34 research outputs found
Optical and radio properties of extragalactic radio sources with recurrent jet activity
We present a sample of 74 radio sources with recurrent jet activity. The
sample consists of 67 galaxies, 2 quasars and 5 unidentified sources, selected
from the published data or are newly recognized. The sample's redshift range is
0.002 < z < 0.7 and the size of inner and outer structures varies from 0.02 to
4248 kpc. We analyse the optical and radio properties of the sample and compare
them with the characteristics of ordinary one-off FRII radio sources. With the
help of stellar population modelling, we derive black hole masses and stellar
masses of host galaxies of 35 restarting radio sources, finding that the black
hole masses in restarting radio sources are comparable to those of typical
single-cycle FRII radio sources. The obtained median values of log M are
8.58 and 8.62 M Unlike the black hole masses, the stellar masses in
restarting radio sources tend to be smaller than in the FRII sources. Although
the stellar populations of the hosts of recurrent activity sources are
dominated by old stars, a significant fraction of young stars can be observed
as well. Based on the Sloan Digital Sky Survey photometric observations, we
also analyse the morphology of the host galaxies and obtained significantly
smaller concentration indices for the restarting radio sources when compared to
the classical FRII hosts. This effect can be interpreted as a result of
frequent merger events in the history of host galaxies of restarting radio
sources.Comment: 21 pages, 17 figure
Hot gas and magnetic arms of NGC 6946 : indications for reconnection heating?
Context. The grand-design face-on spiral galaxy NGC 6946 is remarkable because of its high star formation activity, the massive northern spiral arm, and the magnetic arms, which are observed in polarized radio synchrotron emission and are located between the optical arms and possibly are magnetic reconnection regions. Aims. We used electron densities and temperatures in star-forming (active) and less active regions and compared them to findings from the analysis of the radio data to study the energy budget of NGC 6946. The hot gas above the magnetic arms between the optical arms might suggest gas heating by reconnection. We also study the population of point sources in NGC 6946, including the origin of the puzzling ultra-luminous emission complex MF16. Methods. X-ray observations of NGC 6946 performed with XMM-Newton were used to study the emission from X-ray point sources and diffuse hot gas, including the magnetic arms and the halo. Spectral fitting of the diffuse X-ray emission allowed us to derive temperatures of the hot gas. With assumptions about the emission volume, this allowed us to estimate gas densities, masses, and cooling times. Results. To explain the X-ray emission from the spiral arms of NGC 6946 two-temperature plasma models are needed to account for the disk and halo emission. The interarm regions show only one thermal component. We observe that the temperature of the hot gas in and above the magnetic arm regions increases slightly when compared to the average temperatures in the areas in and above the spiral arms. For the southwestern part of the disk, which is depolarized in the radio range by Faraday rotation, we find more efficient mixing of disk and halo gas. Conclusions. We propose magnetic reconnection in the magnetic arm regions of NGC 6946 as the possible cause of the additional heating of the gas and ordering of the magnetic fields. In the southwestern part of the galactic disk we observed indications of a possible faster outflow of the hot gas. A very hot gas within the MF 16 nebula possibly suggests shock heating by a supernova explosion
The magnetic fields of large Virgo cluster spirals: Paper II
The Virgo cluster of galaxies provides excellent conditions for studying
interactions of galaxies with the cluster environment. Both the high-velocity
tidal interactions and effects of ram pressure stripping by the intracluster
gas can be investigated in detail. We extend our systematic search for possible
anomalies in the magnetic field structures of Virgo cluster spirals in order to
characterize a variety of effects and attribute them to different disturbing
agents. Six angularly large Virgo cluster spiral galaxies (NGC4192, NGC4302,
NGC4303, NGC4321, NGC4388, and NGC4535) were targets of a sensitive total power
and polarization study using the 100-m radio telescope in Effelsberg at 4.85GHz
and 8.35GHz (except for NGC4388 observed only at 4.85GHz, and NGC4535 observed
only at 8.35GHz). Magnetic field structures distorted to various extent are
found in all galaxies. Three galaxies (NGC4302, NGC4303, and NGC4321) show some
signs of possible tidal interactions, while NGC4388 and NGC4535 have very
likely experienced strong ram-pressure and shearing effects, respectively,
visible as distortions and asymmetries of polarized intensity distributions. As
in our previous study, even strongly perturbed galaxies closely follow the
radio-far-infrared correlation. In NGC4303 and NGC4321, we observe symmetric
spiral patterns of the magnetic field and in NGC4535 an asymmetric pattern.
