252 research outputs found
The environments of double radiogalaxies
Discussed here are simple methods by which the densities and pressures of the gas confining double radiogalaxies can be estimated from optical and x ray data. By applying these methods to an unbiased sample of nearby (z less than 0.5) double radiogalaxies, the author quantifies the empirical relation between the external confining pressure and the internal pressure of the lobes as inferred from the minimum energy argument. This relation is explained by an analytic model in which the lobes are statically confined by ambient material pre-heated in the bow-shock of the advancing radiosource. Such a model allows one to estimate source expansion speeds from a combination of radio and environmental data, and estimate properties of the environment from radio data alone, providing expansion speeds can be estimated from multifrequency observations
High-z radio galaxies and the `Youth-Redshift Degeneracy'
We discuss a unifying explanation for many `trends with redshift' of radio
galaxies which includes the relevance of their ages (time since their jet
triggering event), and the marked dependence of their ages on redshift due to
the selection effect of imposing a flux-limit. We briefly describe some
important benefits which this `youth-redshift degeneracy' brings.Comment: to appear in `The Hy-redshift universe: galaxy formation and
evolution at high redshift' eds A.J. Bunker and W.J.M. van Breuge
The TOOT Survey: status and early results
The TexOx-1000 (TOOT) radio source redshift survey is designed to find and
study typical radio-loud active galaxies to high redshift. They are typical in
the same sense that L* galaxies are typical of galaxies in the optical.
Previous surveys have only included the most luminous, rare objects at and
beyond the peak of activity at z~2, but in going a factor of 100 fainter than
the 3C survey, and in assembling a large sample, TOOT probes for the first time
the objects that dominate the radio luminosity density of the universe at high
redshift. Here we describe the current status of the TOOT survey and draw
preliminary conclusions about the redshift distribution of the radio sources.
So far, ~520 of the 1000 radio sources have redshifts, with ~440 of those in
well-defined, complete, sub-regions of the survey. For these we find a median
redshift of z=1, but the measured redshift distribution has a deficit of
objects with z~2, when compared to predictions based on extrapolating
luminosity functions constrained by higher-flux-density samples. These are the
more luminous objects that usually show emission lines, and which should not be
missed in the survey unless they are heavily reddened. The deficit may be real,
but it would not be too surprising to find a population of faint, reddened
radio sources at z~2-3 among the TOOT sources yet to have accurate redshifts.Comment: 6 pages, 2 figures. To appear in the proceedings of the conference
"Radio Galaxies: Past, present and future", Leiden, 11-15 Nov 2002, eds. M.
Jarvis et a
Non-steller light from high-redshift radiogalaxies
With the aid of a new IRCAM image of 3C356, researchers question the common assumption that radiosource-stimulated starbursts are responsible for the extended optical emission aligned with radio structures in high-redshift radiogalaxies. They propose an alternative model in which the radiation from a hidden luminous quasar is beamed along the radio axis and illuminates dense clumps of cool gas to produce both extended narrow emission line regions and, by Thomson scattering, extended optical continua. Simple observational tests of this model are possible and necessary if we are to continue to accept that the color, magnitude and shape evolution of radiogalaxies are controlled by the active evolution of stellar populations
Spectral ageing: a new age perspective
We present an up-to-date critique of the physical basis for the spectral
ageing method. We find that the number of cases where this method may be
meaningfully applied to deduce the ages of classical double radio sources is
small indeed. This critique is much more than merely a re-expression of
anxieties about the calibration of spectral ageing (which have been articulated
by others in the past).Comment: 9 pages, 3 figures, published in `Particles & Fields in Radio
Galaxies', PASP, eds RA Laing & KM Blundel
HST imaging of redshift z>0.5 7C and 3C Quasars
We present preliminary results from HST imaging of radio-loud quasar hosts,
covering a ~x100 range in radio luminosity but in a narrow redshift range (0.5
to 0.65). The sample was selected from our new, spectroscopically complete 7C
survey and the 3CRR catalogue. Despite the very large radio luminosity range,
the host luminosities are only weakly correlated (if at all) with radio power,
perhaps reflecting a predominance of purely central engine processes in the
formation of radio jets, and hence perhaps also in the radio-loud/-quiet
dichotomy at these redshifts. The results also contradict naive expectations
from several quasar formation theories, but the host magnitudes support
radio-loud Unified Schemes.Comment: To appear in the Proceedings of the ESO/IAC Conference on 'Quasar
Hosts' - Tenerife 24-27 September 199
IRAS F10214+4724: the inner 100pc
We use new near-infrared spectroscopy and our published optical spectroscopy
of the gravitationally-lensed Seyfert-2 galaxy F10214+4724 to study both the
links between the starburst and AGN in this object and the properties of the
inner narrow-line clouds. The UV spectrum is consistent with a compact,
moderately- reddened starburst providing about half the UV light. Spectroscopy
of the Halpha /[NII] line blend has enabled us to distinguish emission from the
narrow-line region of the Seyfert-2 and a moderately-reddened emission line
region which we argue is associated with the starburst. Estimates of the star
formation rate from the UV continuum flux and the Halpha flux are broadly
consistent. We can explain the unusual emission line properties of F10214+4724
in terms of conventional models for nearby Seyfert-2 galaxies if lensing is
preferentially magnifying the side of the inner narrow-line region between the
AGN and the observer, and the other side is both less magnified and partially
obscured by the torus. The hydrogen densities of clouds in this region are high
enough to make the Balmer lines optically thick and to suppress forbidden
emission lines with low critical densities. We have deduced the column density
of both ionised and neutral gas in the narrow-line clouds, and the density of
the ionised gas. Using these we have been able to estimate the mass of the
inner narrow-line clouds to be ~ 1 solar mass, and show that the gas:dust ratio
NH/E(B-V) in these clouds must be ~1.3x10^{27}m^{-2}mag^{-1}, significantly
higher than in the Milky Way. The cloud properties are consistent with the
those of the warm absorbers seen in the X-ray spectra of Seyfert-1 galaxies.
Our results favour models in which narrow-line clouds start close to the
nucleus and flow out.Comment: 13 pages, 5 figures. Accepted by MNRA
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