High Rotation Measure Research Articles

Compact steep-spectrum sources and the unified scheme.

The compact steep-spectrum sources (CSSs) are an interesting class of objects which are of subgalactic dimensions; they occur more frequently in high-frequency surveys because their spectra often turn over at lower frequencies. We have estimated the symmetry parameters of a well-defined sample of CSSs and compared these with the larger 3CR sources of similar luminosity to understand the evolution and the consistency of CSSs with the unified scheme. We suggest that the majority of CSSs are likely to be young sources advancing outward through an asymmetric, inhomogeneous environment to form the larger ones. The radio properties of the CSSs are consistent with the unified scheme, where the axes of the quasars are seen closer to the line of sight while the radio galaxies lie closer to the plane of the sky. We discuss how radio polarization observations may be used to probe whether the physical conditions in the central regions of the CSSs are different from the larger ones. We present a simple scenario where the depolarization and high rotation measures seen in many CSSs can be consistent with the low rotation measures of cores in the more extended quasars and suggest further observations to test this scenario.

High Faraday Rotation Measures of the Steep Spectrum Quasar 3C 216

view Abstract Citations (18) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High Faraday Rotation Measures of the Steep Spectrum Quasar 3C 216 Taylor, Gregory B. ; Ge, Jingping ; O'Dea, Christopher P. Abstract We present multifrequency radio observations of the steep spectrum quasar 3C 216 from which we derive subarcsecond resolution images of the total intensity, polarization, spectral index, and rotation measure (RM) structure. These observations confirm the high RMs (~600 rad m^-2^) detected by Aizu et al. (1990) and reveal large gradients in the RMs, a marked asymmetry in the RM structure and in the fractional polarization. This asymmetry may be related to the beaming and superluminal motions of the pc-scale radio jet (Venturi et al., A&A, 271,65(1993)]. We find the high RMs in 3C 216 are consistent with the external densities and magnetic fields derived for lower redshift cluster cooling flows. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117539 Bibcode: 1995AJ....110..522T Keywords: QUASARS: INDIVIDUAL: 3C 216; QUASARS: EMISSION LINES full text sources ADS | data products SIMBAD (1) NED (1)

Optical filaments and global flow in cluster cooling flows

view Abstract Citations (13) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical Filaments and Global Flow in Cluster Cooling Flows Godon, Patrick ; Soker, Noam ; White, Raymond E., III ; Regev, Oded Abstract Many cooling flow clusters have optical filaments with anomalously high luminosities which require that some energy source sustains the filaments at optically emitting temperatures for much longer than their instantaneous cooling times. In an effort to constrain the energy source for powering these optical filaments, we try to supplement previously discovered correlations of filament properties with other cooling flow and cluster characteristics. We find that all cooling flow clusters with high radio rotation measures at their centers also have optical filaments or nuclear emission. Both high rotation measures and filaments may be independent characteristics of cooling flows, but there may also be a causal relationship: for example, magnetic reconnection in magnetized cooling flows may power the optical luminosities of filaments condensing out of the flows. We also find that filament properties are correlated with the peculiar velocities of their associated central dominant galaxies, a more global cluster characteristic. We consider ways in which this correlation could be causal. Publication: The Astronomical Journal Pub Date: December 1994 DOI: 10.1086/117213 Bibcode: 1994AJ....108.2009G Keywords: Cooling Flows (Astrophysics); Cosmic Gases; Galactic Clusters; Luminosity; Prominences; X Ray Astronomy; Faraday Effect; Magnetic Field Reconnection; Rosat Mission; X Ray Imagery; Astronomy; GALAXIES: COOLING FLOWS; GALAXIES: CLUSTERING; GALAXIES: JETS full text sources ADS | data products NED (25) SIMBAD (24)

Magnetic Fields in the Hydra A Cluster

Sensitive, high-resolution VLA polarimetric observations are presented for the radio galaxy Hydra A, an outstanding example of a high-luminosity radio source embedded within a cooling flow cluster. We find extremely high Faraday rotation measures (RMs), throughout the central regions of this source, with a strong asymmetry between the north and south lobes. Simple models for the high RMs are discussed and compared with the observations. To explain the north-south asymmetry in RM requires a magnetic field of strength ∼6 μG organized over a scale of ∼100 kpc. On smaller scales we find a tangled magnetic field with a strength of ∼30 μG

G5.4-1.2 (Milne 56) - a Galactic supernova remnant with a high rotation measure

The radio source G5.4-1.2 (Milne 56), long believed to a supernova remnant, is now of special interest following the recognition of a pulsar-excited nebula just outside the shell and probably associated with the remnant. New polarization observations at 4.6, 4.75, 4.9 and 8.4 GHz are presented, revealing an extremely high rotation measure

What are the gigahertz peaked-spectrum radio sources?

