2,578 research outputs found
Evolution of Compact Groups of Galaxies I. Merging Rates
We discuss the merging rates in compact groups of 5 identical elliptical
galaxies. All groups have the same mass and binding energy. We consider both
cases with individual halos and cases where the halo is common to all galaxies
and enveloping the whole group. In the latter situation the merging rate is
slower if the halo is more massive. The mass of individual halos has little
influence on the merging rates, due to the fact that all galaxies in our
simulations have the same mass, and so the more extended ones have a smaller
velocity dispersion. Groups with individual halos merge faster than groups with
common halos if the configuration is centrally concentrated, like a King
distribution of index 10. On the other hand for less concentrated
configurations the merging is initially faster for individual halo cases, and
slower after part of the group has merged. In cases with common halo, centrally
concentrated configurations merge faster for high halo-to-total mass ratios and
slower for low halo-to-total mass ratios. Groups whose virial ratio is
initially less than one merge faster, while groups that have initially
cylindrical rotation merge slower than groups starting in virial equilibrium.
In order to test how long a virialised group can survive before merging we
followed the evolution of a group with a high halo-to-total mass ratio and a
density distribution with very little central concentration. We find that the
first merging occurred only after a large number of crossing times, which with
areasonable calibration should be larger than a Hubble time. Hence, at least
for appropriate initial conditions, the longevity of compact groups is not
necessarily a problem, which is an alternative explanation to why we observe so
many compact groups despite the fact that their lifetimes seem short.Comment: 15 pages Latex, with 12 figures included, requires mn.sty, accepted
for publication in MNRA
Optimal softening for force calculations in collisionless N-body simulations
In N-body simulations the force calculated between particles representing a
given mass distribution is usually softened, to diminish the effect of
graininess. In this paper we study the effect of such a smoothing, with the aim
of finding an optimal value of the softening parameter. As already shown by
Merritt (1996), for too small a softening the estimates of the forces will be
too noisy, while for too large a softening the force estimates are
systematically misrepresented. In between there is an optimal softening, for
which the forces in the configuration approach best the true forces. The value
of this optimal softening depends both on the mass distribution and on the
number of particles used to represent it. For higher number of particles the
optimal softening is smaller. More concentrated mass distributions necessitate
smaller softening, but the softened forces are never as good an approximation
of the true forces as for not centrally concentrated configurations. We give
good estimates of the optimal softening for homogeneous spheres, Plummer
spheres, and Dehnen spheres. We also give a rough estimate of this quantity for
other mass distributions, based on the harmonic mean distance to the th
neighbour ( = 1, .., 12), the mean being taken over all particles in the
configuration. Comparing homogeneous Ferrers ellipsoids of different shapes we
show that the axial ratios do not influence the value of the optimal softening.
Finally we compare two different types of softening, a spline softening
(Hernquist & Katz 1989) and a generalisation of the standard Plummer softening
to higher values of the exponent. We find that the spline softening fares
roughly as well as the higher powers of the power-law softening and both give a
better representation of the forces than the standard Plummer softening.Comment: 16 pages Latex, 19 figures, accepted for publication in MNRAS,
corrected typos, minor changes mainly in sec.
Statistics of the structure components in S0s: implications for bar induced secular evolution
The fractions and dimension of bars, rings and lenses are studied in the
Near-IR S0 galaxy Survey (NIRS0S). We find evidence that multiple lenses in
some barred S0s are related to bar resonances in a similar manner as the inner
and outer rings, for which the outer/inner length ratio 2. Inner lenses in the
non-barred galaxies normalized to galaxy diameter are clearly smaller than
those in the barred systems. Interestingly, these small lenses in the
non-barred galaxies have similar sizes as barlenses (lens-like structures
embedded in a bar), and therefore might actually be barlenses in former barred
galaxies, in which the outer, more elongated bar component, has been destroyed.
We also find that fully developed inner lenses are on average a factor 1.3
larger than bars, whereas inner rings have similar sizes as bars. The fraction
of inner lenses is found to be constant in all family classes (A, AB, B).
