672 research outputs found
The Effects of Ram-pressure Stripping and Supernova Winds on the Tidal Stirring of Disky Dwarfs: Enhanced Transformation into Dwarf Spheroidals
A conclusive model for the formation of dwarf spheroidal (dSph) galaxies
still remains elusive. Owing to their proximity to the massive spirals Milky
Way (MW) and M31, various environmental processes have been invoked to explain
their origin. In this context, the tidal stirring model postulates that
interactions with MW-sized hosts can transform rotationally supported dwarfs,
resembling present-day dwarf irregular (dIrr) galaxies, into systems with the
kinematic and structural properties of dSphs. Using N-body+SPH simulations, we
investigate the dependence of this transformation mechanism on the gas
fraction, fgas, in the disk of the progenitor dwarf. Our numerical experiments
incorporate for the first time the combined effects of radiative cooling,
ram-pressure stripping, star formation, supernova (SN) winds, and a cosmic UV
background. For a given orbit inside the primary galaxy, rotationally supported
dwarfs with gas fractions akin to those of observed dIrrs (fgas >= 0.5),
demonstrate a substantially enhanced likelihood and efficiency of
transformation into dSphs relative to their collisionless (fgas = 0)
counterparts. We argue that the combination of ram-pressure stripping and SN
winds causes the gas-rich dwarfs to respond more impulsively to tides,
augmenting their transformation. When fgas >= 0.5, disky dwarfs on previously
unfavorable low-eccentricity or large-pericenter orbits are still able to
transform. On the widest orbits, the transformation is incomplete; the dwarfs
retain significant rotational support, a relatively flat shape, and some gas,
naturally resembling transition-type systems. We conclude that tidal stirring
constitutes a prevalent evolutionary mechanism for shaping the structure of
dwarf galaxies within the currently favored CDM cosmological paradigm.Comment: Accepted for publication in ApJ Letters, 8 pages, 2 figures, LaTeX
(uses emulateapj.cls
Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
(Abridged) We conduct a series of high-resolution, dissipationless N-body
simulations to investigate the cumulative effect of substructure mergers onto
thin disk galaxies in the context of the LCDM paradigm of structure formation.
Our simulation campaign is based on a hybrid approach. Substructure properties
are culled directly from cosmological simulations of galaxy-sized cold dark
matter (CDM) halos. In contrast to what can be inferred from statistics of the
present-day substructure populations, accretions of massive subhalos onto the
central regions of host halos, where the galactic disk resides, since z~1
should be common occurrences. One host halo merger history is subsequently used
to seed controlled numerical experiments of repeated satellite impacts on an
initially-thin Milky Way-type disk galaxy. We show that these accretion events
produce several distinctive observational signatures in the stellar disk
including: a ring-like feature in the outskirts; a significant flare; a central
bar; and faint filamentary structures that (spuriously) resemble tidal streams.
The final distribution of disk stars exhibits a complex vertical structure that
is well-described by a standard ``thin-thick'' disk decomposition. We conclude
that satellite-disk encounters of the kind expected in LCDM models can induce
morphological features in galactic disks that are similar to those being
discovered in the Milky Way, M31, and in other disk galaxies. These results
highlight the significant role of CDM substructure in setting the structure of
disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys
and astrometric satellites may be able to distinguish between competing
cosmological models by testing whether the detailed structure of galactic disks
is as excited as predicted by the CDM paradigm.Comment: Accepted version to appear in ApJ, 24 pages, 8 figures, LaTeX (uses
emulateapj.cls). Comparison between the simulated ring-like features and the
Monoceros ring stellar structure in the Milky Way performed; conclusions
unaltere
On the Efficiency of the Tidal Stirring Mechanism for the Origin of Dwarf Spheroidals: Dependence on the Orbital and Structural Parameters of the Progenitor Disky Dwarfs
(Abridged) The tidal stirring model posits the formation of dSph galaxies via
the tidal interactions between rotationally-supported dwarfs and MW-sized host
galaxies. Using a set of collisionless N-body simulations, we investigate the
efficiency of the tidal stirring mechanism. We explore a wide variety of dwarf
orbital configurations and initial structures and demonstrate that in most
cases the disky dwarfs experience significant mass loss and their stellar
components undergo a dramatic morphological and dynamical transformation: from
disks to bars and finally to pressure-supported spheroidal systems with
kinematic and structural properties akin to those of the classic dSphs in the
Local Group (LG). Our results suggest that such tidal transformations should be
common occurrences within the currently favored cosmological paradigm and
