The importance of integral-field spectroscopy in the study of powerful radio galaxies
Introduction
Most powerful radio galaxies possess luminous extended emission-line regions (EELR), which can extend large distances from their nuclei. These EELR are usually aligned along radio axis (McCarthy et al., 1987), and present much more extreme properties at higher redshifts (e.g. Best et al., 2000). Understanding the EELR is crucial to address important issues such as: the origin of extended gas, the triggering of the active galactic nucleus (AGN), and the influence of the AGN activity on the host galaxy.
The majority of spectroscopic studies of radio galaxies that have been carried out to investigate the properties of the extended emission-line gas, are based on long-slit spectroscopic observations, with the slit position usually along the radio axis. These studies concentrate on the shocked structures along the radio axis, and offer a limited view of the EELR properties. To learn about the issues above, the influence of the radio source away from the radio axis must also be understood, and the properties of the intrinsic (pre-shocked) gas at larger radii need to be studied. Thus, full maps of the emission-line kinematics and ionization are required over the entire extent of the haloes in these sources.
Section snippets
Deep narrow-band imaging
Two-dimensional information of the extended nebulae in radio galaxies can be obtained by using narrow-band imaging. Baum et al. (1988) and McCarthy (1988) carried out narrow-band imaging in strong emission-lines of powerful radio galaxies in a wide range of redshifts. They showed that the morphologies of the emission-line nebulae are spatially associated with the radio structures, and the luminosity of the nebulae strongly correlate with the radio source luminosity.
Going beyond this, 2-D
Integral-field spectroscopy
The TTF results presented above clearly demonstrate the vast quantity of information that is lost in single long-slit spectroscopic observations. However, the narrow-band imaging technique does not offer the possibility of studying the kinematics in two spatial dimensions. In order to obtain simultaneously full maps of both kinematics and ionization of the emission-line gas in the extended haloes of these sources, integral-field spectroscopy is required.
The first integral-field spectrographs
Jet-cloud interactions and the expanding radio jet cocoon
Our images provide evidence for existence of jet-cloud interactions in these sources. These include:
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Regions of enhanced emission-line luminosity are found close to the radio hotspots and radio knots (Figs. 2 and 3), indicating that jet-induced shocks are acting as a local ionization source, and/or that the shocks increase the efficiency with which the central AGN photoionizes the gas (e.g. Clark et al., 1998).
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The gas around the radio lobes has the lowest ionization state (Fig. 2), suggesting
Summary
The recent advances in 2-D studies of the properties of the emission-line haloes in radio galaxies are summarized. Integral-field spectroscopic observations of three 3CR radio galaxies are then presented. Evidence for jet-cloud interactions is found in the extended nebulae of these objects, not only near the radio axis but also far away from it, showing for the first time that the expanding radio cocoon plays an important role in the properties of the ambient gas.
References (12)
ApJ
(1989)- et al.
ApJS
(1988) - et al.
MNRAS
(2000) - et al.
ApJ
(1998) - et al.
MNRAS
(1997) - et al.
ApJ
(1987)
Cited by (1)
AGN shocks in 3D
2007, ESO Astrophysics Symposia