The magnetic field structure of the central region in M31

TL;DR

This study investigates the magnetic field structure of M31's central region, revealing a complex 3-D magnetic configuration and evidence of an independent dynamo process distinct from the outer disk.

Contribution

The paper extends the magnetic field model of M31 to its core, demonstrating the necessity of a 3-D dynamo model and providing new insights into the magnetic field orientation and behavior in the galaxy's center.

Findings

The central magnetic field points outward, opposite to the outer disk.

A large pitch angle (~33°) is observed in the core, larger than in the outer regions.

A 3-D dynamo model explains the observed polarization and rotation measure patterns.

Abstract

The Andromeda Galaxy (M31) is the nearest grand-design spiral galaxy. Thus far most studies in the radio regime concentrated on the 10 kpc ring. The central region of M31 has significantly different properties than the outer parts: The star formation rate is low, and inclination and position angle are largely different from the outer disk. The existing model of the magnetic field in the radial range 6<=r<=14 kpc is extended to the innermost part r<=0.5 kpc to ultimately achieve a picture of the entire magnetic field in M31. We combined observations taken with the VLA at 3.6 cm and 6.2 cm with data from the Effelsberg 100-m telescope to fill the missing spacings of the synthesis data. The resulting polarization maps were averaged in sectors to analyse the azimuthal behaviour of the polarized intensity (PI), rotation measure (RM), and apparent pitch angle (\phi_obs). We developed a simplified 3-D model for the magnetic field in the central region to explain the azimuthal behaviour of the three observables. Our 3-D model of a quadrupolar or dipolar dynamo field can explain the observed patterns in PI, RM, and \phi_obs, while a 2-D configuration is not sufficient to explain the azimuthal behaviour. In addition and independent of our model, the RM pattern shows that the spiral magnetic field in the inner 0.5 kpc points outward, which is opposite to that in the outer disk, and has a pitch angle of about 33 degrees, which is much larger than that of 8-19 degrees in the outer disk. The physical conditions in the central region differ significantly from those in the 10 kpc ring. In addition, the orientation of this region with respect to the outer disk is completely different. The opposite magnetic field directions suggest that the central region is decoupled from the outer disk, and we propose that an independent dynamo is active in the central region.