351 research outputs found
Random Matrix Theory and the Sixth Painlev\'e Equation
A feature of certain ensembles of random matrices is that the corresponding
measure is invariant under conjugation by unitary matrices. Study of such
ensembles realised by matrices with Gaussian entries leads to statistical
quantities related to the eigenspectrum, such as the distribution of the
largest eigenvalue, which can be expressed as multidimensional integrals or
equivalently as determinants. These distributions are well known to be
-functions for Painlev\'e systems, allowing for the former to be
characterised as the solution of certain nonlinear equations. We consider the
random matrix ensembles for which the nonlinear equation is the form
of \PVI. Known results are reviewed, as is their implication by way of series
expansions for the distributions. New results are given for the boundary
conditions in the neighbourhood of the fixed singularities at of
\PVI displayed by a generalisation of the generating function for the
distributions. The structure of these expansions is related to Jimbo's general
expansions for the -function of \PVI in the neighbourhood of its
fixed singularities, and this theory is itself put in its context of the linear
isomonodromy problem relating to \PVI.Comment: Dedicated to the centenary of the publication of the Painlev\'e VI
equation in the Comptes Rendus de l'Academie des Sciences de Paris by Richard
Fuchs in 190
The Motions of Clusters of Galaxies and the Dipoles of the Peculiar Velocity Field
In preceding papers of this series, TF relations for galaxies in 24 clusters
with radial velocities between 1000 and 9200 km/s (SCI sample) were obtained, a
Tully-Fisher (TF) template relation was constructed and mean offsets of each
cluster with respect to the template obtained. Here, an estimate of the
line-of-sight peculiar velocities of the clusters and their associated errors
are given. It is found that cluster peculiar velocities in the Cosmic Microwave
Background reference frame do not exceed 600 k/ms and that their distribution
has a line-of-sight dispersion of 300 k/ms, suggesting a more quiescent cluster
peculiar velocity field than previously reported. When measured in a reference
frame in which the Local Group is at rest, the set of clusters at cz > 3000
km/s exhibits a dipole moment in agreement with that of the CMB, both in
amplitude and apex direction. It is estimated that the bulk flow of a sphere of
6000 km/s radius in the CMB reference frame is between 140 and 320 km/s. These
results are in agreement with those obtained from an independent sample of
field galaxies (Giovanelli et al. 1998; see astro-ph/9807274)Comment: 9 pages, 2 tables, 7 figures, uses AAS LaTex; to appear in A
Disk Galaxies in the Outer Local Supercluster: Optical CCD Surface Photometry and Distribution of Galaxy Disk Parameters
We report new B-band CCD surface photometry on a sample of 76 disk galaxies
brighter than B_T = 14.5 mag in the Uppsala General Catalogue of Galaxies,
which are confined within a volume located in the outer part of the Local
Supercluster. With our earlier published I-band CCD and high S/N-ratio 21cm HI
data (Lu et al. 1993), this paper completes our optical surface photometry
campaign on this galaxy sample. As an application of this data set, the B-band
photometry is used here to illustrate two selection effects which have been
somewhat overlooked in the literature, but which may be important in deriving
the distribution function of disk central surface brightness (CSB) of disk
galaxies from a diameter or/and flux limited sample: a Malmquist-type bias
against disk galaxies with small disk scale lengths (DSL) at a given CSB; and a
disk inclination dependent selection effect that may, for example, bias toward
inclined disks near the threshold of a diameter limited selection if disks are
not completely opaque in optical. Taking into consideration these selection
effects, we present a method of constructing a volume sampling function and a
way to interpret the derived distribution function of CSB and DSL. Application
of this method to our galaxy sample implies that if galaxy disks are optically
thin, CSB and DSL may well be correlated in the sense that, up to an
inclination-corrected limiting CSB of about 24.5 mag per square arcsec that is
adequately probed by our galaxy sample, the DSL distribution of galaxies with a
lower CSB may have a longer tail toward large values unless the distribution of
disk galaxies as a function of CSB rises rapidly toward faint values.Comment: 27 pages including 9 figures and 2 tables. To appear in the October
20, 1998 issue of the Astrophysical Journa
The Ensemble Photometric Variability of ~25000 Quasars in the Sloan Digital Sky Survey
Using a sample of over 25000 spectroscopically confirmed quasars from the
Sloan Digital Sky Survey, we show how quasar variability in the rest frame
optical/UV regime depends upon rest frame time lag, luminosity, rest
wavelength, redshift, the presence of radio and X-ray emission, and the
presence of broad absorption line systems. The time dependence of variability
(the structure function) is well-fit by a single power law on timescales from
days to years. There is an anti-correlation of variability amplitude with rest
wavelength, and quasars are systematically bluer when brighter at all
redshifts. There is a strong anti-correlation of variability with quasar
luminosity. There is also a significant positive correlation of variability
amplitude with redshift, indicating evolution of the quasar population or the
variability mechanism. We parameterize all of these relationships. Quasars with
RASS X-ray detections are significantly more variable (at optical/UV
wavelengths) than those without, and radio loud quasars are marginally more
variable than their radio weak counterparts. We find no significant difference
in the variability of quasars with and without broad absorption line troughs.