view Abstract Citations (281) References (120) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS What Are the Gigahertz Peaked-Spectrum Radio Sources? O'Dea, C. P. ; Baum, S. A. ; Stanghellini, C. Abstract We discuss the astrophysical implications of recent radio and optical observations of the powerful, compact gigahertz peaked-spectrum (GPS) radio sources. The defining characteristics of these objects are (1) their peaked radio spectra with narrow spectral shape and steep high- frequency spectrum, (2) low radio (and optical?) polarization, (3) large radio luminosities, and (4) mostly compact radio structure. The nature of these enigmatic objects is still not understood. However, we present some tentative conclusions and suggest a scenario based on the existing data. The turnover in the spectrum is probably due to synchrotron self- absorption, and we assume this as the basis for our inferences from the radio spectral shape. However, as we discuss below, these sources may have higher than normal electron densities in their nuclei, and it is possible that free-free absorption plays a role in at least some sources. The result that some GPS sources have a very narrow spectral shape is consistent with the hypothesis that there is a narrow range of size scales which dominate the radio luminosity; i.e., very extended (~>100 pc) and very compact (<~0.1 pc) emission is apparently very weak if it exists at all. Based on the steep high-frequency radio spectrum and the low variability, there does not appear to be a strong beamed "jetlike" component in these objects. Instead the radio luminosity is dominated by compact (i.e., parsec scale), steep-spectrum emission ("microlobes"?). The highly inverted low-frequency radio spectrum is consistent with the hypothesis that the radio source is tightly confined. We put on a firm foundation the result that the radio polarization of these sources is systematically low. This is consistent either with a very tangled magnetic field or very large Faraday rotation measures (implying high magnetic fields and or very large electron densities). The existence of high rotation measures is tentatively found in two objects (both high-redshift quasars) but it is premature to conclude that this is a general trend in GPS sources. The optical identifications are a mixture of galaxies and quasars. As previously noted (e.g., Phillips & Mutel; Pearson & Readhead) we find that the galaxies tend to have more symmetric milliarcsecond radio structure than the quasars. CCD imaging of small samples of GPS galaxies suggest that they tend to be interacting/merging. Spectroscopy of two GPS galaxies is consistent with the hypothesis that they have unusually dense, highly reddened narrow-line regions. We suggest that GPS radio sources are formed when the radio plasma is confined on the scale of the narrow-line region by an unusually dense and clumpy ISM. In this scenario, the high electron densities are responsible for depolarizing the radio source and in some cases producing very high Faraday rotation measures. The existing (sparse) optical spectroscopic results are also consistent with the existence of a dense and dusty nuclear ISM. The GPS quasars tend to have very high redshifts (half with z ~> 3). The hosts of these radio sources may be protogalaxies with very dense and clumpy interstellar media. The lower redshift GPS galaxies may have recently cannibalized a gas-rich companion, resulting in a very dense and clumpy nuclear ISM. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/170562 Bibcode: 1991ApJ...380...66O Keywords: Astronomical Polarimetry; Extragalactic Radio Sources; Quasars; Radio Spectra; Electron Density (Concentration); Luminosity; Polarization Characteristics; Astrophysics; POLARIZATION; QUASARS; RADIATION MECHANISMS; RADIO SOURCES: GALAXIES; RADIO SOURCES: SPECTRA full text sources ADS | data products SIMBAD (95) NED (95)

Magnetic Fields in Disks and Halos of Spiral Galaxies

Spiral galaxies host interstellar magnetic fields of 4-15 μG total strength. A significant fraction of the field lines shows large-scale structures. At face-on or moderately inclined view, the field lines run generally parallel to the spiral arms, either with uniform direction with respect to azimuthal angle (axisymmetric spiral, ASS), with one reversal along azimuthal angle (bisymmetric spiral, BSS), or with spiral orientation without dominating direction.At edge-on view, the field is concentrated in a thin disk, often surrounded by a thick radio disk with field lines mostly parallel to the plane, similar to the quadrupole-type dynamo field. Radio polarization data from NGC891 indicate that the thermal gas seen in Hα is responsible for Faraday depolarization. The required scaleheight of the field of ~4 kpc is comparable to the value expected in case of energy equipartition between magnetic fields and cosmic rays. The interacting edge-on galaxy NGC 4631 shows a much larger radio halo with field lines perpendicular to the disk, possibly driven by a strong galactic wind or the result of a dipole-type halo field.Field lines bending out of the plane are also visible in face-on galaxies as regions with high rotation measures and low star-formation activity. The resemblance to the phenomenon of the solar corona suggests to call them “galactic coronal holes”.

3 C 191 revisited - Circumquasar shells and radio jets

The quasar 3C 191 is reexamined in the light of recent rotation measure (RM) observations by Kronberg et al. (1990) and recent He II emission data, and an updated model for the intrinsic formation of the narrow shell of this object is developed. It is argued that the extraordinarily high rotation measure in the jet of this quasar is produced by an intrinsic absorption line system at z(abs) = 1.9453, which has a thin shell configuration produced as a wind from the quasar central regions sweeps up interstellar in an elliptical host galaxy. 28 refs.

Magnetic Fields in Cooling Flow Clusters

Faraday rotation measurements of the luminous radio galaxies 3C295, Cygnus A, and Hydra A, all of which are associated with cD galaxies located in massive cooling flow clusters, show very high rotation measures – typically a few thousand radm−2, exceeding 20,000 radm−2 for 3C295. The RMs vary over large ranges and with different scales. The RMs are both positive and negative, implying reversals of the magnetic field – the variations cannot be due to density changes alone. The RMs originate in magnetized cluster gas, but it is unclear what fraction is due to a coccoon surrounding the radio source. If from the cluster as a whole, the magnetic field is , where N is the number of cells along the line of sight. A good case can be made for an enhanced magnetic field localized at or near the surface of the radio source, accounting for a significant fraction of the observed RM.