Annual Report 2011 Max Planck Institute for Astronomy

24 II. Highlights
II. Highlights
II.1 Anchoring Galactic Magnetic Fields in Giant Molecular Clouds:
A Bird’s-eye View
Magnetic fields set the stage for the birth of new stars
Astronomers at MPIA have measured for the first time
the alignment of magnetic fields in gigantic clouds of
gas and dust in a distant galaxy. The results suggest that
such magnetic fields play a key role in channelling matter
to form denser clouds, and thus in setting the stage
for the birth of new stars. The work was published in the
November 24 edition 2011 of the journal Nature.
The relationship between galaxy-wide magnetic fields
(B-fields) and localized molecular clouds is not well
y [kpc]
6.4
6.2
6
5.8
–5.6
Galactocentric Distance [kpc]
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understood. Some models of cloud formation suggest
that the large scale galactic field is largely irrelevant at
the small scale of individual clouds, because turbulence
and rotation of a cloud might randomize the cloud field
orientations (see Dobbs, C., MNRAS 391 844 (2008)
and Fig. II.1.1 bottom). Others suggest that the galactic
B-fields can be strong enough to impose their directions
upon the clouds (See Fig. II.1.1 top and Shetty, R.
& Ostriker, E. ApJ 647, 997 (2006)).
The implication for star formation is that the ordered
cloud B-fields in the latter scenario can regulate cloud
fragmentation and affect star formation rate and efficien-
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Galactocentric Distance [kpc]
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log column density [g / cm 2 ]
200 km s –1
Fig. II.1.1: Two competing scenarios of cloud formation. Top: A
patch from a global galaxy simulation. The solid vectors show
the instantaneous gas velocity in the frame rotating with the
spiral potential. The dotted vectors show the initial velocities
(pure circular motion). The solid lines show B-field orientations.
The gray scale stands for the relative surface density. The
B-fields of the spiral arm are only slightly twisted in the molecular
cloud complexes (dark elongated regions), and in turn
the field tension is strong enough to hinder the cloud rotation.
Bottom: A similar simulation but the well developed cloud
rotation has produced tidal tails extending from the GMC, and
the B-fields (vectors) follow the rotation and lost the “memory”
of the galactic field direction.
Credit: Dobbs, C., MNRAS 391 844 (2008)