Gamma-ray astronomy

Abstract:

The results of a spectroscopic analysis of 3CR and 6C radio galaxies at redshift z ∼ 1 are contrasted with the properties of lower-redshift radio galaxies, chosen to be matched in radio luminosity to the 6C sources studied at z ∼ 1, thus enabling the redshift-radio power degeneracy to be broken. Partial rank correlations and principal component analysis have been used to determine which of redshift and radio power are the critical parameters underlying the observed variation of the ionization state and kinematics of the emission-line gas. [O II]/Hβ is shown to be a useful ionization mechanism diagnostic. Statistical analysis of the data shows that the ionization state of the emission-line gas is strongly correlated with radio power, once the effects of other parameters are removed. No dependence of ionization state on cosmic epoch is observed, implying that the ionization state of the emission-line gas is solely a function of the properties of the active galactic nucleus rather than the host galaxy and/or environment. Statistical analysis of the kinematic properties of the emission-line gas shows that these are strongly correlated independently with both redshift and radio power. The correlation with redshift is the stronger of the two, suggesting that host-galaxy composition or environment may play a role in producing the less extreme gas kinematics observed in the emission-line regions of low-redshift galaxies. For both the ionization and kinematic properties of the galaxies, the independent correlations observed with radio size are stronger than with either radio power or redshift. Radio source age is clearly a determining factor for the kinematics and ionization state of the extended emission-line regions.

Abstract:

The results of deep long-slit optical spectroscopy for a sample of eight 6C radio galaxies at redshift z ∼ 1 are presented. Emission-line ratios are derived for many emission lines with rest-frame wavelengths of 1500-4500 Å and the kinematic properties of the emission-line gas are derived from an analysis of the two-dimensional structure of the [O II] 3727-Å emission line at ≈5 Å spectral resolution. In general, the 6C spectra display many characteristics similar to those of more powerful 3CR sources at the same redshifts. The emission-line region gas kinematics are more extreme for the smaller radio sources in the sample, which often display distorted velocity profiles. The ionization state of the emission-line region also varies with radio size: the spectra of large radio sources (> 120 kpc) are consistent with photoionization by an obscured active galactic nucleus (AGN), whilst smaller (< 120 kpc) sources typically exist in a lower ionization state and have spectra that are better explained by additional ionization due to shocks associated with the expanding radio source. The kinematic and ionization properties of the 6C radio galaxies are clearly linked. As for the 3CR sources, smaller radio sources also typically possess more extensive emission-line regions, with enhanced emission-line luminosities. A high-velocity emission-line gas component is observed in 6C 1019+39, similar to that seen in 3C 265. It is clear that the best interpretation of the spectra of radio sources requires a combination of ionization mechanisms. A simple model is developed, combining AGN photoionization with photoionization from the luminous shock associated with the expanding radio source. The relative contributions of ionizing photons from shocks and the central AGN to an emission-line gas cloud vary with radio source size and the position of the cloud. This model provides a good explanation for both the ionization properties of the emission-line regions and the radio size evolution of the emission-line region extents and luminosities.