320 research outputs found
Highly electronegative metallic contacts to semiconductors using polymeric sulfur nitride
The Schottky barriers formed on nâZnS and nâZnSe by polymeric sulfur nitride have been compared to barriers formed by Au. Barrier energies as determined by photoresponse, currentâvoltage, and capacitanceâvoltage methods show that (SN)_x is approximately 1.0 eV higher than Au on nâZnS and 0.3â0.4 eV higher than Au on nâZnSe. We believe that this is the first report of any metallic contact more electronegative than Au
Tomographic Magnification of Lyman Break Galaxies in The Deep Lens Survey
Using about 450,000 galaxies in the Deep Lens Survey, we present a detection
of the gravitational magnification of z > 4 Lyman Break Galaxies by massive
foreground galaxies with 0.4 < z < 1.0, grouped by redshift. The magnification
signal is detected at S/N greater than 20, and rigorous checks confirm that it
is not contaminated by any galaxy sample overlap in redshift. The inferred
galaxy mass profiles are consistent with earlier lensing analyses at lower
redshift. We then explore the tomographic lens magnification signal by
splitting our foreground galaxy sample into 7 redshift bins. Combining
galaxy-magnification cross-correlations and galaxy angular auto-correlations,
we develop a bias-independent estimator of the tomographic signal. As a
diagnostic of magnification tomography, the measurement of this estimator
rejects a flat dark matter dominated Universe at > 7.5{\sigma} with a fixed
\sigma_8 and is found to be consistent with the expected redshift-dependence of
the WMAP7 {\Lambda}CDM cosmology.Comment: 12 pages, 9 figures, Accepted to MNRA
Exploring Dark Energy with Next-Generation Photometric Redshift Surveys
The coming decade will be an exciting period for dark energy research, during which astronomers will address the question of what drives the accelerated cosmic expansion as first revealed by type Ia supernova (SN) distances, and confirmed by later observations. The mystery of dark energy poses a challenge of such magnitude that, as stated by the Dark Energy Task Force (DETF), nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. The lack of multiple complementary precision observations is a major obstacle in developing lines of attack for dark energy theory. This lack is precisely what next-generation surveys will address via the powerful techniques of weak lensing (WL) and baryon acoustic oscillations (BAO) -- galaxy correlations more generally -- in addition to SNe, cluster counts, and other probes of geometry and growth of structure. Because of their unprecedented statistical power, these surveys demand an accurate understanding of the observables and tight control of systematics. This white paper highlights the opportunities, approaches, prospects, and challenges relevant to dark energy studies with wide-deep multiwavelength photometric redshift surveys. Quantitative predictions are presented for a 20000 sq. deg. ground-based 6-band (ugrizy) survey with 5-sigma depth of r~27.5, i.e., a Stage 4 survey as defined by the DETF
Slow-roll, acceleration, the Big Rip and WKB approximation in NLS-type formulation of scalar field cosmology
Aspects of non-linear Schr\"{o}dinger-type (NLS) formulation of scalar
(phantom) field cosmology on slow-roll, acceleration, WKB approximation and Big
Rip singularity are presented. Slow-roll parameters for the curvature and
barotropic density terms are introduced. We reexpress all slow-roll parameters,
slow-roll conditions and acceleration condition in NLS form. WKB approximation
in the NLS formulation is also discussed when simplifying to linear case. Most
of the Schr\"{o}dinger potentials in NLS formulation are very slowly-varying,
hence WKB approximation is valid in the ranges. In the NLS form of Big Rip
singularity, two quantities are infinity in stead of three. We also found that
approaching the Big Rip, , which is the
same as effective phantom equation of state in the flat case.Comment: [7 pages, no figure, more reference added, accepted by JCAP
A Spectroscopic Survey of Faint Quasars in the SDSS Deep Stripe: I. Preliminary Results from the Co-added Catalog
In this paper we present the first results of a deep spectroscopic survey of
faint quasars in the Sloan Digital Sky Survey (SDSS) Southern Survey, a deep
survey carried out by repeatedly imaging a 270 deg^2 area. Quasar candidates
were selected from the deep data with good completeness over 0<z<5, and 2 to 3
magnitudes fainter than the SDSS main survey. Spectroscopic follow-up was
carried out on the 6.5m MMT with Hectospec. The preliminary sample of this SDSS
faint quasar survey (hereafter SFQS) covers ~ 3.9 deg^2, contains 414 quasars,
and reaches g=22.5. The overall selection efficiency is ~ 66% (~ 80% at
g<21.5); the efficiency in the most difficult redshift range (2<z<3) is better
than 40%. We use the 1/V_{a} method to derive a binned estimate of the quasar
luminosity function (QLF) and model the QLF using maximum likelihood analysis.
