1,065 research outputs found
Space and lunar-based optical telescopes
The growth of space observatories, especially at optical wavelengths, during the next several decades is considered. It is concluded that large aperture optical telescopes on the Moon, possibly constructed of lunar glasses, will be very competitive with and in some instances superior to Earth orbiting telescopes
Accretion shock geometries in the magnetic variables
The first self consistent shock models for the AM Herculis-type systems successfully identified the dominant physical processes and their signatures. These homogenous shock models predict unpolarized, Rayleigh-Jeans optical spectra with sharp cutoffs and rising polarizations as the shocks become optically thin in the ultraviolet. However, the observed energy distributions are generally flat with intermediate polarizations over a broad optical band. These and other observational evidence support a non-homogenous accretion profile which may extend over a considerable fraction of the stellar surface. Both the fundamental assumptions underlying the canonical 1-D shock model and the extension of this model to inhomogenous accretion shocks were identified, for both radial and linear structures. The observational evidence was also examined for tall shocks and little evidence was found for relative shock heights in excess of h/R(1) greater than or equal to 0.1. For several systems, upper limits to the shock height can be obtained from either x ray or optical data. These lie in the region h/R(1) is approximately 0.01 and are in general agreement with the current physical picture for these systems. The quasi-periodic optical variations observed in several magnetic variables may eventually prove to be a major aid in further understanding their accretion shock geometries
The Lyman Break Galaxies: their Progenitors and Descendants
We study the evolution of Lyman Break Galaxies (LBGs) from z=5 to z=0 by
tracing the merger trees of galaxies in a large-scale hydrodynamic simulation
based on a Lambda cold dark matter model. In particular, we emphasize on the
range of properties of the sample selected by the rest-frame V band luminosity,
in accordance with recent near-IR observations. The predicted rest-frame V band
luminosity function agrees well with the observed one when dust extinction is
taken into account. The stellar content and the star formation histories of
LBGs are also studied. We find that the LBGs intrinsically brighter than
Mv=-21.0 at z=3 have stellar masses of at least 10^9\Msun, with a median of
10^{10}h^{-1}\Msun. The brightest LBGs (Mv<-23) at z=3 merge into
clusters/groups of galaxies at z=0, as suggested from clustering studies of
LBGs. Roughly one half of the galaxies with -23<Mv<-22 at z=3 fall into
groups/clusters, and the other half become typical L* galaxies at z=0 with
stellar mass of ~10^{11}\Msun. Descendants of LBGs at the present epoch have
formed roughly 30% of their stellar mass by z=3, and the half of their current
stellar population is 10 Gyr old, favoring the scenario that LBGs are the
precursors of the present day spheroids. We find that the most luminous LBGs
have experienced a starburst within 500 Myr prior to z=3, but also have formed
stars continuously over a period of 1 Gyr prior to z=3 when all the star
formation in progenitors is coadded. We also study the evolution of the mean
stellar metallicity distribution of galaxies, and find that the entire
distribution shifts to lower metallicity at higher redshift. The observed
sub-solar metallicity of LBGs at z=3 is naturally predicted in our simulation.Comment: 29 pages, including 11 figures, ApJ in press. One reference adde
The Hidden Nuclear Spectrum of the Luminous IRAS Source FSC102144724
Optical spectropolarimetry of the luminous IRAS source FSC102144724
(z) reveals that the strong (\twid17\%) linear polarization detected by
Lawrence \etal\/ is shared by both the narrow UV emission lines and the
underlying continuum. This observation and the brightness of the source rule
out synchrotron emission and dichroic extinction by dust as the polarizing
mechanism, leaving scattering as the only plausible cause of the polarized
emission. The narrowness of the lines requires that the scatterers be dust
grains or cool (10~K) electrons. We can recover the spectrum
that is incident on the scattering medium provided we make some reasonable
assumptions regarding the source geometry. The scattered UV spectrum has a
power law index ~ of (), steeper
than what would be expected from a young burst of star formation, but similar
to many AGN.Comment: 10 pages, with figure, uuencoded postscript Institute for Advanced
Study number AST 94/1
From local laboratory data to public domain database in search of indirect association of diseases: AJAX based gene data search engine.
