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Astron. Astrophys. 355, 873-879 (2000)

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The HST view of the FR I / FR II dichotomy *

M. Chiaberge 1, A. Capetti 2 and A. Celotti 1

1 SISSA/ISAS, Via Beirut 2-4, 34014 Trieste, Italy (chiab@sissa.it)
2 Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy

Received 17 June 1999 / Accepted 21 December 1999


In order to explore how the FR I / FR II dichotomy is related to the nuclear properties of radio galaxies, we studied a complete sample of 26 nearby FR II radio galaxies using Hubble Space Telescope (HST) images and compared them with a sample of FR I previously analyzed. FR I nuclei lie in the radio-optical luminosity plane along a tight linear correlation, which argues for a common synchrotron origin. FR II show a more complex behavior, which is however clearly related to their optical spectral classification.

Broad line FR II radio galaxies (BLRG) are located overall well above the FR I correlation, suggesting that a contribution from thermal (disc) emission is present. Three narrow line (NLRG) and one weak line radio galaxy (WLRG), in which no nuclear source is seen, can be interpreted as the obscured counter-parts of BLRG, in agreement with the current unification schemes.

Conversely, in 5 sources of the sample, all of them NLRG or WLRG, optical cores are located on the same correlation defined by FR I and with similar radio and optical luminosities. This suggests that, in analogy to FR I, the emission is dominated by synchrotron radiation and represents the optical counter-part of the non-thermal radio cores. Interestingly, all these galaxies are located in clusters, an environment typical of FR I.

These results imply that, at least at low redshifts, the FR II population is not homogeneous. Furthermore, the traditional dichotomy between edge darkened and brightened radio morphology is not unequivocally connected with the innermost nuclear structure, as we find FR II with FR I-like nuclei and this has interesting bearings from the point of view of the AGN unified models.

Key words: galaxies: active – galaxies: elliptical and lenticular, cD – galaxies: jets – galaxies: nuclei

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 and by STScI grant GO-3594.01-91A

Send offprint requests to: M. Chiaberge

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000