7,700 research outputs found
The Transverse Proximity Effect: A Probe to the Environment, Anisotropy, and Megayear Variability of QSOs
The transverse proximity effect is the expected decrease in the strength of
the Lya forest absorption in a QSO spectrum when another QSO lying close to the
line of sight enhances the photoionization rate above that due to the average
cosmic ionizing background. We select three QSOs from the Early Data Release of
the Sloan Digital Sky Survey that have nearby foreground QSOs, with proper line
of sight tangential separations of 0.50, 0.82, and 1.10 h^{-1} Mpc. We estimate
that the ionizing flux from the foreground QSO should increase the
photoionization rate by a factor (94, 13, 13) in these three cases, which would
be clearly detectable in the first QSO and marginally so in the other two. We
do not detect the transverse proximity effect. Three possible explanations are
provided: an increase of the gas density in the vicinity of QSOs, time
variability, and anisotropy of the QSO emission. We find that the increase of
gas density near QSOs can be important if they are located in the most massive
halos present at high redshift, but is not enough to fully explain the absence
of the transverse proximity effect. Anisotropy requires an unrealistically
small opening angle of the QSO emission. Variability demands that the
luminosity of the QSO with the largest predicted effect was much lower 10^6
years ago, whereas the transverse proximity effect observed in the HeII Lya
absorption in QSO 0302-003 by Jakobsen et al. (2003) implies a lifetime longer
than 10^7 years. A combination of all three effects may better explain the lack
of Lya absorption reduction. A larger sample of QSO pairs may be used to
diagnose the environment, anisotropy and lifetime distribution of QSOs.Comment: 27 pages, 13 figures, accepted by Ap
Communications Biophysics
Contains reports on two research projects.National Science Foundation (Grant G-16526)National Institutes of Health (Grant MH-04737-03)National Aeronautics and Space Administration (Grant NsG-496
Comment on "Giant absorption cross section of ultracold neutrons in Gadolinium"
Rauch et al (PRL 83, 4955, 1999) have compared their measurements of the Gd
cross section for Ultra-cold neutrons with an exptrapolation of the cross
section for thermal neutrons and interpreted the discrepancy in terms of
coherence properties of the neutron. We show the extrapolation used is based on
a misunderstanding and that coherence properties play no role in absorption.Comment: 2 pages, 1 postscript figure, comment on Rauch et al, PRL 83,4955
(1999
The metal absorption systems of the Hubble Deep Field South QSO
The Hubble Deep Field South (HDFS) has been recently selected and the
observations are planned for October 1998. We present a high resolution (FWHM
\kms) spectrum of the quasar J2233--606 () which
is located 5.1 arcmin East of the HDFS. The spectrum obtained with the New
Technology Telescope redward of the Lyman-- emission line covers the
spectral range 4386--8270 \AA. This range corresponds to redshift intervals for
CIV and MgII intervening systems of and
respectively. The data reveal the presence of two complex intervening CIV
systems at redshift and and two complex associated
() systems. Other two CIV systems at and
, suggested by the presence of strong Lyman-- lines in low
resolution ground based and Hubble Space Telescope (HST) STIS observations
(Sealey et al. 1998) have been identified. The system at is also
responsible for the Lyman limit absorption seen in the HST/STIS spectrum. The
main goal of the present work is to provide astronomers interested in the
Hubble Deep Field South program with information related to absorbing
structures at high redshift, which are distributed along the nearby QSO line of
sight. For this purpose, the reduced spectrum, obtained from three hours of
integration time, has been released to the astronomical community.Comment: revisited version accepted for publication by Astronomical Journal;
minor changes; typographical errors corrected; results and discussion
unchange
The Properties of Field Elliptical Galaxies at Intermediate Redshift. I: Empirical Scaling Laws
We present measurements of the Fundamental Plane (FP) parameters (the
effective radius, the mean effective surface brightness, and the central
velocity dispersion) of six field elliptical galaxies at intermediate redshift.
