502 research outputs found
The Asiago-ESO/RASS QSO Survey II. The Southern Sample
This is the second paper of a series describing the Asiago-ESO/RASS QSO
survey, a project aimed at the construction of an all-sky statistically
well-defined sample of very bright QSOs (B_J < 15). Such a survey is required
to remove the present uncertainties about the properties of the local QSO
population and constitutes an homogeneous database for detailed evolutionary
studies of AGN. We present here the complete Southern Sample, which comprises
243 bright (12.60 < B_J < 15.13) QSO candidates at high galactic latitudes
(|b_{gal}| > 30^{\circ}). The area covered by the survey is 5660 sq. deg.
Spectroscopy for the 137 still unidentified objects has been obtained. The
total number of AGN turns out to be 111, 63 of which are new identifications.
The properties of the selection are discussed. The completeness and the success
rate for this survey at the final stage are 63% and 46%, respectively.Comment: 36 pages Latex, with 15 PostScript figures. Accepted for publication
in Astronomical Journa
The Asiago-ESO/RASS QSO Survey. I.The Catalog and the Local QSO Luminosity Function
This paper presents the first results of a survey for bright quasars (V <
14.5 and R30.
The photometric database is derived from the GSC and USNO catalogs. Quasars are
identified on the basis of their X-ray emission measured in the ROSAT All Sky
Survey. The surface density of quasars brighter than 15.5 mag turns out to be
, about 3 times higher than that estimated by
the PG survey. The quasar optical Luminosity Function (LF) at is computed and shown to be consistent with a Luminosity Dependent
Luminosity Evolution of the type derived by La Franca and Cristiani (1997) in
the range . The predictions of semi-analytical models of
hierarchical structure formation agree remarkably well with the present
observations.Comment: 54 pages Latex, with 7 PostScript figures. Some minor changes.
Astronomical Journal, in pres
The power spectrum of the flux distribution in the Lyman-alpha forest of a Large sample of UVES QSO Absorption Spectra (LUQAS)
The flux power spectra of the Lyman-alpha forest from a sample of 27 QSOs
taken with the high resolution echelle spectrograph UVES on VLT are presented.
We find a similar fluctuation amplitude at the peak of the ``3D'' flux power
spectrum at k ~ 0.03 (km/sec)^(-1) as the study by Croft et al. (2002), in the
same redshift range. The amplitude of the flux power spectrum increases with
decreasing redshift if corrected for the increase in the mean flux level as
expected if the evolution of the flux power spectrum is sensitive to the
gravitational growth of matter density fluctuations. This is in agreement with
the findings of McDonald et al. (2000) at larger redshift. The logarithmic
slope of the "3D" flux power spectrum, P_F(k), at large scales k < 0.03
(km/sec)^(-1), is 1.4 +- 0.3, i.e. 0.3 shallower than that found by Croft et
al. (2002) but consistent within the errors.Comment: 18 pages, 9 PS figures, 6 tables. Note that the k-values of the 1D
flux power spectrum had been erroneously shifted by half a bin size (in log
k) in the previous version. All the other results are unaffected. New tables
can be found at http://www.ast.cam.ac.uk/~rtnigm/luqas.ht
The Optical-UV Continuum of a Sample of QSOs
The average optical-UV continuum shape of QSOs has been investigated using
spectra of 62 QSOs having good relative photometric calibrations. The QSO
spectra were extracted from two complete color selected samples in the
magnitude intervals B ~ 16-20. The analysis was performed fitting power-law
continua (f proportional to nu^(alpha)) in well defined rest-frame wavelength
intervals after removing regions of the spectrum affected by strong emission
lines or weak emission bumps. The average slope in the rest-frame optical-UV
region 1200 - 5500 A shows a rapid change around the 3000 A emission bump with
alpha=0.15 longward of it and alpha=-0.65 at shorter wavelengths. Although
these average slopes have been obtained using spectra of QSOs with different
luminosities and redshifts, there are no significant correlations of the
average spectral index with these quantities. For a few QSOs in the sample we
were able to measure the same softening of the spectral shape within the
individual spectrum. These results have significant consequences on the
estimate of the cosmological evolution of the optically selected QSOs as they
affect, for instance, the k-corrections. New k-corrections in the B, V, R and
Gr bands were computed. The derived average spectral shape in the optical-UV
band puts interesting constraints on the expected emission mechanisms.Comment: 12 pages. To appear in the Astronomical Journa
The evolution of the physical state of the IGM