Magnetic fields allow us to trace even weak interactions that are difficult to
detect with other observations. Our results show that the degree of distortions
of a galaxy is not a simple function of the distance to the cluster center but
reflects also the history of its interactions. The angle between the velocity
vector and the rotation vector of a galaxy may be a general parameter that
describes the level of distortions of galactic magnetic fields.Comment: 12 pages, 18 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
Tidal interaction vs. ram pressure stripping effects as seen in X-rays. Hot gas in group and cluster galaxies
The hot intracluster/intragroup medium (ICM/IGM) and a high galaxy density
can lead to perturbations of the galactic interstellar medium (ISM) due to ram
pressure and/or tidal interaction effects. In radio polarimetry observations,
both phenomena may manifest similar features. X-ray data can help to determine
the real origin of the perturbation. We analyse the distribution and physical
properties of the hot gas in the Virgo cluster spiral galaxies NGC 4254 and NGC
4569, which indicate that the cluster environment has had a significant
influence on their properties. By performing both spatial and spectral analyses
of X-ray data, we try to distinguish between two major phenomena: tidal and ram
pressure interactions. We compare our findings with the case of NGC 2276, in
which a shock was reported, by analysing XMM-Newton X-ray data for this galaxy.
We use archival XMM-Newton observations of NGC 4254, NGC 4569, and NGC 2276.
Maps of the soft diffuse emission in the energy band 0.2 - 1 keV are obtained.
For the three galaxies, especially at the position of magnetic field
enhancements we perform a spectral analysis to derive gas temperatures and thus
to look for shock signatures. A shock is a signature of ram pressure resulting
from supersonic velocities; weak tidal interactions are not expected to
influence the temperature of the ionized gas. In NGC 4254, we do not observe
any temperature increase. This suggests tidal interactions rather than ram
pressure stripping. In NGC 4569 the radio polarized ridge shows a higher
temperature, which may indicate ram-pressure effects. For NGC 2276, we do not
find clear indications of a shock. The main driver of the observed distortions
is most likely tidal interaction. Determining gas temperatures via sensitive
X-ray observations seems to be a good method for distinguishing between ram
pressure and tidal interaction effects acting upon a galaxy.Comment: 13 pages, 13 figures, 8 tables, Accepted for publication in Astronomy
and Astrophysic
Magnetic fields and hot gas in M 101
Context. Studies of nearby spiral galaxies in radio and X-ray wavelengths reveal the structure and energy balance of the magnetic fields and the hot interstellar medium (ISM). In some spiral galaxies, large-scale ordered magnetic fields have been found between the spiral stellar arms (the so-called magnetic arms). One of the considered explanations of their origin is magnetic reconnection, which according to theoretical studies can efficiently heat the low-density ISM.
Aims. We present, for the first time, high-resolution C-band (5 GHz) radio maps of the nearby face-on spiral galaxy M 101 to study the magnetic fields and verify the existence of the magnetic arms. The analysis of the archival XMM-Newton X-ray data is performed to search for signatures of gas heating by magnetic reconnection effects in the disk and the halo of this galaxy.
Methods. We combine the Very Large Array (VLA) and Effelsberg radio maps of M 101 to restore the large-scale emission lost in the interferometric observations. From the obtained maps, we derive magnetic field strengths and energy densities, and compare them with the properties of the hot gas found with the spectral analysis of the X-ray data.
