833 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
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
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
Do dwarf galaxies form in tidal tails?
The formation of tidal dwarf galaxies (TDG) inside tidal arms of interacting
disk galaxies has been studied with N-body and N-body/SPH simulations at
different resolutions. In pure N-body simulations no bound objects are formed
at high resolution. At low resolution bound objects can form in tidal tails in
agreement with previous work. We conclude that tidal dwarf galaxies are not
likely to form by pure collisionless collapse in tidal tails. However, the
presence of a sufficiently massive and extended gas component in the progenitor
disk supports the formation of bound stellar objects in the tidal arms. Our
results clearly favor a dissipation supported scenario in which the formation
of TDGs is induced by the local collapse of gas which then triggers the
collapse of the stellar component.Comment: 18 pages, 16 figures, revised version, MNRAS accepte
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
Monster black holes
A combination of ground-based and spacecraft observations has uncovered two
black holes of 10 billion solar masses in the nearby Universe. The finding
sheds light on how these cosmic monsters co-evolve with galaxies.Comment: 2 pages, 1 figure, LaTeX. Published in Nature "News & Views
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