1. T.G.Arshakian MPI für Radioastronomie (Bonn)
Exploring radio galaxies with LOFAR
T.G.Arshakian MPI für Radioastronomie (Bonn)

2. Advantages of low-frequency (LF) radio astronomy
Low frequency emission is purely nonthermal in radio galaxies,
IGM and ISM.
Radio synchrotron emission is a measure of the strength of the total magnetic field (Btot):
Allows detailed studies for few dozens of nearby galaxies … .
Linear polarization – degree of ordering of the magnetic field:
Fully ordered field can polarize the signal up to 75%.
Small Rotation Measures (RM ~  ne B|| dr) can be measured (RM ~ -2) → weak magnetic fields.
RM Synthesis (Brentjens & de Bruyn 2005) – separate RM components from regions along the LOS (from multichannel spectro-polarimetry).

3. Science with Radio Galaxies
Nearby bright radio galaxies (RG):
Detail physical model of FR I / FR II RGs; magnetic field stricture of outer weak regions of radio lobes (M31; Laing).
Extended radio lobes as a polarized background (depolarization silhouettes of foreground galaxies and intergalactic filaments(?) ).
Point-like RGs as a polarized background (magnetic field structure of nearby galaxies and clusters).
Polarization of RGs at 150 and 350 MHz (WSRT observations).
Giant and double-double RGs to constrain the physical characteristics of lobes, cocoons and IGM.

4. Depolarization silhouettes of foreground objects
Fornax A (Ekers et al. 2005) : 1.4 GHz
3C 34 2“
3C 340 5“
Galaxies, interacting systems (~100 rad m-2)
Filaments (RM ~1 rad m-2) ?
high-resolution, International LOFAR
(to minimize the beam depolarization)
RM synthesis to separate RM regions
Johson et al. (2005): 1.4 GHz

5. Number counts of pol. sources at 350 MHz
350 MHz data from Haverkorn 2003, Schnitzeler 2008 (104 RGs)
Array Source/deg2
DFA-1h
(18+18)
IFA-1h
( )
IFA-10h
IFA-100h
1400 MHz
350 MHz
Strong depolarization at 350 MHz and lower sensitivity of LOFAR → low number density of polarized sources at 350 MHz

6. Why giant RGs ?
Polarized at low frequencies (e.g. B ) – polarized calibrators for
LOFAR
Extend to large distances (>700 kpc) – fainter sample of GRSs is
needed (high sensitivity of LOFAR) to test the model of pressure
gradient in the lobes and constrain the pressure in the IGM.
Density structures in the ambient medium and internal properties of the
radio lobes (density of the gas and magnetic field) – detail studies of
RM towards lobes (high resolution - International LOFAR).
Double-double RGs: even more exciting!
Relaxed outer radio lobes – pol. calibrators for LOFAR
Probe of magnetic fields in IGM and IG filaments (?) towards radio lobes – RM measurements towards radio lobes.
Large volume – X-ray emitter (IC) --- amplitude of a magnetic field
Low density environment in the regions of inner lobes – similar studies, test for the model proposed by Kaiser et al. (1999)

7. The radio galaxy B1834+62 Schoenmakers et al (2000)
z = 0.54, 1.2 Mpc size !
all 4 lobes polarized
RM = rad/m2 ( GHz)

8. Depolarization in lobes of B1834+62
WSRT project on low freq polarization at 150 MHz and 350 MHz in giants (Arshakian et al.)
350 MHz:
Inner lobes depolarize at 350 MHz.
Outer lobes still at 13% polarization.
150 MHz (preliminary results):
Outer lobes depolarize: < 5%
very little internal gas in outer lobes but worries about beam depolarization  high resolution International LOFAR (1” at 150 MHz – 400 km baselines).
What is origin of RM difference of 3 rad/m2 between outer lobes ?
Either due to:
- our Galaxy (gradients ?)
- cocoons of lobes (rather high ne, B needed)
- variations in true IGM on scales of ~ 1 Mpc ?
RM ~3 rad/m2 ~ x 1 G x 300 pc
RM ~3 rad/m2 ~ x 0.1 G x pc
de Bruyn
(filaments of cosmic web ?)

9. Intergalactic filaments ?
B : RM = 3 rad m-3
RM = RM1 - RM2 ~ 0 : intrinsic RM1~RM2 for relaxed giant RGs
RM ≠ 0 : contribution by IG fields or IG filaments
Mag. field (B~1 G) of filaments is oriented randomly
ne~0.01 cm-3 ; L=100 pc; 250 pc; 500 pc.
500 pc
250 pc
100 pc
Large sample of DD or giant radio galaxies
at different redshifts is needed !
RM = RM1- RM2 ~ 0
observer
RM1
RM2
RM ≠ 0