MAGNETIC FIELDS - A TRACER OF GALACTIC
CATASTROPHES
Krzysztof T. Chyży
Astronomical Observatory, Jagiellonian University
ul. Orla 171, 30-244 Kraków
Poland
[email protected]
Rainer Beck
Max-Planck-Institut für Radioastronomie
Auf dem Hügel 69, 53121 Bonn
Germany
Abstract
1.
Weak tidal interaction is possibly the cause of the strong magnetic fields we discovered in the nearby irregular dwarf starburst galaxies NGC 4449 and IC 10.
Strong tidal interaction triggered massive star formation in the "Antennae" merging pair of galaxies and enhanced the magnetic fields which locally reach values
of about
G. Our modeling of Faraday rotation data of this system also suggests a magnetized gas outflow into the intergalactic medium along the southern
tidal tail. Thus merging galaxies as well as dwarf irregulars belong to objects
potentially responsible for supplying magnetic fields to the intergalactic space.
Irregulars
Weak tidal interaction is possibly the cause of the unusual strong magnetic fields we discovered in the irregular dwarf starburst galaxies NGC 4449
and IC 10. A detailed analysis of the observed magnetic field orientation in
NGC 4449 (Chyży et al. 2000) shows a well-organized coherent spiral pattern
resembling the dynamo field in spirals. The strengths of the regular and total
magnetic field (assuming equipartition between magnetic field and cosmic ray
energies) reach G and G, respectively. These values are comparable to
those found in radio-bright spirals and are unexpected taking into account the
very slow and almost chaotic rotation of this galaxy.
We are currently conducting a multi-frequency study of the origin and structure of magnetic fields in the nearby galaxy IC 10, the prototype of Blue Com-
2
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Figure 1.
Contours of the total power emission and B-vectors of the polarized intensity of
NGC 4038/39 at 4.86 GHz made from a combination of VLA data in the C and D arrays overlaid
upon the DSS image
pact Galaxies. IC 10 reveals magnetic fields filling the whole galaxy body.
However, the structure of magnetic field does not resemble the spiral-like pattern observed in NGC 4449. IC 10 is much smaller and the generation mechanisms of magnetic fields could be different than in NGC 4449.
Dwarf starburst galaxies are able to eject magnetized plasma into the ambient IGM. This could be possibly a common process in galaxies of the early
cosmic epoch. The studied local cases are a key to understand the structure
and development of the high-redshift Universe.
2.
Merging spirals
Interaction triggered star formation could explain the strong magnetic fields
we found recently in a merging pair of galaxies – NGC 4038/39, “the Antennae” (Chyży and Beck 2004). Our study is based on the VLA radio maps in
total and polarized intensity we obtained at 8.44 GHz, 4.86 GHz and 1.49 GHz.
The radio emission of this interacting system is associated with both galaxy
disks (Fig. 1) and with the “overlapping region” extending between the galax-
3
Magnetic fields - a tracer of galactic catastrophes
NGC 4038/39 Faraday Rotation with errors
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Figure 2.
Faraday rotation measure distribution in NGC 4038/39 computed between
8.44 GHz and 4.86 GHz plotted as symbols (see the Figure legend) overlaid upon a grayscale
plot of the rotation measure errors.
ies where a mixture of dark patches and HII regions (mostly obscured) are
present.
We found that the mean total magnetic field in both merging galaxies is
about two times stronger ( G) than in normal spirals but also more chaotic,
implying tangling of the regular component in regions with interaction-enhanced
star formation. The overlapping region shows a coherent magnetic field structure, probably tracing the line of collision between the arms of the merging
spirals.
The southern part of the overlapping region hosts a particularly intense
merger-triggered starburst. Highly tangled magnetic fields reach strengths of
G there, even larger than in both individual galaxies.
We report a possible feeding of the intergalactic medium by regular magnetic fields of NGC 4038/39 detached from the merging spirals along the southern tidal tail. In this part of the system, away from star-forming regions, the
magnetic field is highly coherent with a strong regular component tracing gas
shearing motions along the tidal tail.
4
We have calculated Faraday rotation measures of the magnetic field vectors
for the whole merging system (Fig. 2). In general, the rotation measures are
low, about 50 rad/m . Higher rotation measures are observed in the northern
part of the system. Our simple modeling of the Faraday rotation measure and
the polarized intensity show that the regular magnetic field of G runs in this
region at an angle of about 60 with respect to the sky plane. In the northeastern
part of the system our model predicts a similar regular field but lying almost
in the sky plane, inclined by 4 . Thus, we are able to determine the 3D curved
structure of the magnetic field which is adjusted to the tidal tail.
Acknowledgments
This work was supported by a grant from the Polish Research Committee
(KBN), grant no. PB249/P03/2001/21.
References
Chyży K.T., & Beck R. 2004, AA, 417, 541
Chyży K.T., Beck R., Kohle S., Klein U., & Urbanik M. 2000, AA, 355, 128