3,740 research outputs found
A comparison of the optical properties of radio-loud and radio-quiet quasars
We have made radio observations of 87 optically selected quasars at 5 GHz
with the VLA in order to measure the radio power for these objects and hence
determine how the fraction of radio-loud quasars varies with redshift and
optical luminosity. The sample has been selected from the recently completed
Edinburgh Quasar Survey and covers a redshift range of 0.3 < z < 1.5 and an
optical absolute magnitude range of -26.5 < M_{B} < -23.5 (h, q_{0} = 1/2). We
have also matched up other existing surveys with the FIRST and NVSS radio
catalogues and combined these data so that the optical luminosity-redshift
plane is now far better sampled than previously. We have fitted a model to the
probability of a quasar being radio-loud as a function of absolute magnitude
and redshift and from this model infer the radio-loud and radio-quiet optical
luminosity functions. The radio-loud optical luminosity function is featureless
and flatter than the radio-quiet one. It evolves at a marginally slower rate if
quasars evolve by density evolution, but the difference in the rate of
evolutions of the two different classes is much less than was previously
thought. We show, using Monte-Carlo simulations, that the observed difference
in the shape of the optical luminosity functions can be partly accounted for by
Doppler boosting of the optical continuum of the radio-loud quasars and explain
how this can be tested in the future.Comment: 33 pages, 9 postscript figures, uses the AAS aaspp4 LaTeX style file,
to appear in the 1 February 1999 issue of The Astrophysical Journa
Particle production and equilibrium properties within a new hadron transport approach for heavy-ion collisions
The microscopic description of heavy-ion reactions at low beam energies is
achieved within hadronic transport approaches. In this article a new approach
SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is introduced
and applied to study the production of non-strange particles in heavy-ion
reactions at GeV. First, the model is described including
details about the collision criterion, the initial conditions and the resonance
formation and decays. To validate the approach, equilibrium properties such as
detailed balance are presented and the results are compared to experimental
data for elementary cross sections. Finally results for pion and proton
production in C+C and Au+Au collisions is confronted with HADES and FOPI data.
Predictions for particle production in collisions are made.Comment: 30 pages, 30 figures, replaced with published version; only minor
change
Integrable string models with constant torsion in terms of chiral invariants of SU(n), SO(n), SP(n) groups
We used the invariant local chiral currents of principal chiral models for
SU(n), SO(n), SP(n) groups to construct new integrable string equations of
hydrodynamic type on the Riemmann space of the chiral primitive invariant
currents and on the chiral non-primitive Casimir operators as Hamiltonians.Comment: 12 pages; 1 figure, This paper is third part of my talk on the "XXII
International Colloquium on Group Theoretical Methods in Physics", Yerevan,
Armenia, August 13-19, 200
Three-Dimensional Integrable Models and Associated Tangle Invariants
In this paper we show that the Boltzmann weights of the three-dimensional
Baxter-Bazhanov model give representations of the braid group, if some suitable
spectral limits are taken. In the trigonometric case we classify all possible
spectral limits which produce braid group representations. Furthermore we prove
that for some of them we get cyclotomic invariants of links and for others we
obtain tangle invariants generalizing the cyclotomic ones.Comment: Number of pages: 21, Latex fil
XY models with disorder and symmetry-breaking fields in two dimensions
The combined effect of disorder and symmetry-breaking fields on the
two-dimensional XY model is examined. The study includes disorder in the
interaction among spins in the form of random phase shifts as well as disorder
in the local orientation of the field. The phase diagrams are determined and
the properties of the various phases and phase transitions are calculated. We
use a renormalization group approach in the Coulomb gas representation of the
model. Our results differ from those obtained for special cases in previous
works. In particular, we find a changed topology of the phase diagram that is
composed of phases with long-range order, quasi-long-range order, and
short-range order. The discrepancies can be ascribed to a breakdown of the
fugacity expansion in the Coulomb gas representation.
