An alignment effect in FR I radio galaxies: U-band polarimetry of the Abell 2597 cluster central galaxy

We have obtained U-band polarimetry of the spatially extended blue optical continuum associated with the FR I radio source PKS 2322-122, located in the Abell 2597 cluster central galaxy. We find a 3 σ upper limit to the degree of optical continuum polarization of less than 6%. The accuracy of the measurement is limited primarily by our ability to measure the amount of diluting galactic starlight. This limit is inconsistent with the blue continuum being primarily scattered light or synchrotron radiation. We can therefore exclude models that attribute the blue continuum to scattered light from an active nucleus that is hidden from direct view. Our result does not support the unification paradigm for BL Lac objects and FR I radio sources. Essentially all of the data pertaining to the blue continuum along the radio source - the "blue lobes" - indicate that they are regions of recent star formation. The spatial coincidence between the blue lobes and disturbances in the radio source suggests that star formation may have been induced by an interaction between the radio source and the cool (less than 10⁴ K) surrounding gas. This result, in addition to the results of a similar study of the A1795 cluster central galaxy, shows that under the appropriate conditions FR I radio sources may be capable of inducing significant episodes of star formation in elliptical galaxies. We compare the rest-frame U-band polarized luminosities and 1.4 GHz radio powers of A2597 and A1795 to those for several high-redshift radio galaxies exhibiting the alignment effect. We find that if the polarized luminosities of radio galaxies scale in proportion to their radio luminosities, we would not have detected a polarized signal in either A2597 or A1795. We suggest that the strength of the active galactic nucleus is the fundamental property distinguishing some powerful high-redshift radio galaxies exhibiting the alignment effect from the lower power FR I radio galaxies in cooling flows that exhibit the blue lobe phenomenon. While both FR I and FR II radio sources seem to be capable of triggering star formation, FR I radio sources seem to be incapable of producing a large enough polarized luminosity to contribute significantly to the aligned continuum radiation.