867 research outputs found
The cosmological origin of the Tully-Fisher relation
We use high-resolution cosmological simulations that include the effects of
gasdynamics and star formation to investigate the origin of the Tully-Fisher
relation in the standard Cold Dark Matter cosmogony. Luminosities are computed
for each model galaxy using their full star formation histories and the latest
spectrophotometric models. We find that at z=0 the stellar mass of model
galaxies is proportional to the total baryonic mass within the virial radius of
their surrounding halos. Circular velocity then correlates tightly with the
total luminosity of the galaxy, reflecting the equivalence between mass and
circular velocity of systems identified in a cosmological context. The slope of
the relation steepens slightly from the red to the blue bandpasses, and is in
fairly good agreement with observations. Its scatter is small, decreasing from
\~0.45 mag in the U-band to ~0.34 mag in the K-band. The particular
cosmological model we explore here seems unable to account for the zero-point
of the correlation. Model galaxies are too faint at z=0 (by about two
magnitudes) if the circular velocity at the edge of the luminous galaxy is used
as an estimator of the rotation speed. The Tully-Fisher relation is brighter in
the past, by about ~0.7 magnitudes in the B-band at z=1, at odds with recent
observations of z~1 galaxies. We conclude that the slope and tightness of the
Tully-Fisher relation can be naturally explained in hierarchical models but
that its normalization and evolution depend strongly on the star formation
algorithm chosen and on the cosmological parameters that determine the
universal baryon fraction and the time of assembly of galaxies of different
mass.Comment: 5 pages, 4 figures included, submitted to ApJ (Letters
The Dynamical State and Mass-Concentration Relation of Galaxy Clusters
We use the Millennium Simulation series to study how the dynamical state of
dark matter halos affects the relation between mass and concentration. We find
that a large fraction of massive systems are identified when they are
substantially out of equilibrium and in a particular phase of their dynamical
evolution: the more massive the halo, the more likely it is found at a
transient stage of high concentration. This state reflects the recent assembly
of massive halos and corresponds to the first pericentric passage of
recently-accreted material when, before virialization, the kinetic and
potential energies reach maximum and minimum values, respectively. This result
explains the puzzling upturn in the mass-concentration relation reported in
recent work for massive halos; indeed, the upturn disappears when only
dynamically-relaxed systems are considered in the analysis. Our results warn
against applying simple equilibrium models to describe the structure of rare,
massive galaxy clusters and urges caution when extrapolating scaling laws
calibrated on lower-mass systems, where such deviations from equilibrium are
less common. The evolving dynamical state of galaxy clusters ought to be
carefully taken into account if cluster studies are to provide precise
cosmological constraints.Comment: 8 Pages. Minor changes to match published versio
Dwarf Galaxies and the Cosmic Web
We use a cosmological simulation of the formation of the Local Group of
Galaxies to identify a mechanism that enables the removal of baryons from
low-mass halos without appealing to feedback or reionization. As the Local
Group forms, matter bound to it develops a network of filaments and pancakes.
This moving web of gas and dark matter drifts and sweeps a large volume,
overtaking many halos in the process. The dark matter content of these halos is
unaffected but their gas can be efficiently removed by ram-pressure. The loss
of gas is especially pronounced in low-mass halos due to their lower binding
energy and has a dramatic effect on the star formation history of affected
systems. This "cosmic web stripping" may help to explain the scarcity of dwarf
galaxies compared with the numerous low-mass halos expected in \Lambda CDM and
the large diversity of star formation histories and morphologies characteristic
of faint galaxies. Although our results are based on a single high-resolution
simulation, it is likely that the hydrodynamical interaction of dwarf galaxies
with the cosmic web is a crucial ingredient so far missing from galaxy
formation models.Comment: Submitted to ApJL. 6 pages, 4 figures. A set of movies showing the
interaction between dwarf galaxies and the Cosmic Web can be found at mirror
1 http://www.astro.uvic.ca/~mario/dwarf-web/ or at mirror 2
http://www.iate.oac.uncor.edu/~alejandro/dwarf-web/ . Comments are welcome
The relation between velocity dispersion and mass in simulated clusters of galaxies: dependence on the tracer and the baryonic physics
[Abridged] We present an analysis of the relation between the masses of
cluster- and group-sized halos, extracted from CDM cosmological N-body
and hydrodynamic simulations, and their velocity dispersions, at different
redshifts from to . The main aim of this analysis is to understand
how the implementation of baryonic physics in simulations affects such
relation, i.e. to what extent the use of the velocity dispersion as a proxy for
cluster mass determination is hampered by the imperfect knowledge of the
baryonic physics. In our analysis we use several sets of simulations with
different physics implemented. Velocity dispersions are determined using three
different tracers, DM particles, subhalos, and galaxies.
We confirm that DM particles trace a relation that is fully consistent with
the theoretical expectations based on the virial theorem and with previous
results presented in the literature. On the other hand, subhalos and galaxies
trace steeper relations, and with larger values of the normalization. Such
relations imply that galaxies and subhalos have a per cent velocity
bias relative to the DM particles, which can be either positive or negative,
depending on halo mass, redshift and physics implemented in the simulation.
We explain these differences as due to dynamical processes, namely dynamical
friction and tidal disruption, acting on substructures and galaxies, but not on
DM particles. These processes appear to be more or less effective, depending on
the halo masses and the importance of baryon cooling, and may create a
non-trivial dependence of the velocity bias and the \soneD--\Mtwo relation
on the tracer, the halo mass and its redshift.
These results are relevant in view of the application of velocity dispersion
as a proxy for cluster masses in ongoing and future large redshift surveys.Comment: 13 pages, 16 figures. Minor modifications to match the version in
press on MNRA
Modeling the Gas Flow in the Bar of NGC 1365
We present new observations of the strongly-barred galaxy NGC 1365, including
new photometric images and Fabry-Perot spectroscopy, as well as a detailed
re-analysis of the neutral hydrogen observations from the VLA archive. We find
the galaxy to be at once remarkably bi-symmetric in its I-band light
distribution and strongly asymmetric in the distribution of dust and in the
kinematics of the gas in the bar region. The velocity field mapped in the
H-alpha line reveals bright HII regions with velocities that differ by 60 to 80
km/s from that of the surrounding gas, which may be due to remnants of
infalling material. We have attempted hydrodynamic simulations of the bar flow
to estimate the separate disk and halo masses, using two different dark matter
halo models and covering a wide range of mass-to-light ratios (Upsilon) and bar
pattern speeds (Omega_p). None of our models provides a compelling fit to the
data, but they seem most nearly consistent with a fast bar, corotation at sim
1.2r_B, and Upsilon_I simeq 2.0 +- 1.0, implying a massive, but not fully
maximal, disk. The fitted dark halos are unusually concentrated, a requirement
driven by the declining outer rotation curve.Comment: 43 pages, 15 figures, accepted to appear in Ap
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