59 research outputs found
Observation of the algebraic localization-delocalization transition in a 1D disordered potential with a bias force
In a one-dimensional (1D) disordered potential, quantum interferences leading to Anderson lo-calization are ubiquitous, such that all wave-functions are exponentially localized. Moreover, no phase transition toward delocalization is expected in 1D. This behavior is strongly modified in the presence of a bias force. We experimentally study this case, launching a non-interacting 39 K Bose-Einstein condensate in a 1D disordered potential induced by a far-off-resonance laser speckle, while controlling a bias force. In agreement with theoretical predictions, we observe a transition between algebraic localization and delocalization as a function of our control parameter that is the relative strength of the disorder against the bias force. We also demonstrate that the initial velocity of the wave-packet only plays a role through an effective disorder strength due to the correlation of the disorder. Adding a bias force is a quite natural way to probe the transport properties of quantum systems, a subject of broad interest that can be in particular addressed with atomic quantum gases thanks to their high degree of control and versatility [1]. For example, Bloch oscillations has been measured through the addition of a constant force to atoms in periodic potential induced by an optical lattice [2]. A force applied to a harmonic trap is equivalent to a trap displacement. The response to such a displacement permits to reveal the fluid or insulating behavior of atomic systems. In 1D interacting Bose gases, the pinning transition by an optical lattice [3] or the insulating transition in quasi-disordered optical lattice [4, 5] have been studied in this manner. More recently, transport in quantum gases is also studied in junction-type setup more analogous to condensed-matter systems: two reservoirs with different chemical potentials are connected through a constriction. For example, in a gas of fermions, the quantization of conductance through a quantum point contact [6] and the superfluid to normal transition in a disordered thin film have been observed [7]. In our work, we focus on the transport of non-interacting particles in disordered media. Without a bias force, quantum interferences between multiple paths lead to Anderson localization [8] whose signature is an exponential decay in space of single particle wave-function [9]. This phenomenon is ubiquitous in wave/quantum physics and it has been observed in many physical contexts [10] including condensed-matter [11] and ultra-cold atoms [12-14]. One-dimensional truly disordered systems are always localized [15], contrary to the 3D case where a phase transition between localized and extended single particle wave-functions takes place as a function of the disorder strength [16-18]. The localization properties of 1D disordered systems are modified in the presence of a bias force. Theoretical studies predict a transition from algebraic localization to delocalization as a function of a single control non-dimensional parameter α which is the ratio of the force to the disorder strength [19, 20]. Physically, α is the relative energy gain ∆E/E of a particle of energy E when moving over a localization length. Interestingly, in a 1D white noise disorder, this quantity is independent of E as the localization length is proportional to E. If α is small, the force does not considerably change the localization behavior of the particle while for large α its dynamics is severely affected leading to delocalization. This localization-delocalization transition is predicted in the infinite time limit for white noise disorder [20]. In a correlated disorder, as the one produced from a far-off-resonance laser speckle [21], the situation is more complicated. Speckles have no Fourier component beyond a spatial frequency 2k c. As a consequence, back-scattering and localization are not expected in the framework of Born approximation for atoms with wavevectors k > k c [12, 22]. Since localized wave-functions always have a small fraction at long distance corresponding to large energies and momenta in the presence of a bias force, we thus expect correlation-induced delocalization at infinite time. However, signatures of the algebraic localization-delocalization transition are predicted to be observable at transient times [20]. In this paper, we report on the observation of the algebraic localization-delocalization transition with cold-atoms propagating in a one dimensional disordered potential in the presence of a controlled bias force. We experimentally show that the non-dimensional parameter α is the only relevant parameter to describe the transition. We notice that the initial velocity of the quantum wave packet only plays a role through the correlation of the disordered potential, showing that the transition is in-trinsically energy independent. In the localized regime, we demonstrate an algebraic decay of the density and measure the corresponding decay exponent as a function of α. At large disorder strength, a saturation of the exp
A neutral hydrogen survey of polar ring galaxies IV. Parkes observations
A total of 33 polar ring galaxies and polar ring galaxy candidates were
observed in the 21-cm HI line with the 64-m Parkes radio telescope. The
objects, selected by their optical morphology, are all south of declination -39
degrees and in only 5 of them HI had been reported previously. HI line emission
