2,103 research outputs found
The Dust Depletion and Extinction of the GRB 020813 Afterglow
The Keck optical spectrum of the GRB 020813 afterglow is the best ever
obtained for GRBs. Its large spectral range and very high S/N ratio allowed for
the first time the detection of a vast variety of absorption lines, associated
with the circumburst medium or interstellar medium of the host. The remarkable
similarity of the relative abundances of 8 elements with the dust depletion
pattern seen in the Galactic ISM suggests the presence of dust. The derived
visual dust extinction A_V=0.40+/-0.06 contradicts the featureless UV spectrum
of the afterglow, very well described by a unreddened power law. The
forthcoming Swift era will open exciting opportunities to explain similar
phenomena in other GRB afterglows.Comment: To be published in "Il Nuovo Cimento", Proceedings of the 4th Rome
Workshop on Gamma-Ray Bursts in the Afterglow Era, eds. L. Piro, L. Amati, S.
Covino, B. Gendr
Orbital Decay and Tidal Disruption of a Star Cluster: Analytical Calculation
The orbital decay and tidal disruption of a star cluster in a galaxy is
studied in an analytical manner. Owing to dynamical friction, the star cluster
spirals in toward the center of the galaxy. Simultaneously, the galactic tidal
field strips stars from the outskirts of the star cluster. Under an assumption
that the star cluster undergoes a self-similar evolution, we obtain the
condition and timescale for the star cluster to reach the galaxy center before
its disruption. The result is used to discuss the fate of so-called
intermediate-mass black holes with >10^3 M(sun) found recently in young star
clusters of starburst galaxies and also the mass function of globular clusters
in galaxies.Comment: 12 pages, 1 PS file for 2 figures, to appear in The Astrophysical
Journa
The Angular Momentum Distribution of Gas and Dark Matter in Galactic Halos
(Abridged) We report results of a series of non radiative N-body/SPH
simulations in a LCDM cosmology. We find that the spin of the baryonic
component is on average larger than that of the dark matter (DM) component and
we find this effect to be more pronounced at lower redshifts. A significant
fraction f of gas has negative angular momentum and this fraction is found to
increase with redshift. We describe a toy model in which the tangential
velocities of particles are smeared by Gaussian random motions. This model is
successful in explaining some of the angular momentum properties. We compare
and contrast various techniques to determine the angular momentum distributions
(AMDs). We show that broadening of velocity dispersions is unsuitable for
making comparisons between gas and DM. We smooth the angular momentum of the
particles over a fixed number of neighbors. We find that an analytical function
based on gamma distribution can be used to describe a wide variety of profiles,
with just one parameter \alpha. The distribution of the shape parameter
for both gas and DM follows roughly a log-normal distribution. The
mean and standard deviation of log(\alpha) for gas is -0.04 and 0.11
respectively. About 90-95% of halos have \alpha<1.3, while exponential disks in
NFW halos would require 1.3<\alpha<1.6. This implies that a typical halo in
simulations has an excess of low angular momentum material as compared to that
of observed exponential disks, a result which is consistent with the findings
of earlier works. \alpha for gas is correlated with that of DM but they have a
significant scatter =1.09 \pm 0.2. \alpha_Gas is also
biased towards slightly higher values compared to \alpha_DM.Comment: 19 pages, 32 figures (replaced to correct a typo in the authors field
in the above line, paper unchanged
Potential formation sites of super star clusters in ultra-luminous infrared galaxies
Recent observational results on high spatial resolution images of
ultra-luminous infrared galaxies (ULIGs) have revealed very luminous, young,
compact, and heavily obscured super star clusters in their central regions,
suggested to be formed by gas-rich major mergers. By using stellar and gaseous
numerical simulations of galaxy mergers, we firstly demonstrate that the
central regions of ULIGs are the most promising formation sites of super star
clusters owing to the rather high gaseous pressure of the interstellar medium.
Based on simple analytical arguments, we secondly discuss the possibility that
super star clusters in an ULIG can be efficiently transferred into the nuclear
region owing to dynamical friction and consequently merge with one another to
form a single compact stellar nucleus with a seed massive black hole. We thus
suggest that multiple merging between super star clusters formed by nuclear
starbursts in the central regions of ULIGs can result in the formation of
massive black holes.Comment: 12 pages 4 figures, 2001, accepted by ApJ
Some Global Characteristics of the Galactic Globular Cluster System
The relations between the luminosities , the metallicities ,
the Galactocentric radii , and the central concentration indices of
Galactic globular clusters are discussed. It is found that the most luminous
clusters rarely have collapsed cores. The reason for this might be that the
core collapse time scales for such populous clusters are greater than the age
of the Galaxy. Among those clusters, for which the structure has not been
modified by core collapse, there is a correlation between central concentration
and integrated luminosity, in the sense that the most luminous clusters have
the strongest central concentration. The outermost region of the Galaxy with
kpc was apparently not able to form metal-rich globular
clusters, whereas such clusters (of which Ter 7 is the prototype) were able to
form in some nearby dwarf spheroidal galaxies. It is not yet clear how the
popular hypothesis that globular clusters were initially formed with a single
power law mass spectrum can be reconciled with the observation that both (1)
Galactic globular clusters with kpc, and (2) the globulars associated
with the Sagittarius dwarf, appear to have bi-modal luminosity functions.Comment: 15 pages, 1 figur
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