291 research outputs found
Cyclotron motion in graphene
We investigate cyclotron motion in graphene monolayers considering both the
full quantum dynamics and its semiclassical limit reached at high carrier
energies. Effects of zitterbewegung due to the two dispersion branches of the
spectrum dominate the irregular quantum motion at low energies and are obtained
as a systematic correction to the semiclassical case. Recent experiments are
shown to operate in the semiclassical regime.Comment: 6 pages, 1 figure include
Redshifts of CLASS Radio Sources
Spectroscopic observations of a sample of 42 flat-spectrum radio sources from
the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of with an RMS spread of 0.95, at a completeness level of 64%. The sample
consists of sources with a 5-GHz flux density of 25-50 mJy, making it the
faintest flat-spectrum radio sample for which the redshift distribution has
been studied. The spectra, obtained with the Willam Herschel Telescope (WHT),
consist mainly of broad-line quasars at and narrow-line galaxies at
. Though the mean redshift of flat-spectrum radio sources exhibits
little variation over more than two orders of magnitude in radio flux density,
there is evidence for a decreasing fraction of quasars at weaker flux levels.
In this paper we present the results of our spectroscopic observations, and
discuss the implications for constraining cosmological parameters with
statistical analyses of the CLASS survey.Comment: 10 pages, AJ accepte
High resolution observations and mass modelling of the CLASS gravitational lens B1152+199
We present a series of high resolution radio and optical observations of the
CLASS gravitational lens system B1152+199 obtained with the Multi-Element
Radio-Linked Interferometer Network (MERLIN), Very Long Baseline Array (VLBA)
and Hubble Space Telescope (HST). Based on the milliarcsecond-scale
substructure of the lensed radio components and precise optical astrometry for
the lensing galaxy, we construct models for the system and place constraints on
the galaxy mass profile. For a single galaxy model with surface mass density
Sigma(r) propto r^-beta, we find that 0.95 < beta < 1.21 at 2-sigma confidence.
Including a second deflector to represent a possible satellite galaxy of the
primary lens leads to slightly steeper mass profiles.Comment: 7 pages, post-referee revision for MNRA
Gauging the dark matter fraction in a S0 galaxy at z=0.47 through gravitational lensing from deep HST/ACS imaging
We analyze a new gravitational lens, OAC-GL J1223-1239, serendipitously found
in a deep I-band image of the Hubble Space Telescope (HST) Advanced Camera for
Surveys (ACS). The lens is a L_*, edge-on S0 galaxy at z=0.4656. The
gravitational arc has a radius of 0.42 arcsec. We have determined the total
mass and the dark matter (DM) fraction within the Einstein radius as a function
of the lensed source redshift, which is presently unknown. For z ~ 1.3, which
is in the middle of the redshift range plausible for the source according to
some external constraints, we find the central velocity dispersion to be ~180
km/s. With this value, close to that obtained by means of the Faber-Jackson
relation at the lens redshift, we compute a 30% DM fraction within the Einstein
radius (given the uncertainty in the source redshift, the allowed range for the
DM fraction is 25-35 % in our lensing model). When compared with the galaxies
in the local Universe, the lensing galaxy, OAC-GL J1223-1239 seems to fall in
the transition regime between massive DM dominated galaxies and lower-mass, DM
deficient systems.Comment: 18 pages, 5 figures; accepted for publication in Ap
The new two-image gravitational lens system CLASS B2319+051
We report the discovery of a new two-image gravitational lens system from the
Cosmic Lens All-Sky Survey, CLASS B2319+051. Radio imaging with the Very Large
Array (VLA) and Multi-Element Radio-Linked Interferometer Network (MERLIN)
shows two compact components with a flux density ratio of 5:1, separated by
1.36 arcsec. Observations with the Very Long Baseline Array (VLBA) resolve each
of the radio components into a pair of parity-reversed subcomponents. Hubble
Space Telescope (HST) observations with the Near-Infrared Camera and
Multi-Object Spectrometer (NICMOS) show a bright elliptical galaxy (G1)
coincident with the radio position, and a second irregular galaxy (G2) 3.4
arcsec to the northwest. Previous spectroscopic studies have indicated that
these galaxies are at different redshifts: z(G1) = 0.624, z(G2) = 0.588.
