990 research outputs found
A survey for large image-separation lensed quasars
The statistics of gravitationally lensed quasars with multiple images in the
0.1''-7'' range have been measured in various surveys. Little is known,
however, about lensed-quasar statistics at larger image separations, which
probe masses on the scale of galaxy clusters. We extend the results of the HST
Snapshot Survey for Lensed Quasars to the 7''-50'' range for a sub-sample of 76
quasars that is free of known selection effects. Using a combination of
multicolor photometry and spectroscopy, we show that none of the point sources
in the entire field of view of the HST observations of these quasars are lensed
images. Large-separation quasar lensing is therefore not common. We carry out a
detailed calculation of the expected statistics of large-separation lensing for
this quasar sample, incorporating realistic input for the mass profiles and
mass function of galaxy clusters. We find that the observational null results
are consistent with the expected effect of galaxy clusters, even if these have
existed in their present form and number since z of about 2. The rarity of
large-separation lensed quasars can rule out some extreme scenarios, e.g. that
the mass-function of clusters has been severely underestimated, or that large
mass concentrations that are not associated with galaxies (i.e. ``failed''
clusters) are common. The rareness of wide lensing also sets limits on the
cosmological constant that are independent of limits derived from galaxy
lensing. The lensing statistics of larger quasar samples can probe the
structure, number, and evolution of clusters, as well as the geometry of space.Comment: LaTex, ApJ, submitte
The KX method for producing K-band flux-limited samples of quasars
The longstanding question of the extent to which the quasar population is
affected by dust extinction, within host galaxies or galaxies along the line of
sight, remains open. More generally, the spectral energy distributions of
quasars vary significantly and flux-limited samples defined at different
wavelengths include different quasars. Surveys employing flux measurements at
widely separated wavelengths are necessary to characterise fully the spectral
properties of the quasar population. The availability of panoramic
near-infrared detectors on large telescopes provides the opportunity to
undertake surveys capable of establishing the importance of extinction by dust
on the observed population of quasars. We introduce an efficient method for
selecting K-band, flux-limited samples of quasars, termed ``KX'' by analogy
with the UVX method. This method exploits the difference between the power-law
nature of quasar spectra and the convex spectra of stars: quasars are
relatively brighter than stars at both short wavelengths (the UVX method) and
long wavelengths (the KX method). We consider the feasibility of undertaking a
large-area KX survey for damped Ly-alpha galaxies and gravitational lenses
using the planned UKIRT wide-field near-infrared camera.Comment: 6 pages, 3 figures, to appear in MNRA
A revised Cepheid distance to NGC 4258 and a test of the distance scale
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope
(HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations
and methods then standard for the Key Project on the Extragalactic Distance
Scale. Here, we reevaluate the Cepheid distance using the revised Key Project
procedures described in Freedman et al. (2001). These revisions alter the zero
points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at
the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and
the treatment of metallicity differences. We also provide herein full
information on the Cepheids described in Maoz et al. 1999. Using the refined
Key Project techniques and calibrations, we determine the distance modulus of
NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag
(systematic uncertainties in Key Project distances), corresponding to a metric
distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of
7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast
(1999), which yield more discrepant results. Additionally, we place weak limits
upon the distance to the LMC and upon the effect of metallicity in Cepheid
distance determinations.Comment: 26 pages in emulateapj5 format, including 6 figures and 5 tables.
