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4.3. Photoionization by Hot Stars

Star formation triggered by the passage of radio jets through a dense interstellar gas appears to be at least partially responsible for the excellent alignment between the radio continuum, optical continuum and emission-line axes in high redshift (z geq 1.0) radio galaxies (e.g., McCarthy et al. 1987; Rees 1989; De Young 1989). Whether this process occurs in low redshift (z leq 0.5) radio galaxies is more controversial. The best case may be "Minkowski's object", a peculiar blue object near the elliptical galaxy NGC 541 in the cluster Abell 194. Minkowski's object exhibits bright optical line and continuum emission associated with a radio jet from NGC 541 (van Breugel et al. 1985b). The optical line emission is found to be brightest near the edges of the jet, correlated with regions of low radio polarization, and filamentary downstream of where the radio jet is deflected and decollimated. The ratios of the optical emission lines in Minkowski's object are similar to those of both extragalactic HII regions and starburst galaxies. van Breugel et al. (1985b) propose that Minkowski's object is an irregular galaxy undergoing a burst of star formation which has been "triggered" by the radio jet. Other examples of possible radio jet- or lobe-induced star formation include NGC 7385 (Hardee, Owen & Eilek 1980), Cen A (which exhibits strings of blue stars and HII regions [e.g., Osmer 1978; Graham & Price 1981] along the radio axis, in addition to the high excitation "optical jet" discussed earlier), and 3C 285 (van Breugel 1992).