low frequency results from the gmrt and the role of the e-lofar dharam vir lal (mpifr, bonn)
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Low frequency results from the GMRT and the role of the E-LOFAR
Dharam Vir Lal (MPIfR, Bonn)
OverviewExpectation:
as the radio emitting plasma flows away from hot-spots in radio galaxies, it ages;
therefore one expects the low frequency observations to show diffuse emission surrounding radio galaxy.
The prime motivation is to test this!
I will present the images (spectral and morphological results for radio sources in cluster environments and field radio sources) and statistics, and will discuss the relevance of these results and the role of E-LOFAR in exploring several unknowns.
Radio galaxies in
cluster environments
versus
Field radio galaxies
Radio galaxies
4.8 GHzFR II radio galaxy
Carilli (1991)
Head-Tail radio galaxy
Lane et al. (2002)325 MHz
3C 405
Synchrotron cooling plays an important part in determining the spectral shape of radio sources (Jenkins & Scheuer 1976; barring (shock / Fermi / …) acceleration mechanisms).
Spectral ageing
How does the synchrotron spectrum evolve with time? Energetic particles
generated in the cores / hot-spots – move to radio lobes, with energy loss via. synchrotron radiation.
Physically, steepening of the spectrum at high frequencies results from the radiative losses of electrons with the highest energy.
Radio sources in clusters
1.4 GHz
low
radio frequencies as
against at high radio
frequencies.
ATLAS of DRAGNs: Leahy et al. 1993
The radio sources in cluster environments show
presence of steep spectrum diffuse emission at
240 MHzLal & Rao (in preparation)
B0314+416
Field radio galaxies
610 MHz240 MHz 4.9 GHz
Remarkably similar radio morphologies at a large range
of radio frequencies (Blundell 2008; Lal & Rao 2007, 2008).
Synchrotron emitting electrons of all energies permeate the lobe
in the same way, despite the fact that high energetic electrons have
shorter radiative lifetimes than the low energy ones!
B1414+110
Field radio galaxies
B0007+124
610 MHz 1.5 GHz240 MHz
ATLAS of DRAGNs: Leahy et al. 1993 and Lal & Rao 2007
It is not true that the low surface
brightness features always
have steeper spectral indices (Lal & Rao 2007, 2008).
SummaryCluster environments show expected behaviour:
Radio sources show steep spectrum diffuse emission at low
radio frequencies.
Field radio galaxies do not show expected behaviour:
Low and high frequency radio images show similar
morphologies (Blundell 2008; Lal & Rao 2007, 2008).
If synchrotron cooling plays a role in determining the spectral
shape of extended lobes, then the lobes should be more
extended at lower frequencies.
THIS RARELY APPEARS TO BE THE CASE!
The low-frequency synchrotron emission fades (nearly) as
rapidly as high-frequency synchrotron emission.
Summary …Both cluster and field:
Some radio sources show low-surface-brightness features that
have flatter spectral indices than high-surface-brightness
features (Lal & Rao 2007, 2008).
The simple picture of spectral electron ageing needs revision
AND / OR
We need to re-examine the formation mechanism of radio
sources.
E-LOFAR will play an important role to unravel (many) such
mysteries! search for low-energy cut-off in the relativistic electron
population, and constrain poorly understood particle
acceleration mechanism(s).
Field radio galaxies
610 MHz240 MHz 4.9 GHz
Remarkably similar radio morphologies at a large range
of radio frequencies (Blundell 2008; Lal & Rao 2007, 2008).
Synchrotron emitting electrons of all energies permeate the lobe
in the same way, despite the fact that high energetic electrons have
shorter radiative lifetimes than the low energy ones!
B1414+110
Field radio galaxies
15.2 GHz32 GHz (grey scale)
610 MHz
240 MHz
8.4 GHz
B0938+399
Remarkably similar
radio morphologies
at a large range of
radio frequencies (Blundell 2008; Lal & Rao 2007,
2008).
1.4 GHz
Radio sources in clusters
B1059+169
1.4 GHz610 MHz
240 MHz
Owen & Ledlow (1997)
Lal & Rao 2007
610 MHzIt is not true that the
low-surface-bright-
ness features alwa-ys
have steeper spectral
indices (Lal & Rao 2007, 2008).
Field radio galaxies - high A
TLA
S o
f D
RA
GN
s:
Leah
y e
t al.
19
93
1.4 GHz
Field radio galaxies - low Lal
& R
ao 2
00
7
610 MHz610 MHz 240 MHz240 MHz
Lal
& R
ao 2
00
8 (
in p
ress)
610 MHz610 MHz 240 MHz 240 MHz
Field radio galaxies - low
GMRT: Introduction Dual Polarised Prime-focus
feeds to cover the six bands,
1420, 610, 325, 240, 150, 50
MHz of operation of GMRT
Simultaneous Dual
Frequency operation in 240
and 610 MHz bands
Mounted on a rotating turret
– RF Band of operation could
be changed in about a
minute
GMRT: System parameters
Observing band (MHz) 150 233 327 610 1420
Primary Beam (degree) 3.8 2.5 1.8 0.9 0.4*1400/f
Synthesized Beam
* Full array (arcsec)
* Central array (arcmin)
20
7
13
4.5
9
3.2
5
1.7
2
0.7
Total Tsys (K) 482 177 108 92 76
Antenna Gain (K/Jy)
RMS noise (mJy)
(6 hr on target, 14 MHz BW)
0.3
0.2
(5 MHz)
0.33
0.07
(5 MHz)
0.330
0.025
0.32
0.02
0.25
0.02
Formation models Backflow from the active lobes into the wings: Diffuse low
surface brightness features are overshoots of the backflow of radio emitting plasma along the active lobes (Leahy & Williams 1984).
Slow, conical precession of the Slow, conical precession of the jet axis jet axis (Parma et al. 1985)(Parma et al. 1985)
PPrecession model requires recession model requires a fortuitous angle between a fortuitous angle between
the precession cone and the precession cone and angle to the line-of-sight,angle to the line-of-sight,
a happy accident of the a happy accident of the positions at which the positions at which the source first switched on, source first switched on, and its current position.and its current position.
((Dennett-Thorpe et al. 2002Dennett-Thorpe et al. 2002))
Reorientation of the jet axis ... Merritt & Ekers (2002) The model suggests that the X-shaped source are formed due to merger of an AGN and a nearby dwarf galaxy. should have age similar to NAT sources. On top,
the slow realignment of jet would cause the jet to deposit its energy into a large volume of space, leading to a FR I source,
rapid realignment would produce an intermediate-luminosity X-shaped sources, perhaps with the radio power near the FR I / FR II break, and
if the realignment occurred long ago ( 108 yr), the jets and the lobes would be well aligned and source could build up to a high-luminosity FR II source.
Formation models …
Radio galaxies in
cluster environments
versus
Field radio galaxies
Radio galaxies
4.8 GHz
Leahy & Perley (1991)
FR IIFR I1.4 GHz
Carilli (1991)
Head-Tail radio galaxy
Lane et al. (2002)325 MHz
3C 296 3C 405
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