7,623 research outputs found
Detecting periodicity in experimental data using linear modeling techniques
Fourier spectral estimates and, to a lesser extent, the autocorrelation
function are the primary tools to detect periodicities in experimental data in
the physical and biological sciences. We propose a new method which is more
reliable than traditional techniques, and is able to make clear identification
of periodic behavior when traditional techniques do not. This technique is
based on an information theoretic reduction of linear (autoregressive) models
so that only the essential features of an autoregressive model are retained.
These models we call reduced autoregressive models (RARM). The essential
features of reduced autoregressive models include any periodicity present in
the data. We provide theoretical and numerical evidence from both experimental
and artificial data, to demonstrate that this technique will reliably detect
periodicities if and only if they are present in the data. There are strong
information theoretic arguments to support the statement that RARM detects
periodicities if they are present. Surrogate data techniques are used to ensure
the converse. Furthermore, our calculations demonstrate that RARM is more
robust, more accurate, and more sensitive, than traditional spectral
techniques.Comment: 10 pages (revtex) and 6 figures. To appear in Phys Rev E. Modified
styl
Routes of salmonellae contamination in pig lairages and the development and evaluation of simple cleaning methods
The aim of this project was to identify, and validate, the best lairage-to-stunning practices to reduce cross-contamination, and to assess the general status of the lairage hygiene and lairage cleaning effectiveness in UK abattoirs
Reconstruction of ionization probabilities from spatially averaged data in N-dimensions
We present an analytical inversion technique which can be used to recover
ionization probabilities from spatially averaged data in an N-dimensional
detection scheme. The solution is given as a power series in intensity. For
this reason, we call this technique a multiphoton expansion (MPE). The MPE
formalism was verified with an exactly solvable inversion problem in 2D, and
probabilities in the postsaturation region, where the intensity-selective
scanning approach breaks down, were recovered. In 3D, ionization probabilities
of Xe were successfully recovered with MPE from simulated (using the ADK
tunneling theory) ion yields. Finally, we tested our approach with
intensity-resolved benzene ion yields showing a resonant multiphoton ionization
process. By applying MPE to this data (which was artificially averaged) the
resonant structure was recovered-suggesting that the resonance in benzene may
have been observable in spatially averaged data taken elsewhere.Comment: 19 pages and 3 figure
Evolution of accretion disks around massive black holes: constraints from the demography of active galactic nuclei
Observations have shown that the Eddington ratios (the ratio of the
bolometric luminosity to the Eddington luminosity) in QSOs/active galactic
nuclei (AGNs) cover a wide range. In this paper we connect the demography of
AGNs obtained by the Sloan Digital Sky Survey with the accretion physics around
massive black holes and propose that the diversity in the Eddington ratios is a
natural result of the long-term evolution of accretion disks in AGNs. The
observed accretion rate distribution of AGNs (with host galaxy velocity
dispersion sigma~70-200 km/s) in the nearby universe (z<0.3) is consistent with
the predictions of simple theoretical models in which the accretion rates
evolve in a self-similar way. We also discuss the implications of the results
for the issues related to self-gravitating disks, coevolution of galaxies and
QSOs/AGNs, and the unification picture of AGNs.Comment: 18 pages, 2 figures; revised, main conclusions not changed; to appear
in ApJ, Oct., 200
Star Formation in AEGIS Field Galaxies since z=1.1 : The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-Forming Galaxies
We analyze star formation (SF) as a function of stellar mass (M*) and
redshift z in the All Wavelength Extended Groth Strip International Survey
(AEGIS). For 2905 field galaxies, complete to 10^10(10^10.8) Msun at z<0.7(1),
with Keck spectroscopic redshifts out to z=1.1, we compile SF rates (SFR) from
emission lines, GALEX, and Spitzer MIPS 24 micron photometry, optical-NIR M*
measurements, and HST morphologies. Galaxies with reliable signs of SF form a
distinct "main sequence (MS)", with a limited range of SFR at a given M* and z
(1 sigma < +-0.3 dex), and log(SFR) approximately proportional to log(M*). The
range of log(SFR) remains constant to z>1, while the MS as a whole moves to
higher SFR as z increases. The range of SFR along the MS constrains the
amplitude of episodic variations of SF, and the effect of mergers on SFR.
