Magnetic fields in the Galaxy via Faraday effect: Future prospects with ASKAP and the SKA Lisa...
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Transcript of Magnetic fields in the Galaxy via Faraday effect: Future prospects with ASKAP and the SKA Lisa...
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,∝
€
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
€
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: [email protected] Web: www.csiro.au
Thank you
CSIRO Astronomy & Space ScienceLisa Harvey-SmithCSIRO SKA Project Scientist
CSIRO. Lisa Harvey-Smith