§6 Alignment effect in radio galaxies

HST observations of a sample of 28 3CR radio galaxies with redshifts $z$ in the interval $0.6 < z < 1.3$ were carried out by Best et al. (1996) and are described in detail by Best (1996). The programme had the intention of imaging almost all 3CR radio galaxies in this redshift interval. These 3CR radio galaxies were observed in wavebands corresponding to rest-frame U and B. Additionally, all images were mapped at $\unit{ 1.8 }{ \giga \hertz }$ using the VLA, with a resolution of $\unit{0.18}{arcsec}$. These galaxies were also observed at $\unit{ 2.2 }{ \micro\meter }$ with the IRCAM3 of the UKIRT at a resolution of $\unit{ 1 }{ arcsec }$. From these set of observations, Best (1996) produced figs.(I.7)-(I.8). The figures show eight 3CR radio galaxies in the interval $1 < z < 1.3$. The reason as to why this redshift interval was chosen is because within it, all radio galaxies have more or less the same intrinsic radio luminosity. Before analysing in more detail those images, it is important to note that, in general terms, a standard $L^\star$ elliptical galaxy at a redshift $z \sim 1$ is a diffuse low surface brightness object and only objects of high surface brightness, or flat spectra will appear clearly on the images in the optical waveband.

The radio images of these galaxies show the standard picture of a classical FR II source. Two radio lobes extend in opposite directions from the central core. The infrared images show the old stellar populations in these galaxies and they indicate that the host galaxy appears to be an elliptical galaxy, as should be expected. However, the optical images show little resemblance to what should be an elliptical host galaxy. Instead, the optical structures observed in the images align to a high degree with the radio jet axis. This effect, the so called radio/optical alignment effect in radio galaxies was first observed by McCarthy et al. (1987); Chambers et al. (1987).

Figure I.7: Five small radio galaxies in the redshift range $1 \lesssim z \lesssim 1.3$ at the same physical scale. The left images show HST images whilst the right ones are UKIRT K-band images. The radio contours had been overlaid on both sets of images. The evolution of the sources with linear size is apparent from the images. These set of images were kindly provided by Philip Best and they are described by Best (1996).

Figure I.8: Three large radio galaxies in the redshift range $1 \lesssim z \lesssim 1.3$ at the same physical scale. For each galaxy, the upper panel shows the HST image, and the lower one the UKIRT K-band image. Radio contours have been overlayed on both images. These images were kindly provided by Philip Best and they are described by Best (1996).

Synchrotron aging arguments have suggested that the greater the separation of the radio source from the nucleus, the older the sources are. This is consistent with a picture in which the sources are powered by a roughly constant energy supply throughout their lifetime. Also, according to the unification scenarios mentioned in section §3, the radio axes of radio galaxies are observed within $\unit{ 45 }{ \degree }$ to the plane of the sky. This implies that the difference between the observed and actual hotspot separations would not be very big and also that the observed structures should lie more or less in the plane of the sky.

As it is seen from the optical images, the oldest radio galaxies do not show a very strong optical structure aligned with the radio jet axis. However, the small, young radio galaxies show a very pronounced effect. This means that the radio/optical alignment effect is relatively short lived. In fact, Best (1996) calculated the ages of the radio galaxies using synchrotron aging arguments and concluded that the activity is completed in about $\unit{ 10 }{ \mega yr }$.

All these observations suggest that the aligned optical structures are a short lived phenomena, which is induced by the passage of the radio jet as it expands through the interstellar and intergalactic medium. It seems that, among all the different models that have been proposed in the literature (see Best, 1996, and references therein for a complete description of the different models), there is no single theory that can account for all of the observed features of the alignment between the optical and radio emission (Longair et al., 1995). The observation of polarised optical emission from some of these sources suggests that scattering of light from an obscured quasar could be responsible for these optical structures. A centrally obscured quasar would be responsible to the illumination of pre-existing dust and gas clouds. However, it seems extremely natural that the primary cause must be the interaction of the jet with cool interstellar clouds surrounding the parent galaxy as well as in the intergalactic medium.