Accepted for publication in the Astrophysical Journa
Minisuperspace Quantization of "Bubbling AdS" and Free Fermion Droplets
We quantize the space of 1/2 BPS configurations of Type IIB SUGRA found by
Lin, Lunin and Maldacena (hep-th/0409174), directly in supergravity. We use the
Crnkovic-Witten-Zuckerman covariant quantization method to write down the
expression for the symplectic structure on this entire space of solutions. We
find the symplectic form explicitly around AdS_5 x S^5 and obtain a U(1)
Kac-Moody algebra, in precise agreement with the quantization of a system of N
free fermions in a harmonic oscillator potential, as expected from AdS/CFT. As
a cross check, we also perform the quantization around AdS_5 x S^5 by another
method, using the known spectrum of physical perturbations around this
background and find precise agreement with our previous calculation.Comment: 22 Pages + 2 Appendices, JHEP3; v3: explanation of factor 2 mismatch
added, references reordered, published versio
Optical Continuum and Emission-Line Variability of Seyfert 1 Galaxies
We present the light curves obtained during an eight-year program of optical
spectroscopic monitoring of nine Seyfert 1 galaxies: 3C 120, Akn 120, Mrk 79,
Mrk 110, Mrk 335, Mrk 509, Mrk 590, Mrk 704, and Mrk 817. All objects show
significant variability in both the continuum and emission-line fluxes. We use
cross-correlation analysis to derive the sizes of the broad Hbeta-emitting
regions based on emission-line time delays, or lags. We successfully measure
time delays for eight of the nine sources, and find values ranging from about
two weeks to a little over two months. Combining the measured lags and widths
of the variable parts of the emission lines allows us to make virial mass
estimates for the active nucleus in each galaxy. The virial masses are in the
range 10^{7-8} solar masses.Comment: 24 pages, 16 figures. Accepted for publication in Ap
Predicting Action Content On-Line and in Real Time before Action Onset - an Intracranial Human Study
The ability to predict action content from neural signals in real time before the action occurs has been long sought in the neuroscientific study of decision-making, agency and volition. On-line real-time (ORT) prediction is important for understanding the relation between neural correlates of decision-making and conscious, voluntary action as well as for brain-machine interfaces. Here, epilepsy patients, implantded with intracranial depth microelectodes or subdural grid electrodes for clinical purposes, participated in a "matching-pennies" game against an opponent. In each trial, subjects were given a 5 s countdown, after which they had to raise their left or right hand immediately as the "go" signal appeared on a computer screen. They won a fixed amount of money if they raised a different hand than their opponent and lost that amount otherwise. The question we here studied was the extent to which neural precursors of the subjects' decisions can be detected in intracranial local field potentials (LFP) prior to the onset of the action. We found that combinded low-frequency (0.1-5 Hz) LFP signals from 10 electrodes were predictive of the intended left-/right-hand movements before the onset of the go signal. Our ORT system predicted which hand the patient would raise 0.5 s before the go signal with 68% accuracy in two patients. Based on these results, we constructed an ORT system that tracked up to 30 electrodes simultaneously, and tested it on retrospective data from 7 patients. On average, we could predict the correct hand choice in 83% of the trials, which rose to 92% if we let the system drop 3/10 of the trials on which it was less confident. Out system demonstrates-for the first time-the feasibility of accurately predicting a binary action on single trials in real time for patients with intracranial recordings, well before the action occurs
Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database
We present improved black hole masses for 35 active galactic nuclei (AGNs)
based on a complete and consistent reanalysis of broad emission-line
reverberation-mapping data. From objects with multiple line measurements, we
find that the highest precision measure of the virial product is obtained by
using the cross-correlation function centroid (as opposed to the
cross-correlation function peak) for the time delay and the line dispersion (as
opposed to full width half maximum) for the line width and by measuring the
line width in the variable part of the spectrum. Accurate line-width
measurement depends critically on avoiding contaminating features, in
particular the narrow components of the emission lines. We find that the
precision (or random component of the error) of reverberation-based black hole
mass measurements is typically around 30%, comparable to the precision attained
in measurement of black hole masses in quiescent galaxies by gas or stellar
dynamical methods. Based on results presented in a companion paper by Onken et
al., we provide a zero-point calibration for the reverberation-based black hole
mass scale by using the relationship between black hole mass and host-galaxy
bulge velocity dispersion. The scatter around this relationship implies that
the typical systematic uncertainties in reverberation-based black hole masses
are smaller than a factor of three. We present a preliminary version of a
mass-luminosity relationship that is much better defined than any previous
attempt. Scatter about the mass-luminosity relationship for these AGNs appears
to be real and could be correlated with either Eddington ratio or object
inclination.Comment: 61 pages, including 8 Tables and 16 Figures. Accepted for publication
in The Astrophysical Journa
Limits on Dust in Rich Clusters of Galaxies from the Color of Background Quasars
I measure the V-I color distribution of two samples of radio-selected
quasars. Quasars from one sample are projected on the sky within 1 degree of a
rich foreground Abell cluster of galaxies, while quasars from the other sample
are more than 3 degrees from any such cluster . There is no significant
difference between the color distributions of the two samples. The 90\% upper
limit on the relative reddening between the two samples is E(B-V)=0.05 mag.
This result limits the allowed quantity of smoothly distributed dust in rich
clusters, and contradicts previous indications for the existence of such a
component.Comment: Submitted to ApJ Letters. 14 pages incl. 2 figures, uuencoded,
compressed postscript. Also available by anonymous ftp to
ftp://wise3.tau.ac.il/pub/dani/dust.ps.
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