16,260 research outputs found
From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs
We compare three different models of clumpy gas disk and show that the
Circumnuclear Disk (CND) in the Galactic Center and a putative, geometrically
thick, obscuring torus are best explained by a collisional model consisting of
quasi-stable, self-gravitating clouds. Kinetic energy of clouds is gained by
mass inflow and dissipated in cloud collisions. The collisions give rise to a
viscosity in a spatially averaged gas dynamical picture, which connects them to
angular momentum transport and mass inflow. It is found that CND and torus
share the same gas physics in our description, where the mass of clouds is 20 -
50 M_sun and their density is close to the limit of disruption by tidal shear.
We show that the difference between a transparent CND and an obscuring torus is
the gas mass and the velocity dispersion of the clouds. A change in gas supply
and the dissipation of kinetic energy can turn a torus into a CND-like
structure and vice versa. Any massive torus will naturally lead to sufficiently
high mass accretion rates to feed a luminous AGN. For a geometrically thick
torus to obscure the view to the center even super-Eddington accretions rates
with respect to the central black hole are required.Comment: 9 pages, no figures. Accepted for publication in A&
Deep HI observations of the surroundings of ram pressure stripped Virgo spiral galaxies - Where is the stripped gas?
Deep Effelsberg 100-m HI observations of 5 HI deficient Virgo spiral galaxies
are presented. No new extended HI tail is found in these galaxies. The already
known HI tail north of NGC 4388 does not significantly extend further than a
WSRT image has shown. Based on the absence of HI tails in a sample of 6 Virgo
spiral galaxies and a balance of previous detections of extraplanar gas in the
targeted galaxies we propose a global picture where the outer gas disk (beyond
the optical radius R_25) is removed much earlier than expected by the classical
ram pressure criterion. Based on the two-phase nature of atomic hydrogen
located in a galactic disk, we argue that the warm diffuse HI in the outer
galactic disk is evaporated much more rapidly than the cold dense HI.
Therefore, after a ram pressure stripping event we can only observe atomic
hydrogen which was cold and dense before it was removed from the galactic disk.
This global picture is consistent with all available observations. We detect
between 0.3% and 20% of the stripped mass assuming an initially non-deficient
galaxy and between 3% and 70% of the stripped mass assuming an initially HI
deficient galaxy (def=0.4). Under the latter assumption we estimate an
evaporation rate by dividing the missing mass by the estimated time to peak ram
pressure from dynamical simulations. We find evaporation rates between 10 and
100 M_solar/yr.Comment: 9 pages, 9 figures, accepted for publication in A&
Ram pressure stripping in a viscous intracluster medium
In the recent literature there is circumstantial evidence that the viscosity
of the intracluster medium may not be too far from the Spitzer value. In this
letter, we present two-dimensional hydrodynamical simulations of ram pressure
stripping of disc galaxies in a viscous intracluster medium. The values of
viscosity explored range between 0.1 and 1.0 times the Spitzer value. We find
that viscosity affects the appearance and the dimensions of the galactic wakes
but has very little effect on the evolution of the gas mass of the galaxy.Comment: 5 pages, 2 figures, accepted by MNRAS letter
A holistic view on ram pressure stripping in the Virgo cluster - The first complete model-based time sequence
Based on a comparison of dynamical models with observations of the
interstellar gas in 6 Virgo cluster spiral galaxies a first complete ram
pressure stripping time sequence has been established. The observational
characteristics of the different stages of ram pressure stripping are
presented. The dynamical models yield the 3D velocity vectors of the galaxies,
peak ram pressures, and times to peak ram pressure. In the case of a smooth,
static, and spherical intracluster medium, peak ram pressure occurs during the
galaxy's closest approach to the cluster center, i.e. when the galaxy's
velocity vector is perpendicular to its distance vector from the cluster center
(M 87). Assuming this condition the galaxy's present line-of-sight distance and
its 3D position during peak ram pressure can be calculated. The linear orbital
segments derived in this way together with the intracluster medium density
distribution derived from X-ray observations give estimates of the ram pressure
that are on average a factor of 2 higher than derived from the dynamical
simulations for NGC 4501, NGC 4330, and NGC 4569. Resolving this discrepancy
would require either a 2 times higher intracluster medium density than derived
from X-ray observations, or a 2 times higher stripping efficiency than assumed
by the dynamical models. Compared to NGC 4501, NGC 4330, and NGC 4569, NGC 4388
requires a still 2 times higher local intracluster medium density or a
direction which is moderately different from that derived from the dynamical
model. A possible scenario for the dynamical evolution of NGC 4438 and M 86
within the Virgo cluster is presented.Comment: 10 pages, 5 figures; accepted for publication in A&
Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438
Ram pressure stripping of the multiphase ISM is studied in the perturbed
Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction
~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA
radio continuum observations at 6 and 20 cm are presented. We detect prominent
extraplanar emission to the west of the galactic center, which extends twice as
far as the other tracers of extraplanar material. The spectral index of the
extraplanar emission does not steepen with increasing distance from the galaxy.
This implies in situ re-acceleration of relativistic electrons. The comparison
with multiwavelength observations shows that the magnetic field and the warm
ionized interstellar medium traced by Halpha emission are closely linked. The
kinematics of the northern extraplanar Halpha emission, which is ascribed to
star formation, follow those of the extraplanar CO emission. In the western and
southern extraplanar regions, the Halpha measured velocities are greater than
those of the CO lines. We suggest that the ionized gas of this region is
excited by ram pressure. The spatial and velocity offsets are consistent with a
scenario where the diffuse ionized gas is more efficiently pushed by ram
pressure stripping than the neutral gas. We suggest that the recently found
radio-deficient regions compared to 24 mum emission are due to this difference
in stripping efficiency.Comment: 8 pages, 6 figures, A&A, accepted for publicatio
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