3,280 research outputs found
Quantum dynamical response of ultracold few boson ensembles in finite optical lattices to multiple interaction quenches
The correlated non-equilibrium quantum dynamics following a multiple
interaction quench protocol for few-bosonic ensembles confined in finite
optical lattices is investigated. The quenches give rise to an interwell
tunneling and excite the cradle and a breathing mode. Several tunneling
pathways open during the time interval of increased interactions, while only a
few occur when the system is quenched back to its original interaction
strength. The cradle mode, however, persists during and in between the
quenches, while the breathing mode possesses dinstinct frequencies. The
occupation of excited bands is explored in detail revealing a monotonic
behavior with increasing quench amplitude and a non-linear dependence on the
duration of the application of the quenched interaction strength. Finally, a
periodic population transfer between momenta for quenches of increasing
interaction is observed, with a power-law frequency dependence on the quench
amplitude. Our results open the possibility to dynamically manipulate various
excited modes of the bosonic system.Comment: 13 pages, 9 figure
Dark Halo and Disk Galaxy Scaling Laws in Hierarchical Universes
We use cosmological N-body/gasdynamical simulations that include star
formation and feedback to examine the proposal that scaling laws between the
total luminosity, rotation speed, and angular momentum of disk galaxies reflect
analogous correlations between the structural parameters of their surrounding
dark matter halos. The numerical experiments follow the formation of
galaxy-sized halos in two Cold Dark Matter dominated universes: the standard
Omega=1 CDM scenario and the currently popular LCDM model. We find that the
slope and scatter of the I-band Tully-Fisher relation are well reproduced in
the simulations, although not, as proposed in recent work, as a result of the
cosmological equivalence between halo mass and circular velocity: large
systematic variations in the fraction of baryons that collapse to form galaxies
and in the ratio between halo and disk circular velocities are observed in our
numerical experiments. The Tully-Fisher slope and scatter are recovered in this
model as a direct result of the dynamical response of the halo to the assembly
of the luminous component of the galaxy. We conclude that models that neglect
the self-gravity of the disk and its influence on the detailed structure of the
halo cannot be used to derive meaningful estimates of the scatter or slope of
the Tully-Fisher relation. Our models fail, however, to match the zero-point of
the Tully-Fisher relation, as well as that of the relation linking disk
rotation speed and angular momentum. These failures can be traced,
respectively, to the excessive central concentration of dark halos formed in
the Cold Dark Matter cosmogonies we explore and to the formation of galaxy
disks as the final outcome of a sequence of merger events. (abridged)Comment: submitted to The Astrophysical Journa
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
The cosmological origin of the Tully-Fisher relation
We use high-resolution cosmological simulations that include the effects of
gasdynamics and star formation to investigate the origin of the Tully-Fisher
relation in the standard Cold Dark Matter cosmogony. Luminosities are computed
for each model galaxy using their full star formation histories and the latest
spectrophotometric models. We find that at z=0 the stellar mass of model
galaxies is proportional to the total baryonic mass within the virial radius of
their surrounding halos. Circular velocity then correlates tightly with the
total luminosity of the galaxy, reflecting the equivalence between mass and
circular velocity of systems identified in a cosmological context. The slope of
the relation steepens slightly from the red to the blue bandpasses, and is in
fairly good agreement with observations. Its scatter is small, decreasing from
\~0.45 mag in the U-band to ~0.34 mag in the K-band. The particular
cosmological model we explore here seems unable to account for the zero-point
of the correlation. Model galaxies are too faint at z=0 (by about two
magnitudes) if the circular velocity at the edge of the luminous galaxy is used
as an estimator of the rotation speed. The Tully-Fisher relation is brighter in
the past, by about ~0.7 magnitudes in the B-band at z=1, at odds with recent
observations of z~1 galaxies. We conclude that the slope and tightness of the
Tully-Fisher relation can be naturally explained in hierarchical models but
that its normalization and evolution depend strongly on the star formation
algorithm chosen and on the cosmological parameters that determine the
universal baryon fraction and the time of assembly of galaxies of different
mass.Comment: 5 pages, 4 figures included, submitted to ApJ (Letters
CIV Absorption From Galaxies in the Process of Formation
We investigate the heavy element QSO absorption systems caused by gas
condensations at high redshift which evolve into galaxies with circular
velocity of 100 to 200 km/s at the present epoch. Artificial QSO spectra were
generated for a variety of lines-of-sight through regions of the universe
simulated with a hydrodynamics code. The CIV and HI absorption features in
these spectra closely resemble observed CIV and HI absorption systems over a
wide range in column density. CIV absorption complexes with multiple-component
structure and velocity spreads up to about 600 km/s are found. The broadest
systems are caused by lines-of-sight passing through groups of protogalactic
clumps with individual velocity dispersions of less than 150 km/s aligned along
filamentary structures. The temperature of most of the gas does not take the
photoionization equilibrium value. This invalidates density and size estimates
derived from thermal equilibrium models. Consequences for metal abundance
determinations are briefly discussed. We predict occasional exceptionally large
ratios of CIV to HI column density (up to a third) for lines-of-sight passing
through compact halos of hot gas with temperature close to 3 10^5 K. Our model
may be able to explain both high-ionization multi-component heavy-element
absorbers and damped Lyman alpha systems as groups of small protogalactic
clumps.Comment: 13 pages, uuencoded postscript file, 4 figures included submitted to
ApJ (Letters); complete version also available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
The Effects of a Photoionizing UV Background on the Formation of Disk Galaxies
We use high resolution N-body/gasdynamical simulations to investigate the
effects of a photoionizing UV background on the assembly of disk galaxies in
hierarchically clustering universes. We focus on the mass and rotational
properties of gas that can cool to form centrifugally supported disks in dark
matter halos of different mass. Photoheating can significantly reduce the
