Cosmic magnetism (KSP of the SKA)

understand the origin and evolution of magnetism in the Galaxy, extragalactic objects, clusters and inter-galactic/-cluster space.

SKADS in the MPI für Radioastronmie (Bonn)the Galaxy and nearby spiral galaxies

M 51 VLA+Eff 6cm M 51 VLA+Eff 6cm (Fletcher & Beck)(Fletcher & Beck)

Galactic foreground simulation

X.H. Sun, W. Reich (MPIfR Bonn),A. Waelkens, T. Enßlin (MPE Garching) submitted to AA

Aim is to develop a Galactic 3D-model which will produce all-sky maps in total intensity, linear polarization and rotation measure

Observations used to constrain the model:– All-sky intensity maps (408 MHz; 1400 MHz; 22.8 GHz - WMAP)

– All-sky polarization maps (1.4 GHz; 22.8 GHz)

– Rotation measures of extragalactic sources (~1800 NVSS sources; Han et al. in prep.)

Galactic foreground simulation

• Simulation code: HAMMURABI (Waelkens 2005)

• HealPix pixelization• for each pixel I/U/Q/RM were calculated by integration along the LOS• Final result: frequency-dependent all-sky maps (20 MHz to 22 GHz)

• Galactic 3D emission model includes the distribution of – B (disk and halo)

– nCR (disk: 2 kpc)

– ne (thick and thin disks and arms; Cordes & Lazio 2002)

Disk fields: ASS (Axisymmetric Spiral) + reversals in arms

Fit of RMs in the plane Fits of 408MHz I

Observational features versus model constrains

Simulated all-sky maps at 15’ resolution (Nside=256)

TI: 408 MHz PI: 1.4 GHz

PI: 22.8 GHz PA: 22.8 GHz

High-latitude RM asymmetry antisymmetric toroidal halo field, regular field strength (to the plane and center) up to 10 µG, CR up to 1 kpc

Synchrotron emission longitude disk CR electron component and 3 µG random field profiles: 408 MHz I/22.8 GHz PI component

Strong depolarization at 1.4 GHz large RM fluctuations along the line of sight by a small Ne filling factor and a coupling of the random field with Ne

Status• Galactic 3D models:

– disk emission well modeled, in agreement with all observations

– strong toroidal halo magnetic field needed

– Faraday depolarization effects not fully understood

• High resolution simulations for the SKA:– possible for a patch of sky (limited by available memory)

Center: (50.6, 0.) Size: 10deg x 10degResolution: 15 arcmin NSIDE: 256

PI @22.8GHz(47.8, 0.)5deg x 5deg1 arcmin4096

(47.1, 0.)1deg x 1deg3 arcsec65536

T.G.Arshakian, R.Beck and M.Krause (MPIfR, Bonn)

R.Stepanov and P.Frick (ICMM, Perm) submitted to A&A

Testing the magnetic field models of spiral galaxies with the SKA

SKADS science simulations

The aim is to estimate the required number density of polarized sources to be detected with the SKA for reliable recognition or reconstruction of the magnetic field structure in nearby spiral galaxies.

Steps• To simulate the RM maps of a typical spiral galaxy for - the regular and turbulent magnetic field models (disk and halo) - the thermal electron density model

2. To recognize or reconstruct the magnetic field structure from the simulated RM map and assess their reliability.

Faraday rotation model for different magnetic fields

model~ RMRMRM turbreg RMreg RMturb RMmodel

BSS model: RMmax = 95 rad m-2, RMmin = -175 rad m-2 for i =100 and RMturb = 30 rad m-2

• Recognition of a magnetic field model by fitting RMreg and RMmodel

• Reconstruction of magnetic field structure from RMmodel map without a priori assumptions about horizontal field

Recognition of regular magnetic field structures for ASS, BSS, QSS, …

Simple magnetic field structures can be reliably recognized from a limited RM measurements (<50).

Recognition method provides a reliable estimate of the pitch angle (~10%) and field amplitude (~10%).

Simulations of a minimum number density of polarized sources (N*) for different: - inclination of a galaxy - turbulence of RM - slope of integral number counts of polarized sources.

f(x,y) (face-on) RM map (i = 700)

x = 0 kpc x = 3 kpc x = 7.5 kpc

1D profiles of f(y)1 kpc

The model of bisymmetric magnetic field Reconstruction for h =1 kpc

Reconstruction of magnetic fields

Needed: >20 RM sources across the galaxy’s minor axis (>1000 sources towards the galaxy).

Perspectives for the SKA at 1.4 GHz• Reconstruction of magnetic field structures is possible from a sample of 1000 RM sources: galaxies at ~1 Mpc with T < 1 h, and ~60 galaxies between 1 to 10 Mpc with tens to hundred hours with the SKA.

• Recognition can be reliably performed from a limited sample of 50 RM measurements: ~600 spiral galaxies (<10 Mpc, p~0.2 µJy) can be recognized within T ~ 15 min SKA observation time, and ~60.000 galaxies (100 Mpc, p~0.015 µJy) with T ~ 100 h.

• The pitch angles are reproduced with accuracy of ±2o for the ASS and ±1o (for the QSS). The field amplitude – about 10% for all field models.

• High modes (BSS and QSS) are easier to recognize.

• The slope of number counts of polarized sources needs to be determined at low flux levels.

• The RM errors are much smaller at low frequencies: RM data can be used for detection and recognition of weak galactic and intergalactic magnetic fields with LOFAR, ASKAP and SKA-AA if background sources are still polarized at low frequencies (<300 MHz) - NEEDS FURTHER INVESTIGATION.