Magnetic fields in merging spirals - the Antennae

We present an extensive study of magnetic fields in a system of merging galaxies. We obtained for NGC4038/39 (the Antennae) radio total intensity and polarization maps at 8.44, 4.86 and 1.49GHz using the VLA in the C and D configurations. The radio thermal fraction was found to be about 50% at 10.45GHz, higher than in normal spirals. The mean total magnetic fields in both galaxies are about two times stronger (20microG) than in normal spirals. However,the degree of field regularity is rather low, implying tangling of the regular component in regions with interaction-enhanced star formation. Our data combined with those in HI, Halpha, X-rays and in far infrared allow us to study local interrelations between different gas phases and magnetic fields. We distinguish several radio-emitting regions with different physical properties and at various evolutionary stages. The whole overlapping region shows a coherent magnetic field structure, probably tracing the line of collision between the arms of merging spirals while the total radio emission reveals hidden star formation nests. The southern part of it is a particularly intense merger-triggered starburst. Highly tangled magnetic fields reach there strengths of 30microG, even larger than in both individual galaxies, possibly due to compression of the original fields pulled out from the parent disks. In the northeastern ridge, away from star-forming regions, the magnetic field is highly coherent with a strong regular component of 10microG tracing gas shearing motions along the tidal tail. Modelling Faraday rotation data show that we deal with a three-dimensionally curved structure of magnetic fields, becoming almost parallel to the sky plane in the southeastern part of the ridge.Comment: Accepted for publication in AA, 16 pages including 16 figures, high-res version at http://www.oa.uj.edu.pl/~chris/publ/4038.ps.g

Magnetic fields and gas in the cluster-influenced spiral galaxy NGC 4254 - II. Structures of magnetic fields

The origin of asymmetric radio polarized emission and properties of magnetic fields in the Virgo Cluster spiral NGC4254 are investigated with help of "magnetic maps" presenting distributions of different magnetic field components over the entire galaxy, free of Faraday rotation and projection effects. We show that the dramatic variation of orientation of magnetic field vectors (from 0deg to more than 40deg) throughout the galaxy cannot arise from the dynamo process alone, but must be dominated by the effects like density waves and local gas flows. We determine within the galaxy the relation between the strength of total magnetic field and the local star-formation rate (SFR) as a power-law with an index of +0.18+-0.01. We find the opposite sense of relation of magnetic field regularity with SFR (-0.32+-0.03) and suggest that it results from efficient production of random field with rising turbulence in the regions with actively forming stars. The distribution of Faraday rotation measures in NGC4254 indicates a perturbed axisymmetrical mean-field dynamo mode or a mixture of axisymmetrical and bisymmetrical ones with regular field directed outwards the disk, which is contrary to most observed galaxies. The magnetic fields within two outer magnetic arms (shifted downstream of a density wave) are strong, up to 13muG in the regular field and 20muG in the total field. Our modeling of cluster influence on different magnetic field components indicates that within the outer magnetic arms the dynamo-induced magnetic fields are modified by stretching and shearing forces rather than by cluster ram pressure. Those forces, which are likely triggered by the galaxy's gravitational interaction, produce an anisotropic component of the regular field and enhance the polarized emission.(abridged)Comment: 15 pages, 7 figures, A&A accepted, minor changes to v

What drives galactic magnetism?

We aim to use statistical analysis of a large number of various galaxies to probe, model, and understand relations between different galaxy properties and magnetic fields. We have compiled a sample of 55 galaxies including low-mass dwarf and Magellanic-types, normal spirals and several massive starbursts, and applied principal component analysis (PCA) and regression methods to assess the impact of various galaxy properties on the observed magnetic fields. According to PCA the global galaxy parameters (like HI, H2, and dynamical mass, star formation rate (SFR), near-infrared luminosity, size, and rotational velocity) are all mutually correlated and can be reduced to a single principal component. Further PCA performed for global and intensive (not size related) properties of galaxies (such as gas density, and surface density of the star formation rate, SSFR), indicates that magnetic field strength B is connected mainly to the intensive parameters, while the global parameters have only weak relationships with B. We find that the tightest relationship of B is with SSFR, which is described by a power-law with an index of 0.33+-0.03. The observed weaker associations of B with galaxy dynamical mass and the rotational velocity we interpret as indirect ones, resulting from the observed connection of the global SFR with the available total H2 mass in galaxies. Using our sample we constructed a diagram of B across the Hubble sequence which reveals that high values of B are not restricted by the Hubble type. However, weaker fields appear exclusively in later Hubble types and B as low as about 5muG is not seen among typical spirals. The processes of generation of magnetic field in the dwarf and Magellanic-type galaxies are similar to those in the massive spirals and starbursts and are mainly coupled to local star-formation activity involving the small-scale dynamo mechanism.Comment: 9 pages, 3 figures, accepted for publication in Astronomy and Astrophysic

Magnetic field evolution in dwarf and Magellanic-type galaxies

Low-mass galaxies radio observations show in many cases surprisingly high levels of magnetic field. The mass and kinematics of such objects do not favour the development of effective large-scale dynamo action. We attempted to check if the cosmic-ray-driven dynamo can be responsible for measured magnetization in this class of poorly investigated objects. We investigated how starburst events on the whole, as well as when part of the galactic disk, influence the magnetic field evolution. We created a model of a dwarf/Magellanic-type galaxy described by gravitational potential constituted from two components: the stars and the dark-matter halo. The model is evolved by solving a three-dimensional (3D) magnetohydrodynamic equation with an additional cosmic-ray component, which is approximated as a fluid. The turbulence is generated in the system via supernova explosions manifested by the injection of cosmic-rays.The cosmic-ray-driven dynamo works efficiently enough to amplify the magnetic field even in low-mass dwarf/Magellanic-type galaxies. The $e$-folding times of magnetic energy growth are 0.50 and 0.25 Gyr for the slow (50 km/s) and fast (100 km/s) rotators, respectively. The amplification is being suppressed as the system reaches the equipartition level between kinetic, magnetic, and cosmic-ray energies. An episode of star formation burst amplifies the magnetic field but only for a short time while increased star formation activity holds. We find that a substantial amount of gas is expelled from the galactic disk, and that the starburst events increase the efficiency of this process.Comment: 8 pages and 5 figures, accepted for publication in Astronomy and Astrophysic