Models involving multiple discrete events or gravitational microlensing are
unlikely by themselves to account for the data. So-called accretion disk
instability models are promising, but more quantitative predictions are needed.Comment: 41 pages, 21 figures, AASTeX, Accepted for publication in Ap
Hubble Space Telescope Observations of Field Ultracool Dwarfs at High Galactic Latitude
We present a sample of 17 newly discovered ultracool dwarf candidates later
than ~M8, drawn from 231.90 arcmin2 of {\it Hubble Space Telescope} Wide Field
Camera 3 infrared imaging. By comparing the observed number counts for
17.5<J_125<25.5 AB mag to an exponential disk model, we estimate a vertical
scale height of z_scl=290 +- 25 (random) +- 30 (systematic) pc for a binarity
fraction of f_b=0. While our estimate is roughly consistent with published
results, we suggest that the differences can be attributed to sample
properties, with the present sample containing far more substellar objects than
previous work. We predict the object counts should peak at J_{125}~24 AB mag
due to the exponentially-declining number density at the edge of the disc. We
conclude by arguing that trend in scale height with spectral type may breakdown
for brown dwarfs since they do not settle onto the main sequence.Comment: 9 pages, 6 figures, 3 tables, accepted to ApJ (v2 is consistent with
Referee changes
The Palomar Testbed Interferometer Calibrator Catalog
The Palomar Testbed Interferometer (PTI) archive of observations between 1998
and 2005 is examined for objects appropriate for calibration of optical
long-baseline interferometer observations - stars that are predictably
point-like and single. Approximately 1,400 nights of data on 1,800 objects were
examined for this investigation. We compare those observations to an
intensively studied object that is a suitable calibrator, HD217014, and
statistically compare each candidate calibrator to that object by computing
both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis
is that the frequency distribution of visibility data associated with
calibrator stars differs from non-calibrator stars such as binary stars.
Spectroscopic binaries resolved by PTI, objects known to be unsuitable for
calibrator use, are similarly tested to establish detection limits of this
approach. From this investigation, we find more than 350 observed stars
suitable for use as calibrators (with an additional being
rejected), corresponding to sky coverage for PTI. This approach
is noteworthy in that it rigorously establishes calibration sources through a
traceable, empirical methodology, leveraging the predictions of spectral energy
distribution modeling but also verifying it with the rich body of PTI's on-sky
observations.Comment: 100 pages, 7 figures, 7 tables; to appear in the May 2008ApJS, v176n
Long-term Optical Variability of Radio-Selected Quasars from the FIRST Survey
We have obtained single-epoch optical photometry for 201 quasars, taken from
the FIRST Bright Quasar Survey, which span a wide range in radio loudness.
Comparison with the magnitudes of these objects on the POSS-I plates provides
by far the largest sample of long-term variability amplitudes for
radio-selected quasars yet produced. We find the quasars to be more variable in
the blue than in the red band, consistent with work on optically selected
samples. The previously noted trend of decreasing variability with increasing
optical luminosity applies only to radio-quiet objects. Furthermore, we do not
confirm a rise in variability amplitude with redshift, nor do we see any
dependence on radio flux or luminosity. The variability over a radio-optical
flux ratio range spanning a factor of 60,000 from radio-quiet to extreme
radio-loud objects is largely constant, although there is a suggestion of
greater variability in the extreme radio-loud objects. We demonstrate the
importance of Malmquist bias in variability studies, and develop a procedure to
correct for the bias in order to reveal the underlying variability properties
of the sample.Comment: 16 pages, 8 figures To be published in 2001 April 1 Astronomical
Journa
Starcounts Redivivus. IV. Density Laws Through Photometric Parallaxes
In an effort to more precisely define the spatial distribution of Galactic
field stars, we present an analysis of the photometric parallaxes of 70,000
stars covering nearly 15 square degrees in seven Kapteyn Selected Areas. We
address the affects of Malmquist Bias, subgiant/giant contamination,
metallicity and binary stars upon the derived density laws. The affect of
binary stars is the most significant. We find that while the disk-like
populations of the Milky Way are easily constrained in a simultaneous analysis
of all seven fields, no good simultaneous solution for the halo is found. We
have applied halo density laws taken from other studies and find that the
Besancon flattened power law halo model (c/a=0.6, r^-2.75) produces the best
fit to our data. With this halo, the thick disk has a scale height of 750 pc
with an 8.5% normalization to the old disk. The old disk scale height is
280-300 pc. Corrected for a binary fraction of 50%, these scale heights are 940
pc and 350-375 pc, respectively. Even with this model, there are systematic
discrepancies between the observed and predicted density distributions. Our
model produces density overpredictions in the inner Galaxy and density
underpredictions in the outer Galaxy. A possible solution is modeling the
stellar halo as a two-component system in which the halo has a flattened inner
distribution and a roughly spherical, but substructured outer distribution.
