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Register a new userMagnetic fields in Clusters of Galaxies
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Authors
L. Feretti
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An important area of study of cosmic magnetic fields is on the largest scales, those of clusters of galaxies. In the last decade it has become clear that the intra-cluster medium (ICM) in clusters of galaxies is magnetized and that magnetic fields play a critical role in the cluster formation and evolution. The observational evidence for the existence of cluster magnetic fields is obtained by the diffuse cluster-wide synchrotron radio emission and from rotation measure (RM) studies of extragalactic radio sources located within or behind the clusters. A significant breakthrough in the knowledge of the cluster magnetic fields will be reached through the SKA, owing to its capabilities, in particular the deep sensitivity and the polarization purity.
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We assess the principal statistical and physical uncertainties associated with the determination of magnetic field strengths in clusters of galaxies from measurements of Faraday rotation (FR) and Compton-synchrotron emissions. In the former case a basic limitation is noted, that the relative uncertainty in the estimation of the mean-squared FR will generally be at least one third. Even greater uncertainty stems from the crucial dependence of the Faraday-deduced field on the coherence length scale characterizing its random orientation; we further elaborate this dependence, and argue that previous estimates of the field are likely to be too high by a factor of a few. Lack of detailed spatial information on the radio emission--and the recently deduced nonthermal X-ray emission in four clusters--has led to an underestimation of the mean value of the field in cluster cores. We conclude therefore that it is premature to draw definite quantitative conclusions from the previously-claimed seemingly-discrepant values of the field determined by these two methods.
We study the influence of intracluster large scale magnetic fields on the thermal Sunyaev-Zeldovich (SZ) effect. In a macroscopic approach we complete the hydrostatic equilibrium equation with the magnetic field pressure component. Comparing the resulting mass distribution with a standard one, we derive a new electron density profile. For a spherically symmetric cluster model, this new profile can be written as the product of a standard ($beta$-) profile and a radius dependent function, close to unity, which takes into account the magnetic field strength. For non-cooling flow clusters we find that the observed magnetic field values can reduce the SZ signal by $sim 10%$ with respect to the value estimated from X-ray observations and the $beta$-model. If a cluster harbours a cooling flow, magnetic fields tend to weaken the cooling flow influence on the SZ-effect.
Magnetic fields are an important component of the interstellar medium, especially in low-mass galaxies like irregulars where the magnetic pressure may be significant. However, few irregular galaxies have observed magnetic field structures. Using the VLA, the GBT, and the ATCA, we have observed several irregular galaxies in the radio continuum to determine their magnetic field structures. Here we report on our results for the galaxies NGC 4214 and NGC 1569.
Radio synchrotron emission is a powerful tool to study the strength and structure of magnetic fields in galaxies. Unpolarized synchrotron emission traces isotropic turbulent fields which are strongest in spiral arms and bars (20-30 mu G) and in central starburst regions (50-100 mu G). Such fields are dynamically important; they affect gas flows and drive gas inflows in central regions. -- Polarized emission traces ordered fields, which can be regular or anisotropic turbulent, where the latter originates from isotropic turbulent fields by the action of compression or shear. The strongest ordered fields (10-15 mu G) are generally found in interarm regions. In galaxies with strong density waves, ordered fields are also observed at the inner edges of spiral arms. Ordered fields with spiral patterns exist in grand-design, barred and flocculent galaxies, and in central regions. Ordered fields in interacting galaxies have asymmetric distributions and are a tracer of past interactions between galaxies or with the intergalactic medium. In radio halos around edge-on galaxies, ordered magnetic fields with X-shaped patterns are observed. -- Faraday rotation measures of the diffuse polarized radio emission from galaxy disks reveal large-scale spiral patterns that can be described by the superposition of azimuthal modes; these are signatures of regular fields generated by mean-field dynamos. Magnetic arms between gaseous spiral arms may also be products of dynamo action, but need a stable spiral pattern to develop. Helically twisted field loops winding around spiral arms were found in two galaxies so far. Large-scale field reversals, like the one found in the Milky Way, could not yet be detected in external galaxies. -- The origin and evolution of cosmic magnetic fields will be studied with forthcoming radio telescopes like the Square Kilometre Array.
Many galaxies contain magnetic fields supported by galactic dynamo action. However, nothing definitive is known about magnetic fields in ring galaxies. Here we investigate large-scale magnetic fields in a previously unexplored context, namely ring galaxies, and concentrate our efforts on the structures that appear most promising for galactic dynamo action, i.e. outer star-forming rings in visually unbarred galaxies. We use tested methods for modelling $alpha-Omega$ galactic dynamos, taking into account the available observational information concerning ionized interstellar matter in ring galaxies. Our main result is that dynamo drivers in ring galaxies are strong enough to excite large-scale magnetic fields in the ring galaxies studied. The variety of dynamo driven magnetic configurations in ring galaxies obtained in our modelling is much richer than that found in classical spiral galaxies. In particular, various long-lived transients are possible. An especially interesting case is that of NGC 4513 where the ring counter-rotates with respect to the disc. Strong shear in the region between the disc and the ring is associated with unusually strong dynamo drivers for the counter-rotators. The effect of the strong drivers is found to be unexpectedly moderate. With counter-rotation in the disc, a generic model shows that a steady mixed parity magnetic configuration, unknown for classical spiral galaxies, may be excited, although we do not specifically model NGC 4513. We deduce that ring galaxies constitute a morphological class of galaxies in which identification of large-scale magnetic fields from observations of polarized radio emission, as well as dynamo modelling, may be possible. Such studies have the potential to throw additional light on the physical nature of rings, their lifetimes and evolution.
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