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