A Survey of the Global Magnetic Fields of Giant Molecular Clouds

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



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°


101 100

50°

20°

10°

30°

5°

mod. C.F.


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:




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




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.

observing magnetic fields in molecular clouds

( Bourke et al. 2001 )

-- polarized dust emission: submm telescopes; Mauna Keamm-wave interferometersKuiper Airborne Obs. (early days)

-- Zeeman splitting observ.:

- radio freq. molecular lines (typ. OH absorption)

- gives Bl.o.s.

SPARO B-vectors on IRAS 100 SPARO B-vectors on IRAS 100 m mapsm maps(equatorial coordinates)(equatorial coordinates)

NGC 6334 d ~ 1.7 kpc Carina d ~ 2.7 kpc

G 333.6 d ~ 3.0 kpc G 331.5 d ~ 5.3 kpc

SPARO B-vectors on PAH maps (equat. coords)

-- Carina: B parallel to edges of bubbles: flux-freezing

MSX Band A

(7-11 μm)

Kuiper et al. (1987) survey of 65 prominent GMCs in southern sky

-- analysis by Kuiper et al. is based on IRAS maps-- analysis by Kuiper et al. is based on IRAS maps

Carina Nebula

Polycyclic Aromatic

Hydrocarbons (PAHs)

in Carina Nebula(Galactic coords)

Smith et al. (2000)MSX Band A (7-11 μm)

-- PAHs trace boundaries of HII bubbles



ββt t = M= M22ββ = 50 = 50

weak fieldweak field

(also did(also did ββtt = 5) = 5)

Dotson et al. (in prep.)

Dotson et al. (in prep.)

OP = 0.73

OP = 0.97 OP = 0.06OP = 0.97 OP = 0.06

OP = 0.80

OP = 0.98 OP = 0.14

NGC 6334 region

bias- corrected

BGMC from SPARO

NGC 6334cloud

NGC 6334region

Foreground correction gives polarization residueForeground correction gives polarization residue(Marraco et al. 1993)(Marraco et al. 1993)

Foreground correction gives polarization residueForeground correction gives polarization residue(Marraco et al. 1993)(Marraco et al. 1993)

-- reject cells with order parameter (O.P.) < 0.3 -- reject cells with fewer than five residues

Polarization residues for 3 of our 75 cells

-- reject cells with order parameter (O.P.) < 0.3

-- reject cells with fewer than five stars

-- save the (equal-weight Stokes’) mean B-field angle for each surviving cell

O.P.= 0.41

O.P.= 0.28

O.P.= 0.21

Rosolowsky et al. 2003; M33 Rosolowsky et al. 2003; M33

Rotation axis parallel to Galactic plane

0.8

0.6

0.4

1

0.2 simulations

observation

Rotation axis perp. to Galactic plane

Rosolowsky et al. 2003 Rosolowsky et al. 2003

Turbulent Beta

10

0.8

1 100

0.6

0.1

0.4

1

0.2

0

simulations

observation

A Survey of the Global Magnetic Fields of Giant Molecular Clouds

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