1,815 research outputs found
Linear statistics for zeros of Riemann's zeta function
We consider a smooth counting function of the scaled zeros of the Riemann
zeta function, around height T. We show that the first few moments tend to the
Gaussian moments, with the exact number depending on the statistic considered
Testing the Modern Merger Hypothesis via the Assembly of Massive Blue Elliptical Galaxies in the Local Universe
The modern merger hypothesis offers a method of forming a new elliptical
galaxy through merging two equal-mass, gas-rich disk galaxies fuelling a
nuclear starburst followed by efficient quenching and dynamical stabilization.
A key prediction of this scenario is a central concentration of young stars
during the brief phase of morphological transformation from highly-disturbed
remnant to new elliptical galaxy. To test this aspect of the merger hypothesis,
we use integral field spectroscopy to track the stellar Balmer absorption and
4000\AA\ break strength indices as a function of galactic radius for 12 massive
(), nearby (),
visually-selected plausible new ellipticals with blue-cloud optical colours and
varying degrees of morphological peculiarities. We find that these index values
and their radial dependence correlate with specific morphological features such
that the most disturbed galaxies have the smallest 4000\AA\ break strengths and
the largest Balmer absorption values. Overall, two-thirds of our sample are
inconsistent with the predictions of the modern merger hypothesis. Of these
eight, half exhibit signatures consistent with recent minor merger
interactions. The other half have star formation histories similar to local,
quiescent early-type galaxies. Of the remaining four galaxies, three have the
strong morphological disturbances and star-forming optical colours consistent
with being remnants of recent, gas-rich major mergers, but exhibit a weak,
central burst consistent with forming of their stars. The final
galaxy possesses spectroscopic signatures of a strong, centrally-concentrated
starburst and quiescent core optical colours indicative of recent quenching
(i.e., a post-starburst signature) as prescribed by the modern merger
hypothesis.Comment: 25 pages, 37 figures, accepted to MNRA
Extragalactic Radio Sources and the WMAP Cold Spot
We detect a dip of 20-45% in the surface brightness and number counts of NVSS
sources smoothed to a few degrees at the location of the WMAP cold spot. The
dip has structure on scales of approximately 1-10 degrees. Together with
independent all-sky wavelet analyses, our results suggest that the dip in
extragalactic brightness and number counts and the WMAP cold spot are
physically related, i.e., that the coincidence is neither a statistical anomaly
nor a WMAP foreground correction problem. If the cold spot does originate from
structures at modest redshifts, as we suggest, then there is no remaining need
for non-Gaussian processes at the last scattering surface of the CMB to explain
the cold spot. The late integrated Sachs-Wolfe effect, already seen
statistically for NVSS source counts, can now be seen to operate on a single
region. To create the magnitude and angular size of the WMAP cold spot requires
a ~140 Mpc radius completely empty void at z<=1 along this line of sight. This
is far outside the current expectations of the concordance cosmology, and adds
to the anomalies seen in the CMB.Comment: revised version, ApJ, in pres
Mock-Gaussian Behaviour for Linear Statistics of Classical Compact Groups
We consider the scaling limit of linear statistics for eigenphases of a
matrix taken from one of the classical compact groups. We compute their moments
and find that the first few moments are Gaussian, whereas the limiting
distribution is not. The precise number of Gaussian moments depends upon the
particular statistic considered
A Comparison of X-ray and Radio Emission from the Supernova Remnant Cassiopeia A
We compare the radio and soft X-ray brightness as a function of position
within the young supernova remnant Cassiopeia A. A moderately strong
correlation (r = 0.7) was found between the X-ray emission (corrected for
interstellar absorption) and radio emission, showing that the thermal and
relativistic plasmas occupy the same volumes and are regulated by common
underlying parameters. The logarithmic slope of the relationship, ln(Sx-ray) =
1.2 x Sradio + ln(k) implies that the variations in brightness are primarily
due to path length differences. The X-ray and radio emissivities are both high
in the same general locations, but their more detailed relationship is poorly
constrained and probably shows significant scatter. The strongest radio and
X-ray absorption is found at the western boundary of Cas A. Based on the
properties of Cas A and the absorbing molecular cloud, we argue that they are
physically interacting. We also compare ASCA derived column densities with 21
cm H I and 18 cm OH optical depths in the direction of Cas A, in order to
provide an independent estimate of ISM properties. We derive an average value
for the H I spin temperature of about 40 K and measure the ratio of OH to
molecular hydrogen to be nominally larger than previous estimates.
Keywords: Cas A, Cassiopeia A, interstellar medium, molecular clouds, radio
astronomy, supernova remnants, X-ray astronomyComment: To appear in Vol. 446 of The Astrophysical Journal on Aug. 1, 1996;
10 pages with 5 embedded figures; replaced because of updated reference
Reaction-Diffusion Process Driven by a Localized Source: First Passage Properties
We study a reaction-diffusion process that involves two species of atoms,
immobile and diffusing. We assume that initially only immobile atoms, uniformly
distributed throughout the entire space, are present. Diffusing atoms are
injected at the origin by a source which is turned on at time t=0. When a
diffusing atom collides with an immobile atom, the two atoms form an immobile
stable molecule. The region occupied by molecules is asymptotically spherical
with radius growing as t^{1/d} in d>=2 dimensions. We investigate the survival
probability that a diffusing atom has not become a part of a molecule during
the time interval t after its injection and the probability density of such a
particle. We show that asymptotically the survival probability (i) saturates in
one dimension, (ii) vanishes algebraically with time in two dimensions (with
exponent being a function of the dimensionless flux and determined as a zero of
a confluent hypergeometric function), and (iii) exhibits a stretched
exponential decay in three dimensions.Comment: 7 pages; version 2: section IV is re-written, references added, 8
pages (final version
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