1,844 research outputs found
Inflow/outflow boundary conditions and global dynamics of spatial mixing layers
The numerical simulation of incompressible spatially-developing shear flows poses a special challenge to computational fluid dynamicists. The Navier-Stokes equations are elliptic and boundary equations need to be specified at the inflow and outflow boundaries in order to compute the fluid properties within the region of interest. It is, however, difficult to choose inflow and outflow conditions corresponding to a given experimental situation. Furthermore the effects that changes in the boundary conditions or in the size of the computational domain may induce on the global dynamics of the flow are presently unknown. These issues are examined in light of recent developments in hydrodynamic stability theory. The particular flow considered is the spatial mixing layer but it was expected that similar phenomena were bound to occur in other cases such as channel flow, the boundary layer, etc. A short summary of local/global and absolute/convective instability concepts is given. The results of numerical simulations are presented which strongly suggest that global resonances may be triggered in domains of finite streamwise extent although the evolution of the perturbation vorticity field is everywhere locally convective. A relationship between finite domains and pressure sources which might help in devising a scheme to eliminate these difficulties is discussed
Room temperature electron spin coherence in telecom-wavelength quaternary quantum wells
Time-resolved Kerr rotation spectroscopy is used to monitor the room
temperature electron spin dynamics of optical telecommunication wavelength
AlInGaAs multiple quantum wells lattice-matched to InP. We found that electron
spin coherence times and effective g-factors vary as a function of aluminum
concentration. The measured electron spin coherence times of these multiple
quantum wells, with wavelengths ranging from 1.26 microns to 1.53 microns,
reach approximately 100 ps at room temperature, and the measured electron
effective g-factors are in the range from -2.3 to -1.1.Comment: 4 pages, 4 figure
Wave-number Selection by Target Patterns and Side Walls in Rayleigh-Benard Convection
We present experimental results for Rayleigh-Benard convection patterns in a
cylindrical container with static side-wall forcing induced by a heater. This
forcing stabilized a pattern of concentric rolls (a target pattern) with the
central roll (the umbilicus) at the center of the cell after a jump from the
conduction to the convection state. A quasi-static increase of the control
parameter (epsilon) beyond 0.8 caused the umbilicus of the pattern to move off
center. As observed by others, a further quasi-static increase of epsilon up to
15.6 caused a sequence of transitions. Each transition began with the
displacement of the umbilicus and then proceeded with the loss of one
convection roll at the umbilicus and the return of the umbilicus to a location
near the center of the cell. Alternatively, with decreasing epsilon new rolls
formed at the umbilicus but large umbilicus displacements did not occur. In
addition to quantitative measurements of the umbilicus displacement, we
determined and analyzed the entire wave-director field of each image. The wave
numbers varied in the axial direction, with minima at the umbilicus and at the
cell wall and a maximum at a radial position close to 2/3 Gamma. The wave
numbers at the maximum showed hysteretic jumps at the transitions, but on
average agreed well with the theoretical predictions for the wave numbers
selected in the far field of an infinitely extended target pattern.Comment: ReVTeX, 11 pages, 16 eps figures include
The CIV-MgII Kinematics Connection in <z>~0.7 Galaxies
We have examined Faint Object Spectrograph data from the Hubble Space
Telescope Archive for CIV 1548,1550 absorption associated with 40 MgII
2796,2803 absorption-selected galaxies at 0.4 < z < 1.4. We report a strong
correlation between MgII kinematics, measured in 6 km/s resolution HIRES/Keck
spectra, and W_r(1548); this implies a physical connection between the
processes that produce "outlying velocity" MgII clouds and high ionization
galactic/halo gas. We found no trend in ionization condition,
W_r(1548)/W_r(2796), with galaxy-QSO line-of-sight separation for 13 systems
with confirmed associated galaxies, suggesting no obvious ionization gradient
with galactocentric distance in these higher redshift galaxies. We find
tentative evidence (2-sigma) that W_r(1548)/W_r(2796) is anti-correlated with
galaxy color; if further data corroborate this trend, in view of the
strong CIV-MgII kinematics correlation, it could imply a connection between
stellar populations, star formation episodes, and the kinematics and ionization
conditions of halo gas at z~1.Comment: Accepted to Astrophysical Journal Letters; 4 pages; 3 figures;
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First Results from the Large Area Lyman Alpha Survey
We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman
Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume
and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD
Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National
Observatory.
Well-detected sources with flux and equivalent width matching known high
redshift Lyman alpha galaxies (i.e., observed equivalent width above 80
Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in
an 80 Angstrom filter) have an observed surface density corresponding to 11000
+- 700 per square degree per unit redshift at z=4.5. Spatial variation in this
surface density is apparent on comparison between counts in 6561 and 6730
Angstrom filters.
