214 research outputs found
The large scale magnetic field structure of the spiral galaxy NGC 5775
In order to better determine the large-scale 3D-structure of magnetic fields
in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral
galaxy NGC 5775. Deep radio-continuum observations in total power and linear
polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg
telescope. They were analyzed together with archival 4.86 and 1.49 GHz
VLA-data. We thus can derive rotation measures from a comparison of three
frequencies and determine the intrinsic magnetic field structure. A very
extended halo is detected in NGC 5775, with magnetic field lines forming an
X-shaped structure. Close to the galactic disk the magnetic field is
plane-parallel. The scaleheights of the radio emission esimated for NGC 5775
are comaprable with other galaxies. The rotation measure distribution varies
smoothly on both sides along the major axis from positive to negative values.
From the derived distribution of rotation measures and the plane-parallel
intrinsic magnetic field orientation along the galactic midplane we conclude
that NGC 5775 has an 'even axisymmetric' large-scale magnetic field
configuration in the disk as generated by an \alpha \Omega -dynamo which is
accompanied by a quadrupolar poloidal field. The magnetic field lines of the
plane-parallel component are pointing 'outwards'. The observed X-shaped halo
magnetic field, however, cannot be explained by the action of the disk's
mean-field dynamo alone. It is probably due to the influence of the galactic
wind together with the dynamo action.Comment: 11 pages, 13 figures, A&A accepte
Discovery of a tidal dwarf galaxy in the Leo Triplet
We report discovery of a dwarf galaxy in the Leo Triplet. Analysis of the
neutral hydrogen distribution shows that it rotates independently of the tidal
tail of NGC 3628, with a radial velocity gradient of 35-40 km s over
approximately 13 kpc. The galaxy has a very high neutral gas content,
explaining large part of its total dynamic mass - suggesting small dark matter
content. As it is located at the tip of the gaseous tail, this strongly
suggests its tidal origin. Should it be the case, it would be one of the most
confident and closest (to the Milky Way) detections of a tidal dwarf galaxy
and, at the same time, a most detached from its parent galaxy (140
kpc) object of this type.Comment: 6 pages, 4 figures; The Astrophysical Journal, in pres
NGC 4654: polarized radio continuum emission as a diagnostic tool for a galaxy--cluster interaction
A recent comparison between deep VLA HI observations and dynamical models of
the Virgo cluster spiral galaxy NGC 4654 has shown that only a model involving
a combination of a tidal interaction and ram pressure can reproduce the data.
Deep radio polarization studies, together with detailed MHD modeling, can
independently verify those conclusions, that are based on HI observations and
dynamical models. We performed deep polarized radio-continuum observations of
the Virgo cluster spiral galaxy NGC 4654 with the Effelsberg 100m telescope at
8.35 GHz and the VLA at 4.85 GHz. Detailed 3D MHD simulations were made to
determine the large-scale magnetic field and the emission distribution of the
polarized radio continuum in the model, during the galaxy evolution within the
cluster environment. This direct comparison between the observed and simulated
polarized radio continuum emission corroborates the earlier results, that the
galaxy had a recent rapid close encounter with NGC 4639 and is undergoing weak
ram pressure by the intracluster medium. This combination of deep radio
polarization studies and detailed MHD modeling thus gives us unique insight
into the interactions of a galaxy with its cluster environment. It represents a
diagnostic tool that is complementary to deep HI observations.Comment: Corrected galaxy name in captions of figures (1 & 2
Multiwavelength study of the radio emission from a tight galaxy pair Arp 143
We present results of the recent low-frequency radio observations of a tight
galaxy pair Arp 143 at 234 and 612 MHz. These data are analysed together with
the archive data at 1490, 4860, 8440, and 14940 MHz. From the analysis of the
radio emission we derive constraints on the age of the radio emitting
structures as well as on the properties of their magnetic field. We show that
the collisional ring of NGC 2445 hosts strong magnetic fields (reaching 12
G in its northwestern part) manifesting as a steep--spectrum, nonthermal
radiation at radio frequencies. The spectral age of this structure is higher
than estimates derived for the star-forming regions from the H
distribution, suggesting that the radio emission might have a different origin.
The galactic core is of a very young spectral age, suggesting an ongoing
starburst activity. Additionally we identify a possible ridge of emission
between the ring galaxy and its elliptical companion NGC 2444.Comment: 10 pages, 5 figures, 4 tables. Accepted for publication in MNRA
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&
Unusual magnetic fields in the interacting spiral NGC 3627
By observing the interacting galaxy NGC 3627 in radio polarization we try to
answer the question to which degree the magnetic field follows the galactic gas
flows. We obtained total power and polarized intensity maps at 8.46 GHz and
4.85 GHz using the VLA in its compact D-configuration. In order to overcome the
zero-spacing problems, the interferometric data were combined with single-dish
measurements obtained with the Effelsberg 100-m radio telescope. The observed
magnetic field structure in NGC 3627 suggests that two field components are
superposed. One component smoothly fills the interarm space and shows up also
in the outermost disk regions, the other component follows a symmetric S-shaped
structure. In the western disk the latter component is well aligned with an
optical dust lane, following a bend which is possibly caused by external
interactions. However, in the SE disk the magnetic field crosses a heavy dust
lane segment, apparently being insensitive to strong density-wave effects. We
suggest that the magnetic field is decoupled from the gas by high turbulent
diffusion, in agreement with the large \ion{H}{i} line width in this region. We
discuss in detail the possible influence of compression effects and
non-axisymmetric gas flows on the general magnetic field asymmetries in NGC
3627. On the basis of the Faraday rotation distribution we also suggest the
existence of a large ionized halo around this galaxy.Comment: 11 pages, 11 figure
Cosmic-ray driven dynamo in the interstellar medium of irregular galaxies
Irregular galaxies are usually smaller and less massive than their spiral,
S0, and elliptical counterparts. Radio observations indicate that a magnetic
field is present in irregular galaxies whose value is similar to that in spiral
galaxies. However, the conditions in the interstellar medium of an irregular
galaxy are unfavorable for amplification of the magnetic field because of the
slow rotation and low shearing rate. We investigate the cosmic-ray driven
dynamo in the interstellar medium of an irregular galaxy. We study its
efficiency under the conditions of slow rotation and weak shear. The star
formation is also taken into account in our model and is parametrized by the
frequency of explosions and modulations of activity. The numerical model
includes a magnetohydrodynamical dynamo driven by cosmic rays that is injected
into the interstellar medium by randomly exploding supernovae. In the model, we
also include essential elements such as vertical gravity of the disk,
differential rotation approximated by the shearing box, and resistivity leading
to magnetic reconnection. We find that even slow galactic rotation with a low
shearing rate amplifies the magnetic field, and that rapid rotation with a low
value of the shear enhances the efficiency of the dynamo. Our simulations have
shown that a high amount of magnetic energy leaves the simulation box becoming
an efficient source of intergalactic magnetic fields.Comment: 9 pages, 6 figure
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