120 research outputs found
Extragalactic radio sources with hybrid morphology: implications for the Fanaroff-Riley dichotomy
We provide observational and theoretical perspectives on the currently much
debated issue of the Fanaroff-Riley (FR) morphological dichotomy of
extragalactic radio sources. In this context we introduce a new, albeit rare,
class of double radio sources in which the two lobes exhibit clearly different
FR morphologies. It is argued that such `HYbrid MOrphology Radio Sources', or
HYMORS, could be used to effectively constrain the theoretical mechanisms
proposed for the FR dichotomy. Basically, the existence of HYMORS supports
explanations for the FR dichotomy based upon jet interaction with the medium
external to the central engine, and appears quite difficult to reconcile with
the class of explanations that posit fundamental differences in the central
engine, such as black hole spin or jet composition, to be responsible for the
two FR classes of double radio sources.Comment: 11 pages including 4 figures. Accepted for publication in A&
Influence of the jet opening angle on the derived kinematical parameters of blazar jets having uniform and stratified bulk motion
We present analytical modelling of conical relativistic jets, in order to
evaluate the role of the jet opening angle on certain key parameters that are
inferred from VLBI radio observations of blazar nuclear jets. The key
parameters evaluated are the orientation angle (i.e., the viewing angle) of the
jet and the apparent speed and Doppler factor of the radio knots on parsec
scales. Quantitative comparisons are made of the influence of the jet opening
angle on the above parameters of the radio knots, as would be estimated for two
widely discussed variants of relativistic nuclear jets, namely, those having
uniform bulk speed and those in which the bulk Lorentz factor of the flow
decreases with distance from the jet axis (a `spine--sheath' flow). Our
analysis shows that for both types of jet velocity distributions the
expectation value of the jet orientation angle at first falls dramatically with
increases in the (central) jet Lorentz factor, but it levels off at a fraction
of the opening angle for extremely relativistic jets. We also find that the
effective values of the apparent speeds and Doppler factors of the knots always
decline substantially with increasing jet opening angle, but that this effect
is strongest for ultra-relativistic jets with uniform bulk speed. We suggest
that the paucity of highly superluminal parsec-scale radio components in TeV
blazars can be understood if their jets are highly relativistic and, being
intrinsically weaker, somewhat less well collimated, in comparison to the jets
in other blazars.Comment: 8 pages, 4 figures, official version appeared in MNRA
Expansion of radio galaxies in a cosmologically evolving medium: Possible implications for the cosmic star-formation history
We compare earlier estimates of the volumes filled by lobes of radio galaxies
during the quasar era based upon non-evolving ambient media with new ones
assuming a strong cosmological evolution of the ambient medium. If the sources
remain active for over 10^8 years the volumes filled by them are found to be
comparable for the two scenarios. This strengthens our earlier inference that
much of the cosmic web of gaseous filaments, the site of galaxy formation, was
probably permeated by radio lobes during the quasar era and this could have
triggered extensive star formation and made large contributions to the spread
of magnetic fields and metals through the universe by z ~ 2.Comment: To appear in Bull. Astron. Soc. India; 10 pages, 3 Figures, basi.sty
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Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars
We present new clues to the problem of the radio loudness dichotomy arising
from an extensive search for intranight optical variability in seven sets of
optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects,
which are matched in optical luminosity and redshift. Our monitoring of
radio-quiet quasars has for the first time clearly detected such intranight
variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~
1/6. The matched BL Lacs have both higher variability amplitudes and duty
cycles when observed in the same fashion. We show that the much less pronounced
intranight variability of the radio-quiet quasars relative to BL Lacs can be
understood in terms of a modest misalignment of the jets in radio-quiet quasars
from the line-of-sight. We thus infer that relativistic particle jets may well
also emerge from radio-quiet quasars, but while traversing the short
optical-emitting distances, they could be snuffed out, possibly through inverse
Compton losses in the nuclear region.Comment: 13 pages, 3 figures, in press in ApJ Letters (20 March 2003
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