Early spectroscopic followup results from the Keck telescope included three
sources meeting our criteria for good Lyman alpha candidates. Of these, one is
confirmed as a z=4.52 source, while another remains consistent with either
z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide
Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies
per square degree per unit redshift.Comment: 10 pages, 2 figures (3 .ps files), uses AASTeX 4. Submitted to The
Astrophysical Journal Letter
Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey
Narrow-band searches for Lyman alpha emission are an efficient way of
identifying star-forming galaxies at high redshifts. We present Keck telescope
spectra confirming redshifts z = 5.7 for three objects discovered in the Large
Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory.
All three spectra show strong, narrow emission lines with the asymmetric
profile that is characteristically produced in high redshift Lyman alpha
emitters by preferential HI absorption in the blue wing of the line. These
objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the
NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and
Lyman alpha forest absorption at redshift z = 5.7. All three objects show large
equivalent widths (>= 150A in the rest-frame), suggesting at least one of the
following: a top-heavy initial mass function, very low stellar metallicity, or
the presence of an active nucleus. We consider the case for an active nucleus
to be weak in all three objects due to the limited width of the Lyman alpha
emission line (< 500 km/s) and the absence of any other indicator of quasar
activity.
The three confirmed high redshift objects were among four spectroscopically
observed targets drawn from the sample of 18 candidates presented by Rhoads and
Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population
statistics from our earlier photometric study, which imply little evolution in
number density from z=5.7 to z=4.5 and provide strong evidence that the
reionization redshift is greater than 5.7.Comment: Submitted to AJ, June 2002. 15 pages, AASTe
A Search for the Damped Ly-alpha Absorber at z=1.86 toward QSO 1244+3443 with NICMOS
We have carried out a high-resolution imaging search for the galaxy
associated with the damped Lyman-alpha (DLA) absorber at z=1.859 toward the
z_{em}=2.48 quasar QSO 1244+3443, using the HST and the NICMOS. Images were
obtained in the broad filter F160W and the narrow filter F187N with camera 2 on
NICMOS with the goal of detecting the rest-frame optical continuum and the
H-alpha line emission from the DLA. After PSF subtraction, two weak features
are seen at projected separations of 0.16-0.24" from the quasar. Parts of these
features may be associated with the DLA absorber, although we cannot completely
rule out that they could be artifacts of the point spread function (PSF). If
associated with the DLA, the objects would be ~1-2 h_{70}^{-1} kpc in size with
integrated flux densities of 2.5 and 3.3 mu Jy in the F160W filter, implying
luminosities at lambda_{central}=5600 A in the DLA rest frame of 4.4-5.9 x
10^{9} h_{70}^{-2} L_{solar} at z=1.86, for q0=0.5. However, no significant
H-alpha line emission is seen from these objects, suggesting low star formation
rates (SFRs). Our 3 sigma upper limit on the SFR in the DLA is 1.3 h_{70}^{-2}
M_{solar}/yr for q0 = 0.5 (2.4 h_{70}^{-2} M_{solar} yr^{-1} for q0 = 0.1).
This together with our earlier result for LBQS 1210+1731 mark a significant
improvement over previous constraints on the star formation rates of DLAs. A
combination of low SFR and some dust extinction is likely to be responsible for
the lack of H-alpha emission. Alternatively, the objects, may be associated
with the quasar host galaxy. In any case, our observations suggest that the DLA
is not a large bright proto-disk, but a compact object or a low-surface
brightness galaxy. If the two features are PSF artifacts then the constraints
on DLA properties are even more severe.Comment: 28 pages, 9 figures. Accepted for publication in The Astrophysical
Journal. Figures are given at a slightly lower resolution here, to decrease
file sizes. The higher resolution versions can be found in the Ap
Gravitational instability of Einstein-Gauss-Bonnet black holes under tensor mode perturbations
We analyze the tensor mode perturbations of static, spherically symmetric
solutions of the Einstein equations with a quadratic Gauss-Bonnet term in
dimension . We show that the evolution equations for this type of
perturbations can be cast in a Regge-Wheeler-Zerilli form, and obtain the exact
potential for the corresponding Schr\"odinger-like stability equation. As an
immediate application we prove that for and , the sign
choice for the Gauss-Bonnet coefficient suggested by string theory, all
positive mass black holes of this type are stable. In the exceptional case , we find a range of parameters where positive mass asymptotically flat
black holes, with regular horizon, are unstable. This feature is found also in
general for .Comment: 7 pages, 1 figure, minor corrections, references adde
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