916 research outputs found
The Variance of QSO Counts in Cells
{}From three quasar samples with a total of 1038 objects in the redshift
range we measure the variance of counts in cells of
volume . By a maximum likelihood analysis applied separately on these
samples we obtain estimates of , with .
The analysis from a single catalog for Mpc and from a
suitable average over the three catalogs for and
Mpc, gives , ,
and , respectively, where the
confidence ranges account for both sampling errors and statistical
fluctuations in the counts. This allows a comparison of QSO clustering on large
scales with analogous data recently obtained both for optical and IRAS
galaxies: QSOs seem to be more clustered than these galaxies by a biasing
factor .Comment: 13 pages in plain Tex, 5 figures available in postscript in a
separate file, submitted to ApJ, DAPD-33
High resolution spectroscopy of the three dimensional cosmic web with close QSO groups
We study the three-dimensional distribution of matter at z~2 using high
resolution spectra of QSO pairs and simulated spectra drawn from cosmological
hydro-dynamical simulations. We present a sample of 15 QSOs, corresponding to
21 baselines of angular separations evenly distributed between ~1 and 14
arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at
the European Southern Observatory-Very Large Telescope (ESO-VLT). The observed
correlation functions of the transmitted flux in the HI Lya forest transverse
to and along the line of sight are in agreement, implying that the distortions
in redshift space due to peculiar velocities are relatively small and - within
the relatively large error bars - not significant. The clustering signal is
significant up to velocity separations of ~300 km/s, corresponding to about 5
h^{-1} comoving Mpc. Compatibility at the 2 sigma level has been found both for
the Auto- and Cross-correlation functions and for the set of the Cross
correlation coefficients. The analysis focuses in particular on two QSO groups
of the sample. Searching for alignments in the redshift space between Lya
absorption lines belonging to different lines of sight, it has been possible to
discover the presence of a wide HI structures extending over about ten Mpc in
comoving space, and give constraints on the sizes of two cosmic under-dense
regions in the intergalactic medium.Comment: Accepted by MNRAS, version matching the published on
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 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 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
- …