75 research outputs found
Tidal Dwarf Galaxies and missing baryons
Tidal dwarf galaxies form during the interaction, collision or merger of
massive spiral galaxies. They can resemble "normal" dwarf galaxies in terms of
mass, size, and become dwarf satellites orbiting around their massive
progenitor. They nevertheless keep some signatures from their origin, making
them interesting targets for cosmological studies. In particular, they should
be free from dark matter from a spheroidal halo. Flat rotation curves and high
dynamical masses may then indicate the presence of an unseen component, and
constrain the properties of the "missing baryons", known to exist but not
directly observed. The number of dwarf galaxies in the Universe is another
cosmological problem that can be significantly impacted if tidal dwarf galaxies
formed frequently at high redshift, when the merger rate was high, and many of
them survived until today.Comment: Tutorial Review for the special issue "Dwarf galaxies and Cosmology"
in Advances in Astronomy. (10 pages, 4 figures
Polar ring galaxies: formation and properties
Formation scenarios for polar ring galaxies are studied through N-body
simulatio ns that are compared with existing observations. It is shown that
polar rings ar e likely to be formed by tidal accretion of the polar material
from a gas rich d onor galaxy. The distribution of dark matter in polar ring
galaxies is studied: dark halos seem to be flattened towards the polar rings.Comment: 2 pages, 1 figure, Proceedings SF2A-2002, Paris, ed. F. Combes and D.
Barret, EDP-Science
A diversity of progenitors and histories for isolated spiral galaxies
We analyze a suite of 33 cosmological simulations of the evolution of Milky
Way-mass galaxies in low-density environments. Our sample spans a broad range
of Hubble types at z=0, from nearly bulgeless disks to bulge-dominated
galaxies. Despite the fact that a large fraction of the bulge is typically in
place by z=1, we find no significant correlation between the morphology at z=1
and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies,
including smooth disks, unstable disks, interacting galaxies and
bulge-dominated systems. By z=0.5, spiral arms and bars are largely in place
and the progenitor morphology is correlated with the final morphology. We next
focus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. These
show a correlation between B/T at z=0 and the mass ratio of the largest merger
at z1. We find that the
galaxies with the lowest B/T tend to have a quiet baryon input history, with no
major mergers at z<2, and with a low and constant gas accretion rate that keeps
a stable angular-momentum direction. More violent merger or gas accretion
histories lead to galaxies with more prominent bulges. Most disk galaxies have
a bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tend
to have histories of intense gas accretion and disk instability rather than
active mergers.Comment: Accepted for publication in ApJ. 29 pages, 32 figure
Non-linear violent disc instability with high Toomre's Q in high-redshift clumpy disc galaxies
We utilize zoom-in cosmological simulations to study the nature of violent
disc instability (VDI) in clumpy galaxies at high redshift, --. Our
simulated galaxies are not in the ideal state assumed in Toomre instability, of
linear fluctuations in an isolated, uniform, rotating disk. There, instability
is characterised by a parameter below unity, and lower when the disk is
thick. Instead, the high-redshift discs are highly perturbed. Over long periods
they consist of non-linear perturbations, compact massive clumps and extended
structures, with new clumps forming in inter-clump regions. This is while the
galaxy is subject to frequent external perturbances. We compute the local,
two-component parameter for gas and stars, smoothed on a
scale to capture clumps of . The regions are
confined to collapsed clumps due to the high surface density there, while the
inter-clump regions show significantly higher than unity. Tracing the
clumps back to their relatively smooth Lagrangian patches, we find that
prior to clump formation typically ranges from unity to a few. This is unlike
the expectations from standard Toomre instability. We discuss possible
mechanisms for high- clump formation, e.g. rapid turbulence decay leading to
small clumps that grow by mergers, non-axisymmetric instability, or clump
formation induced by non-linear perturbations in the disk. Alternatively, the
high- non-linear VDI may be stimulated by the external perturbations such as
mergers and counter-rotating streams. The high may represent excessive
compressive modes of turbulence, possibly induced by tidal interactions.Comment: Accepted for publication in MNRAS. 20 pages, 21 figure
ISM properties in hydrodynamic galaxy simulations: Turbulence cascades, cloud formation, role of gravity and feedback
We study the properties of ISM substructure and turbulence in hydrodynamic
(AMR) galaxy simulations with resolutions up to 0.8 pc and 5x10^3 Msun. We
analyse the power spectrum of the density distribution, and various components
of the velocity field. We show that the disk thickness is about the average
Jeans scale length, and is mainly regulated by gravitational instabilities.
From this scale of energy injection, a turbulence cascade towards small-scale
is observed, with almost isotropic small-scale motions. On scales larger than
the disk thickness, density waves are observed, but there is also a full range
of substructures with chaotic and strongly non-isotropic gas velocity
dispersions. The power spectrum of vorticity in an LMC-sized model suggests
that an inverse cascade of turbulence might be present, although energy input
over a wide range of scales in the coupled gaseous+stellar fluid could also
explain this quasi-2D regime on scales larger than the disk scale height.
Similar regimes of gas turbulence are also found in massive high-redshift disks
with high gas fractions. Disk properties and ISM turbulence appear to be mainly
regulated by gravitational processes, both on large scales and inside dense
clouds. Star formation feedback is however essential to maintain the ISM in a
steady state by balancing a systematic gas dissipation into dense and small
clumps. Our galaxy simulations employ a thermal model based on a barotropic
Equation of State (EoS) aimed at modelling the equilibrium of gas between
various heating and cooling processes. Denser gas is typically colder in this
approach, which is shown to correctly reproduce the density structures of a
star-forming, turbulent, unstable and cloudy ISM down to scales of a few
parsecs.Comment: MNRAS in pres
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