Magnetic field in the Galactic Centre: Rotation Measure observations of extragalactic sources

Abstract

We have estimated the Faraday Rotation Measure (RM) towards 45 extragalactic sources seen through the central –6° < l < 6°, –2° < b < 2° region of the Galaxy using the ATCA and the VLA. This is the first systematic study of RM in the central kpc of the Galaxy and has lead to the detection of a large scale magnetic field in the region. The magnetic field does not undergo any reversal of sign across the rotation axis of the Galaxy, which is consistent with the bisymmetric spiral model for the magnetic field in our Galaxy. If this component prevails over the central 2 kpc region of the Galaxy, and the average electron density in the region is taken to be 0.4 cm−3, then the line of sight component of this regular magnetic field is estimated to be 0.7 μG. In addition to this large scale field, there are also small scale fluctuations, whose coherence scale length is estimated to be ∼20 pc. If the scattering material is uniformly distributed in the GC, then we estimate the random component of the field to have a strength of 10 μG. On the other hand, if the scattering medium is clumpy, the random component will have similar strength as the regular component. Constraining the filling factor of the ionised medium is required for estimating the random component of the magnetic field from the RM data.