amount of gas that can cool in galactic halos. Depending on the strength of the
UV background field, the amount of cooled gas can be reduced by up to in
systems with circular speeds in the range - \kms. The magnitude of the
effect, however, is not enough to solve the ``overcooling'' problem that
plagues hierarchical models of galaxy formation if the UV background is chosen
to be consistent with estimates based on recent observations of QSO absorption
systems. Photoionization has little effect on the collapse of gas at high
redshift and affects preferentially gas that is accreted at late times. Since
disks form inside-out, accreting higher angular momentum gas at later times,
disks formed in the presence of a UV background have spins that are even
smaller than those formed in simulations that do not include the effects of
photoionization. This exacerbates the angular momentum problem that afflicts
hierarchical models of disk formation. We conclude that photoionization cannot
provide the heating mechanism required to reconcile hierarchically clustering
models with observations. Energy feedback and enrichment processes from the
formation and evolution of stars must therefore be indispensable ingredients
for any successful model of the formation of disk galaxies.Comment: 36 pages, w/ embedded figures, submitted to ApJ. Also available at
http://penedes.as.arizona.edu/~jfn/preprints/dskform.ps.g
Absolute velocity measurements in sunspot umbrae
In sunspot umbrae, convection is largely suppressed by the strong magnetic
field. Previous measurements reported on negligible convective flows in umbral
cores. Based on this, numerous studies have taken the umbra as zero reference
to calculate Doppler velocities of the ambient active region. To clarify the
amount of convective motion in the darkest part of umbrae, we directly measured
Doppler velocities with an unprecedented accuracy and precision. We performed
spectroscopic observations of sunspot umbrae with the Laser Absolute Reference
Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency
comb enabled the calibration of the high-resolution spectrograph and absolute
wavelength positions. A thorough spectral calibration, including the
measurement of the reference wavelength, yielded Doppler shifts of the spectral
line Ti i 5713.9 {\AA} with an uncertainty of around 5 m s-1. The measured
Doppler shifts are a composition of umbral convection and magneto-acoustic
waves. For the analysis of convective shifts, we temporally average each
sequence to reduce the superimposed wave signal. Compared to convective
blueshifts of up to -350 m s-1 in the quiet Sun, sunspot umbrae yield a
strongly reduced convective blueshifts around -30 m s-1. {W}e find that the
velocity in a sunspot umbra correlates significantly with the magnetic field
strength, but also with the umbral temperature defining the depth of the
titanium line. The vertical upward motion decreases with increasing field
strength. Extrapolating the linear approximation to zero magnetic field
reproduces the measured quiet Sun blueshift. Simply taking the sunspot umbra as
a zero velocity reference for the calculation of photospheric Dopplergrams can
imply a systematic velocity error.Comment: 10 pages, 7 figures, 2 tables, Appendix with 5 figure
Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections
We perform a variety of tests to determine the numerical resolution of the
cosmological TVD eulerian code developed by Ryu et al (1993). Tests include
512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for
self-similarity and comparison of results with those from higher resolution SPH
and grid-based calculations (Frenk et al 1998). We conclude that in regions
where density gradients are not produced by shocks the code degrades resolution
with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused
gradients (for which the code was designed) the smoothing length is 1.1 cells.
Finally, for \beta model fit clusters, we can approximately correct numerical
resolution by the transformation R^2_{core}\to R^2_{core}-(C\Delta l)^2, where
\Delta l is the cell size and C=1.1-1.7. When we use these corrections on our
previously published computations for the SCDM and \Lambda CDM models we find
luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86,
280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with
observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected
core radii, the COBE normalized SCDM model predicts the number of bright
L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the \Lambda CDM
model is consistent with observations.Comment: ApJ in press (1999
A Unified Scaling Law in Spiral Galaxies
We investigate the origin of a unified scaling relation in spiral galaxies.
Observed spiral galaxies are spread on a plane in the
three-dimensionallogarithmic space of luminosity L, radius R and rotation
velocity V. The plane is expressed as in I-passband,
where is a constant. On the plane, observed galaxies are distributed
in an elongated region which looks like the shape of a surfboard. The
well-known scaling relations, L-V (Tully-Fisher relation), V-R (also the
Tully-Fisher relation) and R-L (Freeman's law), can be understood as oblique
projections of the surfboard-like plane into 2-D spaces. This unified
interpretation of the known scaling relations should be a clue to understand
the physical origin of all the relations consistently. Furthermore, this
interpretation can also explain why previous studies could not find any
correlation between TF residuals and radius.
In order to clarify the origin of this plane, we simulate formation and
evolution of spiral galaxies with the N-body/SPH method, including cooling,
star formation and stellar feedback. Initial conditions are set to isolated 14
spheres with two free parameters, such as mass and angular momentum. The CDM
(h=0.5, ) cosmology is considered as a test case. The simulations
provide the following two conclusions: (a) The slope of the plane is well
reproduced but the zero-point is not. This zero-point discrepancy could be
solved in a low density ($\Omega_00.5) cosmology.
(b) The surfboard-shaped plane can be explained by the control of galactic mass
and angular momentum.Comment: Accepted for publication in ApJ Letters. 6 pages including 2 figure
Subharmonics and Aperiodicity in Hysteresis Loops
We show that it is possible to have hysteretic behavior for magnets that does
not form simple closed loops in steady state, but must cycle multiple times
before returning to its initial state. We show this by studying the
zero-temperature dynamics of the 3d Edwards Anderson spin glass. The specific
multiple varies from system to system and is often quite large and increases
with system size. The last result suggests that the magnetization could be
aperiodic in the large system limit for some realizations of randomness. It
should be possible to observe this phenomena in low-temperature experiments.Comment: 4 pages, 3 figure
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