1. Radio halos and relics in galaxy clusters with MWA: projections for SKA
Lijo Thomas George, K. S. Dwarakanath
Raman Research Institute, Bangalore
SKA 2016, Goa, India

2. Intra-Cluster Medium (ICM)
Introduction
Galaxy Clusters
Dark Matter
80%
Intra-Cluster Medium (ICM)
15-17%
Luminous Matter
(stars etc.)
3-5%

3. Intra-Cluster Medium (ICM)
Introduction
Galaxy Clusters
Dark Matter
80%
Intra-Cluster Medium (ICM)
15-17%
Luminous Matter
(stars etc.)
3-5%
Radio emission
Non-thermal, synchrotron
B ~ μG, E ~ GeV
Radio halos
Radio relics
X-ray emission
T~ K, ρ~10-3–10-4 cm-3
Thermal bremsstrahlung

4. Radio halos
Cluster wide emission (~Mpc)
Diffuse (~mJy/arcmin2 at 1.4 GHz)
Central regions of the cluster
Very little polarization (few %)
Rare (~5% of all, ~35% of high Lx)
Radio relics
Elongated, arc-shaped
Peripheries of the cluster
Significant polarization (~30%)

5. HALO
RELIC
A521 ; z = ; 610 MHz
(Brunetti et al. 2008)

6. Kale et al. (2013)
A3376 ; z = ; 325 MHz

7. The issues Diffusion problem (L ~ 1 Mpc, tdiff ~ 10 * tsync)
in-situ acceleration
Models of halo formation
Primary model : turbulence – reacceleration – halos preferentially found in merging clusters
Secondary model : p-p collision – radio, gamma – expected radio emission is weak, gamma ray flux not yet detected
LX-P1.4 empirical relation

8. (Kale et al. 2015)

9. The issues The low detection rate of halos could be due to:
Low radio luminosity in low LX clusters (LX < 1044 erg/s) clusters

10. (Bohringer et al. 2013)

11. The issues The low detection rate of halos could be due to:
Low radio luminosity in low LX clusters (LX < 1044 erg/s) clusters
Exponential cut-off of radio spectrum at higher frequencies

12. A521
(Dallacasa et al. 2009)

13. The issues The low detection rate of halos could be due to:
Low radio luminosity in low LX clusters (LX < 1044 erg/s) clusters
Exponential cut-off of radio spectrum at higher frequencies
Large angular extents of halos (≥10´ for 1 Mpc at z<0.1) which could get resolved out at higher frequencies (>1.4 GHz)
Hence,
Low frequency (<1 GHz) observations
High quality short spacing data

14. Murchison Widefield Array (MWA)
Low Frequency telescope ( MHz) located in a radio-quiet zone in Western Australia
Collaborative effort by institutes in USA, Australia, New Zealand and India (RRI)
Telescope Configuration – 128 tiles over 3 km
Each tile has 16 dipoles in a 4x4 grid
Filled array but poor resolution. At 200 MHz, Θb~2´, RMS~6 mJy/b
Source:

15. Contours start at 3σ and increase by √2 after, Θb ~ 2´, σ = 5.6 mJy/b
PLCK G , z = 0.39, MWA 200 MHz,
Contours start at 3σ and increase by √2 after, Θb ~ 2´, σ = 5.6 mJy/b
UNRELATED
SOURCES
NW RELIC
HALO
SE RELIC
George et al. (submitted)

16. George et al. (submitted)

17. LX-P1.4 relation
George et al. (submitted)

18. Current Limitations and Unanswered Questions
GMRT (RFI especially in short spacing data)
MWA (Resolution, rms (confusion limited), not as sensitive as NVSS)
Emission from all merging clusters? (weak turbulence)
Radio emission in non-merging clusters due to p-p collisions?

19. Future Prospects with SKA
Better sensitivity and resolution compared to current telescopes
Better detection limit for radio halos
Telescope
Resolution(´´)
RMS (μJy/beam)
SKA1-Low (120 MHz) 10 20
SKA1-SUR (1.4 GHz) 15
MWA (200 MHz)
120
6000
GMRT (150 MHz) 25
2000

20. SKA Detection
Limits
(Kale et al. 2016)

21. Future Prospects with SKA
νb = 1.4 GHz
α = -1
νb = 0.6 GHz
Avg. halo
surf. brightness
(25 μJy/100 arcsec2)
Confusion Limit at 0.15, 0.6, 1.4 and 5 GHz
(Kale et al. 2016)

22. Future Prospects with SKA
Magnetic field measurements in the ICM from FRMs. This along with radio observations of halos will help constrain the secondary model
Polarization measurements of relics and halos (SKA-Low and SKA-Mid)
Expect to see ~1000s of halos
Thank You!