993 research outputs found
Black hole variability and the star formation-active galactic nucleus connection : do all star-forming galaxies host an active galactic nucleus?
We investigate the effect of active galactic nucleus (AGN) variability on the observed connection between star formation and black hole accretion in extragalactic surveys. Recent studies have reported relatively weak correlations between observed AGN luminosities and the properties of AGN hosts, which has been interpreted to imply that there is no direct connection between AGN activity and star formation. However, AGNs may be expected to vary significantly on a wide range of timescales (from hours to Myr) that are far shorter than the typical timescale for star formation (gsim100 Myr). This variability can have important consequences for observed correlations. We present a simple model in which all star-forming galaxies host an AGN when averaged over ~100 Myr timescales, with long-term average AGN accretion rates that are perfectly correlated with the star formation rate (SFR). We show that reasonable prescriptions for AGN variability reproduce the observed weak correlations between SFR and L AGN in typical AGN host galaxies, as well as the general trends in the observed AGN luminosity functions, merger fractions, and measurements of the average AGN luminosity as a function of SFR. These results imply that there may be a tight connection between AGN activity and SFR over galaxy evolution timescales, and that the apparent similarities in rest-frame colors, merger rates, and clustering of AGNs compared to "inactive" galaxies may be due primarily to AGN variability. The results provide motivation for future deep, wide extragalactic surveys that can measure the distribution of AGN accretion rates as a function of SFR
Coronal mass ejections as expanding force-free structures
We mode Solar coronal mass ejections (CMEs) as expanding force-fee magnetic
structures and find the self-similar dynamics of configurations with spatially
constant \alpha, where {\bf J} =\alpha {\bf B}, in spherical and cylindrical
geometries, expanding spheromaks and expanding Lundquist fields
correspondingly. The field structures remain force-free, under the conventional
non-relativistic assumption that the dynamical effects of the inductive
electric fields can be neglected. While keeping the internal magnetic field
structure of the stationary solutions, expansion leads to complicated internal
velocities and rotation, induced by inductive electric field. The structures
depends only on overall radius R(t) and rate of expansion \dot{R}(t) measured
at a given moment, and thus are applicable to arbitrary expansion laws. In case
of cylindrical Lundquist fields, the flux conservation requires that both axial
and radial expansion proceed with equal rates. In accordance with observations,
the model predicts that the maximum magnetic field is reached before the
spacecraft reaches the geometric center of a CME.Comment: 19 pages, 9 Figures, accepted by Solar Physic
On the Three-dimensional Central Moment Lattice Boltzmann Method
A three-dimensional (3D) lattice Boltzmann method based on central moments is
derived. Two main elements are the local attractors in the collision term and
the source terms representing the effect of external and/or self-consistent
internal forces. For suitable choices of the orthogonal moment basis for the
three-dimensional, twenty seven velocity (D3Q27), and, its subset, fifteen
velocity (D3Q15) lattice models, attractors are expressed in terms of
factorization of lower order moments as suggested in an earlier work; the
corresponding source terms are specified to correctly influence lower order
hydrodynamic fields, while avoiding aliasing effects for higher order moments.
These are achieved by successively matching the corresponding continuous and
discrete central moments at various orders, with the final expressions written
in terms of raw moments via a transformation based on the binomial theorem.
