944 research outputs found
Comparison of simple mass estimators for slowly rotating elliptical galaxies
We compare the performance of mass estimators for elliptical galaxies that
rely on the directly observable surface brightness and velocity dispersion
profiles, without invoking computationally expensive detailed modeling. These
methods recover the mass at a specific radius where the mass estimate is
expected to be least sensitive to the anisotropy of stellar orbits. One method
(Wolf et al. 2010) uses the total luminosity-weighted velocity dispersion and
evaluates the mass at a 3D half-light radius , i.e., it depends on the
GLOBAL galaxy properties. Another approach (Churazov et al. 2010) estimates the
mass from the velocity dispersion at a radius where the surface
brightness declines as , i.e., it depends on the LOCAL properties. We
evaluate the accuracy of the two methods for analytical models, simulated
galaxies and real elliptical galaxies that have already been modeled by the
Schwarzschild's orbit-superposition technique. Both estimators recover an
almost unbiased circular speed estimate with a modest RMS scatter (). Tests on analytical models and simulated galaxies indicate that the local
estimator has a smaller RMS scatter than the global one. We show by examination
of simulated galaxies that the projected velocity dispersion at could
serve as a good proxy for the virial galaxy mass. For simulated galaxies the
total halo mass scales with as with RMS scatter
.Comment: 19 pages, 14 figures, 4 tables, accepted for publication in MNRA
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
Peanut yield response to micro and macronutrients of a Ferric Lixisol in the Guinea savanna zone of Ghana
abstractA field experiment was conducted on a Ferric Lixisol at the Savanna Agriculture Research Institute, Wa (10° 3I N, 20° 50I W) in the Guinea savannah zone of Ghana in 2010 and 2011, to study the effect of Cu, Mn, Zn, Ca, P and K on elemental uptake, peanut yield and yield components. Nine treatments were arranged in a randomized complete block (RCB) design with four replicates. Results from the study showed that nutrient application had a positive effect on nutrient concentration, nutrient uptake, pod yield, seed yield and total biomass. The PK + gypsum + Zn treatment had the highest pod yield, seed yield and total biomass. Increases in pod yield of the fertilized treatments ranged between 16.6 per cent and 47.8 per cent over the control. Where micronutrients were combined, some antagonism between micronutrients was also observed. Application of the macronutrients P and K and the micronutrient Zn had the greatest impact on yield.Original scientific paper. Received 16 Sep 14; revised 31 Mar 15
Constrained simulations of the Antennae Galaxies: Comparison with Herschel-PACS observations
We present a set of hydro-dynamical numerical simulations of the Antennae
galaxies in order to understand the origin of the central overlap starburst.
Our dynamical model provides a good match to the observed nuclear and overlap
star formation, especially when using a range of rather inefficient stellar
feedback efficiencies (0.01 < q_EoS < 0.1). In this case a simple conversion of
local star formation to molecular hydrogen surface density motivated by
observations accounts well for the observed distribution of CO. Using radiative
transfer post-processing we model synthetic far-infrared spectral energy
distributions (SEDs) and two-dimensional emission maps for direct comparison
with Herschel-PACS observations. For a gas-to-dust ratio of 62:1 and the best
matching range of stellar feedback efficiencies the synthetic far-infrared SEDs
of the central star forming region peak at values of ~65 - 81 Jy at 99 - 116
um, similar to a three-component modified black body fit to infrared
observations. Also the spatial distribution of the far-infrared emission at 70
um, 100 um, and 160 um compares well with the observations: >50% (> 35%) of the
emission in each band is concentrated in the overlap region while only < 30% (<
15%) is distributed to the combined emission from the two galactic nuclei in
the simulations (observations). As a proof of principle we show that parameter
variations in the feedback model result in unambiguous changes both in the
global and in the spatially resolved observable far-infrared properties of
Antennae galaxy models. Our results strengthen the importance of direct,
spatially resolved comparative studies of matched galaxy merger simulations as
a valuable tool to constrain the fundamental star formation and feedback
physics.Comment: 17 pages, 8 figures, 4 tables, submitted to MNRAS, including
revisions after first referee report, comments welcom
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
The flattening and the orbital structure of early-type galaxies and collisionless N-body binary disk mergers
We use oblate axisymmetric dynamical models including dark halos to determine
the orbital structure of intermediate mass to massive Coma early-type galaxies.
We find a large variety of orbital compositions. Averaged over all sample
galaxies the unordered stellar kinetic energy in the azimuthal and the radial
direction are of the same order, but they can differ by up to 40 percent in
individual systems. In contrast, both for rotating and non-rotating galaxies
the vertical kinetic energy is on average smaller than in the other two
directions. This implies that even most of the rotating ellipticals are
flattened by an anisotropy in the stellar velocity dispersions. Using
three-integral axisymmetric toy models we show that flattening by stellar
anisotropy maximises the entropy for a given density distribution.
Collisionless disk merger remnants are radially anisotropic. The apparent lack
of strong radial anisotropy in observed early-type galaxies implies that they
may not have formed from mergers of disks unless the influence of dissipational
processes was significant.Comment: 14 pages, 8 figures; accepted for publication in MNRA
SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation
The intrinsic anisotropy delta and flattening epsilon of simulated merger
remnants is compared with elliptical galaxies that have been observed by the
SAURON collaboration, and that were analysed using axisymmetric Schwarzschild
models. Collisionless binary mergers of stellar disks and disk mergers with an
additional isothermal gas component, neglecting star formation cannot reproduce
the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit
of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is
however found for merger simulations of disks with gas fractions >= 20%,
including star formation and stellar energy feedback. Massive black hole
feedback does not strongly affect this result. Subsequent dry merging of merger
remnants however does not generate the slowly-rotating SAURON ellipticals which
are characterized by low ellipticities epsilon < 0.25 and low anisotropies.
This indicates that at least some ellipticals on the red galaxy sequence did
not form by binary mergers of disks or early-type galaxies. We show that
stellar spheroids resulting from multiple, hierarchical mergers of
star-bursting subunits in a cosmological context are in excellent agreement
with the low ellipticities and anisotropies of the slowly rotating SAURON
ellipticals and their observed trend of delta with epsilon. The numerical
simulations indicate that the SAURON relation might be a result of strong
violent relaxation and phase mixing of multiple, kinematically cold stellar
subunits with the angular momentum of the system determining its location on
the relation.Comment: 13 pages, 3 figures, submitted to Ap
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
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