highlight the key factor responsible for an effective metamorphosis to be the
strength of the tidal shocks at the pericenters of the orbit. We demonstrate
that the combination of short orbital times and small pericenters,
characteristic of dwarfs being accreted at high redshift, induces the strongest
transformations. Our models also indicate that the transformation efficiency is
affected significantly by the structure of the progenitor disky dwarfs. Lastly,
we find that the dwarf remnants satisfy the relation Vmax = \sqrt{3} * sigma,
where sigma is the 1D, central stellar velocity dispersion and Vmax is the
maximum halo circular velocity, with intriguing implications for the missing
satellites problem. Overall, we conclude that the action of tidal forces from
the hosts constitutes a crucial evolutionary mechanism for shaping the nature
of dwarf galaxies in environments such as that of the LG. Environmental
processes of this type should thus be included as ingredients in models of
dwarf galaxy formation and evolution.Comment: submitted to ApJ, 34 pages, 15 figures, LaTeX (uses emulateapj.cls
The shapes of Milky Way satellites: looking for signatures of tidal stirring
We study the shapes of Milky Way satellites in the context of the tidal
stirring scenario for the formation of dwarf spheroidal galaxies. The standard
procedures used to measure shapes involve smoothing and binning of data and
thus may not be sufficient to detect structural properties like bars, which are
usually subtle in low surface brightness systems. Taking advantage of the fact
that in nearby dwarfs photometry of individual stars is available we introduce
discrete measures of shape based on the two-dimensional inertia tensor and the
Fourier bar mode. We apply these measures of shape first to a variety of
simulated dwarf galaxies formed via tidal stirring of disks embedded in dark
matter halos and orbiting the Milky Way. In addition to strong mass loss and
randomization of stellar orbits, the disks undergo morphological transformation
that typically involves the formation of a triaxial bar after the first
pericenter passage. These tidally induced bars persist for a few Gyr before
being shortened towards a more spherical shape if the tidal force is strong
enough. We test this prediction by measuring in a similar way the shape of
nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in
Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of
eleven studied dwarfs show elongated stellar distributions in the outer parts
that may signify transition to tidal tails. We thus find the shapes of Milky
Way satellites to be consistent with the predictions of the tidal stirring
model.Comment: 14 pages, 11 figures, accepted for publication in Ap
Monitoring of UV spectral irradiance at Thessaloniki (1990?2005): data re-evaluation and quality control
International audienceWe present a re-evaluation and quality control of spectral ultraviolet irradiance measurements from two Brewer spectroradiometers operating regularly at Thessaloniki, Greece. The calibration history of the two instruments was re-examined and data flaws were identified by comparing quasi synchronous measurements. Analysis of the sensitivity of both instruments to variations of their internal temperature revealed that they have temperature coefficients of different sign. These coefficients exhibit small variability during the 15-year period. Using averaged temperature coefficients, we corrected both datasets. Corrections were applied for the angular response error using two different approaches depending on the availability of required ancillary data. The uncertainties associated with the measurements have been estimated and presented. Finally, the two datasets are compared using ratios of irradiance integrals at various bands in the UV, in order to assess any dependencies on the internal instrument temperature, solar zenith angle and wavelength
Halo Shapes, Dynamics and Environment
In the hierarchical structure formation model cosmic halos are supposed to
form by accretion of smaller units along anisotropic direction, defined by
large-scale filamentary structures. After the epoch of primary mass aggregation
(which depend on the cosmological model), violent relaxation processes will
tend to alter the halo phase-space configuration producing quasi-spherical
halos with a relatively smooth density profiles.
Here we attempt to investigate the relation between halos shapes, their
environment and their dynamical state. To this end we have run a large ( Mpc, particles) N-body simulation of a flat low-density
cold dark matter model with a matter density , Hubble constant km s
Mpc and a normalization parameter of . The particle mass
is comparable to the mass of
one single galaxy. The halos are defined using a friends-of-friend algorithm
with a linking length given by where is the mean
density. This linking length corresponds to an overdensity at the present epoch () and the total number of halos
with more than 130 particles () is 57524.Comment: To be published in "Groups Of Galaxies In The Nearby Universe", held
in Chile, December 2005, edited by I.Saviane, V.Ivanov and J.Borissova.
