Magnetic fields in the Galaxy via Faraday effect: Future prospects with ASKAP and the SKA

Lisa Harvey-Smith Collaborators: Bryan

CSIRO SKA Project Scientist Gaensler & Greg Madsen

CSIRO Astronomy & Space Science

4th November 2010

CSIRO. Lisa Harvey-Smith

• Motivation:

• What is the strength of magnetic fields in HII regions?

• To assist modelling of star-formation on galactic scales.

• “What is the balance of pressure and gravity in various parts of the the Galaxy disk?” ( star formation rates and SF feedback)

• “How does diffuse matter form into molecular clouds and ultimately, into stars?.”

• “Do magnetic fields couple to matter in the diffuse ISM?”

• “What is the orientation of the magnetic fields in the local galaxy?”

• Method:

• Measure magnetic fields in HII regions, as they ‘light up’ B field

• Measure relationship between B|| and ρ throughout these HII regions

• Calculate thermal, magnetic, turbulent and total pressures.

• Compare results to models of self-regulated star-formation feedback in the Galactic disk (e.g. Ostriker et al. 2010).

Studying magnetic fields in HII regions

Experimental setup

CSIRO. Lisa Harvey-Smith

Astrophysical Measurement of the Faraday Effect

Faraday rotated radiation

Our model of HII regions

CSIRO. Lisa Harvey-Smith

Measurements of Faraday Rotation Measure

Image: NVSS rotation measure catalogue, (Taylor, Stil & Sunstrum, 2009)

CSIRO. Lisa Harvey-Smith

Derivation of electron density

CSIRO. Lisa Harvey-Smith

Image: Hα all sky map, Finkbeiner (2003)

IHα Emission Measure,∝

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EM = ne

2

obs

∞

∫ dl

Results: Elevated RM in HII regions

CSIRO. Lisa Harvey-Smith

Results: magnetic fields in 5 HII regions

CSIRO. Lisa Harvey-Smith

Results

CSIRO. Lisa Harvey-Smith

Calculated mean thermal, magnetic, turbulent, total pressure in each region.

€

Pmagnetic = 4 ×10−12dyn cm−2

€

Pthermal =1.7 ×10−12dyn cm−2

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α =Ptot /Pther = 2.5Input to models of SF feedback (Ostriker 2010)

Magnetic field - density relation

CSIRO. Lisa Harvey-Smith

Slope of B vs. density below 1000 cm-3 is consistent with zero.

For low-density ionised gas, this suggests that matter accumulates along magnetic field lines.

At higher densities (self-gravitating matter) collapse is dominated by gravity, not by the direction of magnetic fields.

Molecular & HI data courtesy of Dick Crutcher.

Magnetic field in local galaxy (d < 1kpc)

CSIRO. Lisa Harvey-Smith

B points towards l ~90° in the local galaxy (Manchester 1972 and others).

No preferred orientation in z direction.

Future work: The ASKAP Polarization survey

• Australian SKA Pathfinder: 36 x 12m dishes at SKA candidate site.

• ASKAP will carry out a 2-year all-sky polarization survey called POSSUM.

• ASKAP will measure 1.5 million rotation measures (100 per deg2).

CSIRO. Lisa Harvey-Smith

Science with ASKAP: The RM grid

• With this improved “RM grid”, we will be able to probe magnetic fields in HII regions, supernova remnants, planetary nebulae, nearby galaxies and clusters of galaxies.

CSIRO. Lisa Harvey-Smith

Rotation Measure Science with SKA

• With SKA survey, we’ll find 20 million RMs (90” between sources).

• Also measure RM of 20 000 pulsars (galaxy structure).

• This will allow very accurate measurement of RM due to the galaxy (for galactic science + foreground removal!).

• RM grid gives magnetic field structure of thousands of galaxies, including haloes of edge-on galaxies. Answer the question: “are galaxy fields dynamos or spun-up primordial fields?” (Sofue poster).

• Deep observations of nearby galaxies with SKA will yield 104 RMs. @10 Mpc most galaxies will have ~50 background RMs.

• Nearby galaxy clusters will typically have 1000 background RMs from the all-sky RM grid survey. Deep pointed searches do better.

CSIRO. Lisa Harvey-Smith

Conclusions

CSIRO. Lisa Harvey-Smith

• Magnetic field strength in evolved HII regions is 2 - 7 micro Gauss.

• Thermal plasma beta = 1 (equipartition of magnetic & thermal pressures)

• In the warm ionized medium, material accumulates along field lines.

• Total/thermal pressure ratio used by Ostriker (2010) too high by factor of 2.

• We have measured the local direction of Galactic magnetic field.

• Important to expand this work – measure z dependence of pressure.

• Improved “RM grid” from ASKAP & SKA surveys will enable us to answer a

vast array of questions related to cosmic magnetic fields.

Contact UsPhone: 1300 363 400 or +61 3 9545 2176

Email: enquiries@csiro.au Web: www.csiro.au

Thank you

CSIRO Astronomy & Space ScienceLisa Harvey-SmithCSIRO SKA Project Scientist

CSIRO. Lisa Harvey-Smith