Radio Galaxies

The advent of radio astronomy saw the detection of a number of unknown radio sources, and advances made in the field in the decade after the Second World War led to optical counterparts to

Figure 3.1: Radio image of Cygnus A at a wavelength of 20 cm. Two narrow radio jets can be seen emanating from the central core, channelling matter and energy into the large radio lobes. Taken from Perley et al. (1984).

some of them being identified. One of these sources was Cygnus A (a galaxy at z = 0.057), which was found to have a similar emission-line spectrum to the Seyfert galaxies previously identified, but unlike those sources, it was radio-loud (Baade & Minkowski, 1954). Identification of other similar sources quickly followed, and they were classified as “radio galaxies”.

Radio galaxies are some of the brightest radio sources in the sky and have radio luminosities of greater than ∼ 108_L

, with the most powerful displaying overall luminosities as high as ∼ 1012L

(Sparke & Gallagher, 2000). Radio galaxies are highly structured. They are seen to have twin radio-bright, optically-thin, lobes on opposite sides of the galaxy, which are related in size to the strength of the radio source at the nucleus; the largest lobes are ∼ 3 Mpc across (Sparke & Gallagher, 2000). To fill out such lobes, a radio galaxy must have been active for at least 10- 50 million years. Within the lobes, there are luminous hot spots with sizes ∼ 1 kpc; these hot spots are observed to emit polarised visible light via the synchrotron process. The core of a radio galaxy is a radio source only a few parsecs across, which is optically thick and varies in luminosity on timescales of years, suggesting an emission region at most a few light years across. The radio galaxies observed with large radio lobes are generally giant elliptical or cD galaxies (giant ellipticals that have a large halo of stars) and are in many cases the brightest galaxies in their clusters. Many radio galaxies have been observed to be blue in colour and show other signs of recent star formation. Strong radio emission appears to be favoured in radio galaxies with relatively low amounts of cool gas (Sparke & Gallagher, 2000).

Emerging from deep within the central core of a radio galaxy are narrow, bright jets. In some cases, these are observed on only one side of the galaxy, while in others two-sided jets are detected. The matter in these jets is relativistic and is focused into a beam within the central parsec of the galaxy. These jets channel energy and matter into the radio lobes of the galaxy. The jets of radio galaxies emit via synchrotron radiation at all wavelengths from radio to X-rays and they have also been observed to emit γ-rays.

Radio galaxies can be divided into narrow-line radio galaxies, which make up around two-thirds of the population, and broad-line radio galaxies, which account for the rest. In narrow-line radio galaxies, the emission lines seen in the spectrum are relatively narrow (between ∼400 km s−1 and ∼800 km s−1_{, characterised using velocities as the broadening is due to to the range of Doppler}

factors observed for the emitting gas), and a very wide range of ionisation states is observed from [OI] (neutral oxygen) to [FeX] (the ninth ionised state of iron). The emission lines detected are very similar to those seen in planetary nebulae and HII regions, suggesting that the elemental abundances, temperatures and densities near the core of narrow-line radio galaxies are similar to such objects. Absorption lines are generally similar to those seen in elliptical galaxies not hosting an AGN, although with stronger HI lines in the ultraviolet, suggesting a larger population of young stars than would be expected for such objects. Broad-line radio galaxies show broad recombination lines, such as HI, HeI and HeII, although the forbidden line widths are similar to those seen in narrow-line galaxies. The narrow-line spectra in broad line radio galaxies are similar in relative intensity to those seen in narrow line radio galaxies. This suggests that all these objects have similar physical conditions in the narrow-line emission region, but that, in the case of broad-line radio galaxies an additional region is observed which contains matter with a much larger range of velocities.

In addition to being classified on the basis of detected emission lines, the cores of radio galaxies can also be separated into two categories depending on the properties of the jet, Fanaroff-Riley class 1 (FRI) and Fanaroff-Riley class 2 (FRII). Radio galaxies hosting FRI cores (henceforth FRI galaxies) have lobes which are brightest in the centre, with the ends showing “edge-darkening” and steeper radio spectra. Generally the jets are double sided, continuous and brighter than the radio lobes. FRI cores appear to be hosted in by the most luminous ellipticals and cD galaxies (giant ellipticals with a large halo of stars) (Phillips, 2005). The brighter radio galaxies, such as Cygnus A, show lobes which are “edge-brightened”, with steeper radio spectra near the centre of the lobes, and are classified as FRII hosting galaxies (henceforth FRII galaxies). Inside the lobes of FRII galaxies are usually a number of smaller, kpc-sized, radio hotspots. The jets of such objects are usually one-sided or at least asymmetric and, although brighter than the jets in FRI galaxies, show

less contrast with the brightness of the radio lobes. FRII cores are hosted at the core of normal giant elliptical galaxies (Phillips, 2005).