45,001 research outputs found
Transistorized Marx bank pulse circuit provides voltage multiplication with nanosecond rise-time
Base-triggered avalanche transistor circuit used in a Marx bank pulser configuration provides voltage multiplication with nanosecond rise-time. The avalanche-mode transistors replace conventional spark gaps in the Marx bank. The delay time from an input signal to the output signal to the output is typically 6 nanoseconds
ACS photometry of extended, luminous globular clusters in the outskirts of M31
A new population of extended, luminous globular clusters has recently been
discovered in the outskirts of M31. These objects have luminosities typical of
classical globular clusters, but much larger half-light radii. We report the
first results from deep ACS imaging of four such clusters, one of which is a
newly-discovered example lying at a projected distance of ~60 kpc from M31. Our
F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the
horizontal branch level, and clearly demonstrate, for the first time, that all
four clusters are composed of >10 Gyr old, metal-poor stellar populations. No
evidence for multiple populations is observed. From a comparison with Galactic
globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H]
< -1.8. The observed horizontal branch morphologies show a clear second
parameter effect between the clusters. Preliminary radial luminosity profiles
suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median
value of the globular cluster luminosity function. Our results confirm that
these four objects are bona fide old, metal-poor globular clusters, albeit with
combined structures and luminosities unlike those observed for any other
globular clusters in the Local Group or beyond.Comment: 12 pages, 3 figures, 1 table; accepted for publication in ApJ Letter
A comparison of calculated and measured background noise rates in hard X-ray telescopes at balloon altitude
An actively shielded hard X-ray astronomical telescope has been flown on stratospheric balloons. An attempt is made to compare the measured spectral distribution of the background noise counting rates over the energy loss range 20-300 keV with the contributions estimated from a series of Monte Carlo and other computations. The relative contributions of individual particle interactions are assessed
On the accretion origin of a vast extended stellar disk around the Andromeda galaxy
We present the discovery of an inhomogenous, low-surface brightness, extended
disk-like structure around the Andromeda galaxy (M31) based on a large
kinematic survey of more than 2800 stars with the Keck/DEIMOS spectrograph. The
stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out
to R ~ 70 kpc. The constituent stars lag the expected velocity of circular
orbits in the plane of the M31 disk by ~40 kms and have a velocity dispersion
of ~30 kms. The color range on the upper RGB shows a large spread indicative of
a population with a significant range of metallicity. The mean metallicity of
the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 +/-
0.2. The morphology of the structure is irregular at large radii, and shows a
wealth of substructures which must be transitory in nature, and are almost
certainly tidal debris. The presence of these substructures indicates that the
global entity was formed by accretion. This extended disk follows smoothly on
from the central parts of M31 disk with an exponential density law of
scale-length of 5.1 +/- 0.1 kpc, similar to that of the bright inner disk. The
population possesses similar kinematic and abundance properties over the entire
region where it is detected in the survey. We estimate that the structure
accounts for approximately 10% of the total luminosity of the M31 disk, and
given the huge scale, contains ~30% of the total disk angular momentum. This
finding indicates that at least some galactic stellar disks are vastly larger
than previously thought and are formed, at least in their outer regions,
primarily by accretion. [abridged]Comment: 20 pages, 30 figures, ApJ submitte
Elastic precursor of the transformation from glycolipid-nanotube to -vesicle
By the combination of optical tweezer manipulation and digital video
microscopy, the flexural rigidity of single glycolipid "nano" tubes has been
measured below the transition temperature at which the lipid tubules are
transformed into vesicles. Consequently, we have found a clear reduction of the
rigidity obviously before the transition as temperature increasing. Further
experiments of infrared spectroscopy (FT-IR) and differential scanning
calorimetry (DSC) have suggested a microscopic change of the tube walls,
synchronizing with the precursory softening of the nanotubes.Comment: 9 pages, 6 figure
Pattern formation by lateral inhibition with feedback: a mathematical model of Delta-Notch intercellular signalling
In many developing tissues, adjacent cells diverge in character so as to create a fine-grained pattern of cells in contrasting states of differentiation. It has been proposed that such patterns can be generated through lateral inhibition—a type cells–cell interaction whereby a cell that adopts a particular fate inhibits its immediate neighbours from doing likewise. Lateral inhibition is well documented in flies, worms and vertebrates. In all of these organisms, the transmembrane proteins Notch and Delta (or their homologues) have been identified as mediators of the interaction—Notch as receptor, Delta as its ligand on adjacent cells. However, it is not clear under precisely what conditions the Delta-Notch mechanism of lateral inhibition can generate the observed types of pattern, or indeed whether this mechanism is capable of generating such patterns by itself. Here we construct and analyse a simple and general mathematical model of such contact-mediated lateral inhibition. In accordance with experimental data, the model postulates that receipt of inhibition (i.e. activation of Notch) diminishes the ability to deliver inhibition (i.e. to produce active Delta). This gives rise to a feedback loop that can amplify differences between adjacent cells. We investigate the pattern-forming potential and temporal behavior of this model both analytically and through numerical simulation. Inhomogeneities are self-amplifying and develop without need of any other machinery, provided the feedback is sufficiently strong. For a wide range of initial and boundary conditions, the model generates fine-grained patterns similar to those observed in living systems
Young accreted globular clusters in the outer halo of M31
We report on Gemini/GMOS observations of two newly discovered globular
clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at
a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc,
onto a field halo substructure known as the South-West Cloud. We measure radial
velocities for the two clusters which confirm that they are almost certainly
physically associated with this feature. Colour-magnitude diagrams reveal
strikingly short, exclusively red horizontal branches in both PA-7 and PA-8;
both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the
morphology of the horizontal branch is maximally sensitive to age, and we use
the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both
objects are very likely to be at least 2 Gyr younger than the oldest Milky Way
globular clusters. Our observations provide strong evidence for young globular
clusters being accreted into the remote outer regions of M31 in a manner
entirely consistent with the established picture for the Milky Way, and add
credence to the idea that similar processes play a central role in determining
the composition of globular cluster systems in large spiral galaxies in
general.Comment: 14 pages, 8 figures, accepted for publication in MNRA
The Andromeda Stream
The existence of a stream of tidally stripped stars from the Sagittarius
Dwarf galaxy demonstrates that the Milky Way is still in the process of
accreting mass. More recently, an extensive stream of stars has been uncovered
in the halo of the Andromeda galaxy (M31), revealing that it too is
cannibalizing a small companion. This paper reports the recent observations of
this stream, determining it spatial and kinematic properties, and tracing its
three-dimensional structure, as well as describing future observations and what
we may learn about the Andromeda galaxy from this giant tidal stream.Comment: 3 Pages. Refereed contribution to the 5th Galacto Chemodynamics
conference held in Swinburne, July 2003. Accepted for publication in PAS
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