2,271 research outputs found
Neuron types in the zebrafish optic tectum labeled by an id2b transgene
The larval zebrafish optic tectum has emerged as a prominent model for understanding how neural circuits control visually guided behaviors. Further advances in this area will require tools to monitor and manipulate tectal neurons with cell type specificity. Here, we characterize the morphology and neurotransmitter phenotype of tectal neurons labeled by an id2b:gal4 transgene. Whole-brain imaging of stable transgenic id2b:gal4 larvae revealed labeling in a subset of neurons in optic tectum, cerebellum, and hindbrain. Genetic mosaic labeling of single neurons within the id2b:gal4 expression pattern enabled us to characterize three tectal neuron types with distinct morphologies and connectivities. The first is a neuron type previously identified in the optic tectum of other teleost fish: the tectal pyramidal neuron (PyrN). PyrNs are local interneurons that form two stratified dendritic arbors and one stratified axonal arbor in the tectal neuropil. The second tectal neuron type labeled by the id2b:gal4 transgene is a projection neuron that forms a stratified dendritic arbor in the tectal neuropil and an axon that exits tectum to form a topographic projection to torus longitudinalis (TL). A third neuron type labeled is a projection neuron with a nonstratified dendritic arbor and a descending axonal projection to tegmentum. These findings establish the id2b:gal4 transgenic as a useful tool for future studies aimed at elucidating the functional role of tectum, TL, and tegmentum in visually guided behaviors
Bright Source of Cold Ions for Surface-Electrode Traps
We produce large numbers of low-energy ions by photoionization of
laser-cooled atoms inside a surface-electrode-based Paul trap. The
isotope-selective trap loading rate of Yb ions/s exceeds
that attained by photoionization (electron impact ionization) of an atomic beam
by four (six) orders of magnitude. Traps as shallow as 0.13 eV are easily
loaded with this technique. The ions are confined in the same spatial region as
the laser-cooled atoms, which will allow the experimental investigation of
interactions between cold ions and cold atoms or Bose-Einstein condensates.Comment: Paper submitted to PRL for review on 2/1/0
Spin Excitations in a Fermi Gas of Atoms
We have experimentally investigated a spin excitation in a quantum degenerate
Fermi gas of atoms. In the hydrodynamic regime the damping time of the
collective excitation is used to probe the quantum behavior of the gas. At
temperatures below the Fermi temperature we measure up to a factor of 2
reduction in the excitation damping time. In addition we observe a strong
excitation energy dependence for this quantum statistical effect.Comment: 4 pages, 3 figure
Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV
We derive stellar masses, ages and star formation histories of massive
early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them
with those measured in a similarly mass-selected sample of field contemporaries
drawn from the GOODS South Field. Robust estimates of these parameters are
obtained by comparing a large grid of composite stellar population models with
8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the
entire relevant domain of emission of the different stellar populations.
Additionally, we present new, deep -band photometry of both fields, giving
access to the critical FUV rest-frame, in order to constrain empirically the
dependence on the environment of the most recent star formation processes. We
find that early-type galaxies, both in the cluster and in the field, show
analogous optical morphologies, follow comparable mass vs. size relation, have
congruent average surface stellar mass densities and lie on the same Kormendy
relation. We also that a fraction of early-type galaxies in the field employ
longer timescales, , to assemble their mass than their cluster
contemporaries. Hence we conclude that, while the formation epoch of early-type
only depends on their mass, the environment does regulate the timescales of
their star formation histories. Our deep -band imaging strongly supports
this conclusions. It shows that cluster galaxies are at least 0.5 mag fainter
than their field contemporaries of similar mass and optical-to-infrared colors,
implying that the last episode of star formation must have happened more
recently in the field than in the cluster.Comment: 20pages, 10 figures. to appear on Ap
Enhancement and suppression of spontaneous emission and light scattering by quantum degeneracy
Quantum degeneracy modifies light scattering and spontaneous emission. For
fermions, Pauli blocking leads to a suppression of both processes. In contrast,
in a weakly interacting Bose-Einstein condensate, we find spontaneous emission
to be enhanced, while light scattering is suppressed. This difference is
attributed to many-body effects and quantum interference in a Bose-Einstein
condensate.Comment: 4 pages 1 figur
Dilute Bose gas with correlated disorder: A Path Integral Monte Carlo study
We investigate the thermodynamic properties of a dilute Bose gas in a
correlated random potential using exact path integral Monte Carlo methods. The
study is carried out in continuous space and disorder is produced in the
