Magnetic fields in low-density ionized gas Insights into star-formation on galaxy scales Lisa Harvey-Smith Collaborators: Bryan CSIRO SKA Project ScientistGaensler, Greg Madsen CSIRO Astronomy & Space Science 16 th September 2010

The big picture CSIRO. Lisa Harvey-Smith

Motivation: To assist modelling of star-formation on galactic scales “What is state of pressure-gravity balance in the Galaxy disk?” To understand how diffuse matter forms into molecular clouds and ultimately, into stars. “Do magnetic fields couple to matter in the diffuse ISM?” Method: Measure magnetic fields in low-density ionized gas in our Galaxy Use large HII regions, as they provide a ‘lit up’ B field (Faraday) Measure relationship between B and ρ at low gas densities Calculate thermal, magnetic, turbulent and total pressures in diffuse component of gas disk Compare results to models of self-regulated star-formation feedback in the Galactic disk. The project

Results: Mean B and ρ in evolved HII regions CSIRO. Lisa Harvey-Smith

Results CSIRO. Lisa Harvey-Smith Slope of B vs ρ in low density gas is consistent with zero. Suggests that low- density ionized gas ‘ignores’ magnetic field. There is also thermal – magnetic pressure balance in the HII regions. We calculated thermal, magnetic & turbulent (from H-alpha line widths) components of the total pressure.

Conclusions CSIRO. Lisa Harvey-Smith Thermal plasma beta = 1 (equipartition of magnetic & thermal pressures) HII regions not thermally expanding – although ionization fronts may be. In the warm ionized medium, material accumulates along field lines. Total/thermal pressure ratio used by Ostriker (2010) too high by factor of 2. Important to expand this work – measure z dependence of pressure. Experimental refinements will lead to a large reduction in uncertainties. ASKAP & SKA will enable measurement of B in compact/uc HII regions.

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