Furthermore, to alleviate the discrete effects with the source terms, they are
treated to be temporally semi-implicit and second-order, with the implicitness
subsequently removed by means of a transformation. As a result, the approach is
frame-invariant by construction and its emergent dynamics describing fully 3D
fluid motion in the presence of force fields is Galilean invariant. Numerical
experiments for a set of benchmark problems demonstrate its accuracy.Comment: 55 pages, 8 figure
Gating a Single Cell:A Label-Free and Real-Time Measurement Method for Cellular Progression
There is an ever-growing need for more advanced methods to study the response of cancer cells to new therapies. To determine cancer cells’ response from a cell-mortality perspective to various cancer therapies, we report a label-free and real time method to monitor the in situ response of individual HeLa cells using a single cell gated transistor (SCGT). As a cell undergoes apoptotic cell death, it experiences changes in morphology and ion concentrations. This change is well in line with the threshold voltage of the SCGT, which has been verified by correlating the data with the cell morphologies by scanning electron microscopy and the ion-concentration analysis by inductively-coupled plasma mass spectrometry (ICPMS). This SCGT could replace patch clamps to study single cell activity via direct measurement in real time. Importantly, this SCGT can be used to study the electrical response of a single cell to stimuli that leaves the membrane intact
Dynamics in Colloidal Liquids near a Crossing of Glass- and Gel-Transition Lines
Within the mode-coupling theory for ideal glass-transitions, the mean-squared
displacement and the correlation function for density fluctuations are
evaluated for a colloidal liquid of particles interacting with a square-well
potential for states near the crossing of the line for transitions to a gel
with the line for transitions to a glass. It is demonstrated how the dynamics
is ruled by the interplay of the mechanisms of arrest due to hard-core
repulsion and due to attraction-induced bond formation as well as by a nearby
higher-order glass-transition singularity. Application of the universal
relaxation laws for the slow dynamics near glass-transition singularities
explains the qualitative features of the calculated time dependence of the
mean-squared displacement, which are in accord with the findings obtained in
molecular-dynamics simulation studies by Zaccarelli et. al [Phys. Rev. E 66,
041402 (2002)]. Correlation functions found by photon-correlation spectroscopy
in a micellar system by Mallamace et. al [Phys. Rev. Lett. 84, 5431 2000)] can
be interpreted qualitatively as a crossover from gel to glass dynamics.Comment: 13 pages, 12 figure
Variation in Care of Inflammatory Bowel Diseases Patients in Crohn's and Colitis Foundation of America Partners: Role of Gastroenterologist Practice Setting in Disease Outcomes and Quality Process Measures
Background: As variation in care has previously been linked to quality, we aimed to describe variations in inflammatory bowel diseases care by gastroenterology (GI) practice setting. Methods: We performed a cross-sectional study within the Crohn's and Colitis Foundation of America Partners and used bivariate analyses to compare patient characteristics by GI practice setting (GI-academic [GIA], GI-private, or GI-other). Regression models were used to describe the effects of provider type on steroid use, disease activity, and the quality of life. Results: The study included 12,083 patients with inflammatory bowel diseases (7576 with Crohn's disease [CD] and 4507 with ulcerative colitis [UC]). Nearly 95% reported visiting a GI provider annually. Also, CD patients seen by GIA were younger, better educated, used less 5-aminosalicylate agents, and had higher biologic and immunomodulator use (P < 0.001 for all). On multivariate analysis of CD patients, GIA used less steroids when compared with GI-private (odds ratio, 0.84; 95% confidence interval, 0.67-1.06) or GI-other (odds ratio, 0.66; 95% confidence interval, 0.49-0.89). GIA patients were more likely to be in remission, have flu vaccine, and have better quality of life. UC patients seen by GIA were younger, had more hospitalizations, and previous surgery (P < 0.001 for all). No differences existed for steroid use, remission, flu vaccine, or quality of life for UC care on bivariate or multivariate analyses. Conclusions: Significant variations in care patterns and quality measures exist for CD across GI provider types, without similar variation in UC care. Interventions to reduce variations in care could improve the quality of care in CD
Constraining Very Heavy Dark Matter Using Diffuse Backgrounds of Neutrinos and Cascaded Gamma Rays
We consider multi-messenger constraints on very heavy dark matter (VHDM) from