Infrared counterparts to the lensed radio components are not detected in the
NICMOS image, and the source redshift has not yet been determined. Preliminary
mass modeling based on the VLBA subcomponent data indicates that the lensing
potential includes a strong external shear contribution. A VLA monitoring
program is currently being undertaken to measure the differential time delay.Comment: 16 pages, 7 figs, several typos corrected, AJ in press (August 2001
B0850+054: a new gravitational lens system from CLASS
We report the discovery of a new gravitational lens system from the CLASS
survey. Radio observations with the VLA, the WSRT and MERLIN show that the
radio source B0850+054 is comprised of two compact components with identical
spectra, a separation of 0.7 arcsec and a flux density ratio of 6:1. VLBA
observations at 5 GHz reveal structures that are consistent with the
gravitational lens hypothesis. The brighter of the two images is resolved into
a linear string of at least six sub-components whilst the weaker image is
radially stretched towards the lens galaxy. UKIRT K-band imaging detects an
18.7 mag extended object, but the resolution of the observations is not
sufficient to resolve the lensed images and the lens galaxy. Mass modelling has
not been possible with the present data and the acquisition of high-resolution
optical data is a priority for this system.Comment: 5 pages, 4 figures, accepted for publication in MNRA
CLASS B2108+213: A new wide separation gravitational lens system
We present observations of CLASS B2108+213, the widest separation
gravitational lens system discovered by the Cosmic Lens All-Sky Survey. Radio
imaging using the VLA at 8.46 GHz and MERLIN at 5 GHz shows two compact
components separated by 4.56 arcsec with a faint third component in between
which we believe is emission from a lensing galaxy. 5-GHz VLBA observations
reveal milliarcsecond-scale structure in the two lensed images that is
consistent with gravitational lensing. Optical emission from the two lensed
images and two lensing galaxies within the Einstein radius is detected in
Hubble Space Telescope imaging. Furthermore, an optical gravitational arc,
associated with the strongest lensed component, has been detected. Surrounding
the system are a number of faint galaxies which may help explain the wide image
separation. A plausible mass distribution model for CLASS B2108+213 is also
presented.Comment: 9 pages, 8 figures, accepted for publication in MNRA
CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey
The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been
completed, bringing the total number of sources imaged to over 15000 in the
CLASS and JVAS combined survey. In the VLA observations carried out in March
and April of 1998, two new candidate lensed systems were discovered: CLASS
B1152+199 and B1359+154. B1152+199 is a 1.6 arcsecond double, with a background
quasar at z=1.019 lensed by a foreground galaxy at z=0.439. The relatively flat
radio spectra of the lensed images, combined with a previous ROSAT detection of
the source, make B1152+199 a strong candidate for time delay studies at both
radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z=3.235,
with a maximum image separation of 1.7 arcseconds. As yet, the redshift of the
lensing object in this system is undetermined. The steep spectral index of the
source suggests that B1359+154 will not exhibit strong variability, and is
therefore unlikely to be useful for determining the Hubble constant from
measured time delays.Comment: accepted for publication in The Astronomical Journa
A New Quadruple Gravitational Lens System: CLASS B0128+437
High resolution MERLIN observations of a newly-discovered four-image
gravitational lens system, B0128+437, are presented. The system was found after
a careful re-analysis of the entire CLASS dataset. The MERLIN observations
resolve four components in a characteristic quadruple-image configuration; the
maximum image separation is 542 mas and the total flux density is 48 mJy at 5
GHz. A best-fit lens model with a singular isothermal ellipsoid results in
large errors in the image positions. A significantly improved fit is obtained
after the addition of a shear component, suggesting that the lensing system is
more complex and may consist of multiple deflectors. The integrated radio
spectrum of the background source indicates that it is a GigaHertz-Peaked
Spectrum (GPS) source. It may therefore be possible to resolve structure within
the radio images with deep VLBI observations and thus better constrain the
lensing mass distribution.Comment: Accepted for publication in MNRAS. 4 pages, 3 included PostScript
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