Results. Most of the X-ray emission likely comes from the hot gas in the halo of M 101. Its temperature is highest above the massive stellar arm and an inter-arm region with enhanced polarised radio emission, as well as in the inter-arm area where neither Hα nor H
Hot magnetic halo of NGC 628 (M 74)
Context. In several spiral galaxies that are observed face-on, large-scale ordered magnetic fields (the so-called magnetic arms) were found. One of the explanations was the action of the magnetic reconnection, which leads to a higher ordering of the magnetic fields. Because it simultaneously converts the energy of the magnetic fields into thermal energy of the surroundings, magnetic reconnection has been considered as a heating mechanism of the interstellar medium for many years. Until recently, no clear observational evidence for this phenomenon was found. Aims. We search for possible signatures of gas heating by magnetic reconnection effects in the radio and X-ray data for the face-on spiral galaxy NGC 628 (M 74), which presents pronounced magnetic arms and evidence for vertical magnetic fields. Methods. The strengths and energy densities of the magnetic field in the spiral and magnetic arms were derived, as were the temperatures and thermal energy densities of the hot gas, for the disk and halo emission. Results. In the regions of magnetic arms, higher order and lower energy density of the magnetic field is found than in the stellar spiral arms. The global temperature of the hot gas is roughly constant throughout the disk. Conclusions. The comparison of the findings with those obtained for the starburst galaxy M 83 suggests that magnetic reconnection heating may be present in the halo of NGC 628. The joint analysis of the properties of the magnetic fields and the hot gas in NGC 628 also provided clues for possible tidal interaction with the companion galaxy
Radio continuum observations of the Leo Triplet at 2.64 GHz
Aims. The magnetic fields of the member galaxies NGC 3628 and NGC 3627 show
morphological peculiarities, suggesting that interactions within the group may
be caused by stripping of the magnetic field. This process could supply the
intergalactic space with magnetised material, a scenario considered as a
possible source of intergalactic magnetic fields (as seen eg. in the Taffy
pairs of galaxies). Additionally, the plumes are likely to be the tidal dwarf
galaxy candidates.
Methods. We performed radio continuum mapping observations at 2.64 GHz using
the 100-m Effelsberg radio telescope. We obtained total power and polarised
intensity maps of the Triplet. These maps were analysed together with the
archive data, and the magnetic field strength (as well as the magnetic energy
density) was estimated.
Results. Extended emission was not detected either in the total power or the
polarised intensity maps. We obtained upper limits of the magnetic field
strength and the energy density of the magnetic field in the Triplet. We
detected emission from the easternmost clump and determined the strength of its
magnetic field. In addition, we measured integrated fluxes of the member
galaxies at 2.64 GHz and estimated their total magnetic field strengths.
Conclusions. We found that the tidal tail hosts a tidal dwarf galaxy
candidate that possesses a detectable magnetic field with a non-zero ordered
component. Extended radio continuum emission, if present, is weaker than the
reached confusion limit. The total magnetic field strength does not exceed 2.8
{\mu}G and the ordered component is lower than 1.6 {\mu}G.Comment: 7 pages, 3 figures, accepted for publication in
Astronomy&Astrophysic
Hot gas in Mach cones around Virgo Cluster spiral galaxies
The detailed comparison between observations and simulations of ram pressure
stripped spiral galaxies in the Virgo cluster has led to a three dimensional
view of the galaxy orbits within the hot intracluster medium. The 3D velocities
and Mach numbers derived from simulations can be used to derive simple Mach
cone geometries for Virgo spiral galaxies. We search for indications of hot gas
within Mach cones in X-ray observations of selected Virgo Cluster spiral
galaxies (NGC 4569, NGC 4388, and NGC 4501). We find extraplanar diffuse X-ray
emission in all galaxies. Based on the 3D velocity vectors from dynamical
modelling a simple Mach cone is fitted to the triangular shape of NGC 4569's
diffuse X-ray emission. Assuming that all extraplanar diffuse X-ray emission
has to be located inside the Mach cone, we also fit Mach cones to NGC 4388's
and NGC 4501's extraplanar X-ray emission. For NGC 4569 it is hard to reconcile
the derived Mach cone opening angle with a Mach number based on the sound speed
alone. Instead, a Mach number involving the Alfv\'enic speed seems to be more
appropriate, yielding a magnetic field strength of -6 G for a
intracluster medium density of cm. Whereas the
temperature of the hot component of NGC 4569's X-ray halo (0.5 keV) is at the
high end but typical for a galactic outflow, the temperature of the hot gas
tails of NGC 4388 and NGC 4501 are significantly hotter (0.7-0.9 keV). In NGC
4569 we find direct evidence for a Mach cone which is filled with hot gas from
a galactic superwind. We suggest that the high gas temperatures in the X-ray
tails of NGC 4388 and NGC 4501 are due to the mixing of the stripped ISM into
the hot intracluster medium of the Virgo cluster.Comment: 11 pages, 10 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysic
NGC 3627: a galaxy-dwarf collision?
