265 research outputs found
Reliability History and Improvements to the ANL 50 MEV H- Accelerator
The H- Accelerator consists of a 750 keV Cockcroft Walton preaccelerator and
an Alvarez type 50 MeV linac. The accelerator has been in operation since 1961.
Since 1981, it has been used as the injector for the Intense Pulsed Neutron
Source (IPNS), a national user facility for neutron scattering. The linac
delivers about 3.5x1012 H- ions per pulse, 30 times per second (30 Hz), for
multi-turn injection to a 450 MeV Rapid Cycling Synchrotron (RCS). IPNS
presently operates about 4,000 hours per year, and operating when scheduled is
critical to meeting the needs of the user community. For many years the IPNS
injector/RCS has achieved an average reliability of 95%, helped in large part
by the preaccelerator/linac which has averaged nearly 99%. To maintain and
improve system reliability, records need to show what each subsystem
contributes to the total down time. The history of source and linac subsystem
reliability, and improvements that have been made to improve reliability, will
be described. Plans to maintain or enhance this reliability for at least
another ten years of operation, will also be discussed.Comment: 3 pages, 1 figur
Control of an atom laser using feedback
A generalised method of using feedback to control Bose-Einstein condensates
is introduced. The condensates are modelled by the Gross-Pitaevskii equation,
so only semiclassical fluctations can be suppressed, and back-action from the
measurement is ignored. We show that for any available control, a feedback
scheme can be found to reduce the energy while the appropriate moment is still
dynamic. We demonstrate these schemes by considering a condensate trapped in a
harmonic potential that can be modulated in strength and position. The
formalism of our feedback scheme also allows the inclusion of certain types of
non-linear controls. If the non-linear interaction between the atoms can be
controlled via a Feshbach resonance, we show that the feedback process can
operate with a much higher efficiency.Comment: 6 pages, 7 figure
The stellar mass assembly of galaxies from z=0 to z=4. Analysis of a sample selected in the rest-frame near-infrared with Spitzer
Using a sample of ~28,000 sources selected at 3.6-4.5 microns with Spitzer
observations of the HDF-N, the CDF-S, and the Lockman Hole (surveyed area: ~664
arcmin^2), we study the evolution of the stellar mass content of the Universe
at 0<z<4. We calculate stellar masses and photometric redshifts, based on
~2,000 templates built with stellar and dust emission models fitting the
UV-to-MIR SEDs of galaxies with spectroscopic redshifts. We estimate stellar
mass functions for different redshift intervals. We find that 50% of the local
stellar mass density was assembled at 0<z<1 (average SFR:0.048 M_sun/yr/Mpc^3),
and at least another 40% at 1<z<4 (average SFR: 0.074 M_sun/yr/Mpc^3). Our
results confirm and quantify the ``downsizing'' scenario of galaxy formation.
The most massive galaxies (M>10^12.0 M_sun) assembled the bulk of their stellar
content rapidly (in 1-2 Gyr) beyond z~3 in very intense star formation events
(producing high specific SFRs). Galaxies with 10^11.5<M/M_sun<10^12.0 assembled
half of their stellar mass before z~1.5, and more than 90% of their mass was
already in place at z~0.6. Galaxies with M<10^11.5 M_sun evolved more slowly
(presenting smaller specific SFRs), assembling half of their stellar mass below
z~1. About 40% of the local stellar mass density of 10^9.0<M/M_sun<10^11.0
galaxies was assembled below z~0.4, most probably through accretion of small
satellites producing little star formation. The cosmic stellar mass density at
z>2.5 is dominated by optically faint (R>25) red galaxies (Distant Red Galaxies
or BzK sources) which account for ~30% of the global population of galaxies,
but contribute at least 60% to the cosmic stellar mass density. Bluer galaxies
(e.g., Lyman Break Galaxies) are more numerous but less massive, contributing
less than 50% to the global stellar mass density at high redshift.Comment: Published in ApJ. 38 pages, 10 figures, 5 tables, 2 appendices. Some
changes to match the final published versio
Quantum corrections to the dynamics of interacting bosons: beyond the truncated Wigner approximation
We develop a consistent perturbation theory in quantum fluctuations around
the classical evolution of a system of interacting bosons. The zero order
approximation gives the classical Gross-Pitaevskii equations. In the next order
we recover the truncated Wigner approximation, where the evolution is still
classical but the initial conditions are distributed according to the Wigner
transform of the initial density matrix. Further corrections can be
characterized as quantum scattering events, which appear in the form of a
nonlinear response of the observable to an infinitesimal displacement of the
field along its classical evolution. At the end of the paper we give a few
numerical examples to test the formalism.Comment: published versio
Dynamically turning off interactions in a two component condensate
We propose a mechanism to change the interaction strengths of a two component
condensate. It is shown that the application of pi/2 pulses allows to alter the
effective interspecies interaction strength as well as the effective
interaction strength between particles of the same kind. This mechanism
provides a simple method to transform spatially stable condensates into
unstable once and vice versa. It also provides a means to store a squeezed spin
state by turning off the interaction for the internal states and thus allows to
gain control over many body entangled states.Comment: 7 pages 5 figures, symbols changed, minor changes, to appear in Phys.
Rev.
