12,631 research outputs found
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
One-center and Two-center Expansions of the Breit-pauli Hamiltonian
One and two center expansions for orbit-orbit, spin-spin, and spin-orbit Hamiltonian
Compensated Current Injection circuit, theory and applications
This paper presents a detailed description, analysis and example of practical
application of a wide frequency band voltage-to-current converter. The
converter is characterized by a combination of positive and negative feedback
loops. This feature allows compensation for parasitic impedance connected in
parallel with the useful load, which in turn keeps an excitation current
flowing through the useful load independent of its impedance. The simplicity of
the circuit and its good electrical properties are additional advantages of the
scheme.Comment: 9 pages and 7 figures in one PDF fil
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
Clustering at high redshift
The addition of deep near infrared images to the database provided by the
HDF-S WFPC2 is essential to monitor the SEDs of the objects on a wide baseline
and address a number of key issues including the total stellar content of
baryonic mass, the effects of dust extinction, the dependence of morphology on
the rest frame wavelength, the photometric redshifts, the detection and nature
of extremely red objects (EROs). For these reasons deep near infrared images
were obtained with the ISAAC instrument at the ESO VLT in the Js, H and Ks
bands reaching, respectively, 23.5, 22.0, 22.0 limiting Vega-magnitude. A
multi-color (F300, F450, F606, F814, Js, H, Ks) photometric catalog of the
HDF-S has been produced. Photometric redshifts have been generated both fitting
templates to the observed SEDs and with neural network techniques.
Spectroscopic observations of the 9 candidates with I_AB <24.25 have confirmed
all of them to be galaxies with 2<z<3.5. The photometric redshifts for all the
galaxies brighter than I_AB< 27.5 have been used to study the evolution of
galaxy clustering in the interval 0<z<4.5.Comment: 2 pages Latex, To appear in the proceedings of "The mass of galaxies
at low and high redshift", Venice, Oct 24-26, 2001,eds. R. Bender and A.
Renzini (ESO Astrophysics Symposia, Springer-Verlag
New distinguished classes of spectral spaces: a survey
In the present survey paper, we present several new classes of Hochster's
spectral spaces "occurring in nature", actually in multiplicative ideal theory,
and not linked to or realized in an explicit way by prime spectra of rings. The
general setting is the space of the semistar operations (of finite type),
endowed with a Zariski-like topology, which turns out to be a natural
topological extension of the space of the overrings of an integral domain,
endowed with a topology introduced by Zariski. One of the key tool is a recent
characterization of spectral spaces, based on the ultrafilter topology, given
in a paper by C. Finocchiaro in Comm. Algebra 2014. Several applications are
also discussed
-optimal saturated designs: a simulation study
In this work we focus on saturated -optimal designs. Using recent results,
we identify -optimal designs with the solutions of an optimization problem
with linear constraints. We introduce new objective functions based on the
geometric structure of the design and we compare them with the classical
-efficiency criterion. We perform a simulation study. In all the test cases
we observe that designs with high values of -efficiency have also high
values of the new objective functions.Comment: 8 pages. Preliminary version submitted to the 7th IWS Proceeding
Well I Wonder, Wonder...Who Wrote the *#!@%#*! Song: A Call for the Abolishment of Frivolous Lawsuits in the Music Copyright Infringement Arena Through the Right Mix of Existing Law
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