5 research outputs found
Environmental effects on associations of dwarf galaxies
We study the properties of associations of dwarf galaxies and their
dependence on the environment. Associations of dwarf galaxies are extended
systems composed exclusively of dwarf galaxies, considering as dwarf galaxies
those galaxies less massive than . We identify these particular systems using a
semi-analytical model of galaxy formation coupled to a dark matter only
simulation in the Cold Dark Matter cosmological model. To classify
the environment, we estimate eigenvalues from the tidal field of the dark
matter particle distribution of the simulation. We find that the majority, two
thirds, of associations are located in filaments ( per cent),
followed by walls ( per cent), while only a small fraction of them
are in knots ( per cent) and voids ( per cent).
Associations located in more dense environments present significantly higher
velocity dispersion than those located in less dense environments, evidencing
that the environment plays a fundamental role in their dynamical properties.
However, this connection between velocity dispersion and the environment
depends exclusively on whether the systems are gravitational bound or unbound,
given that it disappears when we consider associations of dwarf galaxies that
are gravitationally bound. Although less than a dozen observationally detected
associations of dwarf galaxies are currently known, our results are predictions
on the eve of forthcoming large surveys of galaxies, which will enable these
very particular systems to be identified and studied.Comment: 13 pages, 9 figures. Accepted for publication in MNRA
Calibration of semi-analytic models of galaxy formation using Particle Swarm Optimization
We present a fast and accurate method to select an optimal set of parameters
in semi-analytic models of galaxy formation and evolution (SAMs). Our approach
compares the results of a model against a set of observables applying a
stochastic technique called Particle Swarm Optimization (PSO), a self-learning
algorithm for localizing regions of maximum likelihood in multidimensional
spaces that outperforms traditional sampling methods in terms of computational
cost. We apply the PSO technique to the SAG semi-analytic model combined with
merger trees extracted from a standard CDM N-body simulation. The
calibration is performed using a combination of observed galaxy properties as
constraints, including the local stellar mass function and the black hole to
bulge mass relation. We test the ability of the PSO algorithm to find the best
set of free parameters of the model by comparing the results with those
obtained using a MCMC exploration. Both methods find the same maximum
likelihood region, however the PSO method requires one order of magnitude less
evaluations. This new approach allows a fast estimation of the best-fitting
parameter set in multidimensional spaces, providing a practical tool to test
the consequences of including other astrophysical processes in SAMs.Comment: 11 pages, 4 figures, 1 table. Accepted for publication in ApJ.
Comments are welcom
Associations of dwarf galaxies in a ÎCDM Universe
Associations of dwarf galaxies are loose systems composed exclusively of dwarf galaxies. These systems were identified in the Local Volume for the first time more than 30 yr ago. We study these systems in the cosmological framework of the Î cold dark matter (ÎCDM) model. We consider the Small MultiDark Planck simulation and populate its dark matter haloes by applying the semi-analytic model of galaxy formation SAG. We identify galaxy systems using a friends-of-friends algorithm with a linking length equal to b=0.4Mpchâ1 to reproduce the size of dwarf galaxy associations detected in the Local Volume. Our samples of dwarf systems are built up removing those systems that have one or more galaxies with stellar mass larger than a maximum threshold Mmax. We analyse three different samples defined by log10(Mmax[Mâhâ1])=8.5,9.0â , and 9.5. On average, our systems have typical sizes of âŒ0.2Mpchâ1â , velocity dispersion of âŒ30kmsâ1â , and estimated total mass of âŒ1011Mâhâ1â . Such large typical sizes suggest that individual members of a given dwarf association reside in different dark matter haloes and are generally not substructures of any other halo. Indeed, in more than 90 per cent of our dwarf systems their individual members inhabit different dark matter haloes, while only in the remaining 10 per cent members do reside in the same halo. Our results indicate that the ÎCDM model can naturally reproduce the existence and properties of dwarf galaxiesâ associations without much difficulty.Instituto de AstrofĂsica de La PlataFacultad de Ciencias AstronĂłmicas y GeofĂsica