HST Morphologies of z ~ 2 Dust-Obscured Galaxies II: Bump Sources
We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared
galaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscured
galaxies, or DOGs) which have a local maximum in their spectral energy
distribution (SED) at rest-frame 1.6um associated with stellar emission. These
sources, which we call "bump DOGs", have star-formation rates of 400-4000
Msun/yr and have redshifts derived from mid-IR spectra which show strong
polycyclic aromatic hydrocarbon emission --- a sign of vigorous on-going
star-formation. Using a uniform morphological analysis, we look for
quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected
ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are
more typical of obscured active galactic nuclei than starbursts),
sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from
the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than
power-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc)
and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/-
0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent with
expectations from simulations of major mergers in which merging systems during
the peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7.
Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally
peaked, single-object morphologies rather than diffuse and irregular
morphologies. This distinction in morphologies may imply that less obscured
ULIRGs sample the merger near the end of the peak star-formation rate period.
Alternatively, it may indicate that the intense star-formation in these
less-obscured ULIRGs is not the result of a recent major merger.Comment: Accepted to ApJ; 22 pages, 8 Figures, 7 Table
The Mid-IR and X-ray Selected QSO Luminosity Function
We present the J-band luminosity function of 1838 mid-infrared and X-ray
selected AGNs in the redshift range 0<z<5.85. These luminosity functions are
constructed by combining the deep multi-wavelength broad-band observations from
the UV to the mid-IR of the NDWFS Bootes field with the X-ray observations of
the XBootes survey and the spectroscopic observations of the same field by
AGES. Our sample is primarily composed of IRAC-selected AGNs, targeted using
modifications of the Stern et al.(2005) criteria, complemented by MIPS 24
microns and X-ray selected AGNs to alleviate the biases of IRAC mid-IR
selection against z~4.5 quasars and AGNs faint with respect to their hosts.
This sample provides an accurate link between low and high redshift AGN
luminosity functions and does not suffer from the usual incompleteness of
optical samples at z~3. We find that the space density of the brightest quasars
strongly decreases from z=3 to z=0, while the space density of faint quasars is
at least flat, and possibly increasing, over the same redshift range. At z>3 we
observe a decrease in the space density of quasars of all brightnesses. We
model the luminosity function by a double power-law and find that its evolution
cannot be described by either pure luminosity or pure density evolution, but
must be a combination of both. Our best-fit model has bright and faint
power-law indices consistent with the low redshift measurements based on the
2QZ and 2SLAQ surveys and it generally agrees with the number of bright quasars
predicted by other LFs at all redshifts. If we construct the QSO luminosity
function using only the IRAC-selected AGNs, we find that the biases inherent to
this selection method significantly modify the behavior of phi*(z) only for z<1
and have no significant impact upon the characteristic magnitude M*_J(z).Comment: Corrected minor typo in equations (4) and (6). Accepted for
publication in The Astrophysical Journal. 56 pages + 6 tables + 16 figure
Optical Identifications of 230 HIPASS Radio Sources
We present the coordinates, apparent magnitudes, and morphological types for
230 galaxies presumably identified with HIPASS (HI Parkes All-Sky Survey)
sources. The new optical counterparts of the HIPASS sources follow the
well-known statistical relationships between the hydrogen mass, luminosity, and
type of galaxies. Low-surface-brightness galaxies constitute a significant
fraction among these objects. The median value of the hydrogen
mass-to-luminosity ratio for them is a factor of 2 or 3 higher than that for
bright HIPASS galaxies, reaching . A number of our
objects are located near the boundary that
defines the zone of gravitational stability of disk galaxies against
large-scale star formation
The Star-Formation Histories of z~2 DOGs and SMGs
The Spitzer Space Telescope has identified a population of ultra-luminous
infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the
evolution of massive galaxies. We measure the stellar masses of two populations
of Spitzer-selected ULIRGs, both of which have extremely red R-[24] colors
(dust-obscured galaxies, or DOGs) and compare our results with sub-millimeter
selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their
mid-IR spectral energy distribution (SED) at rest-frame 1.6um associated with
stellar emission ("bump DOGs"), while the other set of 51 DOGs has a power-law
dominated mid-IR SED with spectral features typical of obscured AGN ("power-law
DOGs"). We use stellar population synthesis models applied self-consistently to
broad-band photometry in the rest-frame ultra-violet, optical, and
near-infrared of each of these populations and test a variety of stellar
population synthesis codes, star-formation histories (SFHs), and initial mass
functions (IMFs). Assuming a simple stellar population SFH and a Chabrier IMF,
we find that the median and inner quartile stellar masses of SMGs, bump DOGs
and power-law DOGs are given by log(M_*/M_sun) = 10.42_-0.36^+0.42,
10.62_-0.32^+0.36, and 10.71_-0.34^+0.40, respectively. Implementing more
complicated SFHs with multiple age components increases these mass estimates by
up to 0.5 dex. Our stellar mass estimates are consistent with physical
mechanisms for the origin of z~2 ULIRGs that result in high star-formation
rates for a given stellar mass. Such mechanisms are usually driven by a major
merger of two gas-rich systems, rather than smooth accretion of gas and small
satellites.Comment: 17 pages, 7 figures, 3 tables. Plus figures showing SEDs and best-fit
synthesized stellar population model. Accepted to the Ap
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