Group galaxies very often show distinct signs of interaction with both
companion galaxies and the intragroup medium. X-ray observations are
particularly helpful because they provide information on the temperatures and
the densities of the hot gas in galaxies and intergalactic space. This can put
important constraints on the nature and timescales of these interactions. We
use the XMM-Newton X-ray observations of NGC 3627 in the Leo Triplet galaxy
group to explain peculiar features visible in the polarized radio maps. We
analyzed soft X-ray (0.2-1 keV) emission from NGC 3627 to study the
distribution of the hot gas and its temperature in different areas of the
galaxy. Any change throughout the disk can reflect distortions visible in the
radio polarized emission. We also studied two bright point sources that are
probably tightly linked to the evolution of the galaxy. We find an increase in
the temperature of the hot gas in the area of the polarized radio ridge in the
western arm of the galaxy. In the eastern part of the disk we find two
ultra-luminous X-ray sources. We note a large hot gas temperature difference
(by a factor of 2) between the two bar ends. The polarized radio ridge in the
western arm of NGC 3627 is most likely formed by ram-pressure effects caused by
the movement of the galaxy through the intragroup medium. To explain the
distortions visible in the eastern part of the disk in polarized radio maps,
the asymmetry of the bar, and the distortion of the eastern arm, we propose a
recent collision of NGC 3627 with a dwarf companion galaxy.Comment: 8 pages, 6 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysic
The magnetic fields of large Virgo Cluster spirals
Because of its proximity the Virgo Cluster is an excellent target for
studying interactions of galaxies with the cluster environment. Both the
high-velocity tidal interactions and effects of ram pressure stripping by the
intracluster gas can be investigated. Optical and/or \ion{H}{i} observations do
not always show effects of weak interactions between galaxies and their
encounters with the cluster medium. For this reason we searched for possible
anomalies in the magnetic field structure in Virgo Cluster spirals which could
be attributed to perturbations in their gas distribution and kinematics. Five
angularly large Virgo Cluster spiral galaxies (NGC 4501, NGC 4438, NGC 4535,
NGC 4548 and NGC 4654) were the targets for a sensitive total power and
polarization study using the 100-m radio telescope in Effelsberg at 4.85 GHz.
For two objects polarization data at higher frequencies have been obtained
allowing Faraday rotation analysis. Distorted magnetic field structures were
identified in all galaxies. Interaction-induced magnetized outflows were found
in NGC 4438 (due to nuclear activity) and NGC 4654 (a combination of tidal
tails and ram pressure effects). Almost all objects (except the anaemic NGC
4548) exhibit distortions in polarized radio continuum attributable to
influence of the ambient gas. For some galaxies they agree with observations of
other species, but sometimes (NGC 4535) the magnetic field is the only tracer
of the interaction with the cluster environment. The cluster environment
clearly affects the evolution of the galaxies due to ram pressure and tidal
effects. Magnetic fields provide a very long-lasting memory of past
interactions. Therefore, they are a good tracer of weak interactions which are
difficult to detect by other observations.Comment: 13 pages, 12 figure