Nuclear bars appear most frequently among the weakly barred (AB) galaxies,
which is consistent with the theoretical models by Maciejewski & Athanassoula
(2008). Similar sized bars as the nuclear bars were detected in seven
'non-barred' S0s. Galaxy luminosity does not uniquely define the sizes of bars
or bar-related structures, neither is there any upper limit in galaxy
luminosity for bar formation. Although all the family classes cover the same
range of galaxy luminosity, the non-barred (A) galaxies are on average 0.6 mag
brighter than the strongly barred (B) systems. Overall, our results are
consistent with the idea that bars play an important role in the formation of
the structure components of galaxies. The fact that multiple lenses are common
in S0s, and that at least the inner lenses can have very old stellar
populations, implies that the last destructive merger, or major gas accretion
event, must have taken place at a fairly high redshift.Comment: 36 pages (include 13 figures, 11 tables). Accepted to MNRAS 2013 Jan
2
Forming first-ranked early-type galaxies through hierarchical dissipationless merging
We have developed a computationally competitive N-body model of a
previrialized aggregation of galaxies in a flat LambdaCDM universe to assess
the role of the multiple mergers that take place during the formation stage of
such systems in the configuration of the remnants assembled at their centres.
An analysis of a suite of 48 simulations of low-mass forming groups (of about
1E13 solar masses) demonstrates that the gravitational dynamics involved in
their hierarchical collapse is capable of creating realistic first-ranked
galaxies without the aid of dissipative processes. Our simulations indicate
that the brightest group galaxies (BGGs) constitute a distinct population from
other group members, sketching a scenario in which the assembly path of these
objects is dictated largely by the formation of their host system. We detect
significant differences in the distribution of Sersic indices and total
magnitudes, as well as a luminosity gap between BGGs and the next brightest
galaxy that is positively correlated with the total luminosity of the parent
group. Such gaps arise from both the grow of BGGs at the expense of lesser
companions and the decrease in the relevance of second-ranked objects in equal
measure. This results in a dearth of intermediate-mass galaxies which explains
the characteristic central dip detected in their luminosity functions in
dynamically young galaxy aggregations. The fact that the basic global
properties of our BGGs define a thin mass fundamental plane strikingly similar
to that followed giant early-type galaxies in the local universe reinforces
confidence in the results obtained.Comment: 25 pages, 14 figures, 3 tables. Accepted to 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
Performance and accuracy of a GRAPE-3 system for collisionless N-body simulations
The performance and accuracy of a GRAPE-3 system for collisionless N-body
simulations is discussed. After a description of the hardware configurations
available to us at Marseille, and the usefulness of on-line analysis, we
concentrate on the performance and accuracy of the direct summation and tree
code software. For the former we discuss the sources of round-off errors. The
standard Barnes-Hut tree code has been modified for GRAPE-3, by dividing
particles into blocks and performing the tree traversal for the whole block.
The forces are then calculated by direct summation over the whole interaction
list. The performance of the tree code depends on the number of particles in
the block, the optimum number depending on the speed of the front end and the
number of boards. The time per step scales with the number of particles, and
decreases only weakly as the tolerance increases. We discuss the effect of the
front end and compare performances with those of general purpose computers.
The accuracy of both direct summation and the tree code is discussed as
function of the number of particles and the softening. The increased role of
direct summation in the force calculation enhances the accuracy compared to the
standard tree code. Finally, we follow the evolution of an isolated barred
galaxy using different hardware and software in order to assess the reliability
of our results. We find excellent agreement between the pattern speed of the
bar in direct summation simulations run on the high precision GRAPE-4 machines
and on our GRAPE-3 system. The agreement with the tree code is very good for
tolerance values smaller than about 1.0.
