T.G.Arshakian

MPI für Radioastronomie (Bonn)

Exploring the weak magnetic Exploring the weak magnetic fields with LOFARfields with LOFAR

OutlineOutline

Advantages of low frequency radio astronomy

Observations of regular magnetic fields

Faraday rotation is a powerful tool to detect weak magnetic fields

What can LOFAR observe?

Advantages of low-frequency (LF) Advantages of low-frequency (LF) radio astronomyradio astronomy

LF emission is purely nonthermal in nearby 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).

Observing weak magnetic fieldsObserving weak magnetic fields

Synchrotron intensity: I ~ B1+ -

→ 10x smaller B gives same intensity when observing at 100x smaller frequency (for ≈1)

Synchrotron lifetime:

=50 MHz, B=10μG: Ee=0.6 GeV → tsyn≈ 1.5 108 yr

=50 MHz, B= 3μG: Ee=1.8 GeV → tsyn≈ 9 108 yr

Inverse Compton loss from CMB dominates for weaker fields (<3μG for nearby objects)

→ Observing at low frequencies traces old, low-energy cosmic-ray electrons in weak magnetic fields

→ CR electrons may travel to large distances in weak magnetic fields

M 51M 51VLA+Eff 6cm VLA+Eff 6cm total intensitytotal intensity+ B-vectors+ B-vectors(Fletcher & Beck)(Fletcher & Beck)

Regular magnetic fields in the diskRegular magnetic fields in the disk

Weak magnetic fields may Weak magnetic fields may exist in the outer disk regions.exist in the outer disk regions.

NGC5775NGC57756cm total+6cm total+polarizedpolarizedintensityintensity(Tüllmann et al.(Tüllmann et al.2000)2000)

X-shapedX-shaped halo field:halo field:

VerticalVerticalfield componentsfield components

increasingincreasingwith increasingwith increasing

heightheight

Regular fields in the haloRegular fields in the halo

NGC253NGC2536cm polarized6cm polarizedintensityintensity(PhD Heesen 2007)(PhD Heesen 2007)

VLA + EffelsbergVLA + Effelsberg

Disk + haloDisk + halofieldfield

X-shapedX-shaped halo fieldhalo field

NGC253NGC2536cm total+6cm total+polarizedpolarizedintensityintensity(PhD Heesen2007)(PhD Heesen2007)

Weak magnetic Weak magnetic fields may fields may

exist in the outer exist in the outer halo regions.halo regions.

NGC253NGC2536cm polarized6cm polarizedintensityintensity(PhD Heesen(PhD Heesen2007)2007)

VLA + EffelsbergVLA + Effelsberg

Disk + haloDisk + halofieldfield

X-shapedX-shaped halo fieldhalo field

NGC253NGC2536cm total+6cm total+polarizedpolarizedintensityintensity(PhD Heesen2007)(PhD Heesen2007)

A presence of regular magnetic fields in the disks and halos makes Faraday rotation a perfect tool to study the weak magnetic field structure in spiral galaxies

Radio halos Radio halos and their rotation measuresand their rotation measures

are best observed are best observed at low frequencies at low frequencies (LOFAR)

Faraday rotationFaraday rotation

Δψ λ2 RM

Components of Faraday rotationComponents of Faraday rotation

RMRM == RMRMIGMIGM + RM + RMclcl + RM + RMgalgal + RM + RMMWMW + RM + RMionion

<1 ≤10000 ≤1000 ≤1000 ≤10<1 ≤10000 ≤1000 ≤1000 ≤10 rad mrad m-2-2

LOFAR RM Survey (120-LOFAR RM Survey (120-240 MHz)240 MHz)

LOFAR can measure very low Faraday rotation measures (below

~1 rad m-2) and hence very weak magnetic fields:

Face on galaxies (outer disk): RM<10 rad m-2

Galaxy halos, cluster halos, relics, intergalactic filaments

ne=10-3 cm-3, B =1 μG, L=1 kpc: RM~1 rad m-2

ne=10-2 cm-3, B =1 μG, L=100 pc: RM~1 rad m-2

Intergalactic magnetic fields

ne=10-3 cm-3, B =0.1 μG, L=1 kpc: RM~0.1 rad m-2

RM mapping of nearby RM mapping of nearby galaxies: LMC and SMCgalaxies: LMC and SMC