McCarthy, van Breugel & Spinrad, and Chambers, Miley & van Breugel proposed that the alignment of the observed optical structures were produced because of enhanced star formation induced in some way by the passage of the radio jet through the interstellar medium of the host galaxy and the intergalactic medium. According to this jet-induced star formation picture it would be possible to account for the change in structure with increasing physical size of the radio galaxies. The lifetime of the newly formed stars associated with HII regions would amount to $\sim \! \unit{ \power{ 10 }{ 7 } }{ yr }$. After this time the luminosities of these star-forming regions decay. The polarisation of the light would then be attributed to the scattering of the light by the dust or gas associated with the star-forming regions.

In an attempt to understand better this jet-induced star formation phenomenon, de Young (1989) performed numerical simulations of high-energy radio jets passing through dense protogalactic gas. He showed that cooling in the regions behind the shocks associated with radio jets can lead to star formation rates over $\unit{ 100 }{ M_\odot \usk \power{ yr }{ -1 } }$, when the total mass of gas surrounding the galaxies approaches $\unit{ \power{ 10 }{ 14 } }{ M_\odot }$.

Rees (1988) made a more physical proposal about this star-formation idea. He built a model for which the interstellar and intergalactic medium which contained cold ( $\sim \unit{ \power{ 10 }{ 4 } }{ \kelvin }$) clouds in pressure balance with a hot ( $\sim \unit{ \power{ 10 }{ 6 } }{ \kelvin }$ ) medium. These cold clouds are most probably the result of cooling flows (Fabian, 2000). The expansion of the shocks associated with the radio jet expel the hot phase plasma, but leave the clouds within the lobes, where they are squeezed by the higher pressure around them. This compression drives the stable clouds into a gravitational unstable phase, triggering a burst of star formation. After the shock has passed, the clouds find themselves overpressured by a factor of $\sim M^2$, where $M$ is the Mach number of the flow. The value of $M$ is assumed to be $\sim 100$ along the jet and $\sim 10$ for the transverse expansion of the cocoon. Since the Jeans Mass $\mathsf{M}_J \sim p^{3/2}_{\textrm{cloud}} / \rho^2_{\textrm{cloud}}$ (Kron et al., 1995), where $p$ is the pressure and $\rho$ is the density, the above considerations imply that the resultant overpressure over the clouds would trigger collapse of all clouds down to a fraction of any value that lies in the interval $0.01$   -$0.1$ of the previous Jeans Mass. This results in a very well synchronised burst of star formation enhanced by a factor of the order of the Mach number.

(1989) used a similar model to that by Rees, but considered the shock to be radiative. In this case, the cloud's Jeans mass decreases by a factor $( p_{\textrm{ambient}} / p_{\textrm{cloud}} )^{1/2}$. Clouds which initially have masses somewhat less than the Jeans mass are driven gravitationally unstable by the compression.

Despite the fact that the idea of a jet-induced star formation model is very attractive to account for all these results, observations do not seem to favour this (Best et al., 2000). In fact, it was shown by Best et al. (2000), that for small radio sources ( $\lesssim \unit{ 150 }{ \kilo pc }$), the morphology, kinematics and ionisation properties of the emission line gas of the radio sources are dominated by the effects of the bow shock associated with the expansion of the radio source through the interstellar and intergalactic medium. Gas clouds embedded in the interstellar and intergalactic medium are accelerated by the shocks, giving rise to the distorted velocity profiles and extreme velocity dispersions observed. On the other hand, large radio sources have more settled kinematical properties. The shock fronts have passed well beyond the emission line regions and the dominant source of ionising photons appears to be the AGN. The diagram shown in fig.(I.9) by Best et al. (2000) shows the evidence for these different means of explaining the optical aligned structures. In Chapter V an analytic one dimensional model of the interaction of a shock wave with a high density region (a cloud) is developed. This one dimensional analysis provides an insight into the more physical mechanisms that might be acting on the interaction of the jet as it expands and interacts with surrounding clouds in pressure balance with the interstellar and intergalactic medium.