23,878 research outputs found
A 100 micro Kelvin bolometer system for SIRTF
Progress toward a prototype of 100 mK bolometric detection system for the Space Infrared Telescope Facility (SIRTF) is described. Two adiabatic demagnetization refrigerators (ADR's) were constructed and used to investigate the capabilities necessary for orbital operation. The first, a laboratory ADR, demonstrated a hold time at 0.1 K of over 12 hours, with temperature stability approx. 3 micro-K RMS achieved by controlling the magnetic field. A durable salt pill and an efficient support system have been demonstrated. A second ADR, the SIRTF flight prototype, has been built and will be flown on a balloon. Techniques for magnetic shielding, low heat leak current leads, and a mechanical heat switch are being developed in this ADR. Plans for construction of 100 mK bolometers are discussed. Three important cosmological investigations which will be carried out by these longest wavelength SIRTF detectors are described
The Parsec-scale Structure, Kinematics, and Polarization of Radio-Loud Narrow-Line Seyfert 1 Galaxies
Several narrow-line Seyfert 1 galaxies (NLS1s) have now been detected in
gamma rays, providing firm evidence that at least some of this class of active
galactic nuclei (AGN) produce relativistic jets. The presence of jets in NLS1s
is surprising, as these sources are typified by comparatively small black hole
masses and near- or super-Eddington accretion rates. This challenges the
current understanding of the conditions necessary for jet production. Comparing
the properties of the jets in NLS1s with those in more familiar jetted systems
is thus essential to improve jet production models. We present early results
from our campaign to monitor the kinematics and polarization of the
parsec-scale jets in a sample of 15 NLS1s through multifrequency observations
with the Very Long Baseline Array. These observations are complemented by
fast-cadence 15 GHz monitoring with the Owens Valley Radio Observatory 40m
telescope and optical spectroscopic monitoring with with the 2m class telescope
at the Guillermo Haro Astrophysics Observatory in Cananea, Mexico.Comment: 4 pages, 1 figure. To appear in the Proceedings of the IAU Symposium
No. 313: "Extragalactic jets from every angle," Galapagos, Ecuador, 15-19
September 2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska
(Eds.), Cambridge University Pres
The dust emission of high-redshift quasars
The detection of powerful near-infrared emission in high redshift (z>5)
quasars demonstrates that very hot dust is present close to the active nucleus
also in the very early universe. A number of high-redshift objects even show
significant excess emission in the rest frame NIR over more local AGN spectral
energy distribution (SED) templates. In order to test if this is a result of
the very high luminosities and redshifts, we construct mean SEDs from the
latest SDSS quasar catalogue in combination with MIR data from the WISE
preliminary data release for several redshift and luminosity bins. Comparing
these mean SEDs with a large sample of z>5 quasars we could not identify any
significant trends of the NIR spectral slope with luminosity or redshift in the
regime 2.5 < z < 6 and 10^45 < nuL_nu(1350AA) < 10^47 erg/s. In addition to the
NIR regime, our combined Herschel and Spitzer photometry provides full infrared
SED coverage of the same sample of z>5 quasars. These observations reveal
strong FIR emission (L_FIR > 10^13 L_sun) in seven objects, possibly indicating
star-formation rates of several thousand solar masses per year. The FIR excess
emission has unusally high temperatures (T ~ 65 K) which is in contrast to the
temperature typically expected from studies at lower redshift (T ~ 45 K). These
objects are currently being investigated in more detail.Comment: 6 pages, 3 figures, to appear in the proceedings to "The Central
Kiloparsec in Galactic Nuclei (AHAR2011)", Journal of Physics: Conference
Series (JPCS), IOP Publishin
Cortical axon guidance by the glial wedge during the development of the corpus callosum
Growing axons are often guided to their final destination by intermediate targets. In the developing spinal cord and optic nerve, specialized cells at the embryonic midline act as intermediate targets for guiding commissural axons. Here we investigate whether similar intermediate targets may play a role in guiding cortical axons in the developing brain. During the development of the corpus callosum, cortical axons from one cerebral hemisphere cross the midline to reach their targets in the opposite cortical hemisphere. We have identified two early differentiating populations of midline glial cells that may act as intermediate guideposts for callosal axons. The first differentiates directly below the corpus callosum forming a wedge shaped structure (the glial wedge) and the second differentiates directly above the corpus callosum within the indusium griseum. Axons of the corpus callosum avoid both of these populations in vivo. This finding is recapitulated in vitro in three-dimensional collagen gels. In addition, experimental manipulations in organotypic slices show that callosal axons require the presence and correct orientation of these populations to turn toward the midline. We have also identified one possible candidate for this activity because both glial populations express the chemorepellent molecule slit-2, and cortical axons express the slit-2 receptors robo-1 and robo-2. Furthermore, slit-2 repels-suppresses cortical axon growth in three-dimensional collagen gel cocultures
A novel kinematics analysis method using quaternion interpolation – a case study in frog jumping
Spherical Linear Interpolation (SLERP) has long been used in computer animation to interpolate movements between two 3D orientations. We developed a forward kinematics (FK) approach using quaternions and SLERP to predict how frogs modulate jump kinematics between start posture and takeoff. Frog limb kinematics have been studied during various activities, yet the causal link between differences in joint kinematics and locomotor variation remains unknown. We varied 1) takeoff angle from 8 to 60 degrees; 2) turn angle from 0 to 18 degrees; and 3) initial body pitch from 0 to 70 degrees. Simulations were similar to experimentally observed frog kinematics. Findings suggest a fundamental mechanism whereby limb elevation is modulated by thigh and shank adduction. Forward thrust is produced by thigh and proximal foot retraction with little contribution from the shank except to induce asymmetries for turning. Kinematic shifts causing turns were subtle, marked only by slight counter-rotation of the left versus right shank as well as a 10% timing offset in proximal foot adduction. Additionally, inclining initial body tilt influenced the centre of mass trajectory to determine direction of travel at takeoff. Most importantly, our theory suggests firstly that the convergence of leg segment rotation axes toward a common orientation is crucial both for limb extension and for coordinating jump direction; and, secondly, the challenge of simulating 3D kinematics is simplified using SLERP because frog limbs approximately follow linear paths in unit quaternion space. Our methodology can be applied more broadly to study living and fossil frog taxa as well as to inspire new control algorithms for robotic limbs
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