578 research outputs found
NGC 4569: recent evidence for a past ram pressure stripping event
Deep 21-cm HI line observations of the Virgo cluster spiral galaxy NGC 4569
have been obtained with the VLA in its D configuration and with the Effelsberg
100-m telescope. A low surface density arm was discovered in the west of the
galaxy, whose velocity field is distinct from that of the overall disk
rotation. The observed gas distribution, velocity field, and velocity
dispersion are compared to snapshots of dynamical simulations that include the
effects of ram pressure. Two different scenarios were explored: (i) ongoing
stripping and (ii) a major stripping event that took place about 300 Myr ago.
It is concluded that only the post-stripping scenario can reproduce the main
observed characteristics of NGC 4569. It is not possible to determine if the
gas disk of NGC 4569 had already been truncated before it underwent the ram
pressure event that lead to its observed HI deficiency.Comment: 13 pages, 15 figures. Accepted for publication in A&
Magnetic collimation of meridional-self-similar general relativistic MHD flows
We present a model for the spine of relativistic MHD outflows in the Kerr
geometry. Meridional self-similarity is invoked to derive semi-analytical
solutions close to the polar axis. The study of the energy conservation along a
particular field line gives a simple criterion for the collimation of jets.
Such parameter have already been derived in the classical case by Sauty et al.
1999 and also extended to the Schwarzschild metric by Meliani et al. 2006. We
generalize the same study to the Kerr metric. We show that the rotation of the
black hole increases the magnetic self-confinement.Comment: 16 pages, 6 figures, accepted for publication in Physical Review
NGC 6340: an old S0 galaxy with a young polar disc. Clues from morphology, internal kinematics and stellar populations
Lenticular galaxies are believed to form by a combination of environmental
effects and secular evolution. We study the nearby disc-dominated S0 galaxy NGC
6340 photometrically and spectroscopically to understand the mechanisms of S0
formation and evolution in groups. We use SDSS images to build colour maps and
light profile of NGC 6340 which we decompose using a three-component model
including Sersic and two exponential profiles. We also use Spitzer images to
study the morphology of regions containing warm ISM and dust. Then, we
re-process and re-analyse deep long-slit spectroscopic data for NGC 6340 and
recover its stellar and gas kinematics, distribution of age and metallicity
with the NBursts full spectral fitting. We obtain the profiles of internal
kinematics, age, and metallicity out to >2 half-light radii. The three
structural components of NGC 6340 are found to have distinct kinematical and
stellar population properties. We see a kinematical misalignment between inner
and outer regions of the galaxy. We confirm the old metal-rich centre and a
wrapped inner gaseous polar disc (r~1 kpc) having weak ongoing star formation,
counter-rotating in projection with respect to the stars. The central compact
pseudo-bulge of NGC 6340 looks very similar to compact elliptical galaxies. In
accordance with the results of numerical simulations, we conclude that
properties of NGC 6340 can be explained as the result of a major merger of
early-type and spiral galaxies which occurred about 12 Gyr ago. The
intermediate exponential structure might be a triaxial pseudo-bulge formed by a
past bar structure. The inner compact bulge could be the result of a nuclear
starburst triggered by the merger. The inner polar disc appeared recently,
1/3-1/2 Gyr ago as a result of another minor merger or cold gas accretion.Comment: 12 pages, 8 figures, accepted to A&
Counterrotation in magnetocentrifugally driven jets and other winds
Rotation measurement in jets from T Tauri stars is a rather difficult task.
Some jets seem to be rotating in a direction opposite to that of the underlying
disk, although it is not yet clear if this affects the totality or part of the
outflows. On the other hand, Ulysses data also suggest that the solar wind may
rotate in two opposite ways between the northern and southern hemispheres. We
show that this result is not as surprising as it may seem and that it emerges
naturally from the ideal MHD equations. Specifically, counterrotating jets
neither contradict the magnetocentrifugal driving of the flow nor prevent
extraction of angular momentum from the disk. The demonstration of this result
is shown by combining the ideal MHD equations for steady axisymmetric flows.
Provided that the jet is decelerated below some given threshold beyond the
Alfven surface, the flow will change its direction of rotation locally or
globally. Counterrotation is also possible for only some layers of the outflow
at specific altitudes along the jet axis. We conclude that the counterrotation
of winds or jets with respect to the source, star or disk, is not in
contradiction with the magnetocentrifugal driving paradigm. This phenomenon may
affect part of the outflow, either in one hemisphere, or only in some of the
outflow layers. From a time-dependent simulation, we illustrate this effect and
show that it may not be permanent.Comment: To appear in ApJ
Counter-rotation in relativistic magnetohydrodynamic jets
Young stellar object observations suggest that some jets rotate in the
opposite direction with respect to their disk. In a recent study, Sauty et al.
(2012) have shown that this does not contradict the magnetocentrifugal
mechanism that is believed to launch such outflows. Signatures of motions
transverse to the jet axis and in opposite directions have recently been
measured in M87 (Meyer et al. 2013). One possible interpretation of this motion
is the one of counter rotating knots. Here, we extend our previous analytical
derivation of counter-rotation to relativistic jets, demonstrating that
counter-rotation can indeed take place under rather general conditions. We show
that both the magnetic field and a non-negligible enthalpy are necessary at the
origin of counter-rotating outflows, and that the effect is associated with a
transfer of energy flux from the matter to the electromagnetic field. This can
be realized in three cases : if a decreasing enthalpy causes an increase of the
Poynting flux, if the flow decelerates, or, if strong gradients of the magnetic
field are present. An illustration of the involved mechanism is given by an
example of relativistic MHD jet simulation.Comment: Accepted for publication in ApJ
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
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