Annu. Rev. Astron. Astrophys. 1988. 36: 539-598 Copyright © 1998 by Annual Reviews. All rights reserved

5.2. Extended Lobes

The advancing working surface inflates a "cocoon" in the intergalactic medium (IGM) surrounded by a bow shock enveloping the jet channel. The pressure distribution between hot spots and the leading bow shock drives a flow pattern sideways and backward along the jet. The bow shock permits the internal jet pressure and external IGM pressure to balance through appropriate gradients in their macroscopic quantities. In fact, the cocoon wraps the entire radio source (Begelman & Cioffi 1989). Therefore, the cocoon's width is determined by the drive of its internal pressure expanding in the external medium at sonic speeds (its length is given by the hot spots' advancement).

This situation has been modeled by Begelman & Cioffi (1989), Cioffi & Blondin (1992); Figure 7 is a cartoon drawn from these references. The head of the bow shock can have a cross section A_h A_j if the jet direction fluctuates on short time scale; in this case, the advancement velocity v_h , as determined by the modified balance Equation 20, becomes the following for light jets (v_j >> v_h):

(21)

The cocoon pressure P_c drives a sideways shock into the IGM at a speed v_c that is fixed by the balance of P_c and ram pressure _e v_c²; in the approximation that the cocoon inflates at constant L_j and v_h, P_c ~ L_j T / A_c v_h, where A_c is the cocoon's cross section. Then,

(22)

until it decreases below the speed of sound c_s in the IGM. The cocoon is elongated, v_h v_c, when L_{j e} v_j³ A_h³ / A_c², where A_c is the cocoon cross section. The whole body of the radio galaxy becomes embedded in an overpressured region, and this makes the jet collimation easier: Observationally, we cannot measure the gas pressure in the cocoons owing to the limited angular resolution of X-ray telescopes, but jet confinement may actually come from the regions of the cocoon at the interface of the flow. In addition, the transverse dimensions of lobes are due to the bow shock expansion and are made wide through wave propagation.

Figure 7. Schematic diagram of overpressured cocoons around jets (Begelman & Cioffi 1989).