806 research outputs found
On the Formation of Boxy and Disky Elliptical Galaxies
The origin of boxy and disky elliptical galaxies is investigated. The results
of two collisionless N-body simulations of spiral-spiral mergers with mass
ratios of 1:1 and 3:1 are discussed and the projected properties of the merger
remnants are investigated. It is shown that the equal-mass merger leads to an
anisotropic, slowly rotating system with preferentially boxy isophotes and
significant minor axis rotation. The unequal-mass merger results in the
formation of a rotationally supported elliptical with disky isophotes and small
minor axis rotation. The observed scatter in the kinematical and isophotal
properties of both classes of elliptical galaxies can be explained by
projection effects.Comment: 12 pages, incl. 5 figures, accepted by ApJ Letter
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
Nuclear Activity and the Dynamics of Elliptical Galaxies
This paper looks for any correlation between the internal dynamics of
elliptical galaxies and the relatively mild nuclear activity found in many such
systems. We show that there is such a relation in the sense that the active
ellipticals tend to be significantly less rotationally supported than their
inactive cousins. The correlation can partly be related to the galaxies'
luminosities: the brightest galaxies tend to be more active and less
rotationally supported. However, even at lower luminosities the active and
inactive galaxies seem to have systematically different dynamics. This
variation suggests that there are significant large-scale structural
differences between active and inactive elliptical galaxies, and hence that the
existence of both types of system cannot just be the result of random sporadic
nuclear activity.Comment: 5 pages, 3 figures. Accepted for publication in MNRA
Probing for evolutionary links between local ULIRGs and QSOs from NIR spectroscopy
We present a study of the dynamical evolution of Ultraluminous Infrared
Galaxies (ULIRGs), merging galaxies of infrared luminosity >10^12 L_sun. During
our Very Large Telescope large program, we have obtained ISAAC near-infrared,
high-resolution spectra of 54 ULIRGs (at several merger phases) and 12 local
Palomar-Green QSOs to investigate whether ULIRGs go through a QSO phase during
their evolution. One possible evolutionary scenario is that after nuclear
coalescence, the black hole radiates close to Eddington to produce QSO
luminosities. The mean stellar velocity dispersion that we measure from our
spectra is similar (~160 km/s) for 30 post-coalescence ULIRGs and 7 IR-bright
QSOs. The black holes in both populations have masses of order 10^7-10^8 M_sun
(calculated from the relation to the host dispersion) and accrete at rates >0.5
Eddington. Placing ULIRGs and IR-bright QSOs on the fundamental plane of
early-type galaxies shows that they are located on a similar region (that of
moderate-mass ellipticals), in contrast to giant ellipticals and radio-loud
QSOs. While this preliminary comparison of the ULIRG and QSO host kinematical
properties indicates that (some) ULIRGs may undergo a QSO phase in their
evolutionary history before they settle down as ellipticals, further data on
non-IR excess QSOs are necessary to test this scenario.Comment: To appear in the "QSO Host Galaxies: Evolution and Environment"
conference proceedings; meeting held in Leiden, August 200
The SILCC (SImulating the LifeCycle of molecular Clouds) project: I. Chemical evolution of the supernova-driven ISM
The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a
more self-consistent understanding of the interstellar medium (ISM) on small
scales and its link to galaxy evolution. We simulate the evolution of the
multi-phase ISM in a 500 pc x 500 pc x 10 kpc region of a galactic disc, with a
gas surface density of .
The Flash 4.1 simulations include an external potential, self-gravity, magnetic
fields, heating and radiative cooling, time-dependent chemistry of H and CO
considering (self-) shielding, and supernova (SN) feedback. We explore SN
explosions at different (fixed) rates in high-density regions (peak), in random
locations (random), in a combination of both (mixed), or clustered in space and
time (clustered). Only random or clustered models with self-gravity (which
evolve similarly) are in agreement with observations. Molecular hydrogen forms
in dense filaments and clumps and contributes 20% - 40% to the total mass,
whereas most of the mass (55% - 75%) is in atomic hydrogen. The ionised gas
contributes <10%. For high SN rates (0.5 dex above Kennicutt-Schmidt) as well
as for peak and mixed driving the formation of H is strongly suppressed.
Also without self-gravity the H fraction is significantly lower (
5%). Most of the volume is filled with hot gas (90% within 2 kpc).
