3,880 research outputs found
NGC 4654: gravitational interaction or ram pressure stripping?
The Virgo cluster spiral galaxy NGC 4654 is supposed to be a good candidate
for ongoing ram pressure stripping based on its very asymmetric HI
distribution. However, this galaxy also shows an asymmetric stellar
distribution. Numerical simulations using ram pressure as the only perturbation
can produce a tail structure of the gas content, but cannot account for its
kinematical structure. It is shown that a strong edge-on stripping event can
produce an asymmetric stellar distribution up to 800 Myr after the stripping
event, i.e. the galaxy's closest passage to the cluster center. Simulations
using a gravitational interaction with the companion galaxy NGC 4639 can
account for the asymmetric stellar distribution of NGC 4654, but cannot
reproduce the observed extended gas tail. Only a mixed interaction,
gravitational and ram pressure, can reproduce all observed properties of NGC
4654. It is concluded that NGC 4654 had a tidal interaction ~500 Myr ago and is
continuing to experience ram pressure.Comment: 16 pages with 19 PS figures. Accepted for publication in A&
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&
The quenching of star formation in accretion-driven clumpy turbulent tori of active galactic nuclei
Galactic gas-gas collisions involving a turbulent multiphase ISM share common
ISM properties: dense extraplanar gas visible in CO, large linewidths (>= 50
km/s), strong mid-infrared H_2 line emission, low star formation activity, and
strong radio continuum emission. Gas-gas collisions can occur in the form of
ICM ram pressure stripping, galaxy head-on collisions, compression of the
intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the
galaxy group at a high velocity, or external gas accretion on an existing gas
torus in a galactic center. We suggest that the common theme of all these
gas-gas interactions is adiabatic compression of the ISM leading to an increase
of the turbulent velocity dispersion of the gas. The turbulent gas clouds are
then overpressured and star formation is quenched. Within this scenario we
developed a model for turbulent clumpy gas disks where the energy to drive
turbulence is supplied by external infall or the gain of potential energy by
radial gas accretion within the disk. The cloud size is determined by the size
of a C-type shock propagating in dense molecular clouds with a low ionization
fraction at a given velocity dispersion. We give expressions for the expected
volume and area filling factors, mass, density, column density, and velocity
dispersion of the clouds. The latter is based on scaling relations of
intermittent turbulence whose open parameters are estimated for the CND in the
Galactic Center. The properties of the model gas clouds and the external mass
accretion rate necessary for the quenching of the star formation rate due to
adiabatic compression are consistent with those derived from high-resolution
H_2 line observations. Based on these findings, a scenario for the evolution of
gas tori in galactic centers is proposed and the implications for star
formation in the Galactic Center are discussed.Comment: 13 pages, 1 figure, accepted for publication by 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 stripped molecular gas in the Virgo spiral galaxy NGC 4522
IRAM 30m 12CO(1-0) and 12CO(2-1) HERA observations are presented for the
ram-pressure stripped Virgo spiral galaxy NGC 4522. The CO emission is detected
in the galactic disk and the extraplanar gas. The extraplanar CO emission
follows the morphology of the atomic gas closely but is less extended. The CO
maxima do not appear to correspond to regions where there is peak massive star
formation as probed by Halpha emission. The presence of molecular gas is a
necessary but not sufficient condition for star formation. Compared to the disk
gas, the molecular fraction of the extraplanar gas is 30% lower and the star
formation efficiency of the extraplanar gas is about 3 times lower. The
comparison with an existing dynamical model extended by a recipe for
distinguishing between atomic and molecular gas shows that a significant part
of the gas is stripped in the form of overdense arm-like structures. It is
argued that the molecular fraction depends on the square root of the total
large-scale density. Based on the combination of the CO/Halpha and an
analytical model, the total gas density is estimated to be about 4 times lower
than that of the galactic disk. Molecules and stars form within this dense gas
according to the same laws as in the galactic disk, i.e. they mainly depend on
the total large-scale gas density. Star formation proceeds where the local
large-scale gas density is highest. Given the complex 3D morphology this does
not correspond to the peaks in the surface density. In the absence of a
confining gravitational potential, the stripped gas arms will most probably
disperse; i.e. the density of the gas will decrease and star formation will
cease.Comment: 11 pages, 15 figures, A&A accepted for publicatio
The flaring HI disk of the nearby spiral galaxy NGC 2683
New deep VLA D array HI observations of the highly inclined nearby spiral
galaxy NGC 2683 are presented. Archival C array data were processed and added
to the new observations. To investigate the 3D structure of the atomic gas
disk, we made different 3D models for which we produced model HI data cubes.
The main ingredients of our best-fit model are (i) a thin disk inclined by 80
degrees; (ii) a crude approximation of a spiral and/or bar structure by an
elliptical surface density distribution of the gas disk; (iii) a slight warp in
inclination; (iv) an exponential flare; and (v) a low surface-density gas ring.
The slope of NGC 2683's flare is comparable, but somewhat steeper than those of
other spiral galaxies. NGC 2683's maximum height of the flare is also
comparable to those of other galaxies. On the other hand, a saturation of the
flare is only observed in NGC 2683. Based on the comparison between the high
resolution model and observations, we exclude the existence of an extended
atomic gas halo around the optical and thin gas disk. Under the assumption of
vertical hydrostatic equilibrium we derive the vertical velocity dispersion of
the gas. The high turbulent velocity dispersion in the flare can be explained
by energy injection by (i) supernovae, (ii) magneto-rotational instabilities,
(iii) ISM stirring by dark matter substructure, or (iv) external gas accretion.
The existence of the complex large-scale warping and asymmetries favors
external gas accretion as one of the major energy sources that drives
turbulence in the outer gas disk. We propose a scenario where this external
accretion leads to turbulent adiabatic compression that enhances the turbulent
velocity dispersion and might quench star formation in the outer gas disk of
NGC 2683.Comment: 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
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&
Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501
VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The
HI disk is sharply truncated to the southwest, well within the stellar disk. A
region of low surface-density gas, which is more extended than the main HI
disk, is discovered northeast of the galaxy center. These data are compared to
existing 6cm polarized radio continuum emission, Halpha, and optical broad band
images. We observe a coincidence between the western HI and polarized emission
edges, on the one hand, and a faint Halpha emission ridge, on the other. The
polarized emission maxima are located within the gaps between the spiral arms
and the faint Halpha ridge. Based on the comparison of these observations with
a sample of dynamical simulations with different values for maximum ram
pressure and different inclination angles between the disk and the orbital
plane,we conclude that ram pressure stripping can account for the main observed
characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and
is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The
southwestern ridge of enhanced gas surface density and enhanced polarized
radio-continuum emission is due to ram pressure compression. It is argued that
the faint western Halpha emission ridge is induced by nearly edge-on ram
pressure stripping. NGC 4501 represents an especially clear example of early
stage ram pressure stripping of a large cluster-spiral galaxy.Comment: 22 pages, 25 figures, accepted for publication in A&
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&
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