726 research outputs found
Triggered Star Formation in the Environment of Young Massive Stars
Recent observations with the Spitzer Space Telescope show clear evidence that
star formation takes place in the surrounding of young massive O-type stars,
which are shaping their environment due to their powerful radiation and stellar
winds. In this work we investigate the effect of ionising radiation of massive
stars on the ambient interstellar medium (ISM): In particular we want to
examine whether the UV-radiation of O-type stars can lead to the observed
pillar-like structures and can trigger star formation. We developed a new
implementation, based on a parallel Smooth Particle Hydrodynamics code (called
IVINE), that allows an efficient treatment of the effect of ionising radiation
from massive stars on their turbulent gaseous environment. Here we present
first results at very high resolution. We show that ionising radiation can
trigger the collapse of an otherwise stable molecular cloud. The arising
structures resemble observed structures (e.g. the pillars of creation in the
Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of
gravitation we find small regions that can be identified as formation places of
individual stars. We conclude that ionising radiation from massive stars alone
can trigger substantial star formation in molecular clouds.Comment: 4 pages, 2 figures. To appear in: "Triggered Star Formation in a
Turbulent ISM", IAU Symposium 237, Prague, Czech Republic, August 2006; eds.
B.G.Elmegreen & J. Palou
The Growth in Size and Mass of Cluster Galaxies since z=2
We study the formation and evolution of Brightest Cluster Galaxies starting
from a population of quiescent ellipticals and following them to .
To this end, we use a suite of nine high-resolution dark matter-only
simulations of galaxy clusters in a CDM universe. We develop a scheme
in which simulation particles are weighted to generate realistic and
dynamically stable stellar density profiles at . Our initial conditions
assign a stellar mass to every identified dark halo as expected from abundance
matching; assuming there exists a one-to-one relation between the visible
properties of galaxies and their host haloes. We set the sizes of the luminous
components according to the observed relations for massive quiescent
galaxies. We study the evolution of the mass-size relation, the fate of
satellite galaxies and the mass aggregation of the cluster central. From ,
these galaxies grow on average in size by a factor 5 to 10 of and in mass by 2
to 3. The stellar mass growth rate of the simulated BCGs in our sample is of
1.9 in the range consistent with observations, and of 1.5 in the
range . Furthermore the satellite galaxies evolve to the present day
mass-size relation by . Assuming passively evolving stellar populations,
we present surface brightness profiles for our cluster centrals which resemble
those observed for the cDs in similar mass clusters both at and at .
This demonstrates that the CDM cosmology does indeed predict minor and
major mergers to occur in galaxy clusters with the frequency and mass ratio
distribution required to explain the observed growth in size of passive
galaxies since . Our experiment shows that Brightest Cluster Galaxies can
form through dissipationless mergers of quiescent massive galaxies,
without substantial additional star formation.Comment: submitted to MNRAS, 10 pages, 8 figures, 2 table
The Validity of the Adiabatic Contraction Approximation for Dark Matter Halos
We use high resolution numerical simulations to investigate the adiabatic
contraction of dark matter halos with a Hernquist density profile. We test the
response of the halos to the growth of additional axisymmetric disk potentials
with various central concentrations and the spherically symmetric potential of
a softened point mass. Adding the potentials on timescales that are long
compared to the dynamical time scale of the halo, the contracted halos have
density profiles that are in excellent agreement with analytical predictions
based on the conservation of the adiabatic invariant . This is
surprising as this quantity is strictly conserved only for particles on
circular orbits and in spherically symmetric potentials. If the same potentials
are added on timescales that are short compared to the dynamical timescale, the
result depends strongly on the adopted potential. The adiabatic approximation
still works for disk potentials. It does, however, fail for the central
potential.Comment: 7 pages, 3 figures, 1 table. Added reference. Accepted for
publication in ApJ
Testing a prediction of the merger origin of early-type galaxies: a correlation between stellar populations and asymmetry
One of the key predictions of the merger hypothesis for the origin of
early-type (elliptical and lenticular) galaxies is that tidally-induced
asymmetric structure should correlate with signatures of a relatively young
stellar population. Such a signature was found by Schweizer and Seitzer (1992;
AJ, 104, 1039) at roughly 4sigma confidence. In this paper, we revisit this
issue with a nearly ten-fold larger sample of 0.01<z<0.03 galaxies selected
from the Two Micron All-Sky Survey and the Sloan Digital Sky Survey. We
parameterize tidal structure using a repeatable algorithmic measure of
asymmetry, and correlate this with color offset from the early-type galaxy
color-magnitude relation. We recover the color offset-asymmetry correlation;
furthermore, we demonstrate observationally for the first time that this effect
is driven by a highly-significant trend towards younger ages at higher
asymmetry values. We present a simple model for the evolution of early-type
galaxies through gas-rich major and minor mergers that reproduces their
observed build-up from z=1 to the present day and the distribution of
present-day colors and ages. We show using this model that if both stellar
populations and asymmetry were ideal `clocks' measuring the time since last
major or minor gas-rich interaction, then we would expect a rather tight
correlation between age and asymmetry. We suggest that the source of extra
scatter is natural diversity in progenitor star formation history, gas content,
and merger mass ratio, but quantitative confirmation of this conjecture will
require sophisticated modeling. We conclude that the asymmetry-age correlation
is in basic accord with the merger hypothesis, and indicates that an important
fraction of the early-type galaxy population is affected by major or minor
mergers at cosmologically-recent times.Comment: Astrophysical Journal, in press. 20 pages, 18 figure
Multiple minor mergers: formation of elliptical galaxies and constraints for the growth of spiral disks
Multiple, sequential mergers are unavoidable in the hierarchical build-up
picture of galaxies, in particular for the minor mergers that are frequent and
highly likely to have occured several times for most present-day galaxies.
