1,385 research outputs found
The Metagalactic Ionizing Radiation Field at Low Redshift
We compute the ionizing radiation field at low redshift, arising from
Seyferts, QSOs, and starburst galaxies. This calculation combines recent
Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN
database, and a new intergalactic opacity model based on Hubble Space Telescope
and Keck Ly-alpha absorber surveys. At z = 0 for AGN only, our best estimate
for the specific intensity at 1 Ryd is I_0 = 1.3 (+0.8/-0.5) x 10^-23
ergs/cm^2/s/Hz/sr, independent of H_0, Omega_0, and Lambda. The one-sided
ionizing photon flux is Phi_ion = 3400 (+2100/-1300) photons/cm^2/s, and the H
I photoionization rate is Gamma_HI = 3.2 (+2.0/-1.2) x 10^-14 s^-1 for alpha_s
= 1.8. We also derive Gamma_ HI for z = 0 - 4. These error ranges reflect
uncertainties in the spectral indexes for the ionizing EUV (alpha_s = 1.8 +/-
0.3) and the optical/UV (alpha_UV = 0.86 +/- 0.05), the IGM opacity model, the
range of Seyfert luminosities (0.001 - 100 L*) and the completeness of the
luminosity functions. Our estimate is a factor of three lower than the most
stringent upper limits on the ionizing background (Phi_ion < 10^4
photons/cm^2/s) obtained from H-alpha observations in external clouds, and it
lies within the range implied by other indirect measures. Starburst galaxies
with a sufficiently large Lyman continuum escape fraction, f_ esc > 0.05, may
provide a comparable background to AGN, I_0 (z=0) = 1.1 (+1.5/-0.7) x 10^{-23).
An additional component of the ionizing background of this magnitude would
violate neither upper limits from H-alpha observations nor the acceptable range
from other measurements.Comment: 30 pages, 9 figures, accepted for Astronomical J. (Oct. 1999
Interrelations Between the Neutron's Magnetic Interactions and the Magnetic Aharonov-Bohm Effect
It is proved that the phase shift of a polarized neutron interacting with a
spatially uniform time-dependent magnetic field, demonstrates the same physical
principles as the magnetic Aharonov-Bohm effect. The crucial role of inert
objects is explained, thereby proving the quantum mechanical nature of the
effect. It is also proved that the nonsimply connectedness of the field-free
region is not a profound property of the system and that it cannot be regarded
as a sufficient condition for a nonzero phase shift.Comment: 18 pages, 1 postscript figure, Late
Primary structure of rat plasma membrane Ca2+-ATPase isoform 4 and analysis of alternative splicing patterns at splice site A
The Emergence of the Modern Universe: Tracing the Cosmic Web
This is the report of the Ultraviolet-Optical Working Group (UVOWG)
commissioned by NASA to study the scientific rationale for new missions in
ultraviolet/optical space astronomy approximately ten years from now, when the
Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific
theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to
0, occupying over 80% of cosmic time and beginning after the first galaxies,
quasars, and stars emerged into their present form. We considered
high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field
optical imaging (at least 10 arcmin square). The exciting science to be
addressed in the post-HST era includes studies of dark matter and baryons, the
origin and evolution of the elements, and the major construction phase of
galaxies and quasars. Key unanswered questions include: Where is the rest of
the unseen universe? What is the interplay of the dark and luminous universe?
How did the IGM collapse to form the galaxies and clusters? When were galaxies,
clusters, and stellar populations assembled into their current form? What is
the history of star formation and chemical evolution? Are massive black holes a
natural part of most galaxies? A large-aperture UV/O telescope in space
(ST-2010) will provide a major facility in the 21st century for solving these
scientific problems. The UVOWG recommends that the first mission be a 4m
aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field
imaging. In the coming decade, NASA should investigate the feasibility of an 8m
telescope, by 2010, with deployable optics similar to NGST. No high-throughput
UV/Optical mission will be possible without significant NASA investments in
technology, including UV detectors, gratings, mirrors, and imagers.Comment: Report of UV/O Working Group to NASA, 72 pages, 13 figures, Full
document with postscript figures available at
http://casa.colorado.edu/~uvconf/UVOWG.htm
FUSE Observations of Intrinsic Absorption in the Seyfert 1 Galaxy Mrk 509
We present far-ultraviolet spectra of the Seyfert 1 galaxy Mrk 509 obtained
in 1999 November with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our
data span the observed wavelength range 915-1185 A at a resolution of ~20 km/s.
The spectrum shows a blue continuum, broad OVI 1032,1038 emission, and a broad
CIII 977 emission line. Superposed on these emission components, we resolve
associated absorption lines of OVI 1032,1038, CIII 977, and Lyman lines through
Lzeta. Seven distinct kinematic components are present, spanning a velocity
range of -440 to +170 km/s relative to the systemic velocity. The absorption is
clustered in two groups, one centered at -370m km/s and another at the systemic
velocity. The blue-shifted cluster may be associated with the extended line
emission visible in deep images of Mrk 509 obtained by Phillips et al. Although
several components appear to be saturated, they are not black at their centers.
Partial covering or scattering permits ~7% of the broad-line or continuum flux
to be unaffected by absorption. Of the multiple components, only one has the
same ionization state and column density as highly ionized gas that produces
the OVII and OVIII ionization edges in X-ray spectra of Mrk 509.
This paper will appear in a special issue of Astrophysical Journal Letters
devoted to the first scientific results from the FUSE mission.Comment: To appear in the Astrophysical Journal (Letters). 4 pages, 3 color
PostScript figures. Figures are best viewed and printed in color. Added
acknowledgment that this is one of many papers to be published in a special
issue of ApJL devoted to the first scientific results from the FUSE missio
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