1,649 research outputs found
Unravelling the Dodecahedral Spaces
The hyperbolic dodecahedral space of Weber and Seifert has a natural
non-positively curved cubulation obtained by subdividing the dodecahedron into
cubes. We show that the hyperbolic dodecahedral space has a 6-sheeted irregular
cover with the property that the canonical hypersurfaces made up of the
mid-cubes give a very short hierarchy. Moreover, we describe a 60-sheeted cover
in which the associated cubulation is special. We also describe the natural
cubulation and covers of the spherical dodecahedral space (aka Poincar\'e
homology sphere).Comment: 15 pages + 6 pages appendix, 7 figures, 4 table
Lyman-alpha absorption around nearby galaxies
We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption lines
in the outer regions of eight nearby galaxies using background QSOs and AGN as
probes. Lya lines are detected within a few hundred km/s of the systemic
velocity of the galaxy in all cases. We conclude that a background
line-of-sight which passes within 26-200 h-1 kpc of a foreground galaxy is
likely to intercept low column density neutral hydrogen with log N(HI) >~ 13.0.
The ubiquity of detections implies a covering factor of ~ 100% for low N(HI)
gas around galaxies within 200 h-1 kpc. We discuss the difficulty in trying to
associate individual absorption components with the selected galaxies and their
neighbors, but show that by degrading our STIS data to lower resolutions, we
are able to reproduce the anti-correlation of Lya equivalent width and impact
parameter found at higher redshift. We also show that the equivalent width and
column density of Lya complexes (when individual components are summed over ~
1000 km/s) correlate well with a simple estimate of the volume density of
galaxies brighter than M(B) = -17.5 at the same redshift as a Lya complex. We
do not reject the hypothesis that the selected galaxies are directly
responsible for the observed Lya lines, but our analysis indicates that
absorption by clumpy intragroup gas is an equally likely explanation. (Abriged)Comment: Accepted for publication in Nov 20, 2002 issue of ApJ. Paper with all
figures can be found at http://www.astro.princeton.edu/~dvb/lyapaper.ps
(preferable). Minor typos fixe
Generating functions for generalized binomial distributions
In a recent article a generalization of the binomial distribution associated
with a sequence of positive numbers was examined. The analysis of the
nonnegativeness of the formal expressions was a key-point to allow to give them
a statistical interpretation in terms of probabilities. In this article we
present an approach based on generating functions that solves the previous
difficulties: the constraints of nonnegativeness are automatically fulfilled, a
complete characterization in terms of generating functions is given and a large
number of analytical examples becomes available.Comment: PDFLaTex, 27 pages, 5 figure
Quasi-molecular lines in Lyman wings of cool DA white dwarfs; Application to FUSE observations of G231-40
We present new theoretical calculations of the total line profiles of Lyman
alpha and Lyman beta which include perturbations by both neutral hydrogen AND
protons and all possible quasi-molecular states of H_2 and H_2^+. They are used
to improve theoretical modeling of synthetic spectra for cool DA white dwarfs.
We compare them with FUSE observation of G231-40. The appearance of the line
wings between Lyman alpha and Lyman beta is shown to be sensitive to the
relative abundance of hydrogen ions and neutral atoms, and thereby to provide a
temperature diagnostic for stellar atmospheres and laboratory plasmas.Comment: 6 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Cyclic Statistics In Three Dimensions
While 2-dimensional quantum systems are known to exhibit non-permutation,
braid group statistics, it is widely expected that quantum statistics in
3-dimensions is solely determined by representations of the permutation group.
This expectation is false for certain 3-dimensional systems, as was shown by
the authors of ref. [1,2,3]. In this work we demonstrate the existence of
``cyclic'', or , {\it non-permutation group} statistics for a system of n
> 2 identical, unknotted rings embedded in . We make crucial use of a
theorem due to Goldsmith in conjunction with the so called Fuchs-Rabinovitch
relations for the automorphisms of the free product group on n elements.Comment: 13 pages, 1 figure, LaTex, minor page reformattin
Chandra Discovery of a 300 kpc X-ray Jet in the GPS Quasar PKS1127-145
We have discovered an X-ray jet with Chandra imaging of the z=1.187
radio-loud quasar PKS1127-145. In this paper we present the Chandra X-ray data,
follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray
jet contains 273+/-5 net counts in 27ksec and extends ~30 arcsec, from the
quasar core, corresponding to a minimum projected linear size of ~330/h_50 kpc.
