1,478 research outputs found
HST followup observations of two bright z ~ 8 candidate galaxies from the BoRG pure-parallel survey
We present followup imaging of two bright (L > L*) galaxy candidates at z > 8
from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter
on HST/WFC3. The F098M filter provides an additional constraint on the flux
blueward of the spectral break, and the observations are designed to
discriminate between low- and high-z photometric redshift solutions for these
galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly
probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the
brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2
interloper. As this source was substantially brighter than any other z > 8
candidate, removing it from the sample has a significant impact on the derived
UV luminosity function in this epoch. We show that while previous BoRG results
favored a shallow power-law decline in the bright end of the luminosity
function prior to reionization, there is now no evidence for departure from a
Schechter function form and therefore no evidence for a difference in galaxy
formation processes before and after reionization.Comment: Accepted by ApJL, 7 pages, 4 figure
Coulomb blockade of strongly coupled quantum dots studied via bosonization of a channel with a finite barrier
A pair of quantum dots, coupled through a point contact, can exhibit Coulomb
blockade effects that reflect an oscillatory term in the dots' total energy
whose value depends on whether the total number of electrons on the dots is
even or odd. The effective energy associated with this even-odd alternation is
reduced, relative to the bare Coulomb blockade energy for uncoupled dots, by a
factor (1-f) that decreases as the interdot coupling is increased. When the
transmission coefficient for interdot electronic motion is independent of
energy and the same for all channels within the point contact (which are
assumed uncoupled), the factor (1-f) takes on a universal value determined
solely by the number of channels and the dimensionless conductance g of each
individual channel.
This paper studies corrections to the universal value of (1-f) that result
when the transmission coefficent varies over energy scales of the size of the
bare Coulomb blockade energy. We consider a model in which the point contact is
described by a single orbital channel containing a parabolic barrier potential,
and we calculate the leading correction to (1-f) for one-channel (spin-split)
and two-channel (spin-degenerate) point contacts in the limit where the single
orbital channel is almost completely open. By generalizing a previously used
bosonization technique, we find that, for a given value of the dimensionless
conductance g, the value of (1-f) is increased relative to its value for a
zero-thickness barrier, but the absolute value of the increase is small in the
region where our calculations apply.Comment: 13 pages, 3 Postscript figure
Full sphere hydrodynamic and dynamo benchmarks
Convection in planetary cores can generate fluid flow and magnetic fields, and a number of sophisticated codes exist to simulate the dynamic behaviour of such systems. We report on the first community activity to compare numerical results of computer codes designed to calculate fluid flow within a whole sphere. The flows are incompressible and rapidly rotating and the forcing of the flow is either due to thermal convection or due to moving boundaries. All problems defined have solutions that allow easy comparison, since they are either steady, slowly drifting or perfectly periodic. The first two benchmarks are defined based on uniform internal heating within the sphere under the Boussinesq approximation with boundary conditions that are uniform in temperature and stress-free for the flow. Benchmark 1 is purely hydrodynamic, and has a drifting solution. Benchmark 2 is a magnetohydrodynamic benchmark that can generate oscillatory, purely periodic, flows and magnetic fields. In contrast, Benchmark 3 is a hydrodynamic rotating bubble benchmark using no slip boundary conditions that has a stationary solution. Results from a variety of types of code are reported, including codes that are fully spectral (based on spherical harmonic expansions in angular coordinates and polynomial expansions in radius), mixed spectral and finite difference, finite volume, finite element and also a mixed Fourier–finite element code. There is good agreement between codes. It is found that in Benchmarks 1 and 2, the approximation of a whole sphere problem by a domain that is a spherical shell (a sphere possessing an inner core) does not represent an adequate approximation to the system, since the results differ from whole sphere results
