1,564 research outputs found
Improved Ways to Compare Simulations to Data
Theoretical models for structure formation with Gaussian initial fluctuations
have been worked out in considerable detail and compared with observations on
various scales. It is on nonlinear scales \lsim 10 \ h^{-1}\ {\rm Mpc} that
the greatest differences exist between models that have been
normalized to agree on the largest scales with the COBE data; here especially
there is a need for better statistical tests which are simultaneously {\it
robust}, {\it discriminatory}, and {\it interpretable}. The era at which galaxy
and cluster formation occurs is also a critical test of some models. Needs for
the future include faster and cleverer codes, better control of cosmic variance
in simulations, better understanding of processes leading to galaxy formation,
better ways of comparing observational data with models, and better access to
observational and simulation data.Comment: 9 pages, self-extracting uuencoded postscript, encoded with uufile
Simulations of Dust in Interacting Galaxies
A new Monte-Carlo radiative-transfer code, Sunrise, is used to study the
effects of dust in N-body/hydrodynamic simulations of interacting galaxies.
Dust has a profound effect on the appearance of the simulated galaxies. At peak
luminosities, about 90% of the bolometric luminosity is absorbed, and the dust
obscuration scales with luminosity in such a way that the brightness at
UV/visual wavelengths remains roughly constant. A general relationship between
the fraction of energy absorbed and the ratio of bolometric luminosity to
baryonic mass is found. Comparing to observations, the simulations are found to
follow a relation similar to the observed IRX-Beta relation found by Meurer et
al (1999) when similar luminosity objects are considered. The
highest-luminosity simulated galaxies depart from this relation and occupy the
region where local (U)LIRGs are found. This agreement is contingent on the
presence of Milky-Way-like dust, while SMC-like dust results in far too red a
UV continuum slope to match observations. The simulations are used to study the
performance of star-formation indicators in the presence of dust. The
far-infrared luminosity is found to be reliable. In contrast, the H-alpha and
far-UV luminosity suffer severely from dust attenuation, and dust corrections
can only partially remedy the situation.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"The Spectral Energy Distribution of Gas-Rich Galaxies", eds. C.C. Popescu &
R.J. Tuffs (Heidelberg, October 2004
Probing Galaxy Formation with High Energy Gamma-Rays
I discuss how measurements of the absorption of -rays from GeV to TeV
energies via pair production on the extragalactic background light (EBL) can
probe important issues in galaxy formation. We use semi-analytic models (SAMs)
of galaxy formation, based on the flat LCDM hierarchical structure formation
scenario with and Hubble parameter , to obtain
predictions of the EBL from 0.1 to 1000m. SAMs incorporate simplified
physical treatments of the key processes of galaxy formation - including
gravitational collapse and merging of dark matter halos, gas cooling and
dissipation, star formation, supernova feedback and metal production -- and
have been shown to reproduce key observations at low and high redshift. We
investigate the consequences of variations in input assumptions such as the
stellar initial mass function (IMF) and the efficiency of converting cold gas
into stars. We also discuss recent attempts to determine the emitted spectrum
of high energy gamma rays from blazars such as Mrk 501 from the observed X-rays
using the synchrotron self-Compton model, and note that our favorite SAM EBL
plus the observed spectrum of Mrk 501 do not imply unphysical upturns in the
high energy emitted spectrum - thus undermining recent claims of a crisis with
drastic possible consequences such as breaking of Lorentz invariance. We
conclude that observational studies of the absorption of -rays will
help to determine the EBL, and also help to explain its origin by constraining
some of the most uncertain features of galaxy formation theory, including the
IMF, the history of star formation, and the reprocessing of light by dust.Comment: 22 pages, 7 figures -- This paper is an updated version of
astro-ph/00114
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