Magnetic fields in the Galaxy via Faraday effect: Future prospects with ASKAP and the SKA Lisa Harvey-Smith Collaborators: Bryan CSIRO SKA Project ScientistGaensler & 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) I Hα ∝ Emission Measure,

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. Input 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 deg 2 ). 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 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 10 4 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 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 Us Phone: or Web: Thank you CSIRO Astronomy & Space Science Lisa Harvey-Smith CSIRO SKA Project Scientist CSIRO. Lisa Harvey-Smith