Magnetic fields and cosmic rays in M 31. I. Spectral indices, scale lengths, Faraday rotation, and magnetic field pattern

Abstract

Three deep radio continuum surveys of the Andromeda galaxy, M 31, were performed at 11.3, 6.2, and 3.6 cm wavelength with the Effelsberg 100-m telescope. At all wavelengths, the total and polarized emission is concentrated in a ring-like structure between about 7 kpc and 13 kpc radius from the center. Propagation of cosmic rays away from star-forming regions is evident. The ring of synchrotron emission is wider than the ring of the thermal radio emission, and the radial scale length of synchrotron emission is larger than that of thermal emission. The polarized intensity from the ring varies double-periodically with azimuthal angle, indicating that the ordered magnetic field is almost oriented along the ring, with a pitch angle of -14° $\pm$ 2°. Faraday rotation measures (RM) show a large-scale sinusoidal variation with azimuthal angle, signature of an axisymmetric spiral (ASS) regular magnetic field, plus a superimposed double-periodic variation of a bisymmetric spiral (BSS) field with about 6x smaller amplitude. The dominating ASS field of M 31 is the most compelling case so far of a field generated by the action of a mean-field dynamo. The RM amplitude between 6.2 cm and 3.6 cm is about 50% larger than between 11.3 cm and 6.2 cm, indicating that Faraday depolarization at 11.3 cm is stronger than at 6.2 cm and 3.6 cm. The phase of the sinusoidal RM variation of -7° $\pm$ 1° is interpreted as the average spiral pitch angle of the regular field. The average pitch angle of the ordered field, as derived from the intrinsic orientation of the polarized emission (corrected for Faraday rotation), is significantly smaller: -26° $\pm$ 3°. The difference in pitch angle of the regular and the ordered fields indicates that the ordered field contains a significant fraction of an anisotropic turbulent field that has a different pattern than the regular (ASS + BSS) field.

arXiv commentAccepted for publication in Astronomy 6 Astrophysics

References in corpus (38)

• Astrophysical magnetic fields and nonlinear dynamo theory
• Magnetic Fields in Spiral Galaxies
• Distance to M31 With the HST and Hipparcos Red Clump Stars
• Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes and helical fields
• HI kinematics and dynamics of Messier 31
• Spitzer/MIPS Infrared Imaging of M31: Further Evidence for a Spiral/Ring Composite Structure
• Molecular gas in the Andromeda galaxy
• Evolution of magnetic fields in galaxies and future observational tests with the Square Kilometre Array
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Integrated Polarization Properties of 3C48, 3C138, 3C147, and 3C286
• Global galactic dynamo driven by cosmic-rays and exploding magnetized stars
• The magnetic field of M31 from multi-wavelength radio polarization observations
• A Detailed Study of the Radio--FIR Correlation in NGC6946 with Herschel-PACS/SPIRE from KINGFISH
• Faraday tomography of the local interstellar medium with LOFAR: Galactic foregrounds towards IC342
• Linear polarization structures in LOFAR observations of the interstellar medium in the 3C196 field
• The Herschel Exploitation of Local Galaxy Andromeda (HELGA). I: Global far-infrared and sub-mm morphology
• Relating dust, gas and the rate of star formation in M31
• Multi-scale Radio-IR Correlations in M31 and M33: The Role of Magnetic Fields and Star Formation
• WSRT Faraday tomography of the Galactic ISM at λ~ 0.86 m
• How cosmic-ray electron propagation affects radio-far-infrared correlations in M31 and M33
• Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA
• Characteristics of the structure in the Galactic polarized radio background at 350 MHz
• Star formation laws in the Andromeda galaxy: gas, stars, metals and the surface density of star formation
• Resolved magnetic structures in the disk-halo interface of NGC 628
• G181.1+9.5, a new high-latitude low-surface brightness supernova remnant
• Multiscale magnetic fields in spiral galaxies: evolution and reversals
• Optimum frequency band for radio polarisation observations
• The NOD3 software package: A graphical user interface-supported reduction package for single-dish radio continuum and polarisation observations
• Magnetic and gaseous spiral arms in M83
• Rotation measure synthesis at the 2 m wavelength of the FAN region: Unveiling screens and bubbles
• Molecular gas in the inner 0.7kpc-radius ring of M31
• Polarized synchrotron radiation from the Andromeda Galaxy M31 and background sources at 350 MHz
• Radio polarization and magnetic field structure in M 101
• An analytical dynamo solution for large-scale magnetic fields of galaxies
• The magnetic field structure of the central region in M31
• The supernova-regulated ISM -- VI. Magnetic effects on the structure of the interstellar medium
• Nonlinear Galactic Dynamos and the Magnetic Pitch Angle
• A new method to suppress the bias in polarized intensity

Cited by in corpus (14)

• Galactic Dynamos
• Updated constraints on WIMP dark matter annihilation by radio observations of M31
• Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz
• A model for the infrared-radio correlation of main-sequence galaxies at GHz frequencies and its variation on redshift and stellar mass
• The C-Band All-Sky Survey (C-BASS): New Constraints on the Integrated Radio Spectrum of M 31
• Magnetic fields and cosmic rays in M31. II. Strength and distribution of the magnetic field components
• Low radio frequency observations of seven nearby galaxies with GMRT
• Diversity of dust properties in external galaxies confirmed by polarization signals from Type II supernovae
• Application of a helicity proxy to edge-on galaxies
• Galactic magnetic fields II. Applying the model to nearby galaxies
• Wind of Change: Faraday Rotation in a Simulated Large Magellanic Cloud
• A new window into the sub-parsec scale magnetic field in the Milky Way? Unveiling small-scale magneto-ionic structures with Faraday complexity
• The fine structure of the mean magnetic field in M31
• Limits on Dark Matter Annihilation from the Shape of Radio Emission in M31