627 research outputs found
Single-Molecule Force Microscopy: A Potential Tool for the Mapping of Polysaccharides in Plant Cell Walls
A Keck Survey of Gravitational Lens Systems: I. Spectroscopy of SBS 0909+532, HST 1411+5211, and CLASS B2319+051
We present new results from a continuing Keck program to study gravitational
lens systems. We have obtained redshifts for three lens systems, SBS 0909+532,
HST 1411+5211, and CLASS B2319+051. For all of these systems, either the source
or lens redshift (or both) has been previously unidentified. We find (z_l, z_s)
= (0.830, 1.377) for SBS 0909+532; (z_l, z_s) = (0.465, 2.811) for HST
1411+5211, although the source redshift is still tentative; and (z_l1, z_l2) =
(0.624, 0.588) for the two lensing galaxies in CLASS B2319+051. The background
radio source in B2319+051 has not been detected optically; its redshift is,
therefore, still unknown. We find that the spectral features of the central
lensing galaxy in all three systems are typical of an early-type galaxy. The
observed image splittings in SBS 0909+532 and HST 1411+5211 imply that the
masses within the Einstein ring radii of the lensing galaxies are 1.4 x 10^{11}
and 2.0 x 10^{11} h^{-1} M_sun, respectively. The resulting B band
mass-to-light ratio for HST 1411+5211 is 41.3 +/- 1.2 h (M/L)_sun, a factor of
5 times higher than the average early-type lensing galaxy. This large
mass-to-light is almost certainly the result of the additional mass
contribution from the cluster CL 3C295 at z = 0.46. For the lensing galaxy in
SBS 0909+532, we measure (M/L)_B = 4^{+11}_{-3} h (M/L)_sun where the large
errors are the result of significant uncertainty in the galaxy luminosity.
While we cannot measure directly the mass-to-light ratio of the lensing galaxy
in B2319+051, we estimate that (M/L)_B is between 3-7 h (M/L)_sun.Comment: Accepted for publication in Astronomical Journal. 21 pages, including
7 figure
3C 295, a cluster and its cooling flow at z=0.46
We present ROSAT HRI data of the distant and X-ray luminous (L_x(bol)=2.6^
{+0.4}_{-0.2} 10^{45}erg/sec) cluster of galaxies 3C 295. We fit both a
one-dimensional and a two-dimensional isothermal beta-model to the data, the
latter one taking into account the effects of the point spread function (PSF).
For the error analysis of the parameters of the two-dimensional model we
introduce a Monte-Carlo technique. Applying a substructure analysis, by
subtracting a cluster model from the data, we find no evidence for a merger,
but we see a decrement in emission South-East of the center of the cluster,
which might be due to absorption. We confirm previous results by Henry &
Henriksen(1986) that 3C 295 hosts a cooling flow. The equations for the simple
and idealized cooling flow analysis presented here are solely based on the
isothermal beta-model, which fits the data very well, including the center of
the cluster. We determine a cooling flow radius of 60-120kpc and mass accretion
rates of dot{M}=400-900 Msun/y, depending on the applied model and temperature
profile. We also investigate the effects of the ROSAT PSF on our estimate of
dot{M}, which tends to lead to a small overestimate of this quantity if not
taken into account. This increase of dot{M} (10-25%) can be explained by a
shallower gravitational potential inferred by the broader overall profile
caused by the PSF, which diminishes the efficiency of mass accretion. We also
determine the total mass of the cluster using the hydrostatic approach. At a
radius of 2.1 Mpc, we estimate the total mass of the cluster (M{tot}) to be
(9.2 +/- 2.7) 10^{14}Msun. For the gas to total mass ratio we get M{gas}/M{tot}
=0.17-0.31, in very good agreement with the results for other clusters of
galaxies, giving strong evidence for a low density universe.Comment: 26 pages, 7 figures, accepted for publication in Ap
Microscopic theory for quantum mirages in quantum corrals
Scanning tunneling microscopy permits to image the Kondo resonance of a
single magnetic atom adsorbed on a metallic surface. When the magnetic impurity
is placed at the focus of an elliptical quantum corral, a Kondo resonance has
been recently observed both on top of the impurity and on top of the focus
where no magnetic impurity is present. This projection of the Kondo resonance
to a remote point on the surface is referred to as quantum mirage. We present a
quantum mechanical theory for the quantum mirage inside an ideal quantum corral
and predict that the mirage will occur in corrals with shapes other than
elliptical
Tunable-filter imaging of quasar fields at z~1. I. A cluster around MRC B0450-221
Using a combination of multicolour broad- and narrow-band imaging techniques
and follow-up spectroscopy, we have detected an overdensity of galaxies in the
field of quasar MRC B0450-221, whose properties are consistent with a cluster
at the quasar redshift z=0.9. An excess of red galaxies (V-I>2.2, I-K'>3.8) is
evident within 1' of the quasar, with the colours expected for galaxies at
z=0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting
galaxies (nine candidates with equivalent widths EW>70A) are also detected in
the field using the TAURUS Tunable Filter (TTF). Three have been confirmed
spectroscopically to indeed lie at z=0.9. The TTF candidates with the strongest
[O II] line emission cluster in a group which lies 200-700 kpc away from the
