2,284 research outputs found
On the lives of extra-galactic radio sources: the first 100,000 years
In this paper we discuss the early phase of radio source evolution as
represented by Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS)
radio sources. Correlations between their spectral peak and angular size
strongly suggest that the spectral turnovers are caused by synchrotron self
absorption, and indicate that young radio sources evolve in a self similar way.
We argue that the evolution of a radio source during its first 10^5 years is
qualitatively very different from that during the rest of its life-time. This
may be caused by the difference in the density gradient of the intra-galactic
medium inside and outside the core-radius of the host galaxy.Comment: LaTeX, 7 pages & 2 figs. Invited talk at `Lifecycles of Radio
Galaxies' workshop, ed J. Biretta et al., New Astronomy Reviews. More papers
of the authors at http://www.ast.cam.ac.uk/~snelle
Transverse motions in CSOs?
The measurement of proper motions in CSOs is a powerful tool to determine the
dynamical evolution of the newly born extragalactic radio sources. We observed
3 CSOs with the VLBA in 2004 and in 2006 to monitor changes in their structure
and measure the separation velocity of the hot spots. It is important to
increase the size of the samples of CSOs with measured expansion velocity to
test the existance of frustrated objects, and put stringent constraints on the
current models. We found for all the three objects observed a transverse motion
of the hotspots, and we suggest as the more likey explanation a precession in
the jet axis. This behaviour likely inhibits or at least slows down the radio
source growth because the head of the hotspot continuously hits new regions of
the ISM. Therefore these radio sources may represent an old population of
GPS/CSOs.Comment: 4 pages, 3 figures. Accepted for publication in Astronomische
Nachrichte
Constraining the evolution of young radio-loud AGN
GPS and CSS radio sources are the objects of choice to investigate the
evolution of young radio-loud AGN. Previous investigations, mainly based on
number counts and source size distributions, indicate that GPS/CSS sources
decrease significantly in radio power when evolving into old, extended objects.
We suggest this is preceded by a period of increase in radio luminosity, which
lasts as long as the radio source is confined within the core-radius of its
host galaxy. We have selected a sample of nearby compact radio sources,
unbiased by radio spectrum, to determine their radio luminosity function, size
distribution, dynamical ages, and emission line properties in a complete and
homogeneous way. First results indicate that the large majority of objects
(>80%) exhibit classical GPS/CSS radio spectra, and show structures consistent
with the being compact double, or compact symmetric objects. This sample
provides and ideal basis to further test and constrain possible evolution
scenarios, and to investigate the relation between radio spectra and
morphologies, orientation and Doppler boosting in samples of young radio-loud
AGN, in an unbiased way.Comment: LaTeX, 8 pages, 3 figs: Accepted by Publications of the Astronomical
Society of Australia, as part of the proceedings of the 3rd GPS/CSS workshop,
eds. T. Tzioumis et a
Optical spectroscopy of faint gigahertz peaked spectrum sources
We present spectroscopic observations of a sample of faint Gigahertz Peaked
Spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey
(WENSS). Redshifts have been determined for 19 (40%) of the objects. The
optical spectra of the GPS sources identified with low redshift galaxies show
deep stellar absorption features. This confirms previous suggestions that their
optical light is not significantly contaminated by AGN-related emission, but is
dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars,
justifying the use of these (probably) young radio sources as probes of galaxy
evolution. The optical spectra of GPS sources identified with quasars are
indistinguishable from those of flat spectrum quasars, and clearly different
from the spectra of Compact Steep Spectrum (CSS) quasars. The redshift
distribution of the GPS quasars in our radio-faint sample is comparable to that
of the bright samples presented in the literature, peaking at z ~ 2-3. It is
unlikely that a significant population of low redshift GPS quasars is missed
due to selection effects in our sample. We therefore claim that there is a
genuine difference between the redshift distributions of GPS galaxies and
quasars, which, because it is present in both the radio-faint and bright
samples, can not be due to a redshift-luminosity degeneracy. It is therefore
unlikely that the GPS quasars and galaxies are unified by orientation, unless
the quasar opening angle is a strong function of redshift. We suggest that the
GPS quasars and galaxies are unrelated populations and just happen to have
identical observed radio-spectral properties, and hypothesise that GPS quasars
are a sub-class of flat spectrum quasars.Comment: LaTeX, 13 pages. Accepted by MNRAS. For related papers see
http://www.ast.cam.ac.uk/~snelle
The mass of the young planet Pictoris b through the astrometric motion of its host star
The young massive Jupiters discovered with high-contrast imaging provide a
unique opportunity to study the formation and early evolution of gas giant
planets. A key question is to what extent gravitational energy from accreted
gas contributes to the internal energy of a newly formed planet. This has led
to a range of formation scenarios from 'cold' to 'hot' start models. For a
planet of a given mass, these initial conditions govern its subsequent
evolution in luminosity and radius. Except for upper limits from radial
velocity studies, disk modelling, and dynamical instability arguments, no mass
measurements of young planets are yet available to distinguish between these
different models. Here we report on the detection of the astrometric motion of
Beta Pictoris, the 21 Myr-old host star of an archetypical directly-imaged gas
giant planet, around the system's centre of mass. Subtracting the highly
accurate Hipparcos-Gaia proper motion from the internal 3-yr Hipparcos
astrometric data reveals the reflex motion of the star, giving a
model-independent planet mass of M=11+-2 MJup. This is consistent with
scenarios in which the planet is formed in a high-entropy state as assumed by
hot start models. The ongoing data collection by Gaia will in the near future
lead to mass measurements of other young gas giants and form a great asset to
further constrain early evolution scenarios.Comment: Nature Astronomy, Aug 20, 2018; 16 pages, 4 Figs, 2 Table
Spectral variability in faint high frequency peakers
We present the analysis of simultaneous multi-frequency Very Large Array
(VLA) observations of 57 out of 61 sources from the ``faint'' high frequency
peaker (HFP) sample carried out in various epochs. Sloan Digital Sky Survey
(SDSS) data have been used to identify the optical counterpart of each radio
source. From the analysis of the multi-epoch spectra we find that 24 sources do
not show evidence of spectral variability, while 12 objects do not possess a
peaked spectrum anymore at least in one of the observing epochs. Among the
remaining 21 sources showing some degree of variability, we find that in 8
objects the spectral properties change consistently with the expectation for a
radio source undergoing adiabatic expansion. The comparison between the
variability and the optical identification suggests that the majority of radio
sources hosted in galaxies likely represent the young radio source population,
whereas the majority of those associated with quasars are part of a different
population similar to flat-spectrum objects, which possess peaked spectra
during short intervals of their life, as found in other samples of
high-frequency peaking objects. The analysis of the optical images from the
SDSS points out the presence of companions around 6 HFP hosted in galaxies,
suggesting that young radio sources resides in groups.Comment: 16 pages, 5 figures; accepted for publication in MNRA
Optical identifications of High Frequency Peakers
We present CCD observations of 13 objects from a complete sample of 55 bright
High Frequency Peaker (HFP) radio sources, and provide optical identification
for 12 of them. Images in R and V filters have been used to derive some
additional information concerning the host of the radio source. Three hosts are
likely to be galaxies, one resulted slightly extended, while the remaining 8
are likely distant quasars. Based on these identifications and those available
in the literature, we find that the fraction of quasars in our HFP sample is
significantly higher than in samples of Compact Steep-Spectrum and GHz-Peaked
Spectrum radio sources.Comment: 7 pages, A&A accepte
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