PS figures and mn.sty. Accepted in MNRA
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