4,779 research outputs found
The B3-Vla CSS sample. III: Evn & Merlin images at 18 cm
EVN and MERLIN observations at 18 cm are presented for 18 Compact
Steep--spectrum radio Sources (CSSs) from the B3-VLA CSS sample. These sources
were marginally resolved in previous VLA A-configuration observations at 4.9
and 8.4 GHz or had peculiar morphologies, two of them looking like core-jets.
The MERLIN images basically confirm the VLA structures at 8.4 GHz while the EVN
and/or the combined images reveal several additional details.Comment: 17 pages, many low resoltion figures, A&A accepted. A higher
resolution gzipped postscript file can be found at
http://www.ira.cnr.it/~ddallaca/h3443.ps.g
Extended emission around GPS radio sources
Extended radio emission detected around a sample of GHz Peaked Spectrum (GPS)
radio sources is discussed. Evidence for extended emission which is related to
the GPS source is found in 6 objects out of 33. Three objects are associated
with quasars with core-jet pc-scale morphology, and three are identified with
galaxies with symmetric (CSO) radio morphology. We conclude that the core-jet
GPS quasars are likely to be beamed objects with a continuous supply of energy
from the core to the kpc scale. It is also possible that low surface brightness
extended radio emission is present in other GPS quasars but the emission is
below our detection limit due to the high redshifts of the objects. On the
other hand, the CSO/galaxies with extended large scale emission may be
rejuvenated sources where the extended emission is the relic of previous
activity. In general, the presence of large scale emission associated with GPS
galaxies is uncommon, suggesting that in the context of the recurrent activity
model, the time scale between subsequent bursts is in general longer than the
radiative lifetime of the radio emission from the earlier activity.Comment: 18 paged, 18 figures, accepted for publication on A&
A complete sample of GHz-Peaked-Spectrum radio sources and its radio properties
We define a complete sample of thirty-three GHz-Peaked-Spectrum (GPS) radio
sources based on their spectral properties. We present measurements of the
radio spectra and polarization of the complete sample and a list of additional
GPS sources which fail one or more criteria to be included in the complete
sample. The majority of the data have been obtained from quasi-simultaneous
multi-frequency observations at the Very Large Array (VLA) during 3 observing
sessions. Low frequency data from the Westerbork Synthesis Radio Telescope
(WSRT) and from the literature have been combined with the VLA data in order to
better define the spectral shape.
The objects presented here show a rather wide range of spectral indices at
high and low frequencies, including a few cases where the spectral index below
the turnover is close to the theoretical value of 2.5 typical of self-absorbed
incoherent synchrotron emission. Faint and diffuse extended emission is found
in about 10% of the sources. In the majority of the GPS sources, the fractional
polarization is found to be very low, consistent with the residual instrumental
polarization of 0.3%.Comment: 23 pages, 16 figures, uses psfig.sty, A&AS in pres
The B3-VLA CSS sample. II: VLBA images at 18 cm
VLBA observations at 18 cm are presented for 28 Compact Steep-spectrum radio
Sources (CSSs) from the B3-VLA CSS sample. These sources were unresolved in
previous VLA observations at high frequencies or their brightness distribution
was dominated by an unresolved steep spectrum component. More than half of them
also showed a low frequency turnover in their radio spectrum. The VLBA images
display in most cases a compact symmetric structure. Only in a minority of
cases complex morphologies are present.Comment: 11 pages, many low resolution figures, A&A accepted. A higher quality
g-zipped postscript file can be found at
http://www.ira.cnr.it/~ddallaca/h3442.ps.g
ISO observations of a sample of Compact Steep Spectrum and GHz Peaked Spectrum Radio Galaxies
We present results from observations obtained with ISOPHOT, on board the ISO
satellite, of a representative sample of seventeen CSS/GPS radio galaxies and
of a control sample of sixteen extended radio galaxies spanning similar ranges
in redshift (0.2 = 10^26 W/Hz).
The observations have been performed at lambda = 60, 90, 174 and 200 microns.
Seven of the CSS/GPS sources have detections >= 3 sigma at one or more
wavelengths, one of which is detected at >= 5 sigma. By co-adding the data we
have obtained average flux densities at the four wavelengths. We found no
evidence that the FIR luminosities of the CSS/GPS sources are significantly
different from those of the extended objects and therefore there is not any
support for CSS/GPS sources being objects "frustrated" by an abnormally dense
ambient medium. The two samples were then combined, providing FIR information
on a new sample of radio galaxies at intermediate redshifts. We compare this
information with what previously known from IRAS and discuss the average
properties of radio galaxies in the redshift range 0.2 - 0.8. The FIR emission
cannot be accounted for by extrapolation of the synchrotron radio spectrum and
we attribute it to thermal dust emission. The average FIR luminosity is >=
6*10^11 L_sun. Over the observed frequency range the infrared spectrum can be
described by a power law with spectral index alpha >~1.0 +/- 0.2. Assuming the
emission to be due to dust, a range of temperatures is required, from >=80 K to
\~25 K. The dust masses required to explain the FIR emission range from 5*10^5
M_sun for the hotter component up to 2*10^8 M_sun for the colder one.
