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
1.0) radio galaxies (e.g.,
McCarthy et al. 1987;
Rees 1989;
De Young 1989).
Whether this process
occurs in low redshift (z
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).