Magnetic fields in low-density ionized gas Insights into star-formation on galaxy scales Lisa Harvey-Smith Collaborators: Bryan CSIRO SKA Project ScientistGaensler,

Magnetic fields in low-density ionized gasInsights into star-formation on galaxy scales

Lisa Harvey-Smith Collaborators: Bryan

CSIRO SKA Project Scientist Gaensler, Greg Madsen

CSIRO Astronomy & Space Science

16th September 2010

The big picture

CSIRO. Lisa Harvey-Smith

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.

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

Email: [email protected] Web: www.csiro.au

Thank you

CSIRO Astronomy & Space ScienceLisa Harvey-SmithCSIRO SKA Project Scientist

CSIRO. Lisa Harvey-Smith