| dc.description.abstract | (a) An important conclusion regarding the probability of radio emission associated with the brightest galaxy in the centre of a cluster containing a VSSS (Very Steep Spectrum Source) can be drawn from the study of Table 4.1. It is seen from this table that in every case (8 out of 8), the brightest galaxy (designated a cD) is detectable as a radio source with 1400 MHz radio luminosity P?.? > 5 × 10²³ W Hz?¹. All of these 8 clusters were selected with an a priori knowledge of the presence of a very steep spectrum radio source in the centre of the cluster. Thus, it is clear that whatever physical mechanism is responsible for the visibility and retention of a cluster VSSS is also, in some direct or indirect way, causing an enhancement of the probability of occurrence of a radio source associated with the central brightest elliptical galaxy.
In Chapter 2 of this thesis, we studied the radio emission from a large number of cD galaxies located within rich Abell clusters. These clusters were selected purely on the basis of the optical morphology of the central dominant galaxy. Therefore, this sample is likely to be free of any radio bias. We showed that the probability of detecting a radio source with luminosity P?.? > 5 × 10²³ W Hz?¹ associated with the cD is ? 39 ± 10% (see the radio luminosity function presented in Fig. 2.51, Chapter 2). Comparing the above-mentioned result with this result, we conclude that the probability of finding a radio source with P?.? > 5 × 10²³ W Hz?¹ is about twice as large for a cD galaxy occurring in a cluster harbouring a VSSS, as compared to a cD that is randomly selected purely by optical means.
This is quite a remarkable result and indicates that some physical factor found selectively in the clusters harbouring a VSSS is responsible for the enhanced probability of radio emission from the central galaxies. Two possible physical processes that can create such an effect are discussed below.
As a first hypothesis, we assume that the cD galaxy at the cluster centre is responsible for ‘cannibalising’ the progenitor radio galaxy which gave rise to the radio plasma of the VSSS located nearby. This merger of the ‘victim’ galaxy with the cD is further assumed to create conditions favourable for the generation of an active nuclear source at the centre of the cD. If this hypothesis is true, then one expects to see a trail or a bridge of radio emission (possibly tracing the path taken by the merged galaxy) between the VSSS and the cD galaxy. No such radio feature is visible on the radio maps where the VSSS is well resolved from the cD (A85, A133, A2009, 3C318.1, Kim 44, and A13). In the case of the source 4C20.57 in A2626, a precession of the radio jets is likely (possibly due to galactic merger). However, in this case, the radio emission with very steep spectrum is associated with the cD galaxy itself. Thus, we conclude that the hypothesis of galactic mergers enhancing the probability of radio emission from cD galaxies is not supported by our observations.
Alternatively, the physical mechanism that is believed to be responsible for the longevity of a VSSS, namely the confinement due to the pressure of the hot and dense ICM (intra-cluster medium) acting on it, can as well act on the radio source associated with a cD galaxy. Under these conditions, a radio source which would normally expand adiabatically and lose most of its internal energy (thus quickly becoming undetectable), would be retained for a much longer interval of time. A ‘snapshot’ of such confined radio sources should detect more radio objects. This appears to be consistent with our observations.
A better and more certain prediction of this hypothesis is that, along with the VSSS, the radio sources associated with the cD galaxies should also show steep radio spectra. However, their spectra may not be as steep as that of VSSS because the active galactic nuclei of these galaxies may still be contributing to the radio sources. Our spectral index measurements between the frequencies of 1400 MHz and 330 MHz are tabulated in Table 4.1. The median spectral index of the 8 cD galaxies is found to be ??? = 1.35 ± 0.10. On the other hand, the median spectral index of cD galaxies in clusters selected optically (Chapter 2, Section 2.10) was found to be 0.70 ± 0.13. Clearly, the median spectral index of cDs in clusters with known VSSS is much steeper compared to median spectral indices of cDs in clusters selected optically (the radio luminosity ranges of the two samples are similar). This result is in very good agreement with the above-mentioned expectation from this hypothesis.
Thus, concluding, the enhanced probability of detecting radio sources associated with cD galaxies in clusters with VSSS, as well as their steep radio spectra, are caused by the confining influence of the hot and dense intra-cluster medium acting in a similar way on any radio plasma immersed within it.
(b) The median spectral index of the VSSS in 8 clusters is extraordinarily steep (??? ? 2.9, between 1400 MHz and 330 MHz). Seven out of eight VSSS show evidence for pronounced curvature (steepening) in their radio spectra. Their spectra can be modelled with a spectral ‘break’ occurring at a frequency ?_b, which ranges between ? 80 MHz – 800 MHz. Most of the spectral shapes can be well explained by theories of evolution of a radio plasma, presently confined against adiabatic expansion, and losing energy in synchrotron emission and inverse-Compton scattering. The derived spectral ages of these VSSS are found to be in the range of ? 10? yr – 10? yr, demonstrating the VSSS to be quite old.
(c) Except for two cases (A2626 and Kim 21), where the radio source is associated with the cD galaxy, no optical counterpart could be located for any of the other VSSS. The radio morphology of these VSSS is also found to be quite diverse in nature, ranging from diffuse (A85, A133, and A2009), to those with internal ‘knots’ and unusual structures (Kim 21, 3C318.1, Kim 44, and A13), and one with high degree of rotational symmetry (A2626). The calculated minimum energy magnetic field of these VSSS was found to span a small range of B ? (2 – 8) ?G only.
(d) In general, the 1.4 GHz radio luminosity (P?.?) of the VSSS was found not to be very large. The luminosity ranges between P?.? ? (0.3 – 40.0) × 10²³ W Hz?¹, while the median value of their radio luminosity ?P?.?? ? 2.3 × 10²³ W Hz?¹. This weak radio luminosity is likely to be a consequence of losses of energy over the large interval of time since the progenitors of the VSSS ceased their activity.
(e) The VSSS 0038–096, in the cluster A85, besides being quite large, diffuse, and steep-spectrum, is also associated with an excess of X-ray emission. The most likely explanation of this X-ray excess is the inverse-Compton scattering of the photons of cosmic microwave background with the relativistic radio source electrons (the IC/3K process). The magnetic field of this VSSS, calculated using the observations in radio and X-ray bands, checks well with the derived magnetic field assuming minimum energy condition. The spectrum of the VSSS is also in very good agreement with theoretical models of an old, confined radio plasma, suffering radiative losses in synchrotron emission and IC/3K scattering. As is the case with the other ‘relic’ VSSS, the progenitor of this VSSS could not be identified, although an excess of galaxies, superposed in the region of the VSSS, and possibly forming a ‘subcluster’, has been identified. | |
