1,558 research outputs found
Magnetism, coherent many-particle dynamics, and relaxation with ultracold bosons in optical superlattices
We study how well magnetic models can be implemented with ultracold bosonic
atoms of two different hyperfine states in an optical superlattice. The system
is captured by a two-species Bose-Hubbard model, but realizes in a certain
parameter regime actually the physics of a spin-1/2 Heisenberg magnet,
describing the second order hopping processes. Tuning of the superlattice
allows for controlling the effect of fast first order processes versus the
slower second order ones.
Using the density-matrix renormalization-group method, we provide the
evolution of typical experimentally available observables. The validity of the
description via the Heisenberg model, depending on the parameters of the
Hubbard model, is studied numerically and analytically. The analysis is also
motivated by recent experiments [S. Foelling et al., Nature 448, 1029 (2007);
S. Trotzky et al., Sience 319, 295 (2008)] where coherent two-particle dynamics
with ultracold bosonic atoms in isolated double wells were realized. We provide
theoretical background for the next step, the observation of coherent
many-particle dynamics after coupling the double wells. Contrary to the case of
isolated double wells, relaxation of local observables can be observed. The
tunability between the Bose-Hubbard model and the Heisenberg model in this
setup could be used to study experimentally the differences in equilibration
processes for nonintegrable and Bethe ansatz integrable models. We show that
the relaxation in the Heisenberg model is connected to a phase averaging
effect, which is in contrast to the typical scattering driven thermalization in
nonintegrable models. We discuss the preparation of magnetic groundstates by
adiabatic tuning of the superlattice parameters.Comment: 20 pages, 24 figures; minor changes, published versio
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift
Black-hole masses of the highest redshift quasars (4 <~ z <~ 6) are estimated
using a previously presented scaling relationship, derived from reverberation
mapping of nearby quasars, and compared to quasars at lower redshift. It is
shown that the central black holes in luminous z >~ 4 quasars are very massive
(>~ 10^9 solar masses). It is argued that the mass estimates of the high-z
quasars are not subject to larger uncertainties than those for nearby quasars.
Specifically, the large masses are not overestimates and the lack of similarly
large black-hole masses in the nearby Universe does not rule out their
existence at high-z. However, AGN host galaxies do not typically appear fully
formed or evolved at these early epochs. This supports scenarios in which black
holes build up mass very fast in a radiatively inefficient (or obscured) phase
relative to the stars in their galaxies. Additionally, upper envelopes of
black-hole mass of approximately 10^{10} solar masses and bolometric luminosity
of ~ 10^{48} erg/s are observed at all redshifts.Comment: 17 pages including 7 figures (5 in color) and 1 table. To appear in
ApJ, v600, January 1, 200
Probing the BLR in AGNs using time variability of associated absorption line
It is know that most of the clouds producing associated absorption in the
spectra of AGNs and quasars do not completely cover the background source
(continuum + broad emission line region, BLR). We note that the covering factor
derived for the absorption is the fraction of photons occulted by the absorbing
clouds, and is not necessarily the same as the fractional area covered. We show
that the variability in absorption lines can be produced by the changes in the
covering factor caused by the variation in the continuum and the finite light
travel time across the BLR. We discuss how such a variability can be
distinguished from the variability caused by other effects and how one can use
the variability in the covering factor to probe the BLR.Comment: 12 pages, latex(aaspp4.sty), 2 figures, (To appear in ApJ
The Entanglement Entropy of Solvable Lattice Models
We consider the spin k/2 analogue of the XXZ quantum spin chain. We compute
the entanglement entropy S associated with splitting the infinite chain into
two semi-infinite pieces. In the scaling limit, we find S ~ c_k/6
(ln(xi))+ln(g)+... . Here xi is the correlation length and c_k=3k/(k+2) is the
central charge associated with the sl_2 WZW model at level k. ln(g) is the
boundary entropy of the WZW model. Our result extends previous observations and
suggests that this is a simple and perhaps rather general way both of
extracting the central charge of the ultraviolet CFT associated with the
scaling limit of a solvable lattice model, and of matching lattice and CFT
boundary conditions.Comment: 6 pages; connection with boundary entropy of Affleck and Ludwig added
in revised version and notation slightly change
Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band
Radio astronomy is entering the era of large surveys. This paper describes
the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their
synergy with large surveys at higher frequencies (in particular in the 1-2 GHz
band) that will be possible using future facilities like Apertif or ASKAP. The
LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15,
30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of
view and of the unprecedented sensitivity of this instrument. Four topics have
been identified as drivers for these surveys covering the formation of massive
galaxies, clusters and black holes using z>6 radio galaxies as probes, the
study of the intercluster magnetic fields using diffuse radio emission and
Faraday rotation measures in galaxy clusters as probes and the study of star
formation processes in the early Universe using starburst galaxies as probes.
