Letter to the Editor

Dust in 3CR radio galaxies: On the FR 1 – FR 2 difference ^*

¹ Astronomisches Institut, Ruhr-Universität Bochum (AIRUB), Universitätsstr. 150 / NA7, 44780 Bochum, Germany e-mail: smueller@astro.rub.de
² European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Max–Planck–Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile

Received: 6 August 2004
Accepted: 6 September 2004

Abstract

We compare three 3CR samples of 11 FR 1 galaxies, 17 FR 2 galaxies and 18 lobe-dominated quasars contained in the ISO Data Archive. In contrast to the powerful FR 2 galaxies with edge-brightened lobes, the low radio power FR 1 galaxies in our sample do not exhibit any high MIR or FIR dust luminosity, which is typical for a buried, intrinsically more luminous AGN. This consolidates the fact already inferred from optical studies that their AGNs have only a relatively low luminosity. Also the FR 1 galaxies show a high FIR/MIR luminosity ratio, compared to quasars, suggesting that their FIR luminosity is substantially powered by the interstellar radiation field (ISRF) of the giant elliptical hosts. Finally, we discuss the FR 1 – FR 2 morphological dichotomy. FR 1 galaxies do not have more interstellar matter (ISM) than FR 2s as traced – on the large scale – by the cool FIR emitting dust and – in the nuclear region – by the warm MIR emitting dust. Due to the lack of central gas we suggest that the black holes of our FR 1 galaxies are fed at a lower accretion rate than those of the FR 2 galaxies.