Implications for physical systems such as planar Josephson junctions and the
faceting of crystal surfaces are discussed.Comment: 17 pages Latex with 5 eps figures, change: acknowledgment extende
Magic wavelengths for the transition in rubidium
Magic wavelengths, for which there is no differential ac Stark shift for the
ground and excited state of the atom, allow trapping of excited Rydberg atoms
without broadening the optical transition. This is an important tool for
implementing quantum gates and other quantum information protocols with Rydberg
atoms, and reliable theoretical methods to find such magic wavelengths are thus
extremely useful. We use a high-precision all-order method to calculate magic
wavelengths for the transition of rubidium, and compare the
calculation to experiment by measuring the light shift for atoms held in an
optical dipole trap at a range of wavelengths near a calculated magic value
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
The Metagalactic Ionizing Radiation Field at Low Redshift
We compute the ionizing radiation field at low redshift, arising from
Seyferts, QSOs, and starburst galaxies. This calculation combines recent
Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN
database, and a new intergalactic opacity model based on Hubble Space Telescope
and Keck Ly-alpha absorber surveys. At z = 0 for AGN only, our best estimate
for the specific intensity at 1 Ryd is I_0 = 1.3 (+0.8/-0.5) x 10^-23
ergs/cm^2/s/Hz/sr, independent of H_0, Omega_0, and Lambda. The one-sided
ionizing photon flux is Phi_ion = 3400 (+2100/-1300) photons/cm^2/s, and the H
I photoionization rate is Gamma_HI = 3.2 (+2.0/-1.2) x 10^-14 s^-1 for alpha_s
= 1.8. We also derive Gamma_ HI for z = 0 - 4. These error ranges reflect
uncertainties in the spectral indexes for the ionizing EUV (alpha_s = 1.8 +/-
0.3) and the optical/UV (alpha_UV = 0.86 +/- 0.05), the IGM opacity model, the
range of Seyfert luminosities (0.001 - 100 L*) and the completeness of the
luminosity functions. Our estimate is a factor of three lower than the most
stringent upper limits on the ionizing background (Phi_ion < 10^4
photons/cm^2/s) obtained from H-alpha observations in external clouds, and it
lies within the range implied by other indirect measures. Starburst galaxies
with a sufficiently large Lyman continuum escape fraction, f_ esc > 0.05, may
provide a comparable background to AGN, I_0 (z=0) = 1.1 (+1.5/-0.7) x 10^{-23).
An additional component of the ionizing background of this magnitude would
violate neither upper limits from H-alpha observations nor the acceptable range
from other measurements.Comment: 30 pages, 9 figures, accepted for Astronomical J. (Oct. 1999
Radio Properties of z>4 Optically-Selected Quasars
We report on two programs to address differential evolution between the
radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both
programs entail studying the radio properties of optically-selected quasars.
First, we have observed 32 optically-selected, high-redshift (z>4) quasars with
the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and
well-understood sample. We detect four quasars above our 3-sigma limit of ~0.15
mJy, which is sufficiently sensitive to detect all radio-loud quasars at the
probed redshift range. Second, we have correlated 134 z>4 quasars, comprising
all such sources that we are aware of as of mid-1999, with FIRST and NVSS.
These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately
0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, of
which six were not previously known to be radio-loud. The depth of these
surveys does not reach the radio-loud/radio-quiet demarcation luminosity
density (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift range
considered; this correlation therefore only provides a lower limit to the
radio-loud fraction of quasars at high-redshift. The two programs together
identify eight new radio-loud quasars at z>4, a significant increase over the
seven currently in the published literature. We find no evidence for radio-loud
fraction depending on optical luminosity for -25 > M_B > -28 at z~2, or for
-26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud
fraction between z~2 and z>4 (-26>M_B>-28).Comment: 19 pages, 7 figures; to appear in The Astronomical Journal (April
2000
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