Further reconciliation could be provided by a flared thick disk, a structure
consistent with a merger origin for that population. (Abridged)Comment: 66 pages, accepted to Astrophysical journal, some figures compresse
On the cosmic evolution of the scaling relations between black holes and their host galaxies: Broad Line AGN in the zCOSMOS survey
(Abriged) We report on the measurement of the rest frame K-band luminosity
and total stellar mass of the hosts of 89 broad line Active Galactic Nuclei
detected in the zCOSMOS survey in the redshift range 1<z<2.2. The unprecedented
multiwavelength coverage of the survey field allows us to disentangle the
emission of the host galaxy from that of the nuclear black hole in their
Spectral Energy Distributions. We derive an estimate of black hole masses
through the analysis of the broad Mg II emission lines observed in the
medium-resolution spectra taken with VIMOS/VLT as part of the zCOSMOS project.
We found that, as compared to the local value, the average black hole to host
galaxy mass ratio appears to evolve positively with redshift, with a best fit
evolution of the form (1+z)^{0.68 \pm0.12 +0.6 -0.3}, where the large
asymmetric systematic errors stem from the uncertainties in the choice of IMF,
in the calibration of the virial relation used to estimate BH masses and in the
mean QSO SED adopted. A thorough analysis of observational biases induced by
intrinsic scatter in the scaling relations reinforces the conclusion that an
evolution of the MBH-M* relation must ensue for actively growing black holes at
early times: either its overall normalization, or its intrinsic scatter (or
both) appear to increase with redshift. This can be interpreted as signature of
either a more rapid growth of supermassive black holes at high redshift, a
change of structural properties of AGN hosts at earlier times, or a significant
mismatch between the typical growth times of nuclear black holes and host
galaxies.Comment: 47 pages, 8 figures. Accepted for publication in Ap
Evaluando el progreso de la eficiencia con tecnología en una cadena de hoteles española
This paper analyzes the changes in the total factor productivity index of a Spanish hotel chain in the
period from 2007 to 2010 with the purpose of identifying efficiency patterns for the chain in a period of
financial crisis. The data envelopment analysis (DEA) Malmquist productivity index was used to estimate
productivity change in 38 hotels of the AC chain. Results reveal AC hotels’ efficiency trends and,
therefore, their competitiveness in the recession period; they also show the changes experienced in
these hotels’ total productivity and its components: technological and efficiency changes. Positive
efficiency changes were due to positive technical efficiency rather than technological efficiency. The
recession period certainly influenced the performance of AC Hotels, which focused on organizational
changes rather than investing in technology.Este artigo analisa as mudanças no fator total de produtividade de uma cadeia de hotéis na Espanha,
no período de 2007-2010, com o propósito de identificar os padrões da cadeia em um período
de crise financeira. O índice data envelopment analysis (DEA) Malmquist de produtividade foi usado
para estimar a mudança da produtividade nos 38 hotéis da AC Cadeia de Hotéis. Os resultados revelaram
as tendências de eficiência e competitividade da AC Hotéis em um período de recessão, bem
como as mudanças vivenciadas na produtividade total e, consequentemente, em seus componentes
de eficiência e tecnológicos. O período de recessão influenciou, sem dúvida, o comportamento da AC
Hotéis, que buscou mais mudanças organizacionais do que tecnológicas.Este artículo analiza los cambios del índice de productividad del factor total de una cadena de hoteles
españoles en el periodo de 2007 hasta 2010, con el propósito de identificar patrones de eficiencia
para la cadena en un periodo de crisis financiera. El índice de productividad data envelopment analysis
(DEA) Malmquist fue utilizado para estimar el cambio de productividad en 38 hoteles de la cadena
AC. Los resultados revelan las tendencias de la eficiencia de los hoteles AC y, por lo tanto, su competitividad
en el periodo de recisión; ellos también demuestran los cambios experimentados en la
productividad total de eses hoteles y sus componentes: cambios de eficiencia y tecnológicos. Cambios
de eficiencia positivos se debieron más bien a eficiencias técnicas positivas que a eficiencias tecnológicas.
El periodo de recesión ciertamente ha influenciado los Hoteles AC, que enfocaron más en los
cambios organizacionales que en invirtiendo en tecnología
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