The Discovery of Extragalactic Magnetic Fields in OH Megamasers

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Becoming Troland
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Becoming Troland
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Single-Dish Zeeman Observing
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1 Troland 10 yr
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One can learn astronomy from books without ever
looking through a telescope or handling a
photographic plate, but that is not the Berkeley
way. Ron Bracewell, 1955
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Single-Malt Whiskey Drinking
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The Discovery of Extragalactic Magnetic Fields
in OH Megamasers
Tim Robishaw Carl Heiles Eliot Quataert
Berkeley
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The Story

• Minimum energy and equipartition suggest magnetic
  fields in ULIRGs should be between 1 mG to 10 mG
  (Thompson et al. 2006)
• ULIRGs (LFIR/L?gt1012) are powered by starbursts
  or AGN.

Arp 220
Thompson, Rieke, Schneider, Scoville, NASA
NASA, ESA, C. Wilson
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The Story

• Minimum energy and equipartition suggest magnetic
  fields in ULIRGs should be between 1 mG to 10 mG
  (Thompson et al. 2006)
• ULIRGs (LFIR/L?gt1012) are powered by starbursts
  or AGN.
• OH megamasers (LOH/L?101-4) are found in ULIRGs
  (Darling Giovanelli).

Arp 220
Thompson, Rieke, Schneider, Scoville, NASA
NASA, ESA, C. Wilson
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The Story

• Minimum energy and equipartition suggest magnetic
  fields in ULIRGs should be between 1 mG to 10 mG
  (Thompson et al. 2006)
• ULIRGs (LFIR/L?gt1012) are powered by starbursts
  or AGN.
• OH megamasers (LOH/L?101-4) are found in ULIRGs
  (Darling Giovanelli).
• In the Milky Way, OH masers probe magnetic fields
  in star formation regions 1667 MHz OH line
  stronger than 1665 MHz opposite of Galactic
  masers.
• Went to Arecibo to observe the 6 brightest OHMs,
  including the archetypal ULIRG Arp 220, in hopes
  of measuring Zeeman splitting in the 1667 MHz
  line.

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Arp 220
III Zw 35
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Arp 220
1.5" 550 pc
0.25"
0.5"
Rovilos, Diamond, Lonsdale2, Smith 2003
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Lockett Elitzur 2008
Our results suggest that all OH main-line masers
could be pumped by the same mechanism far-IR
radiation. The 53 µm lines. The major
differences between Galactic star forming regions
on one hand and evolved stars and OHMs on the
other can be attributed to differences in line
overlap effects.
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III Zw 35
100 pc
Pihlström, Conway, Booth, Diamond,
Polatidis 2001
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III Zw 35
100 pc
Pihlström, Conway, Booth, Diamond,
Polatidis 2001
Parra, Conway, Elitzur, Pihlström 2005
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Zeeman Splitting
Splitting ltlt linewidth even at 100 mG.
We fit Stokes V to derivative of Gaussians to
determine Blos
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Arp 220
III Zw 35
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Arp 220
III Zw 35
3mG
-3mG
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LOH/L?gt104
Gigamaser!
Yu 2005
Pihlström, Baan, Darling, Klöckner 2005
200 pc
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LOH/L?gt104
Gigamaser!
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LOH/L?gt104
Gigamaser!
18 mG!
Reversal
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Time-Variable Masers Big Fields?
Jeremy Darling
W75N Slysh Migenes Fish Reid
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W3(OH)
Collisional Pumping
Median B ? 3 mG
B 18 mG !
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More Data

• Arecibo survey continuing right now.
• Arecibo C-band absorption of 6 GHz OH transition
  in Arp 220 III Zw 35
• High-sensitivity VLBA observations of
• Arp 220 and
• III Zw 35

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Results

• Detected 14 B fields.
• Detected fields in 5/8 ULIRGs.
• First extragalactic Zeeman splitting detections
  in emission lines.
• Only previous extragalactic Zeeman detection was
  HI in absorption in high-velocity cloud around
  Perseus A (Kazes et al. 1991 Sarma et al. 2005).
• B is similar to local sites of OH masers.
• Conditions in regions of massive star formation
  are similar to those in Milky Way.
• B is consistent with inferred synchrotron fields.
• Probing gas closer to typical ISM density.

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