recent Fermi gamma-ray and IceCube neutrino observations of isotropic
background radiation. Fermi data on the diffuse gamma-ray background (DGB)
shows a possible unexplained feature at very high energies (VHE), which we have
called the "VHE Excess" relative to expectations for an attenuated power law
extrapolated from lower energies. We show that VHDM could explain this excess,
and that neutrino observations will be an important tool for testing this
scenario. More conservatively, we derive new constraints on the properties of
VHDM for masses of 10^3-10^10 GeV. These generic bounds follow from cosmic
energy budget constraints for gamma rays and neutrinos that we developed
elsewhere, based on detailed calculations of cosmic electromagnetic cascades
and also neutrino detection rates. We show that combining both gamma-ray and
neutrino data is essential for making the constraints on VHDM properties both
strong and robust. In the lower mass range, our constraints on VHDM
annihilation and decay are comparable to other results; however, our
constraints continue to much higher masses, where they become relatively
stronger.Comment: 33 pages, 21 figures, accepted for publication in JCA
Implications of the Fermi-LAT diffuse gamma-ray measurements on annihilating or decaying Dark Matter
We analyze the recently published Fermi-LAT diffuse gamma-ray measurements in
the context of leptonically annihilating or decaying dark matter (DM) with the
aim to explain simultaneously the isotropic diffuse gamma-ray and the PAMELA,
Fermi and HESS (PFH) anomalous data. Five different DM
annihilation/decay channels , , , , or (the latter
two via an intermediate light particle ) are generated with PYTHIA. We
calculate both the Galactic and extragalactic prompt and inverse Compton (IC)
contributions to the resulting gamma-ray spectra. To find the Galactic IC
spectra we use the interstellar radiation field model from the latest release
of GALPROP. For the extragalactic signal we show that the amplitude of the
prompt gamma-emission is very sensitive to the assumed model for the
extragalactic background light. For our Galaxy we use the Einasto, NFW and
Isothermal DM density profiles and include the effects of DM substructure
assuming a simple subhalo model. Our calculations show that for the
annihilating DM the extragalactic gamma-ray signal can dominate only if rather
extreme power-law concentration-mass relation is used, while more
realistic relations make the extragalactic component comparable or
subdominant to the Galactic signal. For the decaying DM the Galactic signal
always exceeds the extragalactic one. In the case of annihilating DM the PFH
favored parameters can be ruled out only if power-law relation is
assumed. For DM decaying into or the PFH favored DM parameters
are not in conflict with the Fermi gamma-ray data. We find that, due to the
(almost) featureless Galactic IC spectrum and the DM halo substructure,
annihilating DM may give a good simultaneous fit to the isotropic diffuse
gamma-ray and to the PFH data without being in clear conflict with the
other Fermi-LAT gamma-ray measurements.Comment: Accepted for publication in JCAP, added missing references, new Figs.
9 \& 10, 35 page
Numerical simulations of a non-commutative theory: the scalar model on the fuzzy sphere
We address a detailed non-perturbative numerical study of the scalar theory
on the fuzzy sphere. We use a novel algorithm which strongly reduces the
correlation problems in the matrix update process, and allows the investigation
of different regimes of the model in a precise and reliable way. We study the
modes associated to different momenta and the role they play in the ``striped
phase'', pointing out a consistent interpretation which is corroborated by our
data, and which sheds further light on the results obtained in some previous
works. Next, we test a quantitative, non-trivial theoretical prediction for
this model, which has been formulated in the literature: The existence of an
eigenvalue sector characterised by a precise probability density, and the
emergence of the phase transition associated with the opening of a gap around
the origin in the eigenvalue distribution. The theoretical predictions are
confirmed by our numerical results. Finally, we propose a possible method to
detect numerically the non-commutative anomaly predicted in a one-loop
perturbative analysis of the model, which is expected to induce a distortion of
the dispersion relation on the fuzzy sphere.Comment: 1+36 pages, 18 figures; v2: 1+55 pages, 38 figures: added the study
of the eigenvalue distribution, added figures, tables and references, typos
corrected; v3: 1+20 pages, 10 eps figures, new results, plots and references
added, technical details about the tests at small matrix size skipped,
version published in JHE
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