945 research outputs found
The Cosmic Evolution of Gamma-Ray Burst Host Galaxies
Due to their extreme luminosities, gamma-ray bursts (GRBs) can be detected in
hostile regions of galaxies, nearby and at very high redshift, making them
important cosmological probes. The investigation of galaxies hosting
long-duration GRBs (whose progenitor is a massive star) demonstrated their
connection to star formation. Still, the link to the total galaxy population is
controversial, mainly because of the small-number statistics: ~ 1,100 are the
GRBs detected so far, ~ 280 those with measured redshift, and ~ 70 the hosts
studied in detail. These are typically low-redshift (z < 1.5), low luminosity,
metal poor, and star-forming galaxes. On the other hand, at 1.5< z <4, massive,
metal rich and dusty, interacting galaxies are not uncommon. The most distant
population (z > 4) is poorly explored, but the deep limits reached point
towards very small and star-forming objects, similar to the low-z population.
This `back to the future' behavior is a natural consequence of the connection
of long GRBs to star formation in young regions of the universe.Comment: Invited talk at the Fall 2012 Gamma-Ray Burst Symposium (Marbella,
Oct 2012), revised version after referee's report, to appear in the European
Astronomical Society Publications Serie
Low-Mass and Metal-Poor Gamma-Ray Burst Host Galaxies
Gamma-ray bursts (GRBs) are cosmologically distributed, very energetic and
very transient sources detected in the gamma-ray domain. The identification of
their x-ray and optical afterglows allowed so far the redshift measurement of
150 events, from z = 0.01 to z = 6.29. For about half of them, we have some
knowledge of the properties of the parent galaxy. At high redshift (z > 2),
absorption lines in the afterglow spectra give information on the cold
interstellar medium in the host. At low redshift (z < 1.0) multi-band
optical-NIR photometry and integrated spectroscopy reveal the GRB host general
properties. A redshift evolution of metallicity is not noticeable in the whole
sample. The typical value is a few times lower than solar. The mean host
stellar mass is similar to that of the Large Magellanic Cloud, but the mean
star formation rate is five times higher. GRBs are discovered with gamma-ray,
not optical or NIR, instruments. Their hosts do not suffer from the same
selection biases of typical galaxy surveys. Therefore, they might represent a
fair sample of the most common galaxies that existed in the past history of the
universe, and can be used to better understand galaxy formation and evolution.Comment: Invited contribution, to appear in proceedings of IAU Symposium 255:
"Low-Metal licity Star Formation: From the First Stars to Dwarf Galaxies",
Rapallo June 2008, L.K. Hunt, S. Madden & R. Schneider, ed
Galaxies as seen through the most Energetic Explosions in the Universe
A gamma-ray burst (GRB) is a strong and fast gamma-ray emission from the
explosion of stellar systems (massive stars or coalescing binary compact
stellar remnants), happening at any possible redshift, and detected by space
missions. Although GRBs are the most energetic events after the Big Bang,
systematic search (started after the first localization in 1997) led to only
374 spectroscopic redshift measurements. For less than half, the host galaxy is
detected and studied in some detail. Despite the small number of known hosts,
their impact on our understanding of galaxy formation and evolution is immense.
These galaxies offer the opportunity to explore regions which are
observationally hostile, due to the presence of gas and dust, or the large
distances reached. The typical long-duration GRB host galaxy at low redshift is
small, star-forming and metal poor, whereas, at intermediate redshift, many
hosts are massive, dusty and chemically evolved. Going even farther in the past
of the Universe, at z > 5, long-GRB hosts have never been identified, even with
the deepest NIR space observations, meaning that these galaxies are very small
(stellar mass < 10^7 M_sun). We considered the possibility that some high-z
GRBs occurred in primordial globular clusters, systems that evolved drastically
since the beginning, but would have back then the characteristics necessary to
host a GRB. At that time, the fraction of stellar mass contained in proto
globular clusters might have been orders of magnitude higher than today. Plus,
these objects contained in the past many massive fast rotating binary systems,
which are also regarded as a favorable situation for GRBs. The common factor
for all long GRBs at any redshift is the stellar progenitor: it is a very
massive rare/short-lived star, present in young regions, whose redshift
evolution is closely related to the star-formation history of the Universe.Comment: 12 pages, 10 figures, accepted for publication in the Journal of High
Energy Astrophysics, special issue "Swift: Ten Years of Discovery
Gamma-ray burst host galaxies at low and high redshift
The galaxies hosting the most energetic explosions in the universe, the
gamma-ray bursts (GRBs), are generally found to be low-mass, metal poor, blue
and star forming galaxies. However, the majority of the targets investigated so
far (less than 100) are at relatively low redshift, z < 2. We know that at low
redshift, the cosmic star formation is predominantly in small galaxies.
Therefore, at low redshift, long-duration GRBs, which are associated with
massive stars, are expected to be in small galaxies. Preliminary investigations
of the stellar mass function of z < 1.5 GRB hosts does not indicate that these
galaxies are different from the general population of nearby star-forming
galaxies. At high-z, it is still unclear whether GRB hosts are different.
Recent results indicate that a fraction of them might be associated with dusty
regions in massive galaxies. Remarkable is the a super-solar metallicity
measured in the interstellar medium of a z = 3.57 GRB host.Comment: Highlight talk at the Astronomische Gesellschaft meeting (Heidelberg
2011), to appear in the book series Reviews in Modern Astronomy, volume 2
On multidimensional inequality with variable distribution mean
We compare alternative populations of individuals, who differ for many characteristics besides income, in terms of inequality. In order to achieve our aim, we extend the notion of Generalized Lorenz Preorder to a context of multivariate distributions with different marginals. Finally, we show, by using convex analysis, that some conditions, relevant in the analysis of multidimensional inequality, are equivalent to the ordering we introducedMultidimensional Inequality, Generalized Lorenz Preordering, Price-majorization
Multivariate Differences Ordering
A multivariate dispersion ordering is introduced as a natural extension of a well-known univariate inequality criterion. A characterization of this new ordering is provided.
Six-degree-of-freedom aircraft simulation with mixed-data structure using the applied dynamics simulation language, ADSIM
A realistic simulation of an aircraft in the flight using the AD 100 digital computer is presented. The implementation of three model features is specifically discussed: (1) a large aerodynamic data base (130,00 function values) which is evaluated using function interpolation to obtain the aerodynamic coefficients; (2) an option to trim the aircraft in longitudinal flight; and (3) a flight control system which includes a digital controller. Since the model includes a digital controller the simulation implements not only continuous time equations but also discrete time equations, thus the model has a mixed-data structure
The power spectrum of the Lyman-alpha clouds
We investigate the clustering properties of 13 QSO lines of sight in flat
space, with average redshifts from z~2 to 4. We estimate the 1-D power spectrum
and the integral density of neighbours, and discuss their variation with
respect to redshift and column density. We compare the results with standard
CDM models, and estimate the power spectrum of Lyman-alpha clustering as a
function both of redshift and column density. We find that a) there is no
significant periodicity or characteristic scale; b) the clustering depends both
on column density and redshift; c) the clustering increases linearly only if at
the same time the HI column density decreases strongly with redshift. The
results remain qualitatively the same assuming an open cosmological model.Comment: Accepted for publication in MNRA
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