quasar and the red galaxy excess, and therefore most likely on the outskirts of
the cluster. These observations are the first in a series probing quasar
environments at z~1 with TTF.Comment: Accepted for publication in AJ. 25 pages, 24 figs (large files in jpg
or gif format), uses emulateapj.st
The Kondo Box: A Magnetic Impurity in an Ultrasmall Metallic Grain
We study the Kondo effect generated by a single magnetic impurity embedded in
an ultrasmall metallic grain, to be called a ``Kondo box''. We find that the
Kondo resonance is strongly affected when the mean level spacing in the grain
becomes larger than the Kondo temperature, in a way that depends on the parity
of the number of electrons on the grain. We show that the single-electron
tunneling conductance through such a grain features Kondo-induced Fano-type
resonances of measurable size, with an anomalous dependence on temperature and
level spacing.Comment: 4 Latex pages, 4 figures, submitted to Phys. Rev. Let
Kondo Resonance in a Mesoscopic Ring Coupled to a Quantum Dot: Exact Results for the Aharonov-Bohm/Casher Effects
We study the persistent currents induced by both the Aharonov-Bohm and
Aharonov-Casher effects in a one-dimensional mesoscopic ring coupled to a
side-branch quantum dot at Kondo resonance. For privileged values of the
Aharonov-Bohm-Casher fluxes, the problem can be mapped onto an integrable
model, exactly solvable by a Bethe ansatz. In the case of a pure magnetic
Aharonov-Bohm flux, we find that the presence of the quantum dot has no effect
on the persistent current. In contrast, the Kondo resonance interferes with the
spin-dependent Aharonov-Casher effect to induce a current which, in the
strong-coupling limit, is independent of the number of electrons in the ring.Comment: Replaced with published version; 5 page
Mesoscopic Kondo Effect in an Aharonov-Bohm Ring
An interacting quantum dot inserted in a mesoscopic ring is investigated. A
variational ansatz is employed to describe the ground state of the system in
the presence of the Aharonov-Bohm flux. It is shown that, for even number of
electrons with the energy level spacing smaller than the Kondo temperature, the
persistent current has a value similar to that of a perfect ring with the same
radius. On the other hand, for a ring with odd number electrons, the persistent
current is found to be strongly suppressed compared to that of an ideal ring,
which implies the suppression of the Kondo-resonant transmission. Various
aspects of the Kondo-assisted persistent current are discussed.Comment: 4 pages Revtex, 4 Postscript figures, final version to appear in
Phys. Rev. Lett. 85, No.26 (Dec. 25, 2000
Kondo screening cloud effects in mesoscopic devices
We study how finite size effects may appear when a quantum dot in the Kondo
Coulomb blockade regime is embedded into a mesoscopic device with finite wires.
These finite size effects appear when the size of the mesoscopic device
containing the quantum dot is of the order of the size of Kondo cloud and
affect all thermodynamic and transport properties of the Kondo quantum dot. We
also generalize our results to the experimentally relevant case where the wires
contain several transverse modes/channels. Our results are based on
perturbation theory, Fermi liquid theory and slave boson mean field theory.Comment: 19 pages, 9 figure
Chandra study of an overdensity of X-ray sources around two distant (z~0.5) clusters
We present results from a Chandra X-ray Observatory study of the field X-ray
source populations in 4 different observations: two high-redshift (z~0.5)
clusters of galaxies 3C295 and RXJ003033.2+261819; and two non-cluster fields
with similar exposure time. Surprisingly, the 0.5-2 keV source surface
densities (~900-1200 sources deg**-2 at a flux limit of 1.5x10**-15 erg
cm**-2s**-1) measured in an ~8'x8' area surrounding each cluster exceed by a
factor of ~2 the value expected on the basis of the ROSAT and Chandra
logN-logS, with a significance of ~2 sigma each, or ~3.5 sigma when the 2
fields are combined (i.e. a probability to be a statistical fluctuation of <1%
and <0.04%, respectively). The same analysis performed on the non-cluster
fields and on the outer chips of the cluster fields does NOT show evidence of
such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the
detected objects is well fitted by a power-law with Gamma~1.7 similar to AGNs
and shows no sign of intrinsic absorption. The few (~10 out of 35) optical
identifications available to date confirm that most of them are, as expected,
AGNs but the number of redshifts available is too small to allow conclusions on
their nature. We discuss possible interpretations of the overdensity in terms
of: a statistical variation of Cosmic Background sources; a concentration of
AGNs and/or powerful starburst galaxies associated with the clusters; and g
ravitational lensing of background QSO's by the galaxy clusters. All
explanations are however difficult to reconcile with the large number of excess
sources detected. Deeper X-ray observations and more redshifts measurements are
clearly required to settle the issue.Comment: 22 LateX pages (including Tables and Figures), uses psfig.sty and
emulateapj.sty. Accepted for publication in Astrophysical Journa
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