~200 RMs behind LMC

Mao et al. 2008

Gaensler et al. 2005

Few RMs behind SMC

RM mapping of galaxies & clusters: RM mapping of galaxies & clusters: M 31 and Abell 514M 31 and Abell 514

RMs of 21 polarized at ~1.4GHz sources shining through M31 (Han et al. 1998)

5 RMs through Abell 514 (Govoni et al. 2001) RMs through 30 clusters (Johnston-Hollitt 2003)

RM mapping of the foreground galaxy M31 at 1.4 GHzSKA RM SKA RM surveysurvey(simulation (simulation by Bryan by Bryan Gaensler)Gaensler)

RM mapping of nearby galaxies RM mapping of nearby galaxies with the SKAwith the SKA

SKA RM survey will detect many polarized sources behind nearby galaxies thus allowing the RM mapping of the foreground galaxy and reconstruction of its magnetic field structures.

Number counts of polarized Number counts of polarized background sources at 1.4 GHzbackground sources at 1.4 GHz

With a SKA sensitivity of 0.05 µJy (T~100 h) ~50000 polarized sources behind M 31,tens to hundreds sources behind a galaxy at a distance from 10 Mpc to 100 Mpc (z < 0.025).

Number counts per 1 degNumber counts per 1 deg22 (dotted line; Taylor et al. 2007): 1. observed number counts

(P > 0.5 mJy) 2. extrapolated to 0.01 mJy

(P < 0.5 mJy)

With a sensitivity of 0.01 mJy (T~1m) about 1000 polarized sources will be detected towards nearest spiral galaxy M 31.

1 Mpc (M31)

10 Mpc100 Mpc

Stepanov et al. (2008)

RM patterns and perspectives for RM patterns and perspectives for the SKAthe SKA

SKA perspectives

~600 spiral galaxies (<10 Mpc, p~0.2 µJy) can be recognized within T ~ 15 min SKA observation time at 1.4 GHz ~60.000 galaxies (100 Mpc, p~0.015 µJy) with T ~ 100 h.

ASS+QSS

BSS

QSS

RMreg patterns: p = 20 deg, i = 45 deg

ASS

RM patterns of galaxies with different magnetic field configurations

Recognition of simple structures of regular magnetic fields can be reliably performed from a limited sample of < 50 RM measurements (Stepanov et al. 2008)

RM mapping with LOFARRM mapping with LOFAR

LOFAR (LWA, ASKAP and SKA-AA) can detect smaller RM values and recognize weak galactic and intergalactic magnetic fields

if background sources are still polarized at low frequencies (<300 MHz) ???

Number counts of pol. sources Number counts of pol. sources at 350 MHzat 350 MHz

Strong depolarization at 350 MHz and lower sensitivity of LOFAR → low number density of polarized sources at 350 MHz

Array Source/deg2 DFA-1h 1(18+18)

IFA-1h 4(18+18+14)

IFA-10h 10(18+18+14)

IFA-100h 25(18+18+14)

350 MHz data from Haverkorn 2003, Schnitzeler 2008

Coma cluster and aroundComa cluster and aroundArecibo + DRAO 408 MHz (73cm)Arecibo + DRAO 408 MHz (73cm)

(Kronberg et al. 2007)(Kronberg et al. 2007)

Coma cluster and filamentsComa cluster and filaments

Diffuse emission: filament (?)Diffuse emission: filament (?)

Area~50 deg2; 10h ; NBG~500 Area~3 deg2; 10h ; NBG~30

RM mapping with LOFARRM mapping with LOFARIFA-10h (18+18+14) gives ~10 pol. sources per sq. degree at HB

Is possible only for nearby sources with large angular sizes covering the sky area of several sq. degrees

LOFAR HB frequencies are preferable (DP is lower)

Recognition of magnetic field structures is possible with NBG>20 polarized sources (Stepanov et al 2008)

Galaxies: M 31 ( ~3 deg2 ; NBG~30)Clusters: (~50 deg2; NBG~500) – Coma clusterFilaments(?): (~3 deg2; NBG~30 ) - in the Coma cluster

SummarySummary

RM measurements (120 MHz and 240) MHz provide a powerful tool to measure weak magnetic fields.

Recognition and detection of weak magnetic field structures is possible for nearby objects with large angular sizes; distant objects (or small angular sizes) require a lot of observational time.

For objects with small angular sizes diffuse polarized emission has to be detected to measure RM.