118 research outputs found
Free-Floating HI Clouds in the M 81 Group
Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M
81 group reveal a hitherto hidden population of extremely low mass (~1e5 Msol)
HI clouds with no obvious optical counterparts. We have searched 10 fields in
the M81 group totalling 2.2 square degree, both targeting known dwarf
spheroidal galaxies and blank fields around the central triplet. Our
observations show that the new population of low-mass HI clouds appears to be
confined to a region toward the South-East of the central triplet (at distances
of ~100 kpc from M 81). Possible explanations for these free-floating HI clouds
are that they are related to the dSphs found to the South-East of M 81, that
they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they
are of primordial nature and provide fresh, unenriched material falling into
the M 81 group, or that they are tidal debris from the 3-body interaction
involving M 81-M 82-NGC 3077. Based on circumstantial evidence, we currently
favour the latter explanation.Comment: To appear in the Proceedings of IAU Symp. 244 on "Dark Galaxies and
Lost Baryons", eds. J. I. Davies & M. J. Disne
Star clusters dynamics in a laboratory: electrons in an ultracold plasma
Electrons in a spherical ultracold quasineutral plasma at temperature in the
Kelvin range can be created by laser excitation of an ultra-cold laser cooled
atomic cloud. The dynamical behavior of the electrons is similar to the one
described by conventional models of stars clusters dynamics. The single mass
component, the spherical symmetry and no stars evolution are here accurate
assumptions. The analog of binary stars formations in the cluster case is
three-body recombination in Rydberg atoms in the plasma case with the same
Heggie's law: soft binaries get softer and hard binaries get harder. We
demonstrate that the evolution of such an ultracold plasma is dominated by
Fokker-Planck kinetics equations formally identical to the ones controlling the
evolution of a stars cluster. The Virial theorem leads to a link between the
plasma temperature and the ions and electrons numbers. The Fokker-Planck
equation is approximate using gaseous and fluid models. We found that the
electrons are in a Kramers-Michie-King's type quasi-equilibrium distribution as
stars in clusters. Knowing the electron distribution and using forced fast
electron extraction we are able to determine the plasma temperature knowing the
trapping potential depth.Comment: Submitted to MNRA
Parallel TREE code for two-component ultracold plasma analysis
The TREE method has been widely used for long-range interaction {\it N}-body
problems. We have developed a parallel TREE code for two-component classical
plasmas with open boundary conditions and highly non-uniform charge
distributions. The program efficiently handles millions of particles evolved
over long relaxation times requiring millions of time steps. Appropriate domain
decomposition and dynamic data management were employed, and large-scale
parallel processing was achieved using an intermediate level of granularity of
domain decomposition and ghost TREE communication. Even though the
computational load is not fully distributed in fine grains, high parallel
efficiency was achieved for ultracold plasma systems of charged particles. As
an application, we performed simulations of an ultracold neutral plasma with a
half million particles and a half million time steps. For the long temporal
trajectories of relaxation between heavy ions and light electrons, large
configurations of ultracold plasmas can now be investigated, which was not
possible in past studies
Gravitational stability and dynamical overheating of stellar disks of galaxies
We use the marginal stability condition for galactic disks and the stellar
velocity dispersion data published by different authors to place upper limits
on the disk local surface density at two radial scalelengths .
Extrapolating these estimates, we constrain the total mass of the disks and
compare these estimates to those based on the photometry and color of stellar
populations. The comparison reveals that the stellar disks of most of spiral
galaxies in our sample cannot be substantially overheated and are therefore
unlikely to have experienced a significant merging event in their history. The
same conclusion applies to some, but not all of the S0 galaxies we consider.
