4,627 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
A rotating helical filament in the L1251 dark cloud
(Abridged) Aims. We derive the physical properties of a filament discovered
in the dark cometary-shaped cloud L1251. Methods. Mapping observations in the
NH3(1,1) and (2,2) inversion lines, encompassing 300 positions toward L1251,
were performed with the Effelsberg 100-m telescope at a spatial resolution of
40 arcsec and a spectral resolution of 0.045 km/s. Results. The filament L1251A
consists of three condensations (alpha, beta, and gamma) of elongated
morphology, which are combined in a long and narrow structure covering a 38
arcmin by 3 arcmin angular range. The opposite chirality (dextral and
sinistral) of the alpha+beta and gamma condensations indicates magnetic field
helicities of two types, negative and positive, which were most probably caused
by dynamo mechanisms. We estimated the magnetic Reynolds number Rm > 600 and
the Rossby number R < 1, which means that dynamo action is important.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in A&
The baryon density at z=0.9-1.9 - Tracing the warm-hot intergalactic medium with broad Lyman alpha absorption
We present an analysis of the Lyman alpha forests of five quasar spectra in
the near UV. Properties of the intergalactic medium (IGM) at an intermediate
redshift interval (0.9 < z < 1.9) are studied. The amount of baryons in the
diffuse photoionised IGM and the warm-hot intergalactic medium (WHIM) are
traced to get constraints on the redshift evolution of the different phases of
the intergalactic gas. The baryon density of the diffuse IGM is determined with
photoionisation calculations under the assumption of local hydrostatic
equilibrium. We assume that the gas is ionised by a metagalactic background
radiation with a Haardt & Madau (2001) spectrum. The WHIM is traced with broad
Lyman alpha (BLA) absorption. The properties of a number of BLA detections are
studied. Under the assumption of collisional ionisation equilibrium a lower
limit to the baryon density could be estimated. It is found that the diffuse
photoionised IGM contains at least 25% of the total baryonic matter at
redshifts 1 < z < 2. For the WHIM a lower limit of 2.4% could be determined.
Furthermore the data indicates that the intergalactic gas is in a state of
evolution at z=1.5. We confirm that a considerable part of the WHIM is created
between z=1 and z=2.Comment: 6 pages, 1 figure, accepted for publication in A&
Representations of p-brane topological charge algebras
The known extended algebras associated with p-branes are shown to be
generated as topological charge algebras of the standard p-brane actions. A
representation of the charges in terms of superspace forms is constructed. The
charges are shown to be the same in standard/extended superspace formulations
of the action.Comment: 22 pages. Typos fixed, refs added. Minor additions to comments
sectio
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