17,729 research outputs found
Solar sensor having coarse and fine sensing with matched preirradiated cells and method of selecting cells Patent
Solar sensor with coarse and fine sensing elements for matching preirradiated cells on degradation rate
The Stellar UV Background at z<1.5 and the Baryon Density of Photoionized Gas
We use new studies of the cosmic evolution of star-forming galaxies to
estimate the production rate of ionizing photons from hot, massive stars at low
and intermediate redshifts. The luminosity function of blue galaxies in the
Canada-France Redshift Survey shows appreciable evolution in the redshift
interval z=0-1.3, and generates a background intensity at 1 ryd of J_L~ 1.3 x
10^{-21} f_{esc} ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1} at z~0.5, where f_esc is
the unknown fraction of stellar Lyman-continuum photons which can escape into
the intergalactic space, and we have assumed that the absorption is picket
fence-type. We argue that recent upper limits on the H-alpha surface brightness
of nearby intergalactic clouds constrain this fraction to be <~ 20%. The
background ionizing flux from galaxies can exceed the QSO contribution at z~
0.5 if f_{esc}>~ 6%. We show that, in the general framework of a diffuse
background dominated by QSOs and/or star-forming galaxies, the cosmological
baryon density associated with photoionized, optically thin gas decreases
rapidly with cosmic time. The results of a recent Hubble Space Telescope survey
of OVI absorption lines in QSO spectra suggest that most of this evolution may
be due to the bulk heating and collisional ionization of the intergalactic
medium by supernova events in young galaxy halos.Comment: 6 pages, Latex file, 2 figures, mn.sty, MNRAS in pres
Topological properties of semigroup primes of a commutative ring
A semigroup prime of a commutative ring is a prime ideal of the semigroup
. One of the purposes of this paper is to study, from a topological
point of view, the space \scal(R) of prime semigroups of . We show that,
under a natural topology introduced by B. Olberding in 2010, \scal(R) is a
spectral space (after Hochster), spectral extension of \Spec(R), and that the
assignment R\mapsto\scal(R) induces a contravariant functor. We then relate
-- in the case is an integral domain -- the topology on \scal(R) with the
Zariski topology on the set of overrings of . Furthermore, we investigate
the relationship between \scal(R) and the space
consisting of all nonempty inverse-closed subspaces of \spec(R), which has
been introduced and studied in C.A. Finocchiaro, M. Fontana and D. Spirito,
"The space of inverse-closed subsets of a spectral space is spectral"
(submitted). In this context, we show that \scal( R) is a spectral retract of
and we characterize when \scal( R) is
canonically homeomorphic to , both in general and
when \spec(R) is a Noetherian space. In particular, we obtain that, when
is a B\'ezout domain, \scal( R) is canonically homeomorphic both to
and to the space \overr(R) of the overrings of
(endowed with the Zariski topology). Finally, we compare the space
with the space \scal(R(T)) of semigroup primes
of the Nagata ring , providing a canonical spectral embedding
\xcal(R)\hookrightarrow\scal(R(T)) which makes \xcal(R) a spectral retract
of \scal(R(T)).Comment: 21 page
A topological version of Hilbert's Nullstellensatz
We prove that the space of radical ideals of a ring , endowed with the
hull-kernel topology, is a spectral space, and that it is canonically
homeomorphic to the space of the nonempty Zariski closed subspaces of
Spec, endowed with a Zariski-like topology.Comment: J. Algebra (to appear
Photometric redshifts and selection of high redshift galaxies in the NTT and Hubble Deep Fields
We present and compare in this paper new photometric redshift catalogs of the
galaxies in three public fields: the NTT Deep Field, the HDF-N and the HDF-S.
Photometric redshifts have been obtained for thewhole sample, by adopting a
minimization technique on a spectral library drawn from the Bruzual
and Charlot synthesis models, with the addition of dust and intergalactic
absorption. The accuracy, determined from 125 galaxies with known spectroscopic
redshifts, is in the redshift intervals . The global redshift distribution of I-selected galaxies shows a
distinct peak at intermediate redshifts, z~0.6 at I_{AB}<26 and z~0.8 at
I_{AB}<27.5 followed by a tail extending to z~6. We also present for the first
time the redshift distribution of the total IR-selected sample to faint limits
( and ). It is found that the number density of galaxies
at 1.25<z<1.5 is ~ 0.1 /arcmin^22 at J<=21 and ~1./arcmin^2} at J<22, and drops
to 0.3/arcmin^2 (at J<22) at 1.5<z<2. The HDFs data sets are used to compare
the different results from color selection criteria and photometric redshifts
in detecting galaxies in the redshift range 3.5<z<4.5 Photometric redshifts
predict a number of high z candidates in both the HDF-N and HDF-S that is
nearly 2 times larger than color selection criteria, and it is shown that this
is primarily due to the inclusion of dusty models that were discarded in the
original color selection criteria by Madau et al 1998. In several cases, the
selection of these objects is made possible by the constraints from the IR
bands. Finally, it is shown that galactic M stars may mimic z>5 candidates in
the HDF filter set and that the 4 brightest candidates at in the HDF-S
are indeed most likely M stars. (ABRIDGED)Comment: Version accepted on July, 20, 2000. To appear on Astronomical
Journal, Nov 2000. The data and photometric redshift catalogs presented here
are available on line at http://www.mporzio.astro.it/HIGH
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