was detected towards 18 objects, though in 3 cases the detection may be
confused by another galaxy in the telescope beam, and one is a marginal
detection. Eight objects were detected for the first time in HI, of which 5 did
not have previously known redshifts.Comment: Accepted by Astronomy and Astrophysics, 12/2/2002; new references
added on 20/2/200
The CORALS Survey I: New Estimates of the Number Density and Gas Content of Damped Lyman Alpha Systems Free from Dust Bias
(Abridged) We present the first results from the Complete Optical and Radio
Absorption Line System (CORALS) survey. We have compiled a homogeneous sample
of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman
alpha systems (DLAs) towards every target, irrespective of its optical
magnitude. This approach circumvents selection effects -- particularly from
intervening dust -- which have long been suspected to affect DLA surveys in
optically-selected, magnitude-limited QSO samples. The CORALS data set consists
of 66 z_em > 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 < z_abs <
z_em have been identified over a total redshift interval Delta z = 55.46. In
this first paper of the CORALS series we describe the sample, present
intermediate resolution spectroscopy and determine the population statistics of
DLAs. We deduce a value of the neutral gas mass density traced by DLAs
(expressed as a fraction of the closure density) log Omega_DLA h =
-2.59^{+0.17}_{-0.24}, and a number density of DLAs per unit redshift n(z) =
0.31^{+0.09}_{-0.08}, both at a mean redshift = 2.37. Taking into account
the errors, we conclude that dust-induced bias in previous surveys may have led
to an underestimate of these quantities by at most a factor of two and we have
not uncovered a previously unrecognised population of high column density DLAs
in front of faint QSOs.Comment: 25 pages, accepted for publication in A&
Star formation rate and metallicity of damped Lyman-alpha absorbers in cosmological SPH simulations
We study the distribution of the star formation rate and metallicity of
damped Lyman-alpha absorbers using cosmological SPH simulations of the Lambda
cold dark matter model in the redshift range z=0-4.5. Our approach includes a
phenomenological model of galactic wind. We find that there is a positive
correlation between the projected stellar mass density and the neutral hydrogen
column density (NHI) of DLAs for high NHI systems, and that there is a good
correspondence in the spatial distribution of stars and DLAs in the
simulations. The evolution of typical star-to-gas mass ratios in DLAs can be
characterised by an increase from about 2 at z=4.5 to 3 at z=3, to 10 at z=1,
and finally to 20 at z=0. We also find that the projected SFR density in DLAs
follows the Kennicutt law well at all redshifts, and the simulated values are
consistent with the recent observational estimates of this quantity by Wolfe et
al. (2003a,b). The rate of evolution in the mean metallicity of simulated DLAs
as a function of redshift is mild, and is consistent with the rate estimated
from observations. The predicted metallicity of DLAs is generally sub-solar in
our simulations, and there is a significant scatter in the distribution of DLA
metallicity for a given NHI. However, we find that the median metallicity of
simulated DLAs is close to that of Lyman-break galaxies, which is higher than
the values typically observed for DLAs by nearly an order of magnitude. This
discrepancy with observations could be due to an inadequate treatment of SN
feedback in our current simulations, perhaps indicating that metals are not
expelled efficiently enough from DLAs by outflows. Alternatively, the current
observations might be missing the majority of the high metallicity DLAs due to
selection effects. (abridged)Comment: 18 pages, 15 figures. Accepted to MNRAS. More visual presentations
and the version with high resolution figures are available at
http://cfa-www.harvard.edu/~knagamine/DLA-pics
Metal Abundances at z < 1.5: Fresh Clues to the Chemical Enrichment History of Damped Lyman alpha Systems
We explore the redshift evolution of the metal content of damped Lyman alpha
systems (DLAs) with new observations of four absorbers at z < 1.5 . The main
conclusion is that the column density--weighted mean metallicity, [] =
-1.03 +/- 0.23 (on a logarithmic scale), is not significantly higher at z < 1.5
than at earlier epochs, despite the fact that the comoving star formation rate
density of the universe was near its maximum value at this redshift. For three
of the four DLAs our observations include absorption lines of Si, Mn, Cr, Fe,
and Ni, as well as Zn. We argue that the relative abundances of these elements
are consistent with a moderate degree of dust depletion which, once accounted
for, leaves no room for the enhancement of the alpha-elements over iron seen in
metal poor stars in the Milky Way. This is contrary to previous assertions that
DLAs have been enriched solely by Type II supernovae, but can be understood if
the rate of star formation in the systems studied proceeded more slowly than in
the early history of our Galaxy. These results add to a growing body of data
all pointing to the conclusion that known DLAs do not trace the galaxy
population responsible for the bulk of star formation. Possible reasons are
that sight-lines through metal rich gas are systematically underrepresented
because the background QSOs are reddened, and that the most actively star
forming galaxies are also the most compact, presenting too small a
cross-section to have been probed yet with the limited statistics of current