However, the effect of repeated minor mergers on galactic structure and
evolution has not been studied systematically so far. We present a numerical
study of multiple, subsequent, minor galaxy mergers, with various mass ratios
ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star
formation. We study the morphological and kinematical properties of the
remnants, and show that several so-called "minor" mergers can lead to the
formation of elliptical-like galaxies that have global morphological and
kinematical properties similar to that observed in real elliptical galaxies.
The properties of these systems are compared with those of elliptical galaxies
produced by the standard scenario of one single major merger. We thus show that
repeated minor mergers can theoretically form elliptical galaxies without major
mergers, and can be more frequent than major mergers, in particular at moderate
redshift. This process must then have formed some elliptical galaxies seen
today, and could in particular explain the high boxiness of massive
ellipticals, and some fundamental relations observed in ellipticals. In
addition, because repeated minor mergers, even at high mass ratios, destroy
disks into spheroids, these results indicate that spiral galaxies cannot have
grown only by a succession of minor mergers.Comment: A&A in press, final version to be published with minor corrections
and updated reference lis
Generation of rotationally dominated galaxies by mergers of pressure-supported progenitors
Through the analysis of a set of numerical simulations of major mergers
between initially non-rotating, pressure supported progenitor galaxies with a
range of central mass concentrations, we have shown that: (1) it is possible to
generate elliptical-like galaxies, with v/sigma > 1 outside one effective
radius, as a result of the conversion of orbital- into internal-angular
momentum; (2) the outer regions acquire part of the angular momentum first; (3)
both the baryonic and the dark matter components of the remnant galaxy acquire
part of the angular momentum, the relative fractions depend on the initial
concentration of the merging galaxies. For this conversion to occur the initial
baryonic component must be sufficiently dense and/or the encounter should take
place on a orbit with high angular momentum. Systems with these hybrid
properties have been recently observed through a combination of stellar
absorption lines and planetary nebulae for kinematic studies of early-type
galaxies. Our results are in qualitative agreement with such observations and
demonstrate that even mergers composed of non-rotating, pressure-supported
progenitor galaxies can produce early-type galaxies with significant rotation
at large radii.Comment: 5 pages, 6 figures, 2 tables. Accepted for publication in A&A Letter
Simulating magnetic fields in the Antennae galaxies
We present self-consistent high-resolution simulations of NGC4038/4039 (the
"Antennae galaxies") including star formation, supernova feedback and magnetic
fields performed with the N-body/SPH code Gadget, in which magnetohydrodynamics
are followed with the SPH method. We vary the initial magnetic field in the
progenitor disks from 1 nG to 100 muG. At the time of the best match with the
central region of the Antennae system the magnetic field has been amplified by
compression and shear flows to an equilibrium field of approximately 10 muG,
independent of the initial seed field. These simulations are a proof of the
principle that galaxy mergers are efficient drivers for the cosmic evolution of
magnetic fields. We present a detailed analysis of the magnetic field structure
in the central overlap region. Simulated radio and polarization maps are in
good morphological and quantitative agreement with the observations. In
particular, the two cores with the highest synchrotron intensity and ridges of
regular magnetic fields between the cores and at the root of the southern tidal
arm develop naturally in our simulations. This indicates that the simulations
are capable of realistically following the evolution of the magnetic fields in
a highly non-linear environment. We also discuss the relevance of the
amplification effect for present day magnetic fields in the context of
hierarchical structure formation.Comment: 18 pages, 14 figures, accepte
Participatory gender-sensitive approaches for addressing key climate change- related research issues: Evidence from Bangladesh, Ghana, and Uganda
Getting a better understanding of how climate variability affects rural men and women differently, and in different regions, is challenging. Since their ability to respond to change and take action that will make them more resilient and able to adapt to a changing climate (alongside all the other social and economic change they are dealing with) differs, we need to focus more research efforts on enhancing this understanding and linking this knowledge with actions aimed at enhancing livelihoods and food security. We examine how well existing participatory gender-sensitive research approaches address some key climate change-related research issues that CCAFS has prioritized. Bringing together gender experts and experienced agricultural research teams from Bangladesh, Ghana, and Uganda, multiple methods were tested in the field, and refined through the lessons learned, to help inform future action research and development efforts towards enhancing communities’ and individuals’ (particularly women’s) access to, and use of, information and knowledge to help them adapt to climate variability through more resilient livelihoods and agro-ecosystems
Estudio de la emisión polínica de un cultivo de girasol (Helianthus annuus L. )
The objective of this study was to estimate the pollen emission dynamics of the sunflower crop and its relationship with some environmental variables such as daily mean temperature (DMT) and precipitation. Two sunflower hybrids (ACA 884 and Cargill 5515) were sown under field conditions at the Facultad de Agronomía, La Pampa on 26/10/98. Meteorological data was obtained from an agrometeorological station located near the field experiment. Soil water content was measured al sowing time and its dynamic registered over time. Capitula population in the crop was measured before first anthesis took place. During the anthesis period the number of disc florets per capilulum was counted in randomly selected plants. To measure the production ot pollen grains per flower, florets still closed were taken from sectors of the capitula located at its periphery, middle and center. DMT controlled the flowering progress of both genotypes according with the maintenance or not of its value over the average temperature during the whole process. The total floret number was similar for both hybrids and the concentration of pollen grains per floret showed no difference between hybrids. Significant differences were however found between the sectors of the capitulum, being the peripheral one the most productive.Resúmenes de Trabajos presentados en otras publicaciones (por docentes de la UNLPam.) Presentado en VII Jornadas Pampeanas de Ciencias Naturales, 1-3 de diciembre de 1999. Santa Rosa. La Pampa: 105-113
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