11,509 research outputs found
Observing Ultra High Energy Cosmic Particles from Space: SEUSO, the Super Extreme Universe Space Observatory Mission
The experimental search for ultra high energy cosmic messengers, from eV to beyond eV, at the very end of the known energy
spectrum, constitutes an extraordinary opportunity to explore a largely unknown
aspect of our universe. Key scientific goals are the identification of the
sources of ultra high energy particles, the measurement of their spectra and
the study of galactic and local intergalactic magnetic fields. Ultra high
energy particles might, also, carry evidence of unknown physics or of exotic
particles relics of the early universe. To meet this challenge a significant
increase in the integrated exposure is required. This implies a new class of
experiments with larger acceptances and good understanding of the systematic
uncertainties. Space based observatories can reach the instantaneous aperture
and the integrated exposure necessary to systematically explore the ultra high
energy universe. In this paper, after briefly summarising the science case of
the mission, we describe the scientific goals and requirements of the SEUSO
concept. We then introduce the SEUSO observational approach and describe the
main instrument and mission features. We conclude discussing the expected
performance of the mission
On the Origins of the High-Latitude H-alpha Background
The diffuse high-latitude H-alpha background is widely believed to be
predominantly the result of in-situ recombination of ionized hydrogen in the
warm interstellar medium of the Galaxy. Instead, we show that both a
substantial fraction of the diffuse high-latitude H-alpha intensity in regions
dominated by Galactic cirrus dust and much of the variance in the high-latitude
H-alpha background are the result of scattering by interstellar dust of H-alpha
photons originating elsewhere in the Galaxy. We provide an empirical relation,
which relates the expected scattered H-alpha intensity to the IRAS 100um
diffuse background intensity, applicable to about 81% of the entire sky. The
assumption commonly made in reductions of CMB observations, namely that the
observed all-sky map of diffuse H-alpha light is a suitable template for
Galactic free-free foreground emission, is found to be in need of
reexamination.Comment: 26 pages, 5 figures, Accepted for publication in Ap
Exobiology in Earth orbit: The results of science workshops held at NASA, Ames Research Center
The Workshops on Exobiology in Earth Orbit were held to explore concepts for orbital experiments of exobiological interest and make recommendations on which classes of experiments should be carried out. Various observational and experimental opportunities in Earth orbit are described including those associated with the Space Shuttle laboratories, spacecraft deployed from the Space Shuttle and expendable launch vehicles, the Space Station, and lunar bases. Specific science issues and technology needs are summarized. Finally, a list of recommended experiments in the areas of observational exobiology, cosmic dust collection, and in situ experiments is presented
The Mass of the Black Hole in Arp 151 from Bayesian Modeling of Reverberation Mapping Data
Supermassive black holes are believed to be ubiquitous at the centers of
galaxies. Measuring their masses is extremely challenging yet essential for
understanding their role in the formation and evolution of cosmic structure. We
present a direct measurement of the mass of a black hole in an active galactic
nucleus (Arp 151) based on the motion of the gas responsible for the broad
emission lines. By analyzing and modeling spectroscopic and photometric time
series, we find that the gas is well described by a disk or torus with an
average radius of 3.99 +- 1.25 light days and an opening angle of 68.9 (+21.4,
-17.2) degrees, viewed at an inclination angle of 67.8 +- 7.8 degrees (that is,
closer to face-on than edge-on). The black hole mass is inferred to be 10^(6.51
+- 0.28) solar masses. The method is fully general and can be used to determine
the masses of black holes at arbitrary distances, enabling studies of their
evolution over cosmic time.Comment: Accepted for publication in ApJ Letter
Formation of stars and clusters over cosmological time
The concept that stars form in the modern era began some 60 years ago with
the key observation of expanding OB associations. Now we see that these
associations are an intermediate scale in a cascade of hierarchical structures
that begins on the ambient Jeans length close to a kiloparsec in size and
continues down to the interiors of clusters, perhaps even to binary and
multiple stellar systems. The origin of this structure lies with the dynamical
nature of cloud and star formation, driven by supersonic turbulence and
interstellar gravity. Dynamical star formation is relatively fast compared to
the timescale for cosmic accretion, and then the star formation rate keeps up
with the accretion rate, leading to a sequence of near-equilibrium states
during galaxy formation and evolution. Dynamical star formation also helps to
explain the formation of bound clusters, which require a local efficiency that
exceeds the average by more than an order of magnitude. Efficiency increases
with density in a hierarchically structured gas. Cluster formation should vary
with environment as the relative degree of cloud self-binding varies, and this
depends on the ratio of the interstellar velocity dispersion to the galaxy
rotation speed. As this ratio increases, galaxies become more clumpy, thicker,
and have more tightly bound star-forming regions. The formation of old globular
clusters is understood in this context, with the metal-rich and metal-poor
globulars forming in high-mass and low-mass galaxies, respectively, because of
the galactic mass-metallicity relation. Metal-rich globulars remain in the
disks and bulge regions where they formed, while metal-poor globulars get
captured as parts of satellite galaxies and end up in today's spiral galaxy
halos. Blue globulars in the disk could have formed very early when the whole
Milky Way had a low mass.Comment: 14 pages, 1 figure, in conference "Lessons from the Local Group," ed.
K. Freeman et al., Springer, 201
A Universal Stellar Initial Mass Function? A Critical Look at Variations
Few topics in astronomy initiate such vigorous discussion as whether or not
the initial mass function (IMF) of stars is universal, or instead sensitive to
the initial conditions of star formation. The distinction is of critical
importance: the IMF influences most of the observable properties of stellar
populations and galaxies, and detecting variations in the IMF could provide
deep insights into the process by which stars form. In this review, we take a
critical look at the case for IMF variations, with a view towards whether other
explanations are sufficient given the evidence. Studies of the field, local
young clusters and associations, and old globular clusters suggest that the
vast majority were drawn from a "universal" IMF: a power-law of Salpeter index
() above a few solar masses, and a log normal or shallower
power-law () between a few tenths and a few solar masses
(ignoring the effects of unresolved binaries). The shape and universality of
the IMF at the stellar-substellar boundary is still under investigation and
uncertainties remain large, but most observations are consistent with a IMF
that declines () well below the hydrogen burning limit.
Observations of resolved stellar populations and the integrated properties of
most galaxies are also consistent with a "universal IMF", suggesting no gross
variations in the IMF over much of cosmic time. There are indications of
"non-standard" IMFs in specific local and extragalactic environments, which
clearly warrant further study. Nonetheless, there is no clear evidence that the
IMF varies strongly and systematically as a function of initial conditions
after the first few generations of stars.Comment: 49 pages, 5 figures, to appear in Annual Reviews of Astronomy and
Astrophysics (2010, volume 48
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