608 research outputs found
The cosmological co-evolution of supermassive black holes, AGN and galaxies
We model the cosmological co-evolution of galaxies and their central
supermassive black holes (BHs) within a semi-analytical framework developed on
the outputs of the Millennium Simulation (Croton et al., 2006; De Lucia &
Blaizot, 2007). In this work, we analyze the model BH scaling relations,
fundamental plane and mass function, and compare them with the most recent
observational data. Furthermore, we extend the original code developed by
Croton et al. (2006) to follow the evolution of the BH mass accretion and its
conversion into radiation, and compare the derived AGN bolometric luminosity
function with the observed one. We find, for the most part, a very good
agreement between predicted and observed BH properties. Moreover, the model is
in good agreement with the observed AGN number density in 0<z<5, provided it is
assumed that the cold gas fraction accreted by BHs at high redshifts is larger
than at low redshifts (Marulli et al., 2008).Comment: Proceedings of "The Central Kiloparsec: Active Galactic Nuclei and
Their Hosts", Ierapetra, Crete, 4-6 June, 2008. To appear in Volume 79 of the
Memorie della Societa' Astronomica Italiana. 5 pages, 4 figure
Effects of Large Aspect Ratios and Fluctuations on Hard X-Ray-Detection in Lower Hybrid Driven Divertor Tokamaks
It is shown that lower hybrid wave scattering from fluctuations plays a critical role in large aspect ratio divertor plasmas even through the edge density fluctuation levels are only at 1%. This is seen in the theoretically calculated electron power-density profiles which can be directly correlated to the standard experimental chordal hard x-ray profiles. It thus seems that fluctuation effects must be included in determining rf current-density profiles
Effect of Fluctuations on Lower Hybrid Power Deposition and Hard X-Ray Detection
The hard X-ray intensity radial profiles from lower hybrid current drive experiments are interpreted as being correlated with fluctuations in the bulk plasma. This view seems to be dictated by comparing the hard X-ray data for various nâ with the Monte Carlo solutions of the lower hybrid wave energy deposition on plasma electrons. Information on internal magnetic fluctuations may, under certain conditions, be unfolded from a nâ scan of the hard X-ray profiles
Effect of Magnetic and Density Fluctuations on the Propagation of Lower Hybrid Waves in Tokamaks
Lower hybrid waves have been used extensively for plasma heating, current drive, and ramp-up as well as sawteeth stabilization, The wave kinetic equation for lower hybrid wave propagation is extended to include the effects of both magnetic and density fluctuations. This integral equation is then solved by Monte Carlo procedures for a toroidal plasma. It is shown that even for magnetic/density fluctuation levels on the order of 10-4, there are significant magnetic fluctuation effects on the wave power deposition into the plasma. This effect is quite pronounced if the magnetic fluctuation spectrum is peaked within the plasma. For Alcator-C-Mod [I. H. Hutchinson and the Alcator Group, Proceedings of the IEEE 13th Symposium on Fusion Engineering (IEEE, New York, 1990), Cat. No. 89CH 2820-9, p. 13] parameters, it seems possible to be able to infer information on internal magnetic fluctuations from hard x-ray data-especially since the effects of fluctuations on electron power density can explain the hard x-ray data from the JT-60 tokamak [H. Kishimoto and JT-60 Team, in Plasma Physics and Controlled Fusion (International Atomic Energy Agency, Vienna, 1989), Vol. I, p. 67]
Perpendicular momentum injection by lower hybrid wave in a tokamak
The injection of lower hybrid waves for current drive into a tokamak affects
the profile of intrinsic rotation. In this article, the momentum deposition by
the lower hybrid wave on the electrons is studied. Due to the increase in the
poloidal momentum of the wave as it propagates into the tokamak, the parallel
momentum of the wave increases considerably. The change of the perpendicular
momentum of the wave is such that the toroidal angular momentum of the wave is
conserved. If the perpendicular momentum transfer via electron Landau damping
is ignored, the transfer of the toroidal angular momentum to the plasma will be
larger than the injected toroidal angular momentum. A proper quasilinear
treatment proves that both perpendicular and parallel momentum are transferred
to the electrons. The toroidal angular momentum of the electrons is then
transferred to the ions via different mechanisms for the parallel and
perpendicular momentum. The perpendicular momentum is transferred to ions
through an outward radial electron pinch, while the parallel momentum is
transferred through collisions.Comment: 22 pages, 4 figure
2MASS NIR photometry for 693 candidate globular clusters in M31 and the Revised Bologna Catalogue
We have identified in the 2MASS database 693 known and candidate globular
clusters in M31. The 2MASS J,H,K magnitudes of these objects have been
transformed to the same homogeneous photometric system of existing near
infrared photometry of M31 globulars, finally yielding J,H,K integrated
photometry for 279 confirmed M31 clusters, 406 unconfirmed candidates and 8
objects with controversial classification. Of these objects 529 lacked any
previous estimate of their near infrared magnitudes. The newly assembled near
infrared dataset has been implemented into a revised version of the Bologna
Catalogue of M31 globulars, with updated optical (UBVRI) photometry taken, when
possible, from the most recent sources of CCD photometry available in the
literature and transformed to a common photometric system. The final Revised
