100 research outputs found
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
Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers
We investigate, by means of numerical simulations, the possibility of forming
counter-rotating old stellar components by major mergers between an elliptical
and a spiral galaxy. We show that counter-rotation can appear both in
dissipative and dissipationless retrograde mergers, and it is mostly associated
to the presence of a disk component, which preserves part of its initial spin.
In turn, the external regions of the two interacting galaxies acquire part of
the orbital angular momentum, due to the action of tidal forces exerted on each
galaxy by the companion.Comment: 6 pages, 15 figures. Accepted on Astronomy & Astrophysic
2-Dimensional Kinematics of Simulated Disc Merger Remnants
We present a two-dimensional kinematic analysis for a sample of simulated
binary disc merger remnants with mass ratios 1:1 and 3:1. For the progenitor
discs we used pure stellar models as well as models with 10% of their mass in
gas. A multitude of phenomena also observed in real galaxies are found in the
simulations. These include misaligned rotation, embedded discs, gas rings,
counter-rotating cores and kinematic misaligned discs. Using the 2D maps we
illustrate projection effects and the change in properties of a merger remnant
when gas is included in the merger. We find that kinematic peculiar subsystems
are preferably formed in equal mass mergers. Equal-mass collisionless remnants
can show almost no rotation, regular rotation or strong kinematic misalignment.
The inclusion of gas makes the remnants appear more round(1:1) and
axisymmetric(3:1). Counter-Rotating Cores (CRCs) are almost exclusively formed
in equal-mass mergers with a dissipational component. 3:1 remnants show a much
more regular structure. We quantify these properties by applying the kinemetric
methods recently developed by Krajnovi\'c et al. This work will help to
understand observations of elliptical galaxies with 2D field spectrographs,
like SAURON.Comment: accepted for publication in MNRAS, discussion substantially enlarged,
conclusion unchange
The Orbital Structure of Galaxies and Dark Matter Halos in N-Body Simulations
We examine in this work two different formation mechanism of galaxies in N-body simulations. Under the assumption that particles in a spherical dark matter halo move on circular orbits we can predict the amount of contraction of the dark matter halo during the slow formation of the disk with an adiabatic approximation. We find in N-body simulations that the adiabatic approximation is valid for all realistic mass ratios between dark matter halos and disk galaxies and that deviations from circular orbits cannot play a decisive role. In the second part we focus on the formation of ellipticals through mergers of disk galaxies. We classify the complex orbital structure in a sample of 150 collisions. The classification is correlated with shape parameters of an elliptical galaxy, such as its triaxiality or the ratios of its principal axes. We are deriving a global occupation probability for self-consistent triaxial galaxies which are in agreement with theoretical expectations. Furthermore we find that the isophotal structure of the merger remnants cannot be explained by one orbit class alone, but by a superposition of classes. The dichotomy of observed isophotal shape in boxy and disky elliptical galaxies, cannot be completely explained by the dominance of box-like, respectivley disk-like orbits in those galaxies. Current simulations cannot reproduce observed correlation between the h_3 parameter and the mean velocity. We identify a central box orbit component as the reason for this discrepancy, which are overproduce in dissipationless simulations. The z-tube component follows the correlation very well. It follows also the observed correlation between the h_3 parameter and v/sigma_0. We conclude that only one dynamical component is necessary to explain the observed correlations, which looks like a puffy disk with high velocity dispersion
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
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
Cold gas and young stars in tidally-disturbed ellipticals at z=0
We present an analysis of the neutral hydrogen and stellar populations of
elliptical galaxies in the Tal et al. (2009) sample. Our aim is to test their
conclusion that the continuing assembly of these galaxies at z~0 is essentially
gas-free and not accompanied by significant star formation. In order to do so,
we make use of HI data and line-strength indices available in the literature.
We look for direct and indirect evidence of the presence of cold gas during the
recent assembly of these objects and analyse its relation to galaxy
morphological fine structure.
We find that >25% of ellipticals contain HI at the level of M(HI)>10^8
M(Sun), and that M(HI) is of the order of a few percent of the total stellar
mass. Available data are insufficient to establish whether galaxies with a
disturbed stellar morphology are more likely to contain HI. However, HI
interferometry reveals very disturbed gas morphology/kinematics in all but one
of the detected systems, confirming the continuing assembly of many ellipticals
but also showing that this is not necessarily gas-free. We also find that all
very disturbed ellipticals have a single-stellar-population-equivalent age <4
Gyr. We interpret this as evidence that ~0.5-5% of their stellar mass is
contained in a young population formed during the past ~1 Gyr. Overall, a large
fraction of ellipticals seem to have continued their assembly over the past few
Gyr in the presence of a mass of cold gas of the order of 10% of the galaxy
stellar mass. This material is now observable as neutral hydrogen and young
stars.Comment: 5 pages, 1 table, 3 figures. Accepted for publication in MNRAS
Letter
2D velocity fields of simulated interacting disc galaxies
We investigate distortions in the velocity fields of disc galaxies and their
use to reveal the dynamical state of interacting galaxies at different
redshift. For that purpose, we model disc galaxies in combined
N-body/hydrodynamic simulations. 2D velocity fields of the gas are extracted
from these simulations which we place at different redshifts from z=0 to z=1 to
investigate resolution effects on the properties of the velocity field. To
quantify the structure of the velocity field we also perform a kinemetry
analysis. If the galaxy is undisturbed we find that the rotation curve
extracted from the 2D field agrees well with long-slit rotation curves. This is
not true for interacting systems, as the kinematic axis is not well defined and
does in general not coincide with the photometric axis of the system. For large
(Milky way type) galaxies we find that distortions are still visible at
intermediate redshifts but partly smeared out. Thus a careful analysis of the
velocity field is necessary before using it for a Tully-Fisher study. For small
galaxies (disc scale length ~2 kpc) even strong distortions are not visible in
the velocity field at z~0.5 with currently available angular resolution.
Therefore we conclude that current distant Tully-Fisher studies cannot give
reliable results for low-mass systems. Additionally to these studies we confirm
the power of near-infrared integral field spectrometers in combination with
adaptive optics (such as SINFONI) to study velocity fields of galaxies at high
redshift (z~2).Comment: 12 pages, 18 figures, accepted for publication in A&A, high
resolution version can be found at
http://astro.uibk.ac.at/~thomas/kronberger.pd
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