(abridged)Comment: Astronomy & Astrophysics, in press, 16 pages, 2 Figure
On the Evolution of and High-Energy Emission from GHz-Peaked-Spectrum Sources
Here we discuss evolution and broad-band emission of compact (< kpc) lobes in
young radio sources. We propose a simple dynamical description for these
objects, consisting of a relativistic jet propagating into a uniform gaseous
medium in the central parts of an elliptical host. In the framework of the
proposed model, we follow the evolution of ultrarelativistic electrons injected
from a terminal hotspot of a jet to expanding lobes, taking into account their
adiabatic energy losses as well as radiative cooling. This allows us to discuss
the broad-band lobe emission of young radio sources. In particular, we argue
that the observed spectral turnover in the radio synchrotron spectra of these
objects cannot originate from the synchrotron self-absorption process but is
most likely due to free-free absorption effects connected with neutral clouds
of interstellar medium engulfed by the expanding lobes and photoionized by
active centers. We also find a relatively strong and complex high-energy
emission component produced by inverse-Compton up-scattering of various
surrounding photon fields by the lobes' electrons. We argue that such high
energy radiation is strong enough to account for several observed properties of
GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In
addition, this emission is expected to extend up to GeV (or possibly even TeV)
photon energies and can thus be probed by several modern gamma-ray instruments.
In particular, we suggest that GPS radio galaxies should constitute a
relatively numerous class of extragalactic sources detected by GLAST.Comment: 32 pages, 3 figures included. Revised version, accepted for
publication in Ap
X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources
We present {\it Chandra} X-ray Observatory observations of Giga-Hertz Peaked
Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. The {\it
Chandra} sample contains 13 quasars and 3 galaxies with measured 2-10 keV X-ray
luminosity within erg s. We detect all of the
sources, five of which are observed in X-ray for the first time. We study the
X-ray spectral properties of the sample. The measured absorption columns in the
quasars are different than those in the galaxies in the sense that the quasars
show no absorption (with limits ) while the galaxies
have large absorption columns () consistent with
previous findings. The median photon index of the sources with high S/N is
and it is larger than the typical index of radio loud
quasars. The arcsec resolution of {\it Chandra} telescope allows us to
investigate X-ray extended emission, and look for diffuse components and X-ray
jets. We found X-ray jets in two quasars (PKS 1127-145, B2 0738+32), an X-ray
cluster surrounding a CSS quasar (z=1.1, 3C 186), detected a possible binary
structure in 0941-080 galaxy and an extended diffuse emission in galaxy PKS B2
1345+12. We discuss our results in the context of X-ray emission processes and
radio source evolution. We conclude that the X-ray emission in these sources is
most likely unrelated to a relativistic jet, while the sources' radio-loudness
may suggest a high radiative efficiency of the jet power in these sources.Comment: 15 pages, to be published in Ap
LISA: Mars and the limits of life
We describe the results of the first tests made on LISA, a simulator of
planetary environments designed and built in Padua, dedicated to the study of
the limit of bacterial life on the planet Mars. Tests on the cryogenic circuit,
on the UV illumination and on bacterial coltures at room temperature that shall
be used as references are described.Comment: 4 pages, 3 figures. Mem. SAIt, in pres
X-ray-emitting Atmospheres of B2 Radio Galaxies
We report ROSAT PSPC spatial and spectral analysis of the eight B2 radio
galaxies NGC 315, NGC 326, 4C 35.03, B2 0326+39, NGC 2484, B2 1040+31, B2
1855+37, and 3C 449, expected to be representative of the class of low-power
radio galaxies. Multiple X-ray components are present in each, and the gas
components have a wide range of linear sizes and follow an extrapolation of the
cluster X-ray luminosity/temperature correlation, implying that there is no
relationship between the presence of a radio galaxy and the gas fraction of the
environment. No large-scale cooling flows are found. There is no correlation of
radio-galaxy size with the scale or density of the X-ray atmosphere. This
suggests that it is processes on scales less than those of the overall gaseous
environments which are the major influence on radio-source dynamics. The
intergalactic medium is usually sufficient to confine the outer parts of the
radio structures, in some cases even to within 5 kpc of the core. In the case
of NGC 315, an extrapolation suggests that the pressure of the atmosphere may
match the minimum pressure in the radio source over a factor of about 40 in
linear size (a factor of about 1600 in pressure).Comment: 34 pages, including 10 figures, using aasms4.sty To appear in the Ap
VLBA images of High Frequency Peakers
We propose a morphological classification based on the parsec scale structure
of fifty-one High Frequency Peakers (HFPs) from the ``bright'' HFP sample. VLBA
images at two adjacent frequencies (chosen among 8.4, 15.3, 22.2 and 43.2 GHz)
have been used to investigate the morphological properties of the HFPs in the
optically thin part of their spectrum. We confirm that there is quite a clear
distinction between the pc-scale radio structure of galaxies and quasars: the
78% of the galaxies show a ``Double/Triple'' morphology, typical of Compact
Symmetric Objects (CSOs), while the 87% of the quasars are characterised by
Core-Jet or unresolved structure. This suggests that most HFP candidates
identified with quasars are likely blazar objects in which a flaring
self-absorbed component at the jet base was outshining the remainder of the
source at the time of the selection based on the spectral shape. Among the
sources classified as CSOs or candidates it is possible to find extremely young
radio sources with ages of about 100 years or even less.Comment: 21 pages, 8 figures; accepted for pubblication in A&A. Paper version
with full resolution images is available at
http://www.ira.inaf.it/~ddallaca/orienti.p
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