simulations by a 3D speckle pattern with tunable intensity and correlation
length. We calculate the shift of the superfluid transition temperature due to
disorder and we highlight the role of quantum localization by comparing the
critical chemical potential with the classical percolation threshold. The
equation of state of the gas is determined in the regime of strong disorder,
where superfluidity is suppressed and the normal phase exists down to very low
temperatures. We find a dependence of the energy in agreement with the
expected behavior in the Bose glass phase. We also discuss the major role
played by the disorder correlation length and we make contact with a
Hartree-Fock mean-field approach that holds valid if the correlation length is
very large. The density profiles are analyzed as a function of temperature and
interaction strength. Effects of localization and the depletion of the order
parameter are emphasized in the comparison between local condensate and total
density. At very low temperature we find that the energy and the particle
distribution of the gas are very well described by the T=0 Gross-Pitaevskii
theory even in the regime of very strong disorder.Comment: 27 pages, 20 figure
Quasiparticle spectrum and dynamical stability of an atomic Bose-Einstein condensate coupled to a degenerate Fermi gas
The quasiparticle excitations and dynamical stability of an atomic
Bose-Einstein condensate coupled to a quantum degenerate Fermi gas of atoms at
zero temperature is studied. The Fermi gas is assumed to be either in the
normal state or to have undergone a phase transition to a superfluid state by
forming Cooper pairs. The quasiparticle excitations of the Bose-Einstein
condensate exhibit a dynamical instability due to a resonant exchange of energy
and momentum with quasiparticle excitations of the Fermi gas. The stability
regime for the bosons depends on whether the Fermi gas is in the normal state
or in the superfluid state. We show that the energy gap in the quasiparticle
spectrum for the superfluid state stabilizes the low energy energy excitations
of the condensate. In the stable regime, we calculate the boson quasiparticle
spectrum, which is modified by the fluctuations in the density of the Fermi
gas.Comment: 12 pages, 3 figure
Imprints of Environment on Cluster and Field Late-type Galaxies at z~1
We present a comparison of late-type galaxies (Sa and later) in intermediate
redshift clusters and the field using ACS imaging of four cluster fields:
CL0152-1357, CL1056-0337 (MS1054), CL1604+4304, and CL1604+4321. Concentration,
asymmetry, and clumpiness parameters are calculated for each galaxy in blue
(F606W or F625W) and red (F775W or F814W) filters. Galaxy half-light radii,
disk scale lengths, color gradients, and overall color are compared. We find
marginally significant differences in the asymmetry distributions of spiral and
irregular galaxies in the X-ray luminous and X-ray faint clusters. The massive
clusters contain fewer galaxies with large asymmetries. The physical sizes of
the cluster and field populations are similar; no significant differences are
found in half-light radii or disk scale lengths. The most significant
difference is in rest-frame color. Late-type cluster galaxies are
significantly redder, magnitudes at rest-frame , than their
field counterparts. Moreover, the intermediate-redshift cluster galaxies tend
to have blue inward color gradients, in contrast to the field galaxies, but
similar to late-type galaxies in low redshift clusters. These blue inward color
gradients are likely to be the result of enhanced nuclear star formation rates
relative to the outer disk. Based on the significant rest-frame color
difference, we conclude that late-type cluster members at are not a
pristine infalling field population; some difference in past and/or current
star formation history is already present. This points to high redshift
``groups'', or filaments with densities similar to present-day groups, as the
sites where the first major effects of environment are imprinted.Comment: updated titl
Assuring Integrity in the Residency Match Process
Integrity in medicine is essential. One of the most important competencies a medical student can master is to be honest at all times. Indeed, professionalism is a key competency in the identity formation and development of a medical student. At times, this competency becomes challenged during the resident matching process. The behavior of some students, faculty members, and program directors who participate in the residency program selection process (the Match) often falls short of this ideal when it comes to handling the process that will be most dispositive in directing the future careers of graduating medical students. Violations of both National Resident Matching Program (NRMP) rules and ethical norms have been reported in the literature and experienced by students. In this Invited Commentary, the authors recommend a series of reforms. Substantially more robust enforcement of NRMP rules should be considered, including the creation of an avenue for anonymous reporting by applicants who experience inappropriate pre-Match, post-interview communications
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