Using a new, increased dataset of 7 QSOs from VLT/UVES observations combined with one QSO from the literature, the minimum Doppler parameters as a function of neutral hydrogen column density N(HI), b_c(N), of the Lyman-alpha forest has been derived at three redshifts z = 2.1, 3.3 and 3.8. When incompleteness of the number of the observed lines towards lower N(HI) is accounted for, the derived slopes of b_c(N), (Gamma-1), are consistent with no-redshift evolution with an indication of lower value at z = 3.3, while b_c(N) at a fixed column density N(HI) = 10^(13.6) cm^(-2), b_c(13.6), increases as z decreases. Assuming a QSO-dominated UV background, the slope of the equation of state (gamma-1) shows no z-evolution within large uncertainties and the temperature at the mean density, T_0, decreases as z decreases at three redshift ranges. There is a large fluctuation of (Gamma-1) and b_c(13.6) even at the similar redshifts, in particular at z = 3.3 and 3.8. The lower (Gamma-1) and higher b_c(13.6) values at z ~ 3.1 and 3.6 compared to ones at z ~ 3.4 and 3.9 are caused by a lack of lower-N(HI) and lower-b lines, probably due to the HeII reionization. This result suggests that an impact from the HeII reionization on the forest might be mainly on the lower-N(HI) forest. We find some forest clouds with a high ratio of SiIV column density to CIV column density at z < 2.5, although the bulk of the forest clouds shows lower N(SiIV)/N(CIV). This high N(SiIV)/N(CIV) at z < 2.5 suggests that some forest clouds are exposed to a soft UV background
A survey of UV-excess AGNs in the South Galactic Pole
Spectra, position, magnitudes and colors are presented for 485 faint (B<20.5)
emission line objects selected with the ultraviolet-excess (UVX) criterion on a
area of 24.6 sq. deg in the South Galactic Pole. The objects were selected from
the analysis of pixel-to-pixel stacking of COSMOS scans of UKST U, J and R
plates. The candidates were observed with the Meudon-ESO Fiber Optics System
(MEFOS) at the ESO 3.6m telescope. 429 type 1 AGNs have been identified (373 in
the redshift range 0.3<z<2.2). This sample has allowed the measure of a
difference on the QSO clustering evolution in comparison with that found for
galaxies (La Franca et al 1998). The region is part of the ESO Imaging Survey
(EIS) and of the 2dF QSO redshift survey.Comment: 32 pages, 19 figures. To appear on A&AS, revised after minor comment
Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines
The similarity of quasar line spectra has been taken as an indication that
the emission line clouds have preferred parameters, suggesting that the
environment is subject to a fine tuning process. We show here that the observed
spectrum is a natural consequence of powerful selection effects. We computed a
large grid of photoionization models covering the widest possible range of
cloud gas density and distance from the central continuum source. For each line
only a narrow range of density and distance from the continuum source results
in maximum reprocessing efficiency, corresponding to ``locally
optimally-emitting clouds'' (LOC). These parameters depend on the ionization
and excitation potentials of the line, and its thermalization density. The mean
QSO line spectrum can be reproduced by simply adding together the full family
of clouds, with an appropriate covering fraction distribution. The observed
quasar spectrum is a natural consequence of the ability of various clouds to
reprocess the underlying continuum, and can arise in a chaotic environment with
no preferred pressure, gas density, or ionization parameter.Comment: 9 pages including 1 ps figure. LaTeX format using aaspp4.st
Exploring the intergalactic medium with VLT/UVES
The remarkable efficiency of the UVES spectrograph at the VLT has made it
possible to push high-resolution, high-S/N ground observations of the Ly-a
forest down to z~1.5, gaining new insight into the physical conditions of the
intergalactic medium and its evolution over more than 90% of the cosmic time.
The universal expansion, the UV ionizing background and the gravitational
condensation of structures are the driving factors shaping the number density
and the column density distribution of the absorbers. A (limited) contribution
of UV photons produced by galaxies is found to be important to reproduce the
observed evolutionary pattern at very high and low redshift. The Lyman forest
contains most of the baryons, at least at z>1.5, and acts as a reservoir for
galaxy formation. The typical Doppler parameter at a fixed column density is
measured to slightly increase with decreasing redshift, but the inferred
temperature at the mean density is increasing with redshift. The signatures of
HeII reionization and feedback from the formation of galactic structures have
possibly been detected in the Lyman forest.Comment: 6 pages Latex, with 2 PostScript figures, to appear in: ``Lighthouses
of the Universe'', Springer Verlag, ESO Astrophysics Symposia, Eds:
R.Sunyaev, M.Gilfanov, E.Churazov, August 200
Sympathetic cooling and collisional properties of a Rb-Cs mixture
We report on measurements of the collisional properties of a mixture of
Cs and Rb atoms in a magnetic trap at
temperatures. By selectively evaporating the Rb atoms using a radio-frequency
field, we achieved sympathetic cooling of Cs down to a few . The
inter-species collisional cross-section was determined through rethermalization
measurements, leading to an estimate of for the s-wave scattering
length for Rb in the and Cs in the magnetic
states. We briefly speculate on the prospects for reaching Bose-Einstein
condensation of Cs inside a magnetic trap through sympathetic cooling
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