We conclude that GRAPE-3 systems are well suited for collisionless
simulations, in particular those of galaxies.Comment: 13 pages Latex, with 11 figures, accepted for publication in MNRA
L4Fe2As2Te1-xO4-yFy (L = Pr, Sm, Gd): a layered oxypnictide superconductor with Tc up to 45 K
The synthesis, structural and physical properties of iron lanthanide
oxypnictide superconductors, L4Fe2As2Te1-xO4 (L = Pr, Sm, Gd), with transition
temperature at ~ 25 K are reported. Single crystals have been grown at high
pressure using cubic anvil technique. The crystal structure consists of layers
of L2O2 tetrahedra separated by alternating layers of chains of Te and of
Fe2As2 tetrahedra: -L2O2-Te-L2O2-Fe2As2-L2O2-Te-L2O2- (space group: I4/mmm, a ~
4.0, c ~ 29.6 {\AA}). Substitution of oxygen by fluorine increases the critical
temperature, e.g. in Gd4Fe2As2Te1-xOyF4-y up to 45 K. Magnetic torque
measurements reveal an anisotropy of the penetration depths of ~31.Comment: 8 figures, 4 table
The SKA view of the Neutral Interstellar Medium in Galaxies
Two major questions in galaxy evolution are how star-formation on small
scales leads to global scaling laws and how galaxies acquire sufficient gas to
sustain their star formation rates. HI observations with high angular
resolution and with sensitivity to very low column densities are some of the
important observational ingredients that are currently still missing. Answers
to these questions are necessary for a correct interpretation of observations
of galaxy evolution in the high-redshift universe and will provide crucial
input for the sub-grid physics in hydrodynamical simulations of galaxy
evolutions. In this chapter we discuss the progress that will be made with the
SKA using targeted observations of nearby individual disk and dwarf galaxies.Comment: 22 pages, 4 figures, to appear as part of 'Neutral Hydrogen' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)12
Setting the normalcy level of HI properties in isolated galaxies
Studying the atomic gas (HI) properties of the most isolated galaxies is
essential to quantify the effect that the environment exerts on this sensitive
component of the interstellar medium. We observed and compiled HI data for a
well defined sample of ~ 800 galaxies in the Catalog of Isolated Galaxies, as
part of the AMIGA project (Analysis of the ISM in Isolated GAlaxies,
http://amiga.iaa.es), which enlarges considerably previous samples used to
quantify the HI deficiency in galaxies located in denser environments. By
studying the shape of 182 HI profiles, we revisited the usually accepted result
that, independently of the environment, more than half of the galaxies present
a perturbed HI disk. In isolated galaxies this would certainly be a striking
result if these are supposed to be the most relaxed systems, and has
implications in the relaxation time scales of HI disks and the nature of the
most frequent perturbing mechanisms in galaxies. Our sample likely exhibits the
lowest HI asymmetry level in the local Universe. We found that other field
samples present an excess of ~ 20% more asymmetric HI profiles than that in
CIG. Still a small percentage of galaxies in our sample present large
asymmetries. Follow-up high resolution VLA maps give insight into the origin of
such asymmetries.Comment: 4 pages, 2 figures, Conference 'Galaxies in Isolation: Exploring
Nature vs. Nurture', Granada, 12-15 May 2009. To be published in the ASP
Conference Serie
A ~ 12 kpc HI extension and other HI asymmetries in the isolated galaxy CIG 340 (IC 2487)
HI kinematic asymmetries are common in late-type galaxies irrespective of
environment, although the amplitudes are strikingly low in isolated galaxies.
As part of our studies of the HI morphology and kinematics in isolated
late-type galaxies we have chosen several very isolated galaxies from the AMIGA
sample for HI mapping. Here we present GMRT 21-cm HI line mapping of CIG 340
which was selected because its integrated HI spectrum has a very symmetric
profile, Aflux = 1.03. Optical images of the galaxy hinted at a warped disk in
contrast to the symmetric integrated HI spectrum profile. Our aim is to
determine the extent to which the optical asymmetry is reflected in the
resolved HI morphology and kinematics. GMRT observations reveal significant HI
morphological asymmetries in CIG 340 despite it's overall symmetric optical
form and highly symmetric HI spectrum. The most notable HI features are: 1) a
warp in the HI disk (with an optical counterpart), 2) the HI north/south flux
ratio = 1.32 is much larger than expected from the integrated HI spectrum
profile and 3) a ~ 45" (12 kpc) HI extension, containing ~ 6% of the detected
HI mass on the northern side of the disk. We conclude that in isolated galaxies
a highly symmetric HI spectrum can mask significant HI morphological
asymmetries. The northern HI extension appears to be the result of a recent
perturbation (10^8 yr), possibly by a satellite which is now disrupted or
projected within the disk. This study provides an important step in our ongoing
program to determine the predominant source of HI asymmetries in isolated
galaxies. For CIG 340 the isolation from major companions, symmetric HI
spectrum, optical morphology and interaction timescales have allowed us to
narrow the possible causes the HI asymmetries and identify tests to further
constrain the source of the asymmetries.Comment: 10 page
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