David J. Barnes, Daisuke Kawata, Kinwah Wu

Magnetic Fields in Astrophysical Systems
David J. Barnes, Daisuke Kawata, Kinwah Wu
Mullard Space Science Laboratory
[email protected]
Magnetic fields in our Universe
Magnetic fields are pervasive in our Universe on all scales, see Fig. 1, from the earth and the solar system to
galaxies and clusters of galaxies, with increasing evidence of their presence even in the voids of the cosmic web.
They are known to be critical in many of the phenomena that are important in the formation of our Universe:
•  Star formation – magnetic fields, via the magnetorotational instability, allow material to accrete on to the
forming protostar by transporting angular momentum out from the accretion disc.
•  Jet production – high velocity jets of material are common to many astrophysical systems and, although the
exact mechanism of jet production is still uncertain, magnetic fields are thought to be critical
to the accretion of material on to the central object and to the acceleration and collimation of
the material that forms the jet.
•  Cosmic rays
– are high energy particles that are produced by supernovae and active galactic nuclei (AGN)
and they require a magnetic field to accelerate them to the energy that we observe them at.
However, magnetic fields in astrophysical systems of galaxy size or greater are significantly less well understood.
All spiral galaxies observed show the presence of a magnetic field that is coherent and ordered on the scale of the
galaxy itself. The energy density of this field is similar in magnitude to the turbulent energy of the gas and the
thermal energy of the galaxy and should play an important role in the formation and evolution of the galaxy. On the
scale of clusters of galaxies magnetic fields are present in the gas between the galaxies, the intra cluster medium
(ICM), but even the very origin of this field is uncertain. Our research is directed towards understanding the very
Fig. 1: Image of the Sun’s surface revealing the presence of magne9c ﬁelds large-scale magnetic fields in galaxies and clusters of galaxies by answering:
(upper le5)[1]. Galaxy M51 produces polarized emission due to the existence of magne9c ﬁelds in the galaxy (upper right)[2]. On large-‐scales, 1000’s of galaxies •  What are the impacts of a magnetic field on the evolution of a spiral galaxy?
group to form a galaxy cluster (lower le5)[3]. The ICM emits in X-‐ray (colour) and radio ﬂux (contours) revealing a magne9c ﬁeld permea9ng it (lower right).[4] •  What is the magnetogenesis mechanism for the very largest structures in our Universe?
Magnetic Fields in Spiral Galaxies
Magnetic fields are thought to originate in spiral galaxies from small initial seeds produced
by supernovae and these are then amplified by the turbulence motions of the galaxy. The
large-scale rotation of the galaxy then orders the magnetic field into the global field that is
observed[5]. Although the production of galactic magnetic fields is an intensely studied
area, there has been significantly less work on the impact of the magnetic field on the
evolution of the galaxy.
Recent work has shown that the arms in spiral galaxies are a transient feature, that form
from local instabilities in the disc of the galaxy and dissipate on time scales of roughly
100Myrs[6,7]. However this work does not include a galactic magnetic field and the
magnetic pressure and magnetic tension force may effect the timescales on which the
arms form and the time it takes them to dissipate. To investigate the impact of a galactic
magnetic field on the formation of spiral arms in a galaxy we simulate the evolution of a
spiral galaxy over a billion years with a magnetic field present.
The results of this simulation can be seen in Fig. 2. Regardless of the initial set up of the
magnetic field the global rotation of the galaxy orientates the magnetic field into the plane
of disc. The magnetic field is maintained at a few microgauss, in agreement with the
observed strength of galactic magnetic fields, and the rotation produces many reversals in
the field structure. In agreement with previous work the stars and gas in the simulation
form many spiral arm features during the simulation. The introduction of a magnetic field
to the simulation causes the arm features to exist for a greater period of time and to wind
up for a longer period of time before they dissipate.
Fig. 2: Face-‐on (upper) and edge-‐on (lower) images of the stars (le5), gas (center) and large-‐scale magne9c ﬁeld (right) of a simulated spiral galaxy a5er 1Gyr of evolu9on. The magne9c ﬁeld shows a large number of reversals in the plane of the galaxy. The galaxy cluster forms via the merger of many
smaller objects and the final object is shown in Fig. 3.
This merging of many smaller objects provides the
energy to amplify the initial seed magnetic field to
2.3µG in the final cluster. The simulated cluster has a
magnetic field strength in good agreement with that of
observed galaxy clusters.
Large-scale Magnetogenesis
For the very largest structures in our Universe the
theories of magnetogenesis fall into two categories[8]:
I.  Primordial generation – seed magnetic fields are
generated in the early Universe via inflation or at
phase transitions, e.g. the quark to hadron transition.
•  Naturally produces observed coherence lengths
From the simulated cluster predicted observables can
then be calculated:
•  Naturally produces magnetic fields in voids
•  Seeds possibly too weak to be amplified to the
magnitude observed in large-scale structures
•  The X-ray and radio flux of the simulated cluster can
be calculated. The strength and morphology of the
“synthetic” emission agrees well with the observed
radio and X-ray emission of galaxy clusters.
II.  Baryonic generation – large-scale structure is
seeded by the magnetic fields ejected by galactic
winds and AGNs.
•  A rotational measure (RM) indicates the line of sight
magnetic field. A RM map of the simulated cluster
was calculated and shows that the magnetic field is
a patchwork coherent of the scale a of a few
kiloparsecs. This is good agreement with the RM
maps of observed galaxy clusters.
•  Easily produces the observed field strength
•  Cannot explain observed coherence length
•  Extremely difficult to reconcile with observations
of magnetic fields in voids
We simulated the formation of a galaxy cluster with a
10-11G primordial field to investigate the amplification of
the seed field and if it is capable of reproducing the
observations of magnetic fields in galaxy clusters.
[1] Credit Hinode, JAXA/NASA [2] Fletcher A., Beck R., Shukurov A., Berkhuijsen E. M., Horellou C., 2011, MNRAS, 412, 2396 [3] Credit NASA Fig. 3: Slices through the center of the cluster showing the density (upper le5) and magne9c ﬁeld strength (upper right). The colour image in the bo>om le5 panel shows the predicted X-‐ray ﬂux with contours of the predicted radio ﬂux overlaid (10-‐3, 10-‐4, 10-‐5, 10-‐6 mJy). The bo>om right panel shows the rota9on measure map for the simulated cluster. [4] Brown S., Rudnick L., 2011, MNRAS, 412, 2 [5] Shukurov A., 2004, arXiv e-‐print, 0411739 [6] Sellwood J. A., 2011, MNRAS, 410, 1637 A primordial seed field is capable of reproducing the
observations of large-scale magnetic fields in galaxy
clusters and we hope to extend this work beyond the
cluster scale.
[7] Grand R. J., Kawata D., Cropper M., 2012, MNRAS, 421, 1529 [8] Widrow L. M., 2002, Reviews of Modern Physics, 74, 775

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