This paper presents an extensible schema for capturing laboratory gene variance data with its meta-data properties in a semi-structured environment. This paper also focuses on the issues of creating a local and task specific component database which is a subset of global data resources. An XML based genetic disorder component database schema is developed with adequate flexibilities to facilitate searching of gene mutation data. A web based search engine is developed that allows researchers to query a set of gene parameters obtained from local XML schema and subsequently allow them to automatically establish a link with the public domain gene databases. The application applies AJAX (Asynchronous Javascript and XML), a cutting-edge web technology, to carry out the gene data searching function
Detection of Extended Polarized Ultraviolet Radiation from the z = 1.82 Radio Galaxy 3C 256
We have detected spatially extended linear polarized UV emission from the
high-redshift radio galaxy 3C~256 (). A spatially integrated (
diameter aperture) measurement of the degree of polarization of the band
(rest frame 0.19 m) emission yields a value of 16.4\% (\%) with a
position angle of (),
orthogonal to the position angle on the sky of the major axis of the extended
emission. The peak emission measured with a diameter circular aperture
is 11.7\% (\%) polarized with a position angle of (). An image of the polarized flux is
presented, clearly displaying that the polarized flux is extended and present
over the entire extent of the object. While it has been suggested that the UV
continuum of 3C~256 might be due to star formation (Elston 1988) or a
protogalaxy (Eisenhardt \& Dickinson 1993) based on its extremely blue spectral
energy distribution and similar morphology at UV and visible wavelengths, we
are unable to reconcile the observed high degree of polarization with such a
model. While the detection of polarized emission from HZRGs has been shown to
be a common phenomena, 3C~256 is only the third object for which a measurement
of the extended polarized UV emission has been presented. These data lend
additional support to the suggestion first made by di Serego Alighieri and
collaborators that the ``alignment effect'', the tendency for the extended UV
continuum radiation and line emission from HZRGs to be aligned with the major
axis of the extended radio emission, is in large part due to scattering of
anisotropic nuclear emission.Comment: 11 pages, LaTeX (aaspp style) file. Figure available by request to
[email protected]
Attosecond control of electrons emitted from a nanoscale metal tip
Attosecond science is based on steering of electrons with the electric field
of well-controlled femtosecond laser pulses. It has led to, for example, the
generation of XUV light pulses with a duration in the sub-100-attosecond
regime, to the measurement of intra-molecular dynamics by diffraction of an
electron taken from the molecule under scrutiny, and to novel ultrafast
electron holography. All these effects have been observed with atoms or
molecules in the gas phase. Although predicted to occur, a strong light-phase
sensitivity of electrons liberated by few-cycle laser pulses from solids has
hitherto been elusive. Here we show a carrier-envelope (C-E) phase-dependent
current modulation of up to 100% recorded in spectra of electrons laser-emitted
from a nanometric tungsten tip. Controlled by the C-E phase, electrons
originate from either one or two sub-500as long instances within the 6-fs laser
pulse, leading to the presence or absence of spectral interference. We also
show that coherent elastic re-scattering of liberated electrons takes place at
the metal surface. Due to field enhancement at the tip, a simple laser
oscillator suffices to reach the required peak electric field strengths,
allowing attosecond science experiments to be performed at the 100-Megahertz
repetition rate level and rendering complex amplified laser systems
dispensable. Practically, this work represents a simple, exquisitely sensitive
C-E phase sensor device, which can be shrunk in volume down to ~ 1cm3. The
results indicate that the above-mentioned novel attosecond science techniques
developed with and for atoms and molecules can also be employed with solids. In
particular, we foresee sub-femtosecond (sub-) nanometre probing of (collective)
electron dynamics, such as plasmon polaritons, in solid-state systems ranging
in size from mesoscopic solids via clusters to single protruding atoms.Comment: Final manuscript version submitted to Natur
- …