The Magnetic Milky Way Bryan Gaensler Centre of Excellence for All-sky Astrophysics Brown (2010)

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Overview 1.Early History 2.Faraday Rotation 3.The Galactic Magnetic Field 4.Challenges and recent progress -Electron density models -Coherence -Fluctuations -The vertical magnetic field 5.Conclusions & Future Work

Magnets in the Sky? ›Alfvén (1937): cosmic ray confinement implies “the existence of a magnetic field in interstellar space” ›Fermi (Jan 1949): “the main process of acceleration [of cosmic rays] is due to magnetic fields which occupy interstellar spaces … the magnetic field in the dilute matter is of the order of 5x10 -6 gauss, while its intensity is probably greater in the heavier clouds” ›Hall, Hiltner (Feb 1949): starlight is polarised ›Davis & Greenstein (Mar 1949): “the polarisation is not a property of the star but is produced while the light is traversing extensive regions of interstellar space … non-spherical dust grains produce [this if] there exists a general galactic magnetic field” ›Kiepenheuer (June 1950): Galactic radio emission comes from cosmic rays gyrating in magnetic fields Hall (1949) Lowell Observatory Archives / ASP / Yerkes Observatory

The Dawn of Radio Polarimetry ›Razin (1956, 1958), Thompson (1957), Pawsey & Harting (1960): attempts to detect polarisation & Faraday rotation in Galactic radio emission ›Bolton & Wild (1957): “large radio reflectors [offer] the possibility of determining longitudinal fields in localised interstellar regions by observing the Zeeman splitting of the 21-cm line” ›Westerhout et al. (1962), Wielebinski et al. (1962): detection of polarisation of diffuse Galactic radio emission ›Cooper & Price (1962): Detection of interstellar Faraday rotation against lobes of Centaurus A ›Verschuur (1968): Detection of H I Zeeman splitting; “Fields of the order of 2x10 -5 G exist in the Perseus spiral arm in the direction of the radio source Cassiopeia A.” (shortest abstract of all time?) Parkes polarimetry of Centaurus A (Cooper & Price 1962)

Pulsar B (Gaensler et al. 1998) RM = +495 ± 6 rad m -2  (degrees) [ (metres)] 2 › Optical starlight polarisation › Synchrotron emission / polarisation B ┴ (orientation, but not direction) › Infrared dust polarisation › Zeeman splitting B ║ (weak effect, long observations) › Faraday rotation & rotation measure (RM) are powerful probes of B ║ -provides direction of B -radio wavelengths: no attenuation of radiation Superconductivity Lab, Oslo University Mapping Magnetic Fields }

Faraday Rotation Philipp Kronberg / Physics Today

The Galactic Magnetic Field ›Large- and small-scale components ›Concentrated in disk; follows arms? -local field is clockwise (Manchester 1972) -field in Sagittarius arm is counterclockwise (Thomson & Nelson 1980; Simard & Kronberg 1980)  reversal between arms … but overall geometry unclear (Noutsos et al. 2008; Men et al. 2008; Vallée 2008; Nota & Katgert 2010; Pshirkov et al. 2011) Han (2009) M. Thévenot (1644) / National Library of Australia Starlight polarisation (Fosalba 2001; Han & Wielebinski 2002) Brown (2010)

› Need to assume model for n e (l) - “NE2001” (Cordes & Lazio 2002, 2003)  n e (x, y, z) ; disk, arms, clumps, voids - for extragalactic RMs,  B  depends on n e (l) - for pulsar RMs,  B   RM/DM, but distance is inferred from DM =  n e (l)dl, and so depends on n e (l) also! Faraday Rotation & Electron Models Cordes & Lazio (2002)

In the Thrall of NE2001 Sun, Reich, Waelkens & Enßlin (2008)

›NE2001: thick disk with H DM = 950 pc -implies B halo ≈ 10 μG, CRs truncated at z = 1 kpc (Sun et al. 2008) ›NE2001 calibrated using 112 PSR distances - test of a model is its predictive power - new parallaxes don’t match NE2001 › Example : PSR B d predicted = 1.0 ± 0.2 kpc (NE2001) ; d parallax = 2.1 ± 0.1 kpc (Chatterjee et al. 2009) › A new look at n e vs. z (Gaensler et al. 2008) PSRs w. accurate distances  revised scale height H DM = 1.8 kpc  double previous estimates  implies B halo ≈ 2 μG & H CR ≈ 0.8 kpc (Sun & Reich 2010) How Reliable Is NE2001? NE2001 model Gaensler et al. (2008)

Sampling of Background RMs ›Han et al. (1997): 557 extragalactic RMs › Taylor et al. (2009): extragalactic RMs (reprocessing of NVSS data) - simultaneously shows large-scale coherence & small-scale fluctuations

Blue= axisymmetric ring Red= axisymmetric spiral Green= bisymmetric spiral Sun, Reich, Waelkens & Enßlin (2008) › VLA polarimetry observations of 194 new extragalactic RMs, 17 o < l < 63 o and 205 o < l < 253 o (Van Eck, Brown, Gaensler et al. 2011) Han (2009) New Insights: Coherence Van Eck et al. (2011)

› B random > B ordered, plus many identified & unidentified regions of anomalous RM (Mitra et al. 2003; McClure-Griffiths, Gaensler et al. 2010; Harvey-Smith, Madsen & Gaensler 2011) -impossible to differentiate between models for large-scale field only via χ 2 of RMs (Men et al. 2008; Nota & Katgert 2010) -approaches needed that simultaneously incorporate large- + small-scale B, plus coherent, random & ordered B (Jaffe et al. 2010; Jansson & Farrar 2011) New Insights: Fluctuations Harvey-Smith et al. (2011)McClure-Griffiths et al. (2010)

›Field symmetry is a vital diagnostic ›Han et al. (1994, 1997, 1999): behaviour of RMs at high |b| -B z = +0.4 ± 0.2 μG from S to N  A0 symmetry ›Wolleben, Gaensler et al. (2010): behaviour of RMs at high |b|  RM feature at l > 0 o, b > 0 o seen also in H I : local magnetised bubble ›Mao, Gaensler et al. (2010): RMs of 1000 xgal sources, |b| > 77 o -B z, south = ± 0.03 μG -B z, north = 0.00 ± 0.02 μG  no coherent vertical field at Sun  not pure dipole or quadrupole  overlapping disk/halo dynamos? Mao, Gaensler et al. (2010)Han et al. (1997) The Vertical Field Brown (2010) NORTHSOUTH

Conclusions ›Galactic B is clockwise in outer disk, counterclockwise in (parts of) inner disk (Van Eck et al. 2011; Jansson & Farrar 2011) ›No coherent vertical field structure (Mao et al. 2010) ›No simple fit for large-scale B (Men et al. 2008)  No match to any simple theoretical model ›The future: -new & improved n e model (“NE2008”; Cordes et al., in prep) -GALFACTS, GMIMS, Planck, Auger, LOFAR, MWA, ASKAP-POSSUM, SKA Hall (1949) Brown (2010) CSIRO / Swinburne © Frank R. Paul estate askap.org/possum