3,790 research outputs found
The Escape Fraction of Ionizing Radiation from Galaxies
The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. We present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from galaxies
ranging from primordial systems to disk-type galaxies that are not heavily
dust-obscured. We consider variations in the galaxy density profile, source
type, location, and spectrum, and gas overdensity/distribution factors. For
sufficiently hard first-light sources, the helium ionization fronts closely
track or advance beyond that of hydrogen. Key new results in this work include
calculations of the escape fractions for He I and He II ionizing radiation, and
the impact of partial ionization from X-rays from early AGN or stellar clusters
on the escape fractions from galaxy halos. When factoring in
frequency-dependent effects, we find that X-rays play an important role in
boosting the escape fractions for both hydrogen and helium, but especially for
He II. We briefly discuss the implications of these results for recent
observations of the He II reionization epoch at low redshifts, as well as the
UV data and emission-line signatures from early galaxies anticipated from
future satellite missions.Comment: 43 pages, 9 figures, accepted in ApJ, comments welcom
Fuzzy !-automata and its relationships
A notion of finite ! - automata with single initial state is proposed.
The concept of fuzzy deterministic Buchi automaton and Muller automaton
with full acceptance component which is recognize the same fuzzy language are
studied. We also establish the relationship between fuzzy deterministic Rabin
automaton and Muller automaton. Further, we define the transition fuzzy !
- automata and show that these automata recognize the same fuzzy language
as in the fuzzy ! - automata. Finally, we give some closure properties of fuzzy
deterministic ! - automata
Double heterostructure lasers with facets formed by a hybrid wet and reactive-ion-etching technique
Double heterostructure lasers were fabricated in which one of the laser facets was produced by a hybrid wet and reactive-ion-etching technique. This technique is suitable for GaAs/GaAlAs heterostructure lasers and utilizes the selectivity of the plasma in preferentially etching GaAs over GaAlAs. Lasers fabricated by this technique are compatible with optoelectronic integration and have threshold currents and quantum efficiency comparable to lasers with both mirrors formed by cleaving. The technique enables the use of relatively higher pressures of noncorrosive gases in the etch plasma resulting in smoother mirror surfaces and further eliminates the nonreproducibility inherent in the etching of GaAlAs layers
Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization
We examine the significance of the first metal-free stars (Pop III) for the
cosmological reionization of HI and HeII. These stars have unusually hard
spectra, with the integrated ionizing photon rates from a Pop III stellar
cluster for HI and HeII being 1.6 and times stronger respectively than
those from a Pop II cluster. For the currently favored cosmology, we find that
Pop III stars alone can reionize HI and HeII at redshifts, of about 9 (4.7)
and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More
realistic scenarios involving combinations of Pop III and Pop II stellar
spectra yield similar results for hydrogen. Helium never reionizes completely
in these cases; the ionization fraction of HeIII reaches a maximum of about 60
percent at of about 5.6 if Pop III star formation lasts for yr.
Future data on HI reionization can test the amount of small-scale power
available to the formation of the first objects, and provide a constraint on
values of less than or about 0.7. Since current UV observations
indicate an epoch of reionization for HeII at of about 3, HeII may reionize
more than once. Measurements of the HeII Gunn-Peterson effect in the
intergalactic medium at redshifts exceeding about 3 may reveal the significance
of Pop III stars for HeII reionization, particularly in void regions that may
contain relic ionization from early Pop III stellar activity.Comment: 29 pages, 4 figures. Accepted for publication in Ap.J. (Feb. 20, 2003
issue; v. 584); minor revisions, results unchange
Modal properties of unstable resonator semiconductor lasers with a lateral waveguide
The modal properties of unstable resonator lasers with a lateral waveguide have been analyzed, and an unstable resonator semiconductor laser with a real index lateral waveguide has been demonstrated. Output powers in excess of 400 mW were observed with a stable, highly coherent lateral field distribution. The incorporation of a lateral real index waveguide with the unstable resonator configuration results in an increase in the external quantum efficiency and the appearance of ripples in the lateral field distribution
Unstable resonator cavity semiconductor lasers
GaAs heterostructure lasers with unstable resonator cavities were demonstrated for the first time with both curved mirrors fabricated by etching. Typical output powers of 0.35 W were observed in a stable, highly coherent lateral mode. The laser operated stably in a single longitudinal mode over a large range of injection currents. The external quantum efficiency was 70% of that of a similar laser with both mirror facets cleaved implying good output coupling of the energy from the entire region
Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background
We examine the constraints on high-redshift star formation, ultraviolet and
X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred
from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron
scattering optical depth of the cosmic microwave background (CMB). Half of this
scattering can be accounted for by the optical depth, tau_e = 0.04-0.05, of a
fully ionized intergalactic medium (IGM) at z < z_GP = 6-7, consistent with
Gunn-Peterson absorption in neutral hydrogen. The required additional optical
depth, Delta-tau_e = 0.03 +/- 0.02 at z > z_GP, constrains the ionizing
contributions of first light sources. WMAP-5 also measured a significant
increase in small-scale power, which lowers the required efficiency of star
formation and ionization from mini-halos. Early massive stars (UV radiation)
and black holes (X-rays) can produce a partially ionized IGM, adding to the
residual electrons left from incomplete recombination. Inaccuracies in
computing the ionization history, x_e(z), and degeneracies in cosmological
parameters (Omega_m, Omega_b, sigma_8, n_s) add systematic uncertainty to the
measurement and modeling of . From the additional optical depth from
sources at z > z_GP, we limit the star-formation efficiency, the rate of
ionizing photon production for Pop III and Pop II stars, and the photon escape
fraction, using standard histories of baryon collapse, minihalo star formation,
and black-hole X-ray preionization.Comment: Greatly revised version, based on WMAP-5 results and new models.
Accepted for ApJ (2008
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