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

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 . Studying magnetic fields in HII regions. Motivation:

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

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  1. 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

  2. Studying magnetic fields in HII regions • 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). CSIRO. Lisa Harvey-Smith

  3. Experimental setup Astrophysical Measurement of the Faraday Effect Faraday rotated radiation CSIRO. Lisa Harvey-Smith

  4. Our model of HII regions CSIRO. Lisa Harvey-Smith

  5. Measurements of Faraday Rotation Measure Image: NVSS rotation measure catalogue, (Taylor, Stil & Sunstrum, 2009) CSIRO. Lisa Harvey-Smith

  6. Derivation of electron density Image: Hα all sky map, Finkbeiner (2003) IHα ∝ Emission Measure, CSIRO. Lisa Harvey-Smith

  7. Results: Elevated RM in HII regions CSIRO. Lisa Harvey-Smith

  8. Results: magnetic fields in 5 HII regions CSIRO. Lisa Harvey-Smith

  9. Results Calculated mean thermal, magnetic, turbulent, total pressure in each region. Input to models of SF feedback (Ostriker 2010) CSIRO. Lisa Harvey-Smith

  10. Magnetic field - density relation 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. CSIRO. Lisa Harvey-Smith

  11. Magnetic field in local galaxy (d < 1kpc) B points towardsl ~90° in the local galaxy (Manchester 1972 and others). No preferred orientation in z direction. CSIRO. Lisa Harvey-Smith

  12. 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

  13. 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

  14. 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

  15. Conclusions • 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. CSIRO. Lisa Harvey-Smith

  16. Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au Thank you CSIRO Astronomy & Space Science Lisa Harvey-Smith CSIRO SKA Project Scientist CSIRO. Lisa Harvey-Smith

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