The imaging is taken from the Medium Deep Survey of the Hubble Space Telescope,
while the kinematical data are obtained from long-slit spectroscopy using the
3.6-m ESO telescope. The Fundamental Plane appears well defined in the field
even at redshift 0.3. The data show a shift in the FP zero point with
respect to the local relation, possibly indicating modest evolution, consistent
with the result found for intermediate redshift cluster samples. The FP slopes
derived for our field data, plus other cluster ellipticals at intermediate
redshift taken from the literature, differ from the local ones, but are still
consistent with the interpretation of the FP as a result of homology, of the
virial theorem and of the existence of a relation between luminosity and mass,
. We also derive the surface brightness vs. effective
radius relation for nine galaxies with redshift up to , and data
from the literature; the evolution that can be inferred is consistent with what
is found using the FP.Comment: 17 pages, including 9 figures, MNRAS, accepte
On Teaching Discrete Mathematics to Freshman Computer Science Students
Discrete Mathematics is an inevitable part of any undergraduate computer science degree programme. However, today's computer science student typically finds this to be at best a necessary evil with which they struggle to engage. Twenty years ago, we started to address this issue seriously in our university, and we have instituted a number of innovations throughout the years which have had a positive effect on engagement and, thus, attainment. In this paper, we describe and motivate the innovations which we introduced, and provide a detailed analysis of how and why engagement and attainment levels varied over two decades as a direct result of these innovation
The Serendipitous Discovery of a Group or Cluster of young Galaxies at z=2.40 in Deep Hubble Space Telescope WFPC2 Images
We report the serendipitous discovery of a group or cluster of young galaxies
at 2.40 in a 24-orbit HST/WFPC2 exposure of the field around the weak
radio galaxy 53W002. Potential cluster members were identified on ground-based
narrow-band redshifted Ly images and confirmed via spectroscopy.
In addition to the known weak radio galaxy 53W002 at z=2.390, two other
objects were found to have excess narrow-band Ly emission at
2.40. Both have been spectroscopically confirmed, and one clearly
contains a weak AGN. They are located within one arcminute of 53W002, or
Mpc (=0.5) at 2.40, which is the physical
scale of a group or small cluster of galaxies. Profile fitting of the WFPC2
images shows that the objects are very compact, with scale lengths
0\farcs 1 (kpc), and are rather faint
(luminosities < L*), implying that they may be sub-galactic sized objects. We
discuss these results in the context of galaxy and cluster evolution and the
role that weak AGN may play in the formation of young galaxies.Comment: Accepted for publication in The Astrophysical Journal (Letters). 13
pages of gzip compressed and uuencoded PS. Figures are available at
http://www.phys.unsw.edu.au/~spd/bib.htm
Mathisson's helical motions for a spinning particle --- are they unphysical?
It has been asserted in the literature that Mathisson's helical motions are
unphysical, with the argument that their radius can be arbitrarily large. We
revisit Mathisson's helical motions of a free spinning particle, and observe
that such statement is unfounded. Their radius is finite and confined to the
disk of centroids. We argue that the helical motions are perfectly valid and
physically equivalent descriptions of the motion of a spinning body, the
difference between them being the choice of the representative point of the
particle, thus a gauge choice. We discuss the kinematical explanation of these
motions, and we dynamically interpret them through the concept of hidden
momentum. We also show that, contrary to previous claims, the frequency of the
helical motions coincides, even in the relativistic limit, with the
zitterbewegung frequency of the Dirac equation for the electron
Fullerenelike arrangements in carbon nitride thin films grown by direct ion beam sputtering
Carbon nitride (CNx) thin films were grown by direct N-2/Ar ion beam
sputtering of a graphite target at moderate substrate temperatures
(300-750 K). The resulting microstructure of the films was studied by
high-resolution transmission electron microscopy. The images showed the
presence of curved basal planes in fullerenelike arrangements. The
achievement and evolution of these microstructural features are
discussed in terms of nitrogen incorporation, film-forming flux, and
ion bombardment effects, thus adding to the understanding of the
formation mechanisms of curved graphitic structures in CNx materials.
(C) 2005 American Institute of Physics
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