491 research outputs found
New VLBA Identifications of Compact Symmetric Objects
The class of radio sources known as Compact Symmetric Objects (CSOs) is of
particular interest in the study of the evolution of radio galaxies. CSOs are
thought to be young (probably ~10^4 years), and a very high fraction of them
exhibit HI absorption toward the central parsecs. The HI, which is thought to
be part of a circumnuclear torus of accreting gas, can be observed using the
VLBA with high enough angular resolution to map the velocity field of the gas.
This velocity field provides new information on the accretion process in the
central engines of these young sources.
We have identified 9 new CSOs from radio continuum observations for the VLBA
Calibrator Survey, increasing the number of known CSOs by almost 50%.Comment: 5 pages, 2 figures, for "Lifecycles of Radio Galaxies", eds. J.
Biretta et al., New Astronomy Review
1245+676 - a CSO/GPS source being an extreme case of a double-double structure
AGN with the so-called `double-double' radio structure have been interpreted
as restarted AGN where the inner structure is a manifestation of a new phase of
activity which happened to begin before the outer radio lobes resulting from
the previous one had faded completely. The radio galaxy 1245+676 is an extreme
example of such a double-double object - its outer structure, measuring 970
h^{-1} kpc, is five orders of magnitude larger than the 9.6 h^{-1} pc inner
one. We present a series of VLBI observations of the core of 1245+676 which
appears to be a compact symmetric object (CSO). We have detected the motion of
the CSO's lobes, measured its velocity, and inferred the kinematic age of that
structure.Comment: A contribution to The Third Workshop on Compact Steep Spectrum and
GHz-Peaked Spectrum Radio Sources, Kerastari, Greece May 28-31, 2002.
Refereed and accepted by Publications of the Astronomical Society of
Australia. 4 pages. Final version copyedited by PASA Edito
Spectral Ages of CSOs and CSS Sources
This paper deals with the spectral ageing study of a representative sample of
compact symmetric objects (CSOs) and compact steep spectrum (CSS) sources.
Observations reveal a distinctive high-frequency steepening of the radio
spectra of many of these sources. The existence of such a spectral feature is
expected or may be naturally interpreted in terms of radiative ageing of
synchrotron emitting electrons. The small angular size of CSS sources makes it
relatively easy to measure their integrated spectra over a wide frequency range
for a conspicuous number of objects. For those sources whose emission is
dominated by the mini-lobes, the integrated spectra can be used to constrain
the source age. Assuming equipartition magnetic fields, the spectral ages we
found are in the range from 10^2 to 10^5 yr. Multifrequency VLBA observations
allow us to study the spectral properties of two CSOs: B1323+321 and B1943+546.
The case of B1943+546 is particularly interesting since for this source a
kinematic age has been derived from the proper motion of the hot spots. We
found that spectral and kinematic ages agree within a factor of 2. The overall
results presented here confirm that the CSOs and CSS sources are indeed young
objects. Finally, we show some examples of compact sources characterised by an
extraordinary steep and curved spectrum. It is plausible that these are relic
sources in which the injection of fresh electrons has ceased for a significant
fraction of their lifetime. These observations may indicate either the presence
of intermittent activity or a class of short-living objects.Comment: 11 pages, 5 figures, Refereed and accepted by Publications of the
Astronomical Society of Australia, as part of the proceedings of the 3rd
GPS/CSS workshop, eds. T. Tzioumis, W. de Vries, I. Snellen, A. Koekemoe
Sub-Relativistic Radio Jets and Parsec-Scale Absorption in Two Seyfert Galaxies
The Very Long Baseline Array has been used at 15 GHz to image the
milliarcsecond structure of the Seyfert galaxies Mrk 231 and Mrk 348 at two
epochs separated by about 1.7 yr. Both galaxies contain parsec-scale double
radio sources whose components have brightness temperatures of 10^9-10^{11} K,
implying that they are generated by synchrotron emission. The nuclear
components are identified by their strong variability between epochs,
indicating that the double sources represent apparently one-sided jets.
Relative component speeds are measured to be ~0.1c at separations of 1.1 pc or
less (for H_0 = 65 km/s/Mpc), implying that parsec-scale Seyfert jets are
intrinsically different from those in most powerful radio galaxies and quasars.
The lack of observed counterjets is most likely due to free-free absorption by
torus gas, with an ionized density n_e > 2 X 10^5 cm^{-3} at T~8000 K, or n_e >
10^7 cm^{-3} at T~10^{6.6} K, in the inner parsec of each galaxy. The lower
density is consistent with values found from X-ray absorption measurements,
while the higher temperature and density are consistent with direct radio
imaging of the NGC 1068 torus by Gallimore et al.Comment: 12 pages, 2 postscript figures, LaTeX file in AASTeX format, accepted
by ApJ Letter
Identifying Compact Symmetric Objects in the Southern Sky
We present results of multifrequency polarimetric VLBA observations of 20
compact radio sources. The observations represent the northern and southern
extensions of a large survey undertaken to identify Compact Symmetric Objects
(CSOs) Observed in the Northern Sky (COINS). CSOs are young radio galaxies
whose jet axes lie close to the plane of the sky, and whose appearance is
therefore not dominated by relativistic beaming effects. The small linear sizes
of CSOs make them valuable for studies of both the evolution of radio galaxies
and testing unified schemes for active galactic nuclei (AGN). In this paper we
report on observations made of 20 new CSO candidates discovered in the northern
and southern extremities of the VLBA Calibrator Survey. We identify 4 new CSOs,
and discard 12 core-jet sources. The remaining 4 sources remain candidates
pending further investigation. We present continuum images at 5 GHz and 15 GHz
and, where relevant, images of the polarized flux density and spectral index
distributions for the 8 new CSOs and CSO candidates.Comment: accepted to Ap
- …