69 research outputs found
Giant Radio Sources in View of the Dynamical Evolution of FRII-type Population
The time evolution of giant (D>1 Mpc) lobe-dominated galaxies is analysed on
the basis of dynamical evolution of the entire FRII-type population.Comment: 4 pages, 2 Postscript figures, uses baltlat1.sty To be published in
"Radio astronomy at 70: from Karl Jansky to microjansky", JENAM-2003
Symposium, eds. L.Gurvits and S.Frey, (Baltic Astronomy
Giant Radio Sources in View of the Dynamical Evolution of FRII-type Population. I. The Observational Data, and Basic Physical Parameters of Sources Derived from the Analytical Model
The time evolution of giant lobe-dominated radio galaxies (with projected
linear size D>1 Mpc if H_{0}=50 km/s/Mpc and q_{0}=0.5 is analysed on the basis
of dynamical evolution of the entire FRII-type population. Two basic physical
parameters, namely the jet power Q_{0} and central density of the galaxy
nucleus rho0 are derived for a sample of giants with synchrotron ages reliably
determined, and compared with the relevant parameters in a comparison sample of
normal-size sources consisting of 3C, B2, and other sources. Having the
apparent radio luminosity P and linear size D of each sample source, Q_{0} and
rho_{0} are obtained by fitting the dynamical model of Kaiser et al. (1997). We
find that: (i) there is not a unique factor governing the source size; they are
old sources with temperate jet power (Q_{0}) evolved in a relatively
low-density environment (rho_{0}). The size is dependent, in order of
decreasing partial correlation coefficients, on age; then on Q_{0}; next on
rho_{0}. (ii) A self-similar expansion of the sources' cocoon seems to be
feasible if the power supplied by the jets is a few orders of magnitude above
the minimum-energy value. In other cases the expansion can only initially be
self-similar; a departure from self-similarity for large and old sources is
justified by observational data of giant sources. (iii) An apparent increase of
the lowest internal pressure value observed within the largest sources' cocoon
with redshift is obscured by the intrinsic dependence of their size on age and
the age on redshift, which hinders us from making definite conclusions about a
cosmological evolution of intergalactic medium (IGM) pressure.Comment: 36 pages, 8 figures, 7 table
Large-scale radio continuum properties of 19 Virgo cluster galaxies The influence of tidal interactions, ram pressure stripping, and accreting gas envelopes
Deep scaled array VLA 20 and 6cm observations including polarization of 19
Virgo spirals are presented. This sample contains 6 galaxies with a global
minimum of 20cm polarized emission at the receding side of the galactic disk
and quadrupolar type large-scale magnetic fields. In the new sample no
additional case of a ram-pressure stripped spiral galaxy with an asymmetric
ridge of polarized radio continuum emission was found. In the absence of a
close companion, a truncated HI disk, together with a ridge of polarized radio
continuum emission at the outer edge of the HI disk, is a signpost of ram
pressure stripping. 6 out of the 19 observed galaxies display asymmetric 6cm
polarized emission distributions. Three galaxies belong to tidally interacting
pairs, two galaxies host huge accreting HI envelopes, and one galaxy had a
recent minor merger. Tidal interactions and accreting gas envelopes can lead to
compression and shear motions which enhance the polarized radio continuum
emission. In addition, galaxies with low average star formation rate per unit
area have a low average degree of polarization. Shear or compression motions
can enhance the degree of polarization. The average degree of polarization of
tidally interacting galaxies is generally lower than expected for a given
rotation velocity and star formation activity. This low average degree of
polarization is at least partly due to the absence of polarized emission from
the thin disk. Ram pressure stripping can decrease whereas tidal interactions
most frequently decreases the average degree of polarization of Virgo spiral
galaxies. We found that moderate active ram pressure stripping has no influence
on the spectral index, but enhances the global radio continuum emission with
respect to the FIR emission, while an accreting gas envelope can but not
necessarily enhances the radio continuum emission with respect to the FIR
emission.Comment: 37 pages, 26 figures, accepted for publication in A&
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
Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band
Radio astronomy is entering the era of large surveys. This paper describes
the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their
synergy with large surveys at higher frequencies (in particular in the 1-2 GHz
band) that will be possible using future facilities like Apertif or ASKAP. The
LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15,
30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of
view and of the unprecedented sensitivity of this instrument. Four topics have
been identified as drivers for these surveys covering the formation of massive
galaxies, clusters and black holes using z>6 radio galaxies as probes, the
study of the intercluster magnetic fields using diffuse radio emission and
Faraday rotation measures in galaxy clusters as probes and the study of star
formation processes in the early Universe using starburst galaxies as probes.
The fourth topic is the exploration of new parameter space for serendipitous
discovery taking advantage of the new observational spectral window open up by
LOFAR. Here, we briefly discuss the requirements of the proposed surveys to
address these (and many others!) topics as well as the synergy with other wide
area surveys planned at higher frequencies (and in particular in the 1-2 GHz
band) with new radio facilities like ASKAP and Apertif. The complementary
information provided by these surveys will be crucial for detailed studies of
the spectral shape of a variety of radio sources (down to sub-mJy sources) and
for studies of the ISM (in particular HI and OH) in nearby galaxies.Comment: to appear in the proceedings of "Panoramic Radio Astronomy:
Wide-field 1-2 GHz research on galaxy evolution", G. Heald and P. Serra eds.,
8 pages, 3 figure
The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals
The influence of the environment on the polarized and total power radio
continuum emission of cluster spiral galaxies is investigated. We present deep
scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo
spiral galaxies. These data are combined with existing optical, HI, and Halpha
data. Ram pressure compression leads to sharp edges of the total power
distribution at one side of the galactic disk. These edges coincide with HI
edges. In edge-on galaxies the extraplanar radio emission can extend further
than the HI emission. In the same galaxies asymmetric gradients in the degree
of polarization give additional information on the ram pressure wind direction.
The local total power emission is not sensitive to the effects of ram pressure.
The radio continuum spectrum might flatten in the compressed region only for
very strong ram pressure. This implies that neither the local star formation
rate nor the turbulent small-scale magnetic field are significantly affected by
ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc)
and is primarily detectable in polarized radio continuum emission.Comment: 16 pages, 10 figures, accepted for publication in A&
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