EDP Sciences
Free access
Volume 433, Number 1, April I 2005
Page(s) 73 - 77
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20042106

A&A 433, 73-77 (2005)
DOI: 10.1051/0004-6361:20042106

Evidence of intense hot ( $\mathsf{\simeq}$340 K) dust emission in 3CR radio galaxies

The most dissipative source of cooling in AGNs
B. Rocca-Volmerange1, 2 and M. Remazeilles1

1  Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
    e-mail: brigitte.rocca@iap.fr
2  Université de Paris-Sud XI, I.A.S., 91405 Orsay Cedex, France

(Received 1 October 2004 / Accepted 17 November 2004)

The spectra of the powerful 3CR radio galaxies present a typical distribution in the far-infrared (FIR). From the observed radio to X-ray spectral energy distribution (SED) templates, we propose to subtract the typical energy distributions of, respectively, the elliptical galaxy host and the synchrotron radiation. The resulting SED reveals that the main dust emission is well fitted by the sum of two blackbody components at the respective temperatures 340 K $\pm$ 50 K and 40 K $\pm$ 16 K. When the AGN is active, the energy rate released by hot dust is much more dissipative than cold dust and stellar emission, even when the elliptical galaxy emission is maximum at the age of $\simeq$90 Myr. Hot dust appears as a huge cooling source which implies an extremely short time-scale $t_{{\rm cool}}$, on balance with the short gravitational time-scale $t_{{\rm grav}}$ of massive galaxies. The dissipative self-gravitational models (Rees & Ostriker 1977) are favoured for radio sources. They justify the existence of massive radio galaxies discovered at z = 4 (Rocca-Volmerange et al. 2004). The synchrotron emission is emitted up to the X-ray wavelength range, so that Extreme X-ray Objects (EXO) could be identified with 3CR radio sources. To confirm these results in the infrared, an analysis of larger data samples from ISO and SPITZER is needed.

Key words: galaxies: evolution -- infrared: galaxies -- galaxies: active -- galaxies: formation

© ESO 2005