459 research outputs found
The Evolution and Structure of Early-type Field Galaxies: A Combined Statistical Analysis of Gravitational Lenses
We introduce a framework for simultaneously investigating the structure and
luminosity evolution of early-type gravitational lens galaxies. The method is
based on the fundamental plane, which we interpret using the aperture
mass-radius relations derived from lensed image geometries. We apply this
method to our previous sample of 22 lens galaxies with measured redshifts and
excellent photometry. Modeling the population with a single mass profile and
evolutionary history, we find that early-type galaxies are nearly isothermal
(logarithmic density slope n = 2.06 +/- 0.17, 68% C.L.), and that their stars
evolve at a rate of dlog(M/L)_B/dz = -0.50 +/- 0.19 (68% C.L.) in the rest
frame B band. For a Salpeter IMF and a concordance cosmology, this implies a
mean star formation redshift of > 1.5 at 95% confidence. While this model
can neatly describe the mean properties of early-type galaxies, it is clear
that the scatter of the lens sample is too large to be explained by
observational uncertainties alone. We therefore consider statistical models in
which the galaxy population is described by a distribution of star formation
redshifts. We find that stars must form over a significant range of redshifts
(Delta z_f > 1.7, 68% C.L.), which can extend as low as z_f = 1 for some
acceptable models. However, the typical galaxy will still have an old stellar
population ( > 1.5). The lens sample therefore favors early star formation
in field ellipticals -- even if we make no a priori assumption regarding the
shape of the mass distribution in lenses, and include the range of possible
deviations from homology in the uncertainties. Our evolution results call into
question several recent claims that early-type galaxies in low-density
environments have much younger stars than those in rich clusters.Comment: 36 pages including 6 figures, (re-)submitted to Ap
CFHT AO Imaging of the CLASS Gravitational Lens System B1359+154
We present adaptive optics imaging of the CLASS gravitational lens system
B1359+154 obtained with the Canada-France-Hawaii Telescope (CFHT) in the
infrared K band. The observations show at least three brightness peaks within
the ring of lensed images, which we identify as emission from multiple lensing
galaxies. The results confirm the suspected compound nature of the lens, as
deduced from preliminary mass modeling. The detection of several additional
nearby galaxies suggests that B1359+154 is lensed by the compact core of a
small galaxy group. We attempted to produce an updated lens model based on the
CFHT observations and new 5-GHz radio data obtained with the MERLIN array, but
there are too few constraints to construct a realistic model at this time. The
uncertainties inherent with modeling compound lenses make B1359+154 a
challenging target for Hubble constant determination through the measurement of
differential time delays. However, time delays will offer additional
constraints to help pin down the mass model. This lens system therefore
presents a unique opportunity to directly measure the mass distribution of a
galaxy group at intermediate redshift.Comment: 12 pages including 3 figures; ApJL accepte
Zitterbewegung of relativistic electrons in a magnetic field and its simulation by trapped ions
One-electron 3+1 and 2+1 Dirac equations are used to calculate the motion of
a relativistic electron in a vacuum in the presence of an external magnetic
field. First, calculations are carried on an operator level and exact
analytical results are obtained for the electron trajectories which contain
both intraband frequency components, identified as the cyclotron motion, as
well as interband frequency components, identified as the trembling motion
(Zitterbewegung, ZB). Next, time-dependent Heisenberg operators are used for
the same problem to compute average values of electron position and velocity
employing Gaussian wave packets. It is shown that the presence of a magnetic
field and the resulting quantization of the energy spectrum has pronounced
effects on the electron Zitterbewegung: it introduces intraband frequency
components into the motion, influences all the frequencies and makes the motion
stationary (not decaying in time) in case of the 2+1 Dirac equation. Finally,
simulations of the 2+1 Dirac equation and the resulting electron ZB in the
presence of a magnetic field are proposed and described employing trapped ions
and laser excitations. Using simulation parameters achieved in recent
experiments of Gerritsma and coworkers we show that the effects of the
simulated magnetic field on ZB are considerable and can certainly be observed.Comment: 19 pages, 9 figures, published versio
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
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