The bright-end galaxy candidates at z ~ 9 from 79 independent HST fields
We present a full data analysis of the pure-parallel Hubble Space Telescope
(HST) imaging observations in the Brightest of Reionizing Galaxies Survey
(BoRG[z9]) in Cycle 22. The medium-deep exposures with five HST/WFC3IR+UVIS
filter bands from 79 independent sightlines (~370 arcmin^2) provide the least
biased determination of number density for z>9 bright galaxies against cosmic
variance. After a strict two-step selection for candidate galaxies, including
dropout color and photometric redshift analyses, and revision of previous BoRG
candidates, we identify one source at z~10 and two sources at z~9. The z~10
candidate shows evidence of line-of-sight lens magnification (mu~1.5), yet it
appears surprisingly luminous (MUV ~ -22.6\pm0.3 mag), making it one of the
brightest candidates at z > 8 known (~ 0.3 mag brighter than the z = 8.68
galaxy EGSY8p7, spectroscopically confirmed by Zitrin and collaborators). For z
~ 9 candidates, we include previous data points at fainter magnitudes and find
that the data are well fitted by a Schechter luminosity function with alpha ~
-2.1, MUV ~ -21.5 mag, and log phi ~ -4.5 Mpc^-3mag^-1, for the first time
without fixing any parameters. The inferred cosmic star formation rate density
is consistent with unaccelerated evolution from lower redshift.Comment: 18pages, 7figures, 6tables. accepted to the Astrophysical Journa
Localization in Artificial Disorder - Two Coupled Quantum Dots
Using Single Electron Capacitance Spectroscopy, we study electron additions
in quantum dots containing two potential minima separated by a shallow barrier.
Analysis of addition spectra in magnetic field allows us to distinguish whether
electrons are localized in either potential minimum or delocalized over the
entire dot. We demonstrate that high magnetic field abruptly splits up a
low-density droplet into two smaller fragments, each residing in a potential
minimum. An unexplained cancellation of electron repulsion between electrons in
these fragments gives rise to paired electron additions.Comment: submitted to Phys.Rev.Let
First results from Faint Infrared Grism Survey (FIGS): first simultaneous detection of Lyman-alpha emission and Lyman break from a galaxy at z=7.51
Galaxies at high redshifts provide a valuable tool to study cosmic dawn, and
therefore it is crucial to reliably identify these galaxies. Here, we present
an unambiguous and first simultaneous detection of both the Lyman-alpha
emission and the Lyman break from a z = 7.512+/- 0.004 galaxy, observed in the
Faint Infrared Grism Survey (FIGS). These spectra, taken with G102 grism on
Hubble Space Telescope (HST), show a significant emission line detection (6
sigma) in multiple observational position angles (PA), with total integrated
Ly{\alpha} line flux of 1.06+/- 0.12 e10-17erg s-1cm-2. The line flux is nearly
a factor of four higher than the previous MOSFIRE spectroscopic observations of
faint Ly{\alpha} emission at {\lambda} = 1.0347{\mu}m, yielding z = 7.5078+/-
0.0004. This is consistent with other recent observations implying that
ground-based near-infrared spectroscopy underestimates total emission line
fluxes, and if confirmed, can have strong implications for reionization studies
that are based on ground-based Lyman-{\alpha} measurements. A 4-{\sigma}
detection of the NV line in one PA also suggests a weak Active Galactic Nucleus
(AGN), potentially making this source the highest-redshift AGN yet found. Thus,
this observation from the Hubble Space Telescope clearly demonstrates the
sensitivity of the FIGS survey, and the capability of grism spectroscopy to
study the epoch of reionization.Comment: Published in ApJL; matches published versio
Conductance oscillations in tunnel-coupled quantum dots in the quantum Hall regime
We present measurements of transport through two tunnel-coupled quantum dots
of different sizes connected in series in a strong, variable, perpendicular
magnetic field. Double dot conductance was measured both as a function of
magnetic field, which was varied across the filling factor nu = 4 quantum Hall
plateau, and as a function of charge induced evenly on the two dots. The
conductance peaks undergo position shifts and height modulations as the
magnetic field is varied. These shifts and modulations form a pattern that
repeats over large ranges of magnetic field and with the addition of double dot
charge. The robust pattern repetition is consistent with a frequency locking
effect.Comment: 12 pages, 4 figure
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