585 research outputs found
Dust penetrated morphology in the high redshift Universe
Images from the Hubble Deep Field (HDF) North and South show a large
percentage of dusty, high redshift galaxies whose appearance falls outside
traditional classification systems. The nature of these objects is not yet
fully understood. Since the HDF preferentially samples restframe UV light, HDF
morphologies are not dust or `mask' penetrated. The appearance of high redshift
galaxies at near-infrared restframes remains a challenge for the New
Millennium. The Next Generation Space Telescope (NGST) could routinely provide
us with such images. In this contribution, we quantitatively determine the
dust-penetrated structures of high redshift galaxies such as NGC 922 in their
near-infrared restframes. We show that such optically peculiar objects may
readily be classified using the dust penetrated z ~ 0 templates of Block and
Puerari (1999) and Buta and Block (2001).Comment: 4 pages, 2 figures. Presented at the conference "The Link between
Stars and Cosmology", 26-30 March, 2001, Puerto Vallarta, Mexico. To be
published by Kluwer, eds. M. Chavez, A. Bressan, A. Buzzoni, and D. Mayya.
High-resolution version of Figure 2 can be found at
http://www.inaoep.mx/~puerari/conf_puertovallart
Consistency and Unanimity in the House Allocation Problems I: Collective Initial Endowments
This paper studies allocation correspondences in the house allocation problems with collective initial endowments. We examine the implications of two axioms, namely 'consistency' and 'unanimity.' Consistency requires the allocation correspondence be invariant under reductions of population. Unanimity requires the allocation correspondence respect unanimity, that is, it assigns to every agent the object that ranks best for him whenever possible. We prove that if an allocation correspondence satisfies these two axioms, then it is a subcorrespondence of the Pareto correspondence. Further, we give a characterization of the Pareto correspondence using a version of 'converse consistency.
Nearly optimal solutions for the Chow Parameters Problem and low-weight approximation of halfspaces
The \emph{Chow parameters} of a Boolean function
are its degree-0 and degree-1 Fourier coefficients. It has been known
since 1961 (Chow, Tannenbaum) that the (exact values of the) Chow parameters of
any linear threshold function uniquely specify within the space of all
Boolean functions, but until recently (O'Donnell and Servedio) nothing was
known about efficient algorithms for \emph{reconstructing} (exactly or
approximately) from exact or approximate values of its Chow parameters. We
refer to this reconstruction problem as the \emph{Chow Parameters Problem.}
Our main result is a new algorithm for the Chow Parameters Problem which,
given (sufficiently accurate approximations to) the Chow parameters of any
linear threshold function , runs in time \tilde{O}(n^2)\cdot
(1/\eps)^{O(\log^2(1/\eps))} and with high probability outputs a
representation of an LTF that is \eps-close to . The only previous
algorithm (O'Donnell and Servedio) had running time \poly(n) \cdot
2^{2^{\tilde{O}(1/\eps^2)}}.
As a byproduct of our approach, we show that for any linear threshold
function over , there is a linear threshold function which
is \eps-close to and has all weights that are integers at most \sqrt{n}
\cdot (1/\eps)^{O(\log^2(1/\eps))}. This significantly improves the best
previous result of Diakonikolas and Servedio which gave a \poly(n) \cdot
2^{\tilde{O}(1/\eps^{2/3})} weight bound, and is close to the known lower
bound of (1/\eps)^{\Omega(\log \log (1/\eps))}\} (Goldberg,
Servedio). Our techniques also yield improved algorithms for related problems
in learning theory
Yield Measurements for ^7Be and ^<10>Be Productions from ^<nat>Cu, ^<nat>Ag and ^<197>Au by Bremsstrahlung Irradiation at E_0=200 MeV(II. Radiochemistry)
The yields of ^7Be and ^Be produced by bremsstrahlung having a maximum energy (E_0) of 200 MeV in ^Cu, ^Ag and ^Au targets were investigated by the AMS technique at MALT of the University of Tokyo. It was found that the yields at E_0 = 200 MeV were much lower than those at E_0 ≧250 MeV, obtained in our previous work. A change in the yields of the fragmentation component in the target-mass dependence was observed at E_0=200 MeV when compared with those at E_0≧250 MeV. However, the ratios of the fragmentation yield of ^Be to that of ^7Be remained unchanged throughout the concerned E_0
The dark matter halo shape of edge-on disk galaxies - II. Modelling the HI observations: methods
This is the second paper of a series in which we attempt to put constraints
on the flattening of dark halos in disk galaxies. For this purpose, we observe
the HI in edge-on galaxies, where it is in principle possible to measure the
force field in the halo vertically and radially from gas layer flaring and
rotation curve decomposition respectively. To calculate the force fields, we
need to analyse the observed XV diagrams to accurately measure all three
functions that describe the planar kinematics and distribution of a galaxy: the
radial HI surface density, the rotation curve and the HI velocity dispersion.
In this paper, we discuss the improvements and limitations of the methods
previously used to measure these HI properties. We extend the constant velocity
dispersion method to include determination of the HI velocity dispersion as a
function of galactocentric radius and perform extensive tests on the quality of
the fits. We will apply this 'radial decomposition XV modelling method' to our
HI observations of 8 HI-rich, late-type, edge-on galaxies in the third paper of
this series.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12566.pd
Neuromuscular synaptic function in mice lacking major subsets of gangliosides
Gangliosides are a family of sialylated glycosphingolipids enriched in the outer leaflet of neuronal membranes, in particular at synapses. Therefore, they have been hypothesized to play a functional role in synaptic transmission. We have measured in detail the electrophysiological parameters of synaptic transmission at the neuromuscular junction (NMJ) ex vivo of a GD3-synthase knockout mouse, expressing only the O- and a-series gangliosides, as well as of a GM2/GD2-synthase*GD3-synthase double-knockout (dKO) mouse, lacking all gangliosides except GM3. No major synaptic deficits were found in either null-mutant. However, some extra degree of rundown of acetylcholine release at high intensity use was present at the dKO NMJ and a temperature-specific increase in acetylcholine release at 35 °C was observed in GD3-synthase knockout NMJs, compared with wild-type. These results indicate that synaptic transmission at the NMJ is not crucially dependent on the particular presence of most ganglioside family members and remains largely intact in the sole presence of GM3 ganglioside. Rather, presynaptic gangliosides appear to play a modulating role in temperature- and use-dependent fine-tuning of transmitter output
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