The fourth topic is the exploration of new parameter space for serendipitous
discovery taking advantage of the new observational spectral window open up by
LOFAR. Here, we briefly discuss the requirements of the proposed surveys to
address these (and many others!) topics as well as the synergy with other wide
area surveys planned at higher frequencies (and in particular in the 1-2 GHz
band) with new radio facilities like ASKAP and Apertif. The complementary
information provided by these surveys will be crucial for detailed studies of
the spectral shape of a variety of radio sources (down to sub-mJy sources) and
for studies of the ISM (in particular HI and OH) in nearby galaxies.Comment: to appear in the proceedings of "Panoramic Radio Astronomy:
Wide-field 1-2 GHz research on galaxy evolution", G. Heald and P. Serra eds.,
8 pages, 3 figure
The FRII Broad Line Seyfert 1 Galaxy: PKSJ 1037-2705
In this article, we demonstrate that PKSJ 1037-2705 has a weak accretion flow
luminosity, well below the Seyfert1/QSO dividing line, weak broad emission
lines (BELs) and moderately powerful FRII extended radio emission. It is one of
the few documented examples of a broad-line object in which the time averaged
jet kinetic luminosity, , is larger than the total thermal luminosity
(IR to X-ray) of the accretion flow, . The blazar nucleus dominates
the optical and near ultraviolet emission and is a strong source of hard
X-rays. The strong blazar emission indicates that the relativistic radio jet is
presently active. The implication is that even weakly accreting AGN can create
powerful jets. Kinetically dominated () broad-line objects
provide important constraints on the relationship between the accretion flow
and the jet production mechanism.Comment: To appear in ApJ November 1, 2008, v687n1 issu
Expanding Quasars and the Expansion of the Universe
The μ–z diagram (Figure 1) plots the observed internal proper motion μ versus redshift z for 32 extragalactic radio sources associated with active galactic nuclei. The observed points fall below an upper bound which decreases with redshift; there is a statistically significant anticorrelation between redshift and internal proper motion
Star formation in the hosts of GHz peaked spectrum and compact steep spectrum radio galaxies
AIMS: Search for star formation regions in the hosts of potentially young
radio galaxies (Gigahertz Peaked Spectrum and Compact Steep Spectrum sources).
METHODS: Near-UV imaging with the Hubble Space Telescope Advanced Camera for
Surveys.} RESULTS: We find near-UV light which could be the product of recent
star formation in eight of the nine observed sources, though other explanations
are not currently ruled out. The UV luminosities of the GPS and CSS sources are
similar to those of a sample of nearby large scale radio galaxies. Stellar
population synthesis models are consistent with a burst of recent star
formation occuring before the formation of the radio source. However,
observations at other wavelengths and colors are needed to definitively
establish the nature of the observed UV light. In the CSS sources 1443+77 and
1814-637 the near-UV light is aligned with and is co-spatial with the radio
source. We suggest that in these sources the UV light is produced by star
formation triggered and/or enhanced by the radio source.Comment: 14 pages, 11 figs. Accepted to A&A. Paper with high resolution images
can be found at
http://damir.iem.csic.es/extragalactic/publications/publications.htm
Red and Blue Shifted Broad Lines in Luminous Quasars
We have observed a sample of 22 luminous quasars, in the range 2.0<z<2.5, at
1.6 microns with the near-infrared (NIR) spectrograph FSPEC on the Multiple
Mirror Telescope. Our sample contains 13 radio-loud and 9 radio-quiet objects.
We have measured the systemic redshifts z_(sys) directly from the strong [O
III]5007 line emitted from the narrow-line-region. From the same spectra, we
have found that the non-resonance broad H lines have a systematic mean
redward shift of 520+/-80 km/s with respect to systemic. Such a shift was not
found in our identical analysis of the low-redshift sample of Boroson & Green.
The amplitude of this redshift is comparable to half the expected gravitational
redshift and transverse Doppler effects, and is consistent with a correlation
between redshift differences and quasar luminosity. From data in the
literature, we confirm that the high-ionization rest-frame ultraviolet broad
lines are blueshifted ~550-1050 km/s from systemic, and that these velocity
shifts systematically increase with ionization potential. Our results allow us
to quantify the known bias in estimating the ionizing flux from the
inter-galactic-medium J_(IGM) via the Proximity Effect. Using redshift
measurements commonly determined from strong broad line species, like Ly\alpha
or CIV1549, results in an over-estimation of J_(IGM) by factors of ~1.9-2.3.
Similarly, corresponding lower limits on the density of baryon Omega_b will be
over-estimated by factors of ~1.4-1.5. However, the low-ionization MgII2798
broad line is within ~50 km/s of systemic, and thus would be the line of choice
for determining the true redshift of 1.0<z<2.2 quasars without NIR
spectroscopy, and z>3.1 objects using NIR spectroscopy.Comment: 12 pages, Latex, 2 figures, 2 tables, Accepted for publication in ApJ
Letter
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