A Survey of the Global Magnetic Fields of Giant Molecular Clouds

Giles Novak, Northwestern University

Instrument: SPARO

Collaborators: P. Calisse, D. Chuss, M. Krejny, H. Li

CO J=1-0 emission from GMCs in the Perseus armCO J=1-0 emission from GMCs in the Perseus arm(~ 2 kpc)(~ 2 kpc)

-- low overall star formation efficiency:-- mechanical support by magnetic fields? (e.g., Shu et al. ‘87; Mouschovias & Ciolek ‘99; Basu & Ciolek ‘04)

-- are GMCs dynamic structures? (e.g., Hartmann et al. `01; Elmegreen & Scalo `04;

Mac Low & Klessen `04)

( Brunt & Heyer 2002 )

Ostriker, Stone, & Gammie (2001)

simulated GMCsimulated GMC

3-d MHD code;compressible;has self-gravity;Isothermal

this map assumes strong B-field

can reproduce size/line-width relationship that is observed in real GMCs

Ostriker, Stone, & Gammie (2001)

simulated GMCsimulated GMC

3-d MHD code;compressible;has self-gravity;Isothermal

this map assumes strong B-field

can reproduce size/line-width relationship that is observed in real GMCs

dynamical collapse: role of B-field; transport of angular momentum: (Allen, Li & Shu ’03) turbulence can affect ang. mom. transport: (Ballesteros-Paredes et al. ’06)IMF: from turbulent fragmentation (Padoan & Nordlund ’02) magnetic levitation? (Shu et al. ’04)

data from SPARO 2003 winter-overdata from SPARO 2003 winter-over( Li et al. 2006 Ap.J. )( Li et al. 2006 Ap.J. )

we observed four GMCs in the Galactic disk

each map covers hundreds of sq. arcmin

total number of polarization detections: 130

median degree of polarization: P = 2.0%

-- next: we focus on internal structure of field-- Carina is different; very advanced stage of star formation-- G331.5 lies at 5.3 kpc

Carina Nebula

G 333.6 G 331.5

NGC 6334

histograms of B-field direction for each GMChistograms of B-field direction for each GMC

-- next: we focus on internal structure of field-- Carina is different; very advanced stage of star formation-- G331.5 lies at 5.3 kpc

Carina Nebula

G 333.6 G 331.5

NGC 6334

histograms of B-field direction for each GMChistograms of B-field direction for each GMC

= 22.3°

= 21.6°

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

Problems: 1 - inclination of Bunif

2 - beam dilution

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

Problems: 1 - inclination of Bunif

2 - beam dilution

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

Problems: 1 - inclination of Bunif

2 - beam dilution

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

Problems: 1 - inclination of Bunif

2 - beam dilution

dispersion in field direction vs. dispersion in field direction vs. energy density of uniform fieldenergy density of uniform field

101 100

50°

20°

10°

30°

5°

mod. C.F.

Problems: 1 - inclination of Bunif

2 - beam dilution

conclusions from comparison with models :conclusions from comparison with models :

total magnetic energy density

kinetic energy density

-- consistent with Crutcher et al. (1999)-- not consistent with Padoan et al. (2001), Padoan & Nordlund (2002)

Is there continuity between GMC fields and Is there continuity between GMC fields and larger-scale Galactic fields?larger-scale Galactic fields?

Glenn et al. ‘99 … “…appear randomly oriented wrt plane…”Hildebrand ‘02 … “…apparently random orientation…”

Searches for correlation of BGMC with orientation of Gal. plane:

Is there continuity between GMC fields and Is there continuity between GMC fields and larger-scale Galactic fields?larger-scale Galactic fields?

Glenn et al. ‘99 … “…appear randomly oriented wrt plane…”Hildebrand ‘02 … “…apparently random orientation…”

Searches for correlation of BGMC with orientation of Gal. plane:

BGMC from SPARO( Li et al. `06 ApJ )

Is there continuity between GMC fields and Is there continuity between GMC fields and larger-scale Galactic fields?larger-scale Galactic fields?

Glenn et al. ‘99 … “…appear randomly oriented wrt plane…”Hildebrand ‘02 … “…apparently random orientation…”

Searches for correlation of BGMC with orientation of Gal. plane:

BGMC from SPARO( Li et al. `06 ApJ )

NGC 6334cloud

Conclusions from SPARO GMC surveyConclusions from SPARO GMC survey

i. For typical GMCs, the internal dispersion in magnetic field direction is estimated to be ~ 28°

ii. By comparing this with the model of Ostriker et al. (2001) we infer that the total magnetic energy density is comparable to the kinetic energy density of turbulence.

iii. By considering the distribution of GMC mean field directions, and by comparing with optical polarimetry, we find evidence for continuity between GMC fields and Galactic fields.