1,045 research outputs found
Galacticus: A Semi-Analytic Model of Galaxy Formation
We describe a new, free and open source semi-analytic model of galaxy
formation, Galacticus. The Galacticus model was designed to be highly modular
to facilitate expansion and the exploration of alternative descriptions of key
physical ingredients. We detail the Galacticus engine for evolving galaxies
through a merging hierarchy of dark matter halos and give details of the
specific implementations of physics currently available in Galacticus. Finally,
we show results from an example model that is in reasonably good agreement with
several observational datasets. We use this model to explore numerical
convergence and to demonstrate the types of information which can be extracted
from Galacticus.Comment: 35 pages, submitted to New Astronom
Self-Consistent Theory of Halo Mergers - II: CDM Power Spectra
We place additional constraints on the three parameters of the dark matter
halo merger rate function recently proposed by Parkinson, Cole & Helly by
utilizing Smoluchowski's coagulation equation, which must be obeyed by any
binary merging process which conserves mass. We find that the constraints from
Smoluchowski's equation are degenerate, limiting to a thin plane in the three
dimensional parameter space. This constraint is consistent with those obtained
from fitting to N-body measures of progenitor mass functions, and provides a
better match to the evolution of the overall dark matter halo mass function,
particularly for the most massive halos. We demonstrate that the proposed
merger rate function does not permit an exact solution of Smoluchowski's
equation and, therefore, the choice of parameters must reflect a compromise
between fitting various parts of the mass function. The techniques described
herein are applicable to more general merger rate functions, which may permit a
more accurate solution of Smoluchowski's equation. The current merger rate
solutions are most probably sufficiently accurate for the vast majority of
applications.Comment: 11 pages, submitted to MNRA
Galaxy Formation Spanning Cosmic History
Over the past several decades, galaxy formation theory has met with
significant successes. In order to test current theories thoroughly we require
predictions for as yet unprobed regimes. To this end, we describe a new
implementation of the Galform semi-analytic model of galaxy formation. Our
motivation is the success of the model described by Bower et al. in explaining
many aspects of galaxy formation. Despite this success, the Bower et al. model
fails to match some observational constraints and certain aspects of its
physical implementation are not as realistic as we would like. The model
described in this work includes substantially updated physics, taking into
account developments in our understanding over the past decade, and removes
certain limiting assumptions made by this (and most other) semi-analytic
models. This allows it to be exploited reliably in high-redshift and low mass
regimes. Furthermore, we have performed an exhaustive search of model parameter
space to find a particular set of model parameters which produce results in
good agreement with a wide range of observational data (luminosity functions,
galaxy sizes and dynamics, clustering, colours, metal content) over a wide
range of redshifts. This model represents a solid basis on which to perform
calculations of galaxy formation in as yet unprobed regimes.Comment: MNRAS accepted. Extended version (with additional figures and details
of implementation) is available at http://www.galform.or
Dark-matter decays and Milky Way satellite galaxies
We consider constraints on a phenomenological dark-matter model consisting of
two nearly degenerate particle species using observed properties of the Milky
Way satellite galaxy population. The two parameters of this model, assuming the
particle masses are >~ GeV, are v_k, the recoil speed of the daughter particle,
and tau, the lifetime of the parent particle. The satellite constraint that
spans the widest range of v_k is the number of satellites that have a mass
within 300 pc M300 > 5 x 10^6 solar masses, although constraints based on M300
in the classical dwarfs and the overall velocity function are competitive for
v_k >~ 50 km/s. In general, we find that tau <~ 30 Gyr is ruled out for 20 km/s
<~ v_k <~ 200 km/s, although we find that the limits on tau for fixed v_k can
change constraints by a factor of ~3 depending on the star-formation histories
of the satellites. We advocate using the distribution of M300 in Milky Way
satellites determined by next-generation all-sky surveys and follow-up
spectroscopy as a probe of dark-matter properties.Comment: 17 pages, 9 figures, submitted to Phys. Rev.
The Escape Fraction of Ionizing Radiation from Galaxies
The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. We present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from galaxies
ranging from primordial systems to disk-type galaxies that are not heavily
dust-obscured. We consider variations in the galaxy density profile, source
type, location, and spectrum, and gas overdensity/distribution factors. For
sufficiently hard first-light sources, the helium ionization fronts closely
track or advance beyond that of hydrogen. Key new results in this work include
calculations of the escape fractions for He I and He II ionizing radiation, and
the impact of partial ionization from X-rays from early AGN or stellar clusters
on the escape fractions from galaxy halos. When factoring in
frequency-dependent effects, we find that X-rays play an important role in
boosting the escape fractions for both hydrogen and helium, but especially for
He II. We briefly discuss the implications of these results for recent
observations of the He II reionization epoch at low redshifts, as well as the
UV data and emission-line signatures from early galaxies anticipated from
future satellite missions.Comment: 43 pages, 9 figures, accepted in ApJ, comments welcom
Supermassive Black Hole Merger Rates: Uncertainties from Halo Merger Theory
The merger of two supermassive black holes is expected to produce a
gravitational-wave signal detectable by the satellite LISA. The rate of
supermassive-black-hole mergers is intimately connected to the halo merger
rate, and the extended Press-Schechter formalism is often employed when
calculating the rate at which these events will be observed by LISA. This
merger theory is flawed and provides two rates for the merging of the same pair
of haloes. We show that the two predictions for the LISA
supermassive-black-hole-merger event rate from extended Press-Schechter merger
theory are nearly equal because mergers between haloes of similar masses
dominate the event rate. An alternative merger rate may be obtained by
inverting the Smoluchowski coagulation equation to find the merger rate that
preserves the Press-Schechter halo abundance, but these rates are only
available for power-law power spectra. We compare the LISA event rates derived
from the extended Press-Schechter merger formalism to those derived from the
merger rates obtained from the coagulation equation and find that the extended
Press-Schechter LISA event rates are thirty percent higher for a power spectrum
spectral index that approximates the full Lambda-CDM result of the extended
Press-Schechter theory.Comment: 9 pages, 9 figures. More concise treatment, accepted for publication
in MNRA
- …