146 research outputs found
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
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
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
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
A search for extended radio emission from selected compact galaxy groups
Context. Studies on compact galaxy groups have led to the conclusion that a
plenitude of phenomena take place in between galaxies that form them. However,
radio data on these objects are extremely scarce and not much is known
concerning the existence and role of the magnetic field in intergalactic space.
Aims. We aim to study a small sample of galaxy groups that look promising as
possible sources of intergalactic magnetic fields; for example data from radio
surveys suggest that most of the radio emission is due to extended, diffuse
structures in and out of the galaxies. Methods. We used the Effelsberg 100 m
radio telescope at 4.85 GHz and NRAO VLA Sky Survey (NVSS) data at 1.40 GHz.
After subtraction of compact sources we analysed the maps searching for
diffuse, intergalactic radio emission. Spectral index and magnetic field
properties were derived. Results. Intergalactic magnetic fields exist in groups
HCG 15 and HCG 60, whereas there are no signs of them in HCG 68. There are also
hints of an intergalactic bridge in HCG 44 at 4.85 GHz. Conclusions.
Intergalactic magnetic fields exist in galaxy groups and their energy density
may be comparable to the thermal (X-ray) density, suggesting an important role
of the magnetic field in the intra-group medium, wherever it is detected.Comment: 13 pages, 4 figures, 3 tables, accepted for publication in A&
3D global simulations of a cosmic-ray-driven dynamo in dwarf galaxies
Star-forming dwarf galaxies can be seen as the local proxies of the
high-redshift building blocks of more massive galaxies according to the current
paradigm of the hierarchical galaxy formation. They are low-mass objects, and
therefore their rotation speed is very low. Several galaxies are observed to
show quite strong magnetic fields. These cases of strong ordered magnetic
fields seem to correlate with a high, but not extremely high, star formation
rate. We investigate whether these magnetic fields could be generated by the
cosmic-ray-driven dynamo. The environment of a dwarf galaxy is unfavourable for
the large-scale dynamo action because of the very slow rotation that is
required to create the regular component of the magnetic field. We built a 3D
global model of a dwarf galaxy that consists of two gravitational components:
the stars and the dark-matter halo described by the purely phenomenological
profile proposed previously. We solved a system of magnetohydrodynamic (MHD)
equations that include an additional cosmic-ray component described by the
fluid approximation. We found that the cosmic-ray-driven dynamo can amplify the
magnetic field with an exponential growth rate. The -folding time is
correlated with the initial rotation speed. The final mean value of the
azimuthal flux for our models is of the order of few G and the system
reaches its equipartition level. The results indicate that the
cosmic-ray-driven dynamo is a process that can explain the magnetic fields in
dwarf galaxies.Comment: 6 pages, 4 figures, accepted for publication in A&
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