2,798 research outputs found
The Spectral Energy Distribution of Normal, Starburst and Active Galaxies
We present the results of an extensive literature search of multiwavelength
data for a sample of 59 galaxies, consisting of 26 Starbursts, 15 Seyfert 2's,
5 LINER's, 6 normal spirals and 7 normal elliptical galaxies. The data include
soft X-ray fluxes, ultraviolet and optical spectra, near, mid/far infrared
photometry and radio measurements, selected to match as closely as possible the
IUE aperture (10" X 20"). The galaxies are separated into 6 groups with similar
characteristics, namely, Ellipticals, Spirals, LINER's, Seyfert 2's, Starbursts
of Low and High reddening, for which we create average spectral energy
distributions (SED). The individual groups SED's are normalized to the
7000\AA flux and compared, looking for similarities and differences
among them.The bolometric fluxes of different types of galaxies were calculated
integrating their SED's. These values are compared with individual waveband
flux densities, in order to determine the wavebands which contribute most to
the bolometric flux. Linear regressions were performed between the bolometric
and individual band fluxes for each kind of galaxy. These fits can be used in
the calculation of the bolometric flux for other objects of similar activity
type, but with reduced waveband information. We have also collected
multiwavelength data for 4 HII regions, a thermal supernova remnant, and a
non-thermal supernova remnant (SNR), which are compared with the Starburst
SED's.Comment: 29 pages, 13 postscript figures and 10 tables. To appear in The
Astronomical Journa
Starbursts and Star Clusters in the Ultraviolet
Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies
reveal them to be highly irregular, even after excluding compact sources
(clusters and resolved stars). Most (7/9) are found to have a similar intrinsic
effective surface brightnesses, suggesting that a negative feedback mechanism
is setting an upper limit to the star formation rate per unit area. All
starbursts in our sample contain UV bright star clusters indicating that
cluster formation is an important mode of star formation in starbursts. On
average about 20% of the UV luminosity comes from these clusters. The brightest
clusters, or super star clusters (SSC), are preferentially found at the very
heart of starbursts. The size of the nearest SSCs are consistent with those of
Galactic globular clusters. The luminosity function of SSCs is well represented
by a power law with a slope alpha ~ -2. There is a strong correlation between
the far infrared excess and the UV spectral slope. The correlation is well
modeled by a geometry where much of their dust is in a foreground screen near
to the starburst, but not by a geometry of well mixed stars and dust.Comment: 47 pages, text only, LaTeX with aaspp.sty (version 3.0), compressed
postscript figures available at
ftp://eta.pha.jhu.edu/RecentPublications/meurer
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
Dust and Recent Star Formation in the Core of NGC5253
(Abridged) Ultraviolet and optical narrow and broad band images of NGC5253
obtained with the HST WFPC2 are used to derive the properties of the dust
distribution and the recent star formation history of this metal-poor dwarf
galaxy. Corrections for the effects of dust are important in the center of
NGC5253: dust reddening is markedly inhomogeneous across the galaxy's central
20" region. One of the most obscured regions coincides with the region of
highest star formation activity in the galaxy: clouds of more than 9~magnitudes
of optical depth at V enshroud a young (2.5~Myr old) stellar cluster in the
region. Star formation has been active at least over the past 100 Myr in the
core of the galaxy, as indicated by the age distribution of both the blue
diffuse stellar population and the bright stellar clusters. The star formation
is currently concentrated in a 6" region, about 5 Myr old and with a star
formation intensity 10-100 times higher than the average within the central
20". The 2.5 Myr old cluster coincides with the peak intensity of the star
formation; its mass may be as large as 10^6 solar masses, making this one a
Super-Star-Cluster candidate.Comment: 29 pages, Latex, 3 Tables (Postscript), 9 Figures (Postscript).
Accepted for publication in the Astronomical Journal, July 31st, 199
On the photon polarization in radiative B -> phi K gamma decay
The photon polarization in radiative decays B -> Y gamma is known to be a
subtle probe of the effective Lagrangian structure and possible New Physics
effects. We discuss exclusive decay mode B -> phi K gamma where the
experimentally distinct final state makes analysis especially promising. The
possibility to extract information on the photon polarization out of the data
entirely depends on the partial waves interference pattern in the phi K system.Comment: RevTeX, 6 pages, 1 figure; the journal versio
Sharing Social Network Data: Differentially Private Estimation of Exponential-Family Random Graph Models
Motivated by a real-life problem of sharing social network data that contain
sensitive personal information, we propose a novel approach to release and
analyze synthetic graphs in order to protect privacy of individual
relationships captured by the social network while maintaining the validity of
statistical results. A case study using a version of the Enron e-mail corpus
dataset demonstrates the application and usefulness of the proposed techniques
in solving the challenging problem of maintaining privacy \emph{and} supporting
open access to network data to ensure reproducibility of existing studies and
discovering new scientific insights that can be obtained by analyzing such
data. We use a simple yet effective randomized response mechanism to generate
synthetic networks under -edge differential privacy, and then use
likelihood based inference for missing data and Markov chain Monte Carlo
techniques to fit exponential-family random graph models to the generated
synthetic networks.Comment: Updated, 39 page
Investigation of shock waves in explosive blasts using fibre optic pressure sensors
The published version of this article may be accessed at the link below. Copyright @ IOP Publishing, 2006.We describe miniature all-optical pressure sensors, fabricated by wafer etching techniques, less than 1 mm(2) in overall cross-section with rise times in the mu s regime and pressure ranges typically 900 kPa (9 bar). Their performance is suitable for experimental studies of the pressure-time history for test models exposed to shocks initiated by an explosive charge. The small size and fast response of the sensors promises higher quality data than has been previously available from conventional electrical sensors, with potential improvements to numerical models of blast effects. Results from blast tests are presented in which up to six sensors were multiplexed, embedded within test models in a range of orientations relative to the shock front.Support from the UK Engineering&Physical
Sciences Research Council and Dstl Fort Halstead through the MoD Joint Grants Scheme are acknowledged. WN MacPherson is supported by an EPSRC Advanced Research Fellowship
Back reaction of a long range force on a Friedmann-Robertson-Walker background
It is possible that there may exist long-range forces in addition to gravity.
In this paper we construct a simple model for such a force based on exchange of
a massless scalar field and analyze its effect on the evolution of a
homogeneous Friedmann-Robertson-Walker cosmology. The presence of such an
interaction leads to an equation of state characterized by positive pressure
and to resonant particle production similar to that observed in preheating
scenarios.Comment: 14 pages, 6 color Postscript figures, LaTe
Quantum Gravity and Inflation
Using the Ashtekar-Sen variables of loop quantum gravity, a new class of
exact solutions to the equations of quantum cosmology is found for gravity
coupled to a scalar field, that corresponds to inflating universes. The scalar
field, which has an arbitrary potential, is treated as a time variable,
reducing the hamiltonian constraint to a time-dependent Schroedinger equation.
When reduced to the homogeneous and isotropic case, this is solved exactly by a
set of solutions that extend the Kodama state, taking into account the time
dependence of the vacuum energy. Each quantum state corresponds to a classical
solution of the Hamiltonian-Jacobi equation. The study of the latter shows
evidence for an attractor, suggesting a universality in the phenomena of
inflation. Finally, wavepackets can be constructed by superposing solutions
with different ratios of kinetic to potential scalar field energy, resolving,
at least in this case, the issue of normalizability of the Kodama state.Comment: 18 Pages, 2 Figures; major corrections to equations but prior results
still hold, updated reference
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