2,706 research outputs found
The spherical collapse model with shell crossing
In this work, we study the formation and evolution of dark matter halos by
means of the spherical infall model with shell-crossing. We present a framework
to tackle this effect properly based on the numerical follow-up, with time, of
that individual shell of matter that contains always the same fraction of mass
with respect to the total mass. In this first step, we do not include angular
momentum, velocity dispersion or triaxiality. Within this framework - named as
the Spherical Shell Tracker (SST) - we investigate the dependence of the
evolution of the halo with virial mass, with the adopted mass fraction of the
shell, and for different cosmologies. We find that our results are very
sensitive to a variation of the halo virial mass or the mass fraction of the
shell that we consider. However, we obtain a negligible dependence on
cosmology. Furthermore, we show that the effect of shell-crossing plays a
crucial role in the way that the halo reaches the stabilization in radius and
the virial equilibrium. We find that the values currently adopted in the
literature for the actual density contrast at the moment of virialization,
delta_vir, may not be accurate enough. In this context, we stress the problems
related to the definition of a virial mass and a virial radius for the halo.
The question of whether the results found here may be obtained by tracking the
shells with an analytic approximation remains to be explored.Comment: 15 pages, 4 figures, 9 tables, replaced to match the published MNRAS
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SIDE, a fiber fed spectrograph for the 10.4 m Gran Telescopio Canarias (GTC)
SIDE (Super Ifu Deployable Experiment) will be a second-generation,
common-user instrument for the Grantecan (GTC) on La Palma (Canary Islands,
Spain). It is being proposed as a spectrograph of low and intermediate
resolution, highly efficient in multi-object spectroscopy and 3D spectroscopy.
SIDE features the unique possibility of performing simultaneous visible and NIR
observations for selected ranges. The SIDE project is leaded by the Instituto
de Astrofisica de Andalucia (IAA-CSIC) in Granada (Spain) and the SIDE
Consortium is formed by a total of 10 institutions from Spain, Mexico and USA.
The SIDE Feasibility Study has been completed and currently the project is
under revision by the GTC Project Office.Comment: 9 pages, 6 figures, to appear in "Ground-based and Airborne
Instrumentation for Astronomy II" SPIE conference Proc. 7014, Marseille,
23-28 June 200
Empirical calibration of the near-IR Ca II triplet - I. The stellar library and index definition
A new stellar library at the near-IR spectral region developed for the
empirical calibration of the Ca II triplet and stellar population synthesis
modeling is presented. The library covers the range 8348-9020 Angstrom at 1.5
Angstrom (FWHM) spectral resolution, and consists of 706 stars spanning a wide
range in atmospheric parameters. We have defined a new set of near-IR indices,
CaT*, CaT and PaT, which mostly overcome the limitations of previous
definitions, the former being specially suited for the measurement of the Ca II
triplet strength corrected for the contamination from Paschen lines. We also
present a comparative study of the new and the previous calcium indices, as
well as the corresponding transformations between the different systems. A
thorough analysis of the sources of index errors and the procedure to calculate
them is given. Finally, index and error measurements for the whole stellar
library are provided together with the final spectra.Comment: 24 pages, 10 figures, LaTeX. For associated spectra, tables and
software, see http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.html or
http://www.nottingham.ac.uk/~ppzrfp/CATRIPLET.htm
A Minimalist Model of Characteristic Earthquakes
In a spirit akin to the sandpile model of self-organized criticality, we
present a simple statistical model of the cellular-automaton type which
produces an avalanche spectrum similar to the characteristic-earthquake
behavior of some seismic faults. This model, that has no parameter, is amenable
to an algebraic description as a Markov Chain. This possibility illuminates
some important results, obtained by Monte Carlo simulations, such as the
earthquake size-frequency relation and the recurrence time of the
characteristic earthquake.Comment: 9 pages, 4 figure
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