Bologna Catalogue (available in electronic form) is the most comprehensive list
presently available of confirmed and candidate M31 globular clusters, with a
total of 1164 entries. In particular, it includes 337 confirmed GCs, 688 GC
candidates, 10 objects with controversial classification, 70 confirmed
galaxies, 55 confirmed stars, and 4 HII regions lying within ~3 deg. from the
center of the M31 galaxy. Using the newly assembled database we show that the
V-K color provides a powerful tool to discriminate between M31 clusters and
background galaxies, and we identify a sample of 83 globular cluster
candidates, which is not likely to be contaminated by misclassified galaxies.Comment: 9 pages,5 figures,accepted for publication in Astronomy &
Astrophysics ASCII (commented) version of the tables 2,3,4 are available at
http://www.bo.astro.it/M3
The Kinematics and Metallicity of the M31 Globular Cluster System
With the ultimate aim of distinguishing between various models describing the
formation of galaxy halos (e.g. radial or multi-phase collapse, random
mergers), we have completed a spectroscopic study of the globular cluster
system of M31. We present the results of deep, intermediate-resolution,
fibre-optic spectroscopy of several hundred of the M31 globular clusters using
the Wide Field Fibre Optic Spectrograph (WYFFOS) at the William Herschel
Telescope in La Palma, Canary Islands. These observations have yielded precise
radial velocities (+/-12 km/s) and metallicities (+/-0.26 dex) for over 200
members of the M31 globular cluster population out to a radius of 1.5 degrees
from the galaxy center. Many of these clusters have no previous published
radial velocity or [Fe/H] estimates, and the remainder typically represent
significant improvements over earlier determinations. We present analyses of
the spatial, kinematic and metal abundance properties of the M31 globular
clusters. We find that the abundance distribution of the cluster system is
consistent with a bimodal distribution with peaks at [Fe/H] = -1.4 and -0.5.
The metal-rich clusters demonstrate a centrally concentrated spatial
distribution with a high rotation amplitude, although this population does not
appear significantly flattened and is consistent with a bulge population. The
metal-poor clusters tend to be less spatially concentrated and are also found
to have a strong rotation signature.Comment: 33 pages, 20 figure
Halo stochasticity in global clustering analysis
In the present work we study the statistics of haloes, which in the halo
model determines the distribution of galaxies. Haloes are known to be biased
tracer of dark matter, and at large scales it is usually assumed there is no
intrinsic stochasticity between the two fields. Following the work of Seljak &
Warren (2004), we explore how correct this assumption is and, moving a step
further, we try to qualify the nature of stochasticity. We use Principal
Component Analysis applied to the outputs of a cosmological N-body simulation
to: (1) explore the behaviour of stochasticity in the correlation between
haloes of different masses; (2) explore the behaviour of stochasticity in the
correlation between haloes and dark matter. We show results obtained using a
catalogue with 2.1 million haloes, from a PMFAST simulation with box size of
1000h^{-1}Mpc. In the relation between different populations of haloes we find
that stochasticity is not-negligible even at large scales. In agreement with
the conclusions of Tegmark & Bromley (1999) who studied the correlations of
different galaxy populations, we found that the shot-noise subtracted
stochasticity is qualitatively different from `enhanced' shot noise and,
specifically, it is dominated by a single stochastic eigenvalue. We call this
the `minimally stochastic' scenario, as opposed to shot noise which is
`maximally stochastic'. In the correlation between haloes and dark matter, we
find that stochasticity is minimized, as expected, near the dark matter peak (k
~ 0.02 h Mpc^{-1} for a LambdaCDM cosmology) and, even at large scales, it is
of the order of 15 per cent above the shot noise. Moreover, we find that the
reconstruction of the dark matter distribution is improved when we use
eigenvectors as tracers of the bias. [Abridged]Comment: 9 pages, 12 figures. Submitted to MNRA
Variable Galaxies in the Hubble Deep Field
We present results from a study to detect variable galaxies in the Hubble
Deep Field North. The goal of this project is to investigate the number density
of AGN at z=1 through the detection of variable galaxy nuclei. The advantage of
HST is the ability to do accurate photometry within smaller apertures, thus
allowing us to probe much lower AGN/host galaxy luminosity ratios than can be
done from the ground. The primary data sets analyzed for galactic variability
follow from the original HDF-N observations in December 1995 and a second epoch
obtained two years later.
We have detected nuclear variability at or above the 3 sigma level in 8 of
633 HDF galaxies at I<27. Only 2 detections would be expected by chance in a
normal distribution. At least one of these 8 has been spectroscopically
confirmed as a Seyfert 1 galaxy. Based on the AGN structure function for
variability, the estimated luminosity of the varying component in each galaxy
lies in the range -19.5<M<-15.0. We construct an upper limit to the luminosity
function for the variable nuclei and compare this to the local Seyfert LF and
the LF for QSOs at z=1. Assuming we have detected all Seyfert-like nuclei in
the HDF-N, we find no evidence for an increase in the number density of AGN at
M=-19. We estimate that ~1-3% of field galaxies with I<27 may contain a nuclear
AGN.Comment: 31 pages, 8 figures, accepted to the A
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