Brown (2010)
The Magnetic
Milky Way
Bryan Gaensler
Centre of Excellence for
All-sky Astrophysics
www.caastro.org
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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?
› 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
› Kiepenheuer (June 1950): Galactic radio emission
comes from cosmic rays gyrating in magnetic fields
Hall (1949)
› 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”
Lowell Observatory Archives / ASP / Yerkes Observatory
› Alfvén (1937): cosmic ray confinement implies “the
existence of a magnetic field in interstellar space”
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)
Mapping Magnetic Fields
›
›
›
›
}
Optical starlight polarisation
Synchrotron emission / polarisation
Infrared dust polarisation
Zeeman splitting
B┴ (orientation, but not direction)
B║ (weak effect, long observations)
› Faraday rotation & rotation measure (RM) are powerful probes of B║
-2
RM » 220 rad m
(
RM = K ò ne B × dl
0
L
)
æ B|| öæ L ö
÷
ç
֍
-3
0.03 cm è 3 mG øè 3 kpc ø
ne
- provides direction of B
RM = +495 ± 6 rad m-2
 (degrees)
Q - Q0 = RM l 2
- radio wavelengths:
no attenuation of radiation
[ (metres)]2
Superconductivity
Lab, Oslo
University
Pulsar B1154-62
(Gaensler
et al. 1998)
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
Han (2009)
(Thomson & Nelson 1980; Simard & Kronberg 1980)
 reversal between arms
… but overall geometry unclear
(Noutsos et al. 2008; Men et al. 2008; Vallée 2008;
& Katgert 2010; Pshirkov et al. 2011)
Nota
M. Thévenot (1644) / National Library of Australia
Brown (2010)
Starlight polarisation (Fosalba 2001; Han & Wielebinski 2002)
Faraday Rotation & Electron Models
Cordes & Lazio (2002)
Q - Q0 = RM l
-2
RM » 220 rad m
2
RM = K ò ne B × dl
0
L
(
)
æ B|| öæ L ö
÷
ç
֍
0.03 cm-3 è 3 mG øè 3 kpc ø
ne
› Need to assume model for ne(l)
- “NE2001” (Cordes & Lazio 2002, 2003)
ne(x, y, z) ; disk, arms, clumps, voids
- for extragalactic RMs,
B depends on ne(l)
- for pulsar RMs, B
RM/DM, but distance is
inferred from DM = ne(l)dl , and so depends on ne(l) also!
In the Thrall of NE2001
Sun, Reich, Waelkens & Enßlin (2008)
How Reliable Is NE2001?
› NE2001: thick disk with HDM = 950 pc
- implies Bhalo ≈ 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 B1508+55
dpredicted = 1.0 ± 0.2 kpc (NE2001) ;
dparallax = 2.1 ± 0.1 kpc
NE2001
model
(Chatterjee et al. 2009)
› A new look at ne vs. z
(Gaensler et al. 2008)
- 166 PSRs w. accurate distances
- revised scale height HDM = 1.8 kpc
 double previous estimates
- implies Bhalo ≈ 2 μG & HCR ≈ 0.8 kpc
Gaensler et al. (2008)
(Sun & Reich 2010)
Sampling of Background RMs
› Han et al. (1997): 557 extragalactic RMs
› Taylor et al. (2009): 37543 extragalactic RMs (reprocessing of NVSS data)
- simultaneously shows large-scale coherence & small-scale fluctuations
New Insights: Coherence
Han (2009)
Sun, Reich, Waelkens & Enßlin (2008)
Van Eck et al. (2011)
› VLA polarimetry observations of
194 new extragalactic RMs,
17o < l < 63o and 205o < l < 253o
(Van Eck, Brown, Gaensler et al. 2011)
Blue = axisymmetric ring
Red = axisymmetric spiral
Green = bisymmetric spiral
New Insights: Fluctuations
› Brandom > Bordered , 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 χ2 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)
McClure-Griffiths et al. (2010)
Harvey-Smith et al. (2011)
The Vertical Field
Brown (2010)
› Field symmetry is a vital diagnostic
› Han et al. (1994, 1997, 1999):
behaviour of RMs at high |b|
- Bz = +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 > 0o, b > 0o seen
also in H I: local magnetised bubble
› Mao, Gaensler et al. (2010):
RMs of 1000 xgal sources, |b| > 77o
SOUTH
NORTH
- Bz, south = +0.31 ± 0.03 μG
- Bz, north = 0.00 ± 0.02 μG
no coherent vertical field at Sun
not pure dipole or quadrupole
overlapping disk/halo dynamos?
Han et al. (1997)
Mao, Gaensler et al. (2010)
Hall (1949)
Conclusions
› Galactic B is clockwise in outer disk,
counterclockwise in (parts of) inner disk
(Van Eck et al. 2011; Jansson & Farrar 2011)
Brown (2010)
› 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:
CSIRO / Swinburne
- new & improved ne model
(“NE2008”; Cordes et al., in prep)
- GALFACTS, GMIMS, Planck, Auger,
LOFAR, MWA, ASKAP-POSSUM, SKA
© Frank R. Paul estate
askap.org/possum