Inferring the atmosphere of classical double sources from synthetic radio maps

Details Inferring the atmosphere of classical double sources from synthetic radio maps Inferring the atmosphere of classical double sources from synthetic radio maps

Aug 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003basbr..23r.155l&link_type=abstract

Boletim da Sociedade Astronômica Brasileira (ISSN 0101-3440), vol.23, no.1, p. 155-156

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We use a detailed analytical model of double radio source expansion to investigate the effect of the ambient gas on its radio surface brightness distribution. The model takes into account changes in the radio spectrum due to radiation losses and a magnetic field calculated assuming equipartition of energy between relativistic particles and fields. The effect of the surrounding medium is taken into account by considering either a constant density atmosphere or an isothermal atmosphere with a density gradient according King's law. Synthetic radio maps of the model sources were drawn and compared with observed maps. They show the edge-brightened morphology characteristic of FRII radio galaxies and an elongated bridge extending from the jet head toward the nucleus. The radio map of a source propagating in an isothermal atmosphere with a density gradient shows an excess emission around the center of the source. The radio emissivity is proportional to the pressure inside the cocoon. In a decaying density atmosphere, the cocoon pressure near the galactic nucleus is greater or at least comparable to that near the jet head. This happens because up to a distance from the nucleus of the order of the core radius of King's distribution, the cocoon pressure stays relatively high, that is, larger or of the order of the external pressure. Away from the source center, it starts falling as the external pressure decreases and, finally, increases near the jet head as expected, since it is there where the jet kinetic energy is being deposited. In real source, we do not observe this brightening of the central region of the cocoon. A possible explanation for why we do not see the central brightening of the cocoon is that the density is either fairly constant or does not decrease as fast as given by King's law. Furthermore, the core radius could be much larger than the 5 kpc used here. Therefore, our results are consistent with observed radio images of radio galaxies in a constant or mildly varying atmosphere.

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