The Infrared Spectral Energy Distribution of the Seyfert 2 Prototype NGC 5252

The complete mid- to far-infrared continuum energy distribution collected with the Infrared Space Observatory of the Seyfert 2 prototype NGC 5252 is presented. ISOCAM images taken in the 3-15 μm show a resolved central source that is consistent at all bands with a region of about 1.3 kpc in size. Because of the lack of ongoing star formation in the disk of the galaxy, this resolved emission is associated with either dust heated in the nuclear active region or bremsstrahlung emission from the nuclear and extended ionized gas. The size of the mid-IR emission contrasts with the standard unification scenario envisaging a compact dusty structure surrounding and hiding the active nucleus and the broad-line region. The mid-IR data are complemented by ISOPHOT aperture photometry in the 25-200 μm range. The overall IR spectral energy distribution is dominated by a well-defined component peaking at ~100 μm, a characteristic temperature of T ≃ 20 K, and an associated dust mass of 2.5 × 10⁷ M_☉, which greatly dominates the total dust mass content of the galaxy. The heating mechanism of this dust is probably the interstellar radiation field. After the contribution of this cold dust component is subtracted, the bulk of the residual emission is attributed to dust heated within the nuclear environment. Its luminosity consistently accounts for the reprocessing of the X-ray to UV emission derived for the nucleus of this galaxy. The comparison of NGC 5252's spectral energy distribution with current torus models favors large nuclear disk structure on the kiloparsec scale.