27,504 research outputs found
Telescopes don't make catalogues!
Astronomical instruments make intensity measurements; any precise
astronomical experiment ought to involve modeling those measurements. People
make catalogues, but because a catalogue requires hard decisions about
calibration and detection, no catalogue can contain all of the information in
the raw pixels relevant to most scientific investigations. Here we advocate
making catalogue-like data outputs that permit investigators to test hypotheses
with almost the power of the original image pixels. The key is to provide users
with approximations to likelihood tests against the raw image pixels. We
advocate three options, in order of increasing difficulty: The first is to
define catalogue entries and associated uncertainties such that the catalogue
contains the parameters of an approximate description of the image-level
likelihood function. The second is to produce a K-catalogue sampling in
"catalogue space" that samples a posterior probability distribution of
catalogues given the data. The third is to expose a web service or equivalent
that can re-compute on demand the full image-level likelihood for any
user-supplied catalogue.Comment: presented at ELSA 2010: Gaia, at the frontiers of astrometr
Multiwavelength Observations of Massive Stellar Cluster Candidates in the Galaxy
The Galaxy appears to be richer in young, massive stellar clusters than
previously known, due to advances in infrared surveys which have uncovered
deeply embedded regions of star formation. Young, massive clusters can
significantly impact the surrounding interstellar medium (ISM) and hence radio
observations can also be an important tracer of their activity. Several hundred
cluster candidates are now known by examining survey data. Here we report on
multiwavelength observations of six of these candidates in the Galaxy. We
carried out 4.9 and 8.5 GHz VLA observations of the radio emission associated
with these clusters to obtain the physical characteristics of the surrounding
gas, including the Lyman continuum photon flux and ionized gas mass. Spitzer
Infrared Array Camera observations were also made of these regions, and provide
details on the stellar population as well as the dust continuum and polycyclic
aromatic hydrocarbon emission. When compared to the known young, massive
clusters in the Galaxy, the six cluster candidates have less powerful Lyman
ionizing fluxes and ionize less of the H II mass in the surrounding ISM.
Therefore, these cluster candidates appear to be more consistent with
intermediate-mass clusters (10^3-10^4 Msun).Comment: 39 pages, 20 figures. Accepted in the Astronomical Journal; to be
published Fall 201
Quantum critical behaviour of the plateau-insulator transition in the quantum Hall regime
High-field magnetotransport experiments provide an excellent tool to
investigate the plateau-insulator phase transition in the integral quantum Hall
effect. Here we review recent low-temperature high-field magnetotransport
studies carried out on several InGaAs/InP heterostructures and an InGaAs/GaAs
quantum well. We find that the longitudinal resistivity near the
critical filling factor ~ 0.5 follows the universal scaling law
, where . The critical exponent equals ,
which indicates that the plateau-insulator transition falls in a non-Fermi
liquid universality class.Comment: 8 pages, accepted for publication in Proceedings of the Yamada
Conference LX on Research in High Magnetic Fields (August 16-19, 2006,
Sendai
Invariance of simultaneous similarity and equivalence of matrices under extension of the ground field
We give a new and elementary proof that simultaneous similarity and
simultaneous equivalence of families of matrices are invariant under extension
of the ground field, a result which is non-trivial for finite fields and first
appeared in a paper of Klinger and Levy.Comment: 10 pages (minor corrections
To what extent is a large space of matrices not closed under the product?
Let K denote a field. Given an arbitrary linear subspace V of M_n(K) of
codimension lesser than n-1, a classical result states that V generates the
K-algebra M_n(K). Here, we strengthen this in three ways: we show that M_n(K)
is spanned by the products of the form AB with A and B in V; we prove that
every matrix in M_n(K) can be decomposed into a product of matrices of V;
finally, when V is a linear hyperplane of M_n(K) and n>2, we show that every
matrix in M_n(K) is a product of two elements of V.Comment: 20 pages (v2 : minor typos corrected, title changed
New Insights into the Plateau-Insulator Transition in the Quantum Hall Regime
We have measured the quantum critical behavior of the plateau-insulator (PI)
transition in a low-mobility InGaAs/GaAs quantum well. The longitudinal
resistivity measured for two different values of the electron density follows
an exponential law, from which we extract critical exponents kappa = 0.54 and
0.58, in good agreement with the value (kappa = 0.57) previously obtained for
an InGaAs/InP heterostructure. This provides evidence for a non-Fermi liquid
critical exponent. By reversing the direction of the magnetic field we find
that the averaged Hall resistance remains quantized at the plateau value h/e^2
through the PI transition. From the deviations of the Hall resistance from the
quantized value, we obtain the corrections to scaling.Comment: accepted proceedings of EP2DS-15 (to be published in Physica E
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