Springer-Verlag series "ESO Astrophysics Symposia
Density Profiles of Cold Dark Matter Substructure: Implications for the Missing Satellites Problem
The structural evolution of substructure in cold dark matter (CDM) models is
investigated combining ``low-resolution'' satellites from cosmological N-body
simulations of parent halos with N=10^7 particles with high-resolution
individual subhalos orbiting within a static host potential. We show that, as a
result of mass loss, convergence in the central density profiles requires the
initial satellites to be resolved with N=10^7 particles and parsec-scale force
resolution. We find that the density profiles of substructure halos can be well
fitted with a power-law central slope that is unmodified by tidal forces even
after the tidal stripping of over 99% of the initial mass and an exponential
cutoff in the outer parts. The solution to the missing-satellites problem
advocated by Stoehr et al. in 2002 relied on the flattening of the dark matter
(DM) halo central density cusps by gravitational tides, enabling the observed
satellites to be embedded within DM halos with maximum circular velocities as
large as 60 km/s. In contrast, our results suggest that tidal interactions do
not provide the mechanism for associating the dwarf spheroidal satellites
(dSphs) of the Milky Way with the most massive substructure halos expected in a
CDM universe. We compare the predicted velocity dispersion profiles of Fornax
and Draco to observations, assuming that they are embedded in CDM halos. Models
with isotropic and tangentially anisotropic velocity distributions for the
stellar component fit the data only if the surrounding DM halos have maximum
circular velocities in the range 20-35 km/s. If the dSphs are embedded within
halos this large then the overabundance of satellites within the concordance
LCDM cosmological model is significantly alleviated, but this still does not
provide the entire solution.Comment: Accepted for publication in ApJ, 17 pages, 9 figures, LaTeX (uses
emulateapj5.sty
Tidal evolution of discy dwarf galaxies in the Milky Way potential: the formation of dwarf spheroidals
We conduct high-resolution collisionless N-body simulations to investigate the tidal evolution of dwarf galaxies on an eccentric orbit in the Milky Way (MW) potential. The dwarfs originally consist of a low surface brightness stellar disc embedded in a cosmologically motivated dark matter halo. During 10 Gyr of dynamical evolution and after five pericentre passages, the dwarfs suffer substantial mass loss and their stellar component undergoes a major morphological transformation from a disc to a bar and finally to a spheroid. The bar is preserved for most of the time as the angular momentum is transferred outside the galaxy. A dwarf spheroidal (dSph) galaxy is formed via gradual shortening of the bar. This work thus provides a comprehensive quantitative explanation of a potentially crucial morphological transformation mechanism for dwarf galaxies that operates in groups as well as in clusters. We compare three cases with different initial inclinations of the disc and find that the evolution is fastest when the disc is coplanar with the orbit. Despite the strong tidal perturbations and mass loss, the dwarfs remain dark matter dominated. For most of the time, the one-dimensional stellar velocity dispersion, Ï, follows the maximum circular velocity, Vmax, and they are both good tracers of the bound mass. Specifically, we find that MboundâV3.5max and in agreement with earlier studies based on pure dark matter simulations. The latter relation is based on directly measuring the stellar kinematics of the simulated dwarf, and may thus be reliably used to map the observed stellar velocity dispersions of dSphs to halo circular velocities when addressing the missing satellites proble
Stirring Up the Pot: Can Cooling Flows In Galaxy Clusters Be Quenched By Gas Sloshing?
X-ray observations of clusters of galaxies reveal the presence of edges in
surface brightness and temperature, known as "cold fronts". In relaxed clusters
with cool cores, these commonly observed edges have been interpreted as
evidence for the "sloshing" of the core gas in the cluster's gravitational
potential. Such sloshing may provide a source of heat to the cluster core by
mixing hot gas from the cluster outskirts with the cool core gas. Using
high-resolution -body/Eulerian hydrodynamics simulations, we model gas
sloshing in galaxy clusters initiated by mergers with subclusters. The
simulations include merger scenarios with gas-filled and gasless subclusters.
The effect of changing the viscosity of the intracluster medium is also
explored. We find that sloshing can facilitate heat inflow to the cluster core,
provided that there is a strong enough disturbance. In adiabatic simulations,
we find that sloshing can raise the entropy floor of the cluster core by nearly
an order of magnitude in the strongest cases. If the ICM is viscous, the mixing
of gases with different entropies is decreased and consequently the heat flux
to the core is diminished. In simulations where radiative cooling is included,
we find that though eventually a cooling flow develops, sloshing can prevent
the significant buildup of cool gas in the core for times on the order of a Gyr
for small disturbances and a few Gyr for large ones. If repeated encounters
with merging subclusters sustain the sloshing of the central core gas as is
observed, this process can provide a relatively steady source of heat to the
core, which can help to prevent a significant cooling flow.Comment: 22 pages, 26 figures, "emulateapj" format. The version accepted by
ApJ, with proof correction
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