samples.Comment: 40 pages, LaTex, 9 Postscript Figures. Accepted for publication in
the Astrophysical Journa
Soy Isoflavones Genistein and Daidzein Exert Anti-Apoptotic Actions via a Selective ER-mediated Mechanism in Neurons following HIV-1 Tat1–86 Exposure
HIV-1 viral protein Tat partially mediates the neural dysfunction and neuronal cell death associated with HIV-1 induced neurodegeneration and neurocognitive disorders. Soy isoflavones provide protection against various neurotoxic insults to maintain neuronal function and thus help preserve neurocognitive capacity.We demonstrate in primary cortical cell cultures that 17β-estradiol or isoflavones (genistein or daidzein) attenuate Tat(1-86)-induced expression of apoptotic proteins and subsequent cell death. Exposure of cultured neurons to the estrogen receptor antagonist ICI 182,780 abolished the anti-apoptotic actions of isoflavones. Use of ERα or ERβ specific antagonists determined the involvement of both ER isoforms in genistein and daidzein inhibition of caspase activity; ERβ selectively mediated downregulation of mitochondrial pro-apoptotic protein Bax. The findings suggest soy isoflavones effectively diminished HIV-1 Tat-induced apoptotic signaling.Collectively, our results suggest that soy isoflavones represent an adjunctive therapeutic option with combination anti-retroviral therapy (cART) to preserve neuronal functioning and sustain neurocognitive abilities of HIV-1 infected persons
Clumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4'x4' fields
The Carina region is an excellent astrophysical laboratory for studying the
feedback mechanisms of newly born, very massive stars within their natal giant
molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to
analyse the observed intensities of atomic carbon and CO and to derive the
excitation conditions of the gas. The NANTEN2-4m submillimeter telescope was
used to map the [CI] 3P1-3P0, 3P2-3P1 and CO 4-3, 7-6 lines in two 4'x4'
regions of Carina where molecular material interfaces with radiation from the
massive star clusters. One region is the northern molecular cloud near the
compact OB cluster Tr14, and the second region is in the molecular cloud south
of etaCar and Tr16. These data were combined with 13CO SEST spectra, HIRES/IRAS
60um and 100um maps of the FIR continuum, and maps of 8um IRAC/Spitzer and MSX
emission. We used the HIRES far-infrared dust data to create a map of the FUV
field heating the gas. The northern region shows an FUV field of a few 1000 in
Draine units while the field of the southern region is about a factor 10
weaker. We constructed models consisting of an ensemble of small spherically
symmetric PDR clumps within the 38" beam (0.43pc), which follow canonical
power-law mass and mass-size distributions. We find that an average local clump
density of 2x10**5 cm-3 is needed to reproduce the observed line emission at
two selected interface positions. Stationary, clumpy PDR models reproduce the
observed cooling lines of atomic carbon and CO at two positions in the Carina
Nebula.Comment: accepted by A&
The host of GRB 030323 at z=3.372: a very high column density DLA system with a low metallicity
We present photometry and spectroscopy of the afterglow of GRB 030323. VLT
spectra of the afterglow show damped Lya (DLA) absorption and low- and
high-ionization lines at a redshift z=3.3718+-0.0005. The inferred neutral
hydrogen column density, log N(HI)=21.90+-0.07, is larger than any (GRB- or
QSO-) DLA HI column density inferred directly from Lya in absorption. From the
afterglow photometry, we derive a conservative upper limit to the host-galaxy
extinction: A(V)<0.5 mag. The iron abundance is [Fe/H]=-1.47+-0.11, while the
metallicity of the gas as measured from sulphur is [S/H]=-1.26+-0.20. We derive
an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2)+N(HI))<~10^-6. In
the Lya trough, a Lya emission line is detected, which corresponds to a
star-formation rate (not corrected for dust extinction) of roughly 1 Msun per
year. All these results are consistent with the host galaxy of GRB 030323
consisting of a low metallicity gas with a low dust content. We detect
fine-structure lines of silicon, SiII*, which have never been clearly detected
in QSO-DLAs; this suggests that these lines are produced in the vicinity of the
GRB explosion site. Under the assumption that these fine-structure levels are
populated by particle collisions, we estimate the HI volume density to be
n_HI=100-10000 cm^-3. HST/ACS imaging 4 months after the burst shows an
extended AB(F606W)=28.0+-0.3 mag object at a distance of 0.14" (1kpc) from the
early afterglow location, which presumably is the host galaxy of GRB 030323.Comment: 15 pages, 7 figures, accepted for publication in A&
The effect of ISM absorption on stellar activity measurements and its relevance for exoplanet studies
Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R'HK indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca II lines, broadening b-parameter, and Ca II column density). We find that for relative velocities between the stellar and ISM lines smaller than 30–40 km s−1 and for ISM Ca II column densities log NCaII ⪆ 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca II column density of log NCaII = 12 is typically reached by a distance of about 100 pc along most sight lines.
In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R'HK value larger than 0.05–0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars
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