Mathematics – Probability
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aujph..51..971s&link_type=abstract
Australian Journal of Physics, vol. 51, p. 971
Mathematics
Probability
7
Galactic Clusters, Depolarization, Faraday Effect, Flux Density, High Temperature Gases, Magnetic Fields, Observation, Radiant Flux Density, Spectra, Radio Galaxies, Brightness, Confinement, Electrons, Exponents, Frequencies, Luminosity, Probability Theory, Static Pressure, Stellar Magnitude, Very Large Array (Vla)
Scientific paper
We present a detailed analysis of the results of a two-frequency VLA survey with scaled arrays of 60 fields near 58 Abell clusters. The sample of sources is complete to a flux density of 2.0 mJy at 1.5 GHz, from which we chose two sub-samples of radio galaxies from clusters that had been surveyed completely out to 0 24, and 0 51 times the Abell radius RA respectively. The following conclusions are drawn from these samples: (1) The sources are strongly concentrated towards their cluster centres, showing a power-law variation of surface density with radius, with an exponent of approx. 1 6. Each cluster contains an average of two radio galaxies. (2) The ratios of flux density and surface brightness in the lobes of cluster and background doubles are very similar, as are their spectral indices; the ratios of component area are significantly different. (3) The lobes of cluster doubles have spectra that steepen with radio power, but a less significant relationship is found between spectral index and surface brightness. (4) Relationships are found between source area and radio power, optical luminosity and r/RA, spectral index and r/RA, and between radio power and optical luminosity. (5) We define three polarisation classes on the basis of the presence or absence of observable polarisation at 4 9 and 1 5 GHz; the physical parameters vary systematically across the classes. (6) Depolarisation in sources that are partially polarized at both frequencies (Class 1) is predominantly caused by tangled fields; sources that are polarised at only the higher frequency (Class 2) or at neither frequency (Class 3) are depolarized by a combination of tangled fields and differential Faraday rotation. The rms dispersion in the scale size of the tangled fields within the radio plasma is approx. 5 kpc. (7) Differential Faraday depolarisation in Classes 1 and 2 suggests that thermal electrons are entrained in the radio plasma with densities in the range 1.1 - 2.3xl0-4/cc. (8) The magnetic fields in Class 1 doubles (with minimum Faraday rotation) favour directions predominantly parallel to the major axis of the double. (9) The univariate fractional luminosity function suggests that an early-type cluster galaxy with MR less than -21.0 has a probability of approx. 0.02 of creating a radio source with P1.5 less than 10(sup 24.9) W/Hz. (10) The integrated bivariate fractional luminosity function shows that higher luminosity cluster galaxies are more likely to form radio sources, although in each bin of absolute magnitude the fraction of radio sources decreases with increasing radio power. (11) The average lifetime of a cluster source of approx. 2 X 106 yr is consistent with the travel-time of the plasma at approx. 3000 km/s from a once-only acceleration site near the centre of the host galaxy. (12) The confinement of the radio lobes can be easily accomplished by the static pressure of the hot intra-cluster gas and/or ram pressure generated by the passage of the galaxy through the gas. (13) Radio powers of cluster sources and optical luminosities of their hosts are both independently correlated with cluster X-ray luminosity. This suggests that radio power is maintained by the confining pressure of the hot gas, but the cause of the optical correlation is unknown. 15 Finally, core powers in cluster radio galaxies are of the same order as those in non-cluster sources, indicating that the hot intra-cluster gas does not provide an extra fuel supply for the nuclear engine.
Andernach Heinz
Roy Alan L.
Slee Owen Bruce
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