However, a substantial part of the early type galaxies do show the stellar
velocity dispersion well in excess of the gravitational stability threshold
suggesting a major merger event in the past. We find dynamically overheated
disks among both seemingly isolated galaxies and those forming pairs. The ratio
of the marginal stability disk mass estimate to the total galaxy mass within
four radial scalelengths remains within a range of 0.4---0.8. We see no
evidence for a noticeable running of this ratio with either the morphological
type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter
More evidence for hidden spiral and bar features in bright early-type dwarf galaxies
Following the discovery of spiral structure in IC3328 (Jerjen et al.~2000),
we present further evidence that a sizable fraction of bright early-type dwarfs
in the Virgo cluster are genuine disk galaxies, or are hosting a disk
component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed
with the Very Large Telescope in and , we found another four systems
exhibiting non-axisymmetric structures, such as a bar and/or spiral arms,
indicative of a disk (IC0783, IC3349, NGC4431, IC3468). Particularly remarkable
are the two-armed spiral pattern in IC0783 and the bar and trailing arms in
NGC4431. For both galaxies the disk nature has recently been confirmed by a
rotation velocity measurement (Simien & Prugniel 2002). Our photometric search
is based on a Fourier decomposition method and a specific version of unsharp
masking. Some ``early-type'' dwarfs in the Virgo cluster seem to be former
late-type galaxies which were transformed to early-type morphology, e.g. by
``harassment'', during their infall to the cluster, while maintaining part of
their disk structure.Comment: A&A accepte
The Evolution of Early-Type Galaxies in Distant Clusters II: Internal Kinematics of 55 Galaxies in the z=0.33 Cluster CL1358+62
We define a large sample of galaxies for use in a study of the fundamental
plane in the intermediate redshift cluster CL1358+62 at . We have
analyzed high resolution spectra for 55 members of the cluster. The data were
acquired with the Low Resolution Imaging Spectrograph on the Keck I 10m
telescope. A new algorithm for measuring velocity dispersions is presented and
used to measure the internal kinematics of the galaxies. This algorithm has
been tested against the Fourier Fitting method so the data presented here can
be compared with those measured previously in nearby galaxies. We have measured
central velocity dispersions suitable for use in a fundamental plane analysis.
The data have high and the resulting random errors on the dispersions are
very low, typically . Uncertainties due to mismatch of the stellar
templates has been minimized through several tests and the total systematic
error is of order \about 5%. Good seeing enabled us to measure velocity
dispersion profiles and rotation curves for most of the sample and although a
large fraction of the galaxies display a high level of rotation, the gradients
of the total second moment of the kinematics are all very regular and similar
to those in nearby galaxies. We conclude that the data therefore can be
reliably corrected for aperture size in a manner consistent with nearby galaxy
samples.Comment: 30 pages, 13 figures; for publication in the ApJ (accepted on 23
August 1999
Central K-band kinematics and line strength maps of NGC 1399
In this paper we present for the first time high spatial resolution K-band
maps of the central kinematical and near-infrared spectral properties of the
giant cD galaxy in the Fornax cluster, NGC 1399. We confirm the presence of a
central velocity dispersion dip within radius < 0.2" seen in previous long-slit
studies. Our velocity dispersion maps give evidence for a non-symmetric
structure in this central area by showing three sigma peaks to the north-east,
south-east and west of the galaxy centre. Additionally we measure near-IR line
strength indices at unprecedented spatial resolution in NGC 1399. The most
important features we observe in our 2-dimensional line strength maps are drops
in Na I and CO(2-0) line strength in the nuclear region of the galaxy,
coinciding spatially with the drop in sigma. The observed line strength and
velocity dispersion changes suggest a scenario where the centre of NGC 1399
harbours a dynamically cold subsystem with a distinct stellar population.Comment: 9 pages, 5 figures, accepted for publication in A&
Absorption Imaging and Spectroscopy of Ultracold Neutral Plasmas
Absorption imaging and spectroscopy can probe the dynamics of an ultracold
neutral plasma during the first few microseconds after its creation.
Quantitative analysis of the data, however, is complicated by the inhomogeneous
density distribution, expansion of the plasma, and possible lack of global
thermal equilibrium for the ions. In this article we describe methods for
addressing these issues. Using simple assumptions about the underlying
temperature distribution and ion motion, the Doppler-broadened absorption
spectrum obtained from plasma images can be related to the average temperature
in the plasma.Comment: 14 pages, 8 figure
On the Specific Features of Temperature Evolution in Ultracold Plasmas
A theoretical interpretation of the recent experimental studies of
temperature evolution in the course of time in the freely-expanding ultracold
plasma bunches, released from a magneto-optical trap, is discussed. The most
interesting result is finding the asymptotics of the form T_e ~ t^{-(1.2 +/-
0.1)} instead of t^{-2}, which was expected for the rarefied monatomic gas
during inertial expansion. As follows from our consideration, the substantially
decelerated decay of the temperature can be well explained by the specific
features of the equation of state for the ultracold plasmas with strong
Coulomb's coupling, whereas a heat release due to inelastic processes (in
particular, three-body recombination) does not play an appreciable role in the
first approximation. This conclusion is confirmed both by approximate
analytical estimates, based on the model of "virialization" of the
charged-particle energies, and by the results of "ab initio" numerical
simulation. Moreover, the simulation shows that the above-mentioned law of
temperature evolution is approached very quickly--when the virial criterion is
satisfied only within a factor on the order of unity.Comment: LaTeX + 3 eps figures, 16 pages. Plasma Physics Reports, v.37, in
press (2011
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