288 research outputs found
Radio-loud and radio-quiet quasars: one population, different epochs of observation
I bring together evidence for the rapidity with which quasars' radio
synchrotron lobe emission fades and for the intermittency with which jet plasma
is ejected from individual quasars and radio galaxies and affirm the picture
presented by Nipoti et al (2005) that the radio-loudness of quasars is a
function of the epoch at which they are observed. I briefly illustrate this
account with examples of successive episodes of jet activity where the axis
along which jet plasma is launched appears to have precessed. A new model for
the weak core radio emission from radio-quiet quasars, that is not any kind of
jet ejecta, is also briefly described.Comment: Invited talk at Alaska meeting on "Extragalactic Jets: theory and
observation from radio to gamma-ray". Editors: Travis Rector and David De
Youn
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
SS433's circumbinary ring and accretion disc viewed through its attenuating disc wind
We present optical spectroscopy of the microquasar SS433 covering a
significant fraction of a precessional cycle of its jet axis. The components of
the prominent stationary H-alpha and H-beta lines are mainly identified as
arising from three emitting regions: (i) a super-Eddington accretion disc wind,
in the form of a broad component accounting for most of the mass loss from the
system, (ii) a circumbinary disc of material that we presume is being excreted
through the binary's L2 point, and (iii) the accretion disc itself as two
remarkably persistent components. The accretion disc components move with a
Keplerian velocity of ~600 km/s in the outer region of the disc. A direct
result of this decomposition is the determination of the accretion disc size,
whose outer radius attains ~8 R_sun in the case of Keplerian orbits around a
black hole mass of 10 M_sun. We determine an upper limit for the accretion disc
inner to outer radius ratio in SS433, R_in/R_out ~ 0.2, independent of the mass
of the compact object. The Balmer decrements, H-alpha/H-beta, are extracted
from the appropriate stationary emission lines for each component of the
system. The physical parameters of the gaseous components are derived. The
circumbinary ring decrement seems to be quite constant throughout precessional
phase, implying a constant electron density of log N_e(cm^-3) ~ 11.5 for the
circumbinary disc. The accretion disc wind shows a larger change in its
decrements exhibiting a clear dependence on precessional phase, implying a
sinusoid variation in its electron density log N_e(cm^-3) along our
line-of-sight between 10 and 13. This dependence of density on direction
suggests that the accretion disc wind is polloidal in nature.Comment: 7 pages, 5 figures. Accepted for publication in MNRAS Main Journal
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
Fluctuations and symmetry in the speed and direction of the jets of SS433 on different timescales
ABRIDGED We present new results on the variations in speed and direction of
the jet bolides in the Galactic microquasar SS433, from high resolution
spectra, taken with the ESO 3.6-m New Technology Telescope almost nightly over
0.4 of a precession cycle. We find: (i) These data exhibit multiple ejections
within most 24-hour periods and, throughout the duration of the observing
campaign, the weighted means of the individual bolides, in both the red jet and
the blue jet, clearly exhibit the pronounced nodding known in this system. (ii)
We present further evidence for a 13-day periodicity in the jet speed, and show
this cannot be dominated by Doppler shifts from orbital motion. (iii) We show
the phase of this peak jet speed has shifted by a quarter of a cycle in the
last quarter-century. (iv) We show that the two jets ejected by SS433 are
highly symmetric on timescales measured thus far. (v) We demonstrate that the
anti-correlation between variations in direction and in speed is not an
artifact of an assumption of symmetry. (vi) We show that a recently proposed
mechanism (Begelman et al 2006) for varying the ejection speed and
anti-correlating it with polar angle variations is ruled out. (vii) The speed
of expansion of the plasma bolides in the jets is approximately 0.0024c. These
novel data carry a clear signature of speed variations. They have a simple and
natural interpretation in terms of both angular and speed fluctuations which
are identical on average in the two jets. They complement archival optical data
and recent radio imaging.Comment: to appear in A&A (8 pages
The evolution of classical doubles: clues from complete samples
We describe the inter-dependence of four properties of classical double radio
sources - spectral index, linear size, luminosity and redshift - from an
extensive study based on spectroscopically-identified complete samples. We use
these relationships to discuss aspects of strategies for searching for radio
galaxies at extreme redshifts, in the context of possible capabilities of the
new generation of proposed radio telescopes.Comment: To appear in `Perspectives in Radio Astronomy: scientific imperatives
at cm and m wavelengths.' eds: M.P. van Haarlem and J.M. van der Hulst
Version with colour figures available at
http://www-astro.physics.ox.ac.uk/~km
SS433: Observation of the circumbinary disc and extraction of the system mass
The so-called "stationary" H-alpha line of SS433 is shown to consist of three
components. A broad component is identified as emitted in that wind from the
accretion disc which grows in speed with elevation above the plane of the disc.
There are two narrow components, one permanently redshifted and the other
permanently to the blue. These are remarkably steady in wavelength and must be
emitted from a circumbinary ring, orbiting the centre of mass of the system
rather than orbiting either the compact object or its companion: perhaps the
inner rim of an excretion disc. The orbiting speed (approximately 200 km/s) of
this ring material strongly favours a large mass for the enclosed system
(around 40 solar masses), a large mass ratio for SS433, a mass for the compact
object plus accretion disc of ~16 solar masses and hence the identity of the
compact object as a rather massive stellar black hole.Comment: to appear in ApJ Let
The central engines of radio-quiet quasars
Two rival hypotheses have been proposed for the origin of the compact radio
flux observed in radio-quiet quasars (RQQs). It has been suggested that the
radio emission in these objects, typically some two or three orders of
magnitude less powerful than in radio-loud quasars (RLQs), represents either
emission from a circumnuclear starburst or is produced by radio jets with bulk
kinetic powers 10^3 times lower than those of RLQs with similar luminosity
ratios in other wavebands. We describe the results of high resolution
(parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very
Long Baseline Array (VLBA). We find strong evidence for jet-producing central
engines in 8 members of our sample.Comment: 7 pages, 8 figures. Accepted for publication in MNRA
Extremely red galaxy counterparts to 7C radio sources
We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey
(7CRS) which lack strong emission lines and we use these data to investigate
their spectral energy distributions (SEDs) with models which constrain their
redshifts. Six of these seven galaxies have extremely red colours (R-K>5.5) and
we find that almost all of them lie in the redshift range 1<z<2. We also
present near-infrared spectroscopy of these galaxies which demonstrate that
their SEDs are not dominated by emission lines, although tentative lines,
consistent with H-alpha at z=1.45 and z=1.61, are found in two objects.
Although the red colours of the 7CRS galaxies can formally be explained by
stellar populations which are either very old or young and heavily reddened,
independent evidence favours the former hypothesis. At z~1.5 at least 1/4 of
powerful radio jets are triggered in massive (>L*) galaxies which formed the
bulk of their stars several Gyr earlier, that is at epochs corresponding to
redshifts z>5. If a similar fraction of all z~1.5 radio galaxies are old, then
extrapolation of the radio luminosity function shows that, depending on the
radio source lifetimes, between 10-100% of the near-IR selected extremely red
object (ERO) population undergo a radio outburst at epochs corresponding to
1<z<2. An ERO found serendipitously in the field of one of the 7CRS radio
sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission
line likely to be [OII] at z=1.20. The implication is that some of the most
massive elliptical galaxies formed the bulk of their stars at z>5 and these
objects probably undergo at least two periods of AGN activity: one at high
redshift during which the black hole forms and another one at an epoch
corresponding to z~1.5.Comment: 20 pages, 10 figures, accepted for publication in MNRA
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