178 research outputs found
Do Quasars Lens Quasars?
If the unexpectedly high frequency of quasar pairs with very different
component redshifts is due to the lensing of a population of background quasars
by the foreground quasar, typical lens masses must be \sim10^{12}M_{\sun} and
the sum of all such quasar lenses would have to contain times the
closure density of the Universe. It then seems plausible that a very high
fraction of all \sim10^{12} M_{\sun} gravitational lenses with redshifts
contain quasars. Here I propose that these systems have evolved to
form the present population of massive galaxies with M and M
>5\times10^{11} M_{\sun}.Comment: 6 pages, aas style, ams symbols, ApJL (accepted
Changes in life-style after liver transplantation
Sixty-five pediatric patients who received liver transplants between May 1981 and May 1984 were observed for as many as 5 years and examined for changes in life-style. Children were less frequently hospitalized, spent less time hospitalized, required fewer medications, and generally had excellent liver and renal function after hepatic transplantation as compared with their pretransplantation status. Most children were in age-appropriate and standard school classes or were only 1 year behind. Cognitive abilities remained unchanged. Children improved in gross motor function and patients' behavior significantly improved according to parents' perceptions. Enuresis was more prevalent, however, than in the population of children who had not received liver transplants. Parental divorce rates were no greater than those reported for other families with chronically ill children. Overall, objective changes in life-style as well as parents' perceptions of behavior of children appear to be improved after liver transplantation
PVF2 Transducers for NDE
We report on recent calculations and experiments on the broadband properties and impulse characteristics of PVF2 transducers and arrays. Experimental wedge transducers show bandwidths of approximately 100% in the excitation of surface acoustic waves at 7 MHz on nonpiezoelectric silicon nitride ceramic substrates. Computer calculations predict similar bandwidths for interdigital transducer arrays on PVF2 films for surface acoustic wave excitation on similar substrates. Insertion loss versus frequency measurements on bulk longitudinal wave transducers in water at frequencies in the 1 to 30 MHz range show good agreement with theory. A computer program for multilayer piezoelectric films predicts included angles of acceptance exceeding 60°, and control of acceptance angle profiles, in face plates using multilayer PVF2 films
NICMOS Imaging of the Host Galaxies of z ~ 2 - 3 Radio-Quiet Quasars
We have made a deep NICMOS imaging study of a sample of 5 z ~ 2 - 3
radio-quiet quasars with low absolute nuclear luminosities, and we have
detected apparent host galaxies in all of these. Most of the hosts have
luminosities approximately equal to present-day L*, with a range from 0.2 L* to
about 4 L*. These host galaxies have magnitudes and sizes consistent with those
of the Lyman break galaxies at similar redshifts and at similar rest
wavelengths, but are about two magnitudes fainter than high-z powerful radio
galaxies. The hosts of our high-z sample are comparable to or less luminous
than the hosts of the low-z RQQs with similar nuclear absolute magnitudes.
However, the high z galaxies are more compact than the hosts of the low z
quasars, and probably have only 10 - 20% of the stellar mass of their low-z
counterparts. Application of the M(bulge)/M(BH) relation found for present-day
spheroids to the stellar masses implied for the high z host galaxies would
indicate that they contain black holes with masses around 10^8 Msolar.
Comparison to their nuclear magnitudes implies accretion rates that are near or
at the Eddington limit. Although these high z hosts already contain
supermassive black holes, the galaxies will need to grow significantly to
evolve into present-day L* galaxies. These results are basically consistent
with theoretical predictions for the hierarchical buildup of the galaxy host
and its relation to the central supermassive black hole.Comment: 25 pages, 13 figures, accepted for publication in Ap
PVF2 Transducers for NDT
We have investigated the spatial dependence of the longitudinal piezoelectric stress constant e of PVF2. Experiments were performed on a series of no.minally identical brass-backed PVF2 longitudinal wave transducers in water using commercial PVF2 film. Computer programs were designed to predict the performance of the transducers as a function of ezz (Z) , the thickness mode piezoelectric stress constant as a function of position Z through the thickness of the film. Our experiments indicate that the coupling coefficient is uniform across the film thickness. These computer programs were also used to model the insertion loss and bandwidth performance of the transducers. High voltage pulses were applied to PVF2 transducers to determine the region of linearity and to find the maximum nondestructive voltage that can be used. Transducers of this type were also made using PVF films fabricated in the Stanford Center for Materials Research using commercial resin, and found to pertorm as well as transducers using commercial film. Transducers of the above kind have also been used as bulk wave sources in wedge transducer assemblies for the production of surface acoustic waves on ceramic plates, and initial observations of reflections from surface cracks have been made
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
Quasar clustering: evidence for an increase with redshift and implications for the nature of AGNs
The evolution of quasar clustering is investigated with a new sample of 388
quasars with 0.3<z<=2.2, B<=20.5 and Mb<-23, selected over an area of 24.6 sq.
deg. in the South Galactic Pole. Assuming a two-point correlation function of
the form xi(r) = (r/r_o)^-1.8, we detect clustering with r_0=6.2 +/- 1.6 h^-1
comoving Mpc at an average redshift of z=1.3. We find a 2 sigma significant
increase of the quasar clustering between z=0.95 and z=1.8, independent of the
quasar absolute magnitude and inconsistent with recent evidence on the
evolution of galaxy clustering. If other quasar samples are added (resulting in
a total data-set of 737 quasars) the increase of the quasar clustering is still
favoured although it becomes less significant. We find epsilon=-2.5.
Evolutionary parameters epsilon>0.0 are excluded at a 0.3% probability level,
to be compared with epsilon=0.8 found for galaxies. The observed clustering
properties appear qualitatively consistent with a scenario of Omega=1 CDM in
which a) the difference between the quasar and the galaxy clustering can be
explained as a difference in the effective bias and redshift distributions, and
b) the quasars, with a lifetime of t~10^8 yr, sparsely sample halos of mass
greater than M_min~10^12-10^13 h^-1 M_sun. We discuss also the possibility that
the observed change in the quasar clustering is due to an increase in the
fraction of early-type galaxies as quasar hosts at high z.Comment: 8 pages including 2 eps figures, LaTeX (AAS v4.0), ApJ in pres
CONSENSUS CONFERENCE REPORT ON LIVER* TRANSPLANTATION.
Liver transplantation has been developed to the point of a service operation, the exploitation of which depends upon the establishment of multiple regional centers. The increased use of this procedure will permit the delivery of optimum health care to victims of end stage liver disease
Near-infrared thermal emissivity from ground based atmospheric dust measurements at ORM
We present an analysis of the atmospheric content of aerosols measured at
Observatorio del Roque de los Muchachos (ORM; Canary Islands). Using a laser
diode particle counter located at the Telescopio Nazionale Galileo (TNG) we
have detected particles of 0.3, 0.5, 1.0, 3.0, 5.0 and 10.0 um size. The
seasonal behavior of the dust content in the atmosphere is calculated. The
Spring has been found to be dustier than the Summer, but dusty conditions may
also occur in Winter. A method to estimate the contribution of the aerosols
emissivity to the sky brightness in the near-infrared (NIR) is presented. The
contribution of dust emission to the sky background in the NIR has been found
to be negligible comparable to the airglow, with a maximum contribution of
about 8-10% in the Ks band in the dusty days.Comment: 6 pages, 3 figures, 6 tables, accepted for publication in MNRA
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