1. Low frequency results from the GMRT and the role of the E-LOFAR Dharam Vir Lal (MPIfR, Bonn)

2. Overview Expectation:  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.

3. Radio galaxies in cluster environments versus Field radio galaxies Radio galaxies 4.8 GHz FR II radio galaxy Carilli (1991) Head-Tail radio galaxy Lane et al. (2002) 325 MHz 3C 405

4. 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.

5. 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 MHz Lal & Rao (in preparation) B0314+416

6. Field radio galaxies 610 MHz240 MHz4.9 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

7. 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).

8. Summary Cluster 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.

9. 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).

11. Field radio galaxies 610 MHz240 MHz4.9 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

12. 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

13. Radio sources in clusters B1059+169 1.4 GHz 610 MHz 240 MHz Owen & Ledlow (1997) Lal & Rao 2007 610 MHz It is not true that the low-surface-bright- ness features alwa- ys have steeper spectral indices (Lal & Rao 2007, 2008).

14. Field radio galaxies - high ATLAS of DRAGNs: Leahy et al. 1993 1.4 GHz

15. Field radio galaxies - low Lal & Rao 2007 610 MHz 240 MHz

16. Lal & Rao 2008 (in press) 610 MHz 240 MHz Field radio galaxies - low

17. 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

18. GMRT: System parameters Observing band (MHz)1502333276101420 Primary Beam (degree)3.82.51.80.90.4*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)4821771089276 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

19. 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 jet axis (Parma et al. 1985) Slow, conical precession of the jet axis (Parma et al. 1985) Precession model requires a fortuitous angle between the precession cone and angle to the line-of-sight, a fortuitous angle between the precession cone and angle to the line-of-sight, a happy accident of the positions at which the source first switched on, and its current position. a happy accident of the positions at which the source first switched on, and its current position. (Dennett-Thorpe et al. 2002)

20. 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 (  10 8 yr), the jets and the lobes would be well aligned and source could build up to a high- luminosity FR II source. Formation models …

21. Radio galaxies in cluster environments versus Field radio galaxies Radio galaxies 4.8 GHz Leahy & Perley (1991) FR II FR I 1.4 GHz Carilli (1991) Head-Tail radio galaxy Lane et al. (2002) 325 MHz 3C 296 3C 405