SIMBAD references

We have begun an investigation of how often and for how long galaxies undergo episodes of nonthermal nuclear activity. Radio images of galaxies at high frequencies (∼1 GHz and above) reveal nuclear activity over only a limited time interval preceding the current epoch. However, extended radio emission from earlier epochs of activity should be more prominent at lower radio frequencies because the much longer synchrotron radiation lifetimes of low-energy electrons result in spectra that steepen with time. After ∼10⁸ yr, radio emission from early epochs of activity may only be detectable at lower radio frequencies. To begin our investigation, we have chosen fields for observation that contain currently active galaxies because these are known to be capable of nuclear activity. We observed three 10° fields with the VLA at 74 MHz that contained seven active galaxies from the Pearson-Readhead sample plus a rich cluster of galaxies. Three of the active galaxies and the galaxy cluster were also observed at 327 MHz. None of the targeted active galaxies showed extended emission that we interpret as evidence of earlier activity. However, 10 background radio sources (out of ∼100) in the VLA fields of view did appear to have extended low-frequency emission, with the best case for fossil radio lobes being 3C 338. Our results suggest that extended low-frequency radio emission on scales of several arcminutes may be quite common in radio sources detectable with the VLA at 74 MHz. With adequate statistics, the study of fossil lobes can improve our understanding of the number and duration of active epochs, the history of black hole accretion, and galactic evolution. More sensitive, higher resolution, and lower frequency observations with future instruments will greatly enhance this study.