Only for random or clustered driving, a vertically expanding warm component of
atomic hydrogen indicates a fountain flow. Magnetic fields have little impact
on the final disc structure. However, they affect dense gas () and delay H formation. We highlight that individual chemical
species, in particular atomic hydrogen, populate different ISM phases and
cannot be accurately accounted for by simple temperature-/density-based phase
cut-offs.Comment: 30 pages, 23 figures, submitted to MNRAS. Comments welcome! For
movies of the simulations and download of selected Flash data see the SILCC
website: http://www.astro.uni-koeln.de/silc
Dynamical evolution of massive black holes in galactic-scale N-body simulations - Introducing the regularized tree code 'rVINE'
We present a hybrid code combining the OpenMP-parallel tree code VINE with an
algorithmic chain regularization scheme. The new code, called "rVINE", aims to
significantly improve the accuracy of close encounters of massive bodies with
supermassive black holes in galaxy-scale numerical simulations. We demonstrate
the capabilities of the code by studying two test problems, the sinking of a
single massive black hole to the centre of a gas-free galaxy due to dynamical
friction and the hardening of a supermassive black hole binary due to close
stellar encounters. We show that results obtained with rVINE compare well with
NBODY7 for problems with particle numbers that can be simulated with NBODY7. In
particular, in both NBODY7 and rVINE we find a clear N-dependence of the binary
hardening rate, a low binary eccentricity and moderate eccentricity evolution,
as well as the conversion of the galaxy's inner density profile from a cusp to
a a core via the ejection of stars at high velocity. The much larger number of
particles that can be handled by rVINE will open up exciting opportunities to
model stellar dynamics close to SMBHs much more accurately in a realistic
galactic context. This will help to remedy the inherent limitations of commonly
used tree solvers to follow the correct dynamical evolution of black holes in
galaxy scale simulations
The SILCC project: III. Regulation of star formation and outflows by stellar winds and supernovae
We study the impact of stellar winds and supernovae on the multi-phase
interstellar medium using three-dimensional hydrodynamical simulations carried
out with FLASH. The selected galactic disc region has a size of (500 pc) x
5 kpc and a gas surface density of 10 M/pc. The simulations
include an external stellar potential and gas self-gravity, radiative cooling
and diffuse heating, sink particles representing star clusters, stellar winds
from these clusters which combine the winds from indi- vidual massive stars by
following their evolution tracks, and subsequent supernova explosions. Dust and
gas (self-)shielding is followed to compute the chemical state of the gas with
a chemical network. We find that stellar winds can regulate star (cluster)
formation. Since the winds suppress the accretion of fresh gas soon after the
cluster has formed, they lead to clusters which have lower average masses
(10 - 10 M) and form on shorter timescales (10 -
10 Myr). In particular we find an anti-correlation of cluster mass and
accretion time scale. Without winds the star clusters easily grow to larger
masses for ~5 Myr until the first supernova explodes. Overall the most massive
stars provide the most wind energy input, while objects beginning their
evolution as B-type stars contribute most of the supernova energy input. A
significant outflow from the disk (mass loading 1 at 1 kpc) can be
launched by thermal gas pressure if more than 50% of the volume near the disc
mid-plane can be heated to T > 3x10 K. Stellar winds alone cannot create a
hot volume-filling phase. The models which are in best agreement with observed
star formation rates drive either no outflows or weak outflows.Comment: 23 pages; submitted to MNRA
The Cool ISM in Elliptical Galaxies. II. Gas Content in the Volume - Limited Sample and Results from the Combined Elliptical and Lenticular Surveys
We report new observations of atomic and molecular gas in a volume limited
sample of elliptical galaxies. Combining the elliptical sample with an earlier
and similar lenticular one, we show that cool gas detection rates are very
similar among low luminosity E and SO galaxies but are much higher among
luminous S0s. Using the combined sample we revisit the correlation between cool
gas mass and blue luminosity which emerged from our lenticular survey, finding
strong support for previous claims that the molecular gas in ellipticals and
lenticulars has different origins. Unexpectedly, however, and contrary to
earlier claims, the same is not true for atomic gas. We speculate that both the
AGN feedback and merger paradigms might offer explanations for differences in
detection rates, and might also point towards an understanding of why the two
gas phases could follow different evolutionary paths in Es and S0s. Finally we
present a new and puzzling discovery concerning the global mix of atomic and
molecular gas in early type galaxies. Atomic gas comprises a greater fraction
of the cool ISM in more gas rich galaxies, a trend which can be plausibly
explained. The puzzle is that galaxies tend to cluster around
molecular-to-atomic gas mass ratios near either 0.05 or 0.5.Comment: 37 pages, including 4 tables and 12 figures. Accepted for publication
in the Astrophysical Journa
The Formation of Disks in Elliptical Galaxies
We investigate detailed kinematical properties of simulated collisionless
merger remnants of disk galaxies with mass ratios of 1:1 and 3:1. The
simulations were performed by direct summation using the new special hardware
device GRAPE-5. In agreement with observations, the shape of the line-of-sight
velocity distribution (LOSVD) is Gaussian with small deviations. For most cases
we find that the retrograde wings of the LOSVD are steeper than the prograde
ones. This is in contradiction with observations which show broad retrograde
and steep prograde wings. This serious problem in the collisionless formation
scenario of massive elliptical galaxies can be solved if all rotating
ellipticals, even boxy ones, contain an additional stellar disk component with
of the total stellar mass and a scale length of order the
effective radius of the spheroid. We propose that the progenitor galaxies of
massive ellipticals must have contained a significant amount of gas that did
not condense into stars during the merger process itself but formed an extended
gaseous disk before the star formation epoch. The heating source that prevented
the gas from forming stars early and the origin of the large specific angular
momentum required for the gas component to form an extended disk are still
unsolved problems.Comment: 16 pages, 4 figures, submitted to ApJ Letter
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