Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003basbr..23q.146c&link_type=abstract
Boletim da Sociedade Astronômica Brasileira (ISSN 0101-3440), vol.23, no.1, p. 146-146
Astronomy and Astrophysics
Astronomy
Scientific paper
We have carried out hydrodynamical numerical simulations of light jets in order to compare with radio observations of Cygnus A. Numerical simulations studies indicate that light jets will inflate a cocoon whose width is inversely proportional to the density ratio (h) between the jet and the ambient gas. The over expanded bridge in Cyg A implies a very low value for h and an order-of-magnitude estimate of the jet density gives h~4´10-5. We have run simulations where we used h = 2´10-4 and a jet internal Mach number M = 10. A plot of the source advance speed shows that the source decelerates rapidly with its Mach number decreasing by a factor of ~10. This is due to the progressive increase of the jet head radius. The dynamical age of Cyg A determined using the ram pressure of the hotspot is larger than the synchrotron age and it has been suggested that this discrepancy can be alleviated if one relaxes the equipartition hypothesis when estimating the magnetic field. The decelaration of the source head observed in the simulations can, at least in part, be responsible for the discrepancy and departure from equipartition may be smaller than previously thought. We have calculated the volume radio emissivity of the source and integrated to obtain the surface brightness which allows us to draw synthetic radio maps. We have also taken into account radiation loss of the relativistic electron population. The calculated radio map shows the characteristic edge-brightened shape of powerful FR-II sources and an aspect ratio resembling that of Cygnus A. The surface brightness distribution both across the bridge and along the source axis also shows a good agreement with the one observed in Cyg A. These results allows us to make a good estimate of the physical parameters of the radio source.
Carvalho João C.
Daly Ruth
O'Dea Christopher P.
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