The best model fits confirm previous results showing that the QLF has steep
slopes at the bright end and much flatter slopes (-1.25 at z<2.0 and -1.55 at
z>2.0) at the faint end, indicating a break in the QLF slope. Using a
luminosity-dependent density evolution model, we find that the quasar density
at M_{g}<-22.5 peaks at z~2, which is later in cosmic time than the peak of
z~2.5 found from surveys of more luminous objects. The SFQS QLF is consistent
with the results of the 2dF QSO Redshift Survey, the SDSS, and the 2dF-SDSS LRG
and QSO Survey, but probes fainter quasars. We plan to obtain more quasars from
future observations and establish a complete faint quasar sample with more than
1000 objects over 10 deg^2.Comment: 25 pages, 13 figures, accepted for publication in A
Quasinormal modes of a black hole surrounded by quintessence
Using the third-order WKB approximation, we evaluate the quasinormal
frequencies of massless scalar field perturbation around the black hole which
is surrounded by the static and spherically symmetric quintessence. Our result
shows that due to the presence of quintessence, the scalar field damps more
rapidly. Moreover, we also note that the quintessential state parameter
(the ratio of pressure to the energy density ) play an
important role for the quasinormal frequencies. As the state parameter
increases the real part increases and the absolute value of the
imaginary part decreases. This means that the scalar field decays more slowly
in the larger quintessence case.Comment: 7 pages, 3 figure
Quasinormal modes of gravitational perturbation around a Schwarzschild black hole surrounded by quintessence
In this paper, the quasinormal modes of gravitational perturbation around a
Schwarzschild black hole surrounded by quintessence were evaluated by using the
third-order WKB approximation. Due to the presence of quintessence, the
gravitational wave damps more slowly
Photometric Redshifts of Quasars
We demonstrate that the design of the Sloan Digital Sky Survey (SDSS) filter
system and the quality of the SDSS imaging data are sufficient for determining
accurate and precise photometric redshifts (``photo-z''s) of quasars. Using a
sample of 2625 quasars, we show that photo-z determination is even possible for
z<=2.2 despite the lack of a strong continuum break that robust photo-z
techniques normally require. We find that, using our empirical method on our
sample of objects known to be quasars, approximately 70% of the photometric
redshifts are correct to within delta z = 0.2; the fraction of correct
photometric redshifts is even better for z>3. The accuracy of quasar
photometric redshifts does not appear to be dependent upon magnitude to nearly
21st magnitude in i'. Careful calibration of the color-redshift relation to
21st magnitude may allow for the discovery of on the order of 10^6 quasars
candidates in addition to the 10^5 quasars that the SDSS will confirm
spectroscopically. We discuss the efficient selection of quasar candidates from
imaging data for use with the photometric redshift technique and the potential
scientific uses of a large sample of quasar candidates with photometric
redshifts.Comment: 29 pages, 8 figures, submitted to A
Stability of a vacuum nonsingular black hole
This is the first of series of papers in which we investigate stability of
the spherically symmetric space-time with de Sitter center. Geometry,
asymptotically Schwarzschild for large and asymptotically de Sitter as
, describes a vacuum nonsingular black hole for and
particle-like self-gravitating structure for where a critical
value depends on the scale of the symmetry restoration to de Sitter
group in the origin. In this paper we address the question of stability of a
vacuum non-singular black hole with de Sitter center to external perturbations.
We specify first two types of geometries with and without changes of topology.
Then we derive the general equations for an arbitrary density profile and show
that in the whole range of the mass parameter objects described by
geometries with de Sitter center remain stable under axial perturbations. In
the case of the polar perturbations we find criteria of stability and study in
detail the case of the density profile
where is the density of de Sitter vacuum at the center, is de
Sitter radius and is the Schwarzschild radius.Comment: 18 pages, 8 figures, submitted to "Classical and Quantum Gravity
An Empirical Characterization of Extended Cool Gas Around Galaxies Using MgII Absorption Features
We report results from a survey of MgII absorbers in the spectra of
background QSOs that are within close angular distances to a foreground galaxy
at z<0.5, using the Magellan Echellette Spectrograph. We have established a
spectroscopic sample of 94 galaxies at a median redshift of = 0.24 in
fields around 70 distant background QSOs (z_QSO>0.6), 71 of which are in an
'isolated' environment with no known companions and located at rho <~ 120 h^-1
kpc from the line of sight of a background QSO. The rest-frame absolute B-band
magnitudes span a range from M_B-5log h=-16.4 to M_B-5log h=-21.4 and
rest-frame B_AB-R_AB colors range from B_AB-R_AB~0 to B_AB-R_AB~1.5. Of these
'isolated' galaxies, we find that 47 have corresponding MgII absorbers in the
spectra of background QSOs and rest-frame absorption equivalent width
W_r(2796)=0.1-2.34 A, and 24 do not give rise to MgII absorption to sensitive
upper limits. Our analysis shows that (1) Wr(2796) declines with increasing
distance from 'isolated' galaxies but shows no clear trend in 'group'
environments; (2) more luminous galaxies possess more extended MgII absorbing
halos with the gaseous radius scaled by B-band luminosity according to
R_gas=75x(L_B/L_B*)^(0.35+/-0.03) h^{-1} kpc; (3) there is little dependence
between the observed absorber strength and galaxy intrinsic colors; and (4)
within R_gas, we find a mean covering fraction of ~70% for absorbers
of Wr(2796)>=0.3 A and ~80% for absorbers of Wr(2796)>=0.1 A. The
lack of correlation between Wr(2796) and galaxy colors suggests a lack of
physical connection between the origin of extended MgII halos and recent star
formation history of the galaxies. Finally, we discuss the total gas mass in
galactic halos as traced by MgII absorbers. We also compare our results with
previous studies.Comment: 20 pages, 13 figures; to appear in the Astrophysical Journal 2010 May
10 issue; a version with higher resolution figures can be found at
http://lambda.uchicago.edu/public/tmp/mage_apj.pd
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