Typical galaxies spend ~67(95)% of their lifetime since z=1 within a factor of
<~ 2(4) of their average SFR at a given M* and z. The dominant mode of the
evolution of SF since z~1 is apparently a gradual decline of the average SFR in
most individual galaxies, not a decreasing frequency of starburst episodes, or
a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z~1 seem
to mostly reflect the high SFR typical for massive galaxies at that epoch. The
smooth MS may reflect that the same set of few physical processes governs star
formation prior to additional quenching processes. A gradual process like gas
exhaustion may play a dominant role.Comment: 5 pages, 1 figure, emulateapj; ApJ Letters, accepted; AEGIS special
issue; proof-level corrections added; title change
Quasar Luminosity Functions from Joint Evolution of Black Holes and Host Galaxies
We show that our previously proposed anti-hierarchical baryon collapse
scenario for the joint evolution of black holes and host galaxies predicts
quasar luminosity functions at redshifts 1.5<z<6 and local properties in nice
agreement with observations. In our model the quasar activity marks and
originates the transition between an earlier phase of violent and heavily
dust-enshrouded starburst activity promoting rapid black hole growth, and a
later phase of almost passive evolution; the former is traced by the
submillimeter-selected sources, while the latter accounts for the high number
density of massive galaxies at substantial redshifts z>1.5, the population of
Extremely Red Objects, and the properties of local ellipticals.Comment: 15 pages, 8 figures, uses REVTeX 4 + emulateapj.cls and apjfonts.sty.
Version revised following referee's comments. Accepted on Ap
Toward precise constraints on growth of massive black holes
Growth of massive black holes (MBHs) in galactic centers comes mainly from
gas accretion during their QSO/AGN phases. In this paper we apply an extended
Soltan argument, connecting the local MBH mass function with the time-integral
of the QSO luminosity function, to the demography of MBHs and QSOs from recent
optical and X-ray surveys, and obtain robust constraints on the luminosity
evolution (or mass growth history) of individual QSOs (or MBHs). We find that
the luminosity evolution probably involves two phases: an initial exponentially
increasing phase set by the Eddington limit and a following phase in which the
luminosity declines with time as a power law (with a slope of -1.2--1.3) set by
a self-similar long-term evolution of disk accretion. Neither an evolution
involving only the increasing phase with a single Eddington ratio nor an
exponentially declining pattern in the second phase is likely. The period of a
QSO radiating at a luminosity higher than 10% of its peak value is about
(2-3)x10^8 yr, during which the MBH obtains ~80% of its mass. The
mass-to-energy conversion efficiency is , with the
latter error accounting for the maximum uncertainty due to Compton-thick AGNs.
The expected Eddington ratios in QSOs from the constrained luminosity evolution
cluster around a single value close to 0.5-1 for high-luminosity QSOs and
extend to a wide range of lower values for low-luminosity ones. The Eddington
ratios for high luminosity QSOs appear to conflict with those estimated from
observations (~0.25) by using some virial mass estimators for MBHs in QSOs
unless the estimators systematically over-estimate MBH masses by a factor of
2-4. We also infer the fraction of optically obscured QSOs ~60-80%. Further
applications of the luminosity evolution of individual QSOs are also discussed.Comment: 25 pages, 13 figures, ApJ in pres
On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars
We consider a class of models for the redshift evolution (between 0\lsim z
\lsim 4) of the observed optical and X-ray quasar luminosity functions (LFs),
with the following assumptions: (i) the mass-function of dark matter halos
follows the Press-Schechter theory, (ii) the black hole (BH) mass scales
linearly with the halo mass, (iii) quasars have a constant universal lifetime,
and (iv) a thin accretion disk provides the optical luminosity of quasars,
while the X-ray/optical flux ratio is calibrated from a sample of observed
quasars. The mass accretion rate onto quasar BHs is a free parameter
of the models, that we constrain using the observed LFs. The accretion rate
inferred from either the optical or X-ray data under these assumptions
generally decreases as a function of cosmic time from to . We find that a comparable accretion rate is inferred from the X-ray and
optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near
, drops to substantially sub-Eddington values at which
advection-dominated accretion flows (ADAFs) exist. Such a decline of ,
possibly followed by a transition to radiatively inefficient ADAFs, could
explain both the absence of bright quasars in the local universe and the
faintness of accreting BHs at the centers of nearby galaxies. We argue that a
decline of the accretion rate of the quasar population is indeed expected in
cosmological structure formation models.Comment: Latex, 23 pages, 9 figures, accepted for publication in Ap
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