Direct Measurement of a Direct Measurement of a Magnetic Field at z=0.692Magnetic Field at z=0.692
Art WolfeArt Wolfe
Regina Jorgenson: IOATim Robishaw: UCBCarl Heiles:UCBJason X. Prochaska:UCSC
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Evidence for B fields at z ≈ 1
Far IR vs Radio Correlation
Dust vs Synchrotron Emission
Correlation Independent of z
Appleton etal ’04
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Evidence for B fields at z ≈ 1:Statistical Evidence for Faraday Rotation in Mg IIAbsorption Systems at <z>1.3 (Bernet etal. ‘08)
Mg II
Control
Parameters from 21 cm Absorption
• z=0.6921526±0.0000008
• =3.75±0.20 km s-1
• 0=0.095±0.006
•|Blos|=83.9±8.8G
completely unexpected:
Dynamo theory predicts
weaker B fields in the past!
Optical AbsorptionLines in DLA-3C286
Physical Parameters fromHIRES spectra
• v21-vopt=3.80.2 km s-1
• =3.08±0.13 km s-1
•[M/H]=-1.3
•Dust-to-Gas = 1% Galactic
Optical AbsorptionLines in DLA-3C286
Physical Parameters fromHIRES spectra
• v21-vopt=3.8±0.2 km s-1
• =3.080.13 km s-1
•[M/H]=-1.3
•Dust-to-Gas = 1% Galactic
FOS spectrum
• [C II] 158 m cooling rate < 1% Galaxy cooling rate
• SFR < (1/3)(SFR)Milky Way
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Upper Limits on Faraday rotation in 3C 286 at =1332-1434 MHz (Gaensler & Ekers ‘08)
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Upper Limits on Faraday rotation in 3C 286 at =1332-1434 MHz (Gaensler & Ekers ‘08)
• RMobs=-0.1±0.1 rad m-2
• RMobs=RMDLA+RMMW
• RMMW=1.4±0.2 rad m-2
• RMDLA<1.9 rad m-2 (95% c.l.)
Since
RMDLA=2.6(Ne/1019)Blos(1+z)-2
the electron fraction
xe≤1.410-4 (95% c.l.)
Consequences of Strong B Field
• Since (B2plane/8π) >> (2/2), B field dominates
midplane pressure.
• Magnetostatic Equilibrium of Gaseous Disk
Predicted: (B2plane/8midplane=G/2
Observed: (B2plane/8midplane≥(715)G2/2
•Therefore magnetized H I gas cannot be confined by its self-gravity
• Minimum l.o.s. gas surface Density min = 490 M pc-2
required to confine B field
Zeeman Splitting in Molecular Cloud (NGC 2024)
•OH and H I Zeeman Splitting Results-Blos > 100 G
- N(H2)>1022 cm-2
•But, no H2 absorption in HST spectraQuickTime™ and a
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Inferred surface density: =1450 Mpc-2
Maximum l.o.s B field: Blos=87 G
Does B field arise in a molecular cloud ?
•Absence of Lyman and Werner absorption implies f(H2)<710-4
•Perhaps radio photons traverse molecular gas, while optical Photons only go through atomic gas
H II
H I
H2
H I
=21 cm=1216Å
Does B field arise in a molecular cloud ?
•Absence of Lyman and Werner absorption implies f(H2)<710-4
•Perhaps radio photons traverse molecular gas, while optical Photons only go through atomic gas
•But, absence of OH 1612 MHz absorption implieslos < (1/3)min
H II
H I
H2
H I
=21 cm=1216Å
Interpretation(Transient Configuration)
•B field Enhancement by Merger-induced Shock (F. Shu) If preshock (B1)= 5 G in disk of galaxy
Then postshock (B2) = 100 G if ushock = 250 km s-1
Image does not rule out two foreground galaxies
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WFPC2 Image of 3C 286 (LeBrun etal ‘97; Chen ‘08)
• PSF-Subtracted smoothed (0.2)
• Diffuse object on top of QSO
-Asymmetry foreground galaxy
• Suggests sightline to QSO passes within a few kpc of a galaxy
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WFPC2 Image of 3C 286 (LeBrun etal ‘97; Chen ‘08)
• Filament 2.5 SE of QSO
• AB(F702W)=23.6 mag
• Is the filament (a) Outer spiral arm at zabs ?
(b) A tidal tail at zabs ?
Conclusions1. First measurement of galactic B field at z >> 0 results in surprise, since BDLA 20<BISM> at 6.4 Gyr Look BackTime
-Field is average over neutral gas with (a) velocity dispersion v =3.75 kms-1 and (b) linear scales from 50 to 200 pc -No evidence for strong SFRs usually associated with large fields
2. Consequences -Magnetic Pressure >> self-gravity of H I gas
-Magnetic Pressure may be confined by gravity of molecular gas or -B field enhanced by shocks generated by galaxy merger: merger probability p=Rmergetduration=0.006 to 0.03 -Star formation rates in DLAs may be suppressed by strong B fields
Stokes I and V spectra of 3C 286 (N. Kanekar ‘08)
•2 hr. integration on GBT
•Same ‘S Curve’ shape as Wolfe etal (‘08)
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