The evaluation of the X-ray emission processes is complicated by the observed
offsets between X-ray and radio brightness peaks. We discuss the problems posed
by these observations to jet models. In addition, PKS1127-145 is a Giga-Hertz
Peaked Spectrum radio source, a member of the class of radio sources suspected
to be young or ``frustrated'' versions of FRI radio galaxies. However the
discovery of an X-ray and radio jet extending well outside the host galaxy of
PKS1127-145 suggests that activity in this and other GPS sources may be
long-lived and complex.Comment: 22 pages, 11 ps figures, 1 figure in a JPG file, 3 tables. AASTEX.
Accepted by The Astrophysical Journa
XMM-Newton observation of the Lockman Hole II. Spectral analysis
We present the results of the X-ray spectral analysis of the first deep X-ray
survey with the XMM-Newton observatory during Performance Verification. We
restrict the analysis to the sample of 98 sources with more than 70 net counts
(flux limit in the [0.5-7] keV band of 1.6 10^{-15} erg cm^{-2} s^{-1}) of
which 61 have redshift identification. We find no correlation between the
spectral index Gamma and the intrinsic absorption column density N_H and, for
both the Type-1 and Type-2 AGN populations, we obtain ~2. The
progressive hardening of the mean X-ray source spectrum with decreasing flux is
essentially due to an increase in intrinsic absorption. The marked separation
between the two AGN populations in several diagnostics diagrams, involving
X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is
also a consequence of different absorption column densities and enables the
classification of optically faint obscured AGN. About 27% of the subsample with
R-K' colour are EROs (R-K>5) and most of these 18 X-ray selected EROs contain
anobscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios.
There are six sources in our sample with L_X[0.5-10]>10^44 erg s^{-1} and
log(N_H)>10^22 cm^{-2}: which are likely Type-2 QSOs and we thus derive a
density of ~69 objects of this class per square degree.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 10 figure
Highly Ionized High Velocity Clouds: Intergalactic Gas in the Local Group or Distant Gas in the Galactic Halo?
We have recently identified several high velocity (V < -100 km/s) clouds in
the directions of Mrk 509 and PKS 2155-304 that have unusual ionization
properties. The clouds exhibit strong C IV absorption with little or no
detectable low ion (C II, Si II) absorption or H I 21cm emission. As the
closest known analog to the outer diffuse halos of damped Ly-alpha absorbers
and the low H I column density metal line absorption systems seen in the
spectra of high redshift quasars, these "C IV-HVCs" present unique
opportunities for relating the conditions within the Milky Way halo and nearby
intergalactic gas to the properties of galactic halos at higher redshift.
The C IV-HVCs have ionization properties consistent with photoionization by
extragalactic background radiation, though some contribution by collisional
ionization within a hot plasma cannot be ruled out. The clouds are probably low
density [n(H) ~ 10^-4 cm^-3], large [greater than several kiloparsecs], and
mostly ionized [n(HI)/n(H) ~ 10^-3] regions located well beyond the neutral gas
layer of the Galaxy. The presence of weak H I-HVCs detected through 21cm
emission near both sight lines indicates that the C IV-HVCs trace the extended,
ionized, low density regions of the H I-HVCs. Several lines of evidence,
including very low thermal pressures (P/k ~ 2 cm^-3 K), favor a location for
the C IV-HVCs in the Local Group or very distant Galactic halo.Comment: AASTEX manuscript and tables, 55 pages, 8 postscript figures.
Astrophysical Journal, in pres
Constraint structure of O(3) nonlinear sigma model revisited
We study the constraint structure of the O(3) nonlinear sigma model in the
framework of the Lagrangian, symplectic, Hamilton-Jacobi as well as the
Batalin-Fradkin-Tyutin embedding procedure.Comment: 17 page
New Results from the X-ray and Optical Survey of the Chandra Deep Field South: The 300ks Exposure
We present results from 300 ks of X-ray observations of the Chandra Deep
Field South. The field of the four combined exposures is now 0.1035 deg^2 and
we reach a flux limit of 10^{-16} erg s^{-1} cm^{-2} in the 0.5-2 keV soft band
and 10^{-15} erg s^{-1} cm^{-2} in the 2-10 keV hard band, thus a factor 2
fainter than the previous 120 ks exposure. The total catalogue is composed of
197 sources including 22 sources detected only in the hard band, 51 only in the
soft band, and 124 detected in both bands. We have now the optical spectra for
86 optical counterparts. We compute the total contribution to the X-ray
background in the 2-10 keV band, which now amounts to (1.45\pm 0.15)*10^{-11}
erg cm^{-2} s^{-1} deg^{-2} (after the inclusion of the ASCA sources to account
for the bright end) to a flux limit of 10^{-15} erg s^{-1} cm^{-2}. This
corresponds to 60-90% of the unresolved hard X-ray background (XRB), given the
uncertainties on its actual value.[ABRIDGED]Comment: 26 pages including 10 figures, ApJ accepted (scheduled for v560, Oct
10, 2001). Figure 10 replace
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