Transcript The Radio Jets of AGN

Studying the Relativistic Jets of
Active Galactic Nuclei
Denise Gabuzda
Radio Astronomy Group
Colm
Coughlan
Eoin
Murphy
Fiona
Healy
Active Galactic Nuclei (AGN): extremely compact,
generate much more energy than a normal galaxy.
Activity due to accretion onto a supermassive (~109 solar
masses! ) black hole
Sometimes eject “jets” of radio-emitting plasma extending
far beyond optical (visible) galaxy.
This radio emission is
SYNCHROTRON RADIATION —
electromagetic radiation given off by
energetic electrons during their
acceleration by local magnetic fields.
Radio Interferometry - using an array of
radio telescopes with synchronized
signals, provides resolution
R ~ l/D
Where D is maximum distance between
telescopes used.
Can use different radio telescope arrays
to study jets on different scales.
In connected-element arrays, the
telescopes are linked electronically
Images of AGN
jets obtained with
the VLA, scales
of kiloparsec
(1000’s of light
years)
Very Large Array (VLA), max baseline
36 km
Very Long Baseline Array
(VLBA)
In Very Long Baseline
Interferometry, the data
are usually recorded on
disc and processed after
the observations
Images of AGN jets obtained
with the American VLBA —
one-sided structure due to
Doppler beaming
The linear polarization
of the radio emission
tells us about the B field
giving rise to the
synchrotron radiation.
The direction
of the linear
polarization is
perpendicular
to the B field.
In these images, B is perpendicular to the jets.
This orthogonal B field may be the toroidal component of an
intrinsic underlying helical B field, due to rotation of the
central supermassive black hole and its accretion disc +
relativistic jet outflow.
Meier, Koide &
Uchida 2001
Faraday rotation of the plane of polarisation occurs when an
EM wave passes through a magnetised plasma, due to
different propagation velocities of the RCP and LCP
components of the EM wave in the plasma.
The rotation is proportional to the square of the wavelength,
and its sign is determined by the direction of the line-ofsight B field:
= o + RM l2
RM = (constants)  ne B•dl
A helical jet B field should give rise to a gradient in the
Faraday rotation across the jet, due to the systematic change
in the line-of-sight component of the helical field.

LOS B away
from observer
B
(RM < 0)
Jet axis
LOS B towards
observer
•
(RM > 0)
10-credit projects (PY4115)
Gabuzda, Cantwell & Cawthorne 2013
1) Searching for new AGN jets
displaying evidence for helical jet B
fields Making and analyzing
Faraday rotation maps of AGN to
look for statistically significant
transverse gradients.
2) Analysis of transverse profiles of
AGN jets The presence of helical B
fields can also give rise to characteristic transverse polarization
structures; fitting models can give
estimates for the pitch angle and
viewing angle of the helical field.
Murphy, Cawthorne & Gabuzda 2013
5-credit projects (PY4114)
1) Calculating the polarization and
Faraday rotation of AGN cores using
a simple model The core regions of
AGN sometimes show clear Faraday
rotation gradients – but are these
optical depth effects?
2) Composing a database of AGN
images and posting them on the web
The UCC AGN group has obtained
data for a sample of AGN at 18, 20,
21 and 22cm. The calibration is
done, but images need to be made
and put on our website.
Unpolarized (tangled) cats.