IR and X-ray cores in low-redshift active galaxies

1. IR and X-ray cores in low-redshift active galaxies M. Birkinshaw, D.M. Worrall (U. Bristol), H.A. Smith, B.J. Wilkes, S.P. Willner (Harvard-Smithsonian CfA), C.R. Lawrence (JPL), D.C. Hines (U. Arizona), E.J. Hooper (U. Texas), I. van Bemmel (Sterrewacht Leiden) Abstract Spitzer and Chandra observations of low-redshift 3CRR radio galaxies are used to examine the relationship between the IR and X-ray emissions from their active nuclei. Although we have complete coverage of this well-known radio-selected sample in the IR, only about 70% of the sources have adequate X-ray coverage. While this limits the quality of the inferences that can be drawn, we find a close relationship between the IR and X-ray spectra of the cores. We interpret the results in terms of the energy contents of the active nuclei and the physical processes occurring in them. Table 1. Core properties for the subsample plotted in Figure 1 Figure 2. A three-colour IR image of NGC 6251, showing the extensive galaxy halo, the compact core, and the jet to the NW (Birkinshaw et al. 2006a, in preparation). • Seven-band Spitzer imaging observations have been made for the complete sample of thirty-six low-redshift (z < 0.1) 3CRR galaxies (Birkinshaw et al. 2006b, in preparation). • IR spectral distributions for ten of these objects (Table 1) are shown in Fig. 1. These are typical of the objects with good X-ray imaging spectroscopy from XMM-Newton and/or Chandra (Evans et al. 2006, ApJ 642, 96). • The different types of radio galaxy show different core IR properties. • FR1 (two-sided, “jetty”, low-excitation) objects show a long- IR peak from cold dust, a minimum at about 24 μm, and near-IR emission stellar emission with a hint of PAH features. • FR2 (two-sided, lobe-plus-hotspot, narrow-line) sources have much broader dust continua. Hot dust in the cores dominates over the near-IR stellar emission and fills the 24 μm minimum. • Strong hot dust emission is associated with the presence of high NH column and an Fe K line in the core X-ray spectra. Hot dust and a dense absorbing column are always present together. • IR jets are seen in several of the radio galaxies (e.g., NGC 6251; Fig. 2). Figure 3. 3C33 at 3.6 m (left) and 8 m (right). The FR2 core is prominent relative to the galaxy, and dominates at  > 5.6 m (Birkinshaw et al. 2006b, in preparation). • Summary • The difference between FR1 and FR2 galaxies is intrinsic, rather than set by jet/IGM interactions – the core properties differ in the IR and X-ray. • The total dust mass is higher in the higher-power radio galaxies. The IR images and the relationship with the X-ray absorption suggest that much of the IR emission originates close to the AGN. • The bolometric IR outputs of the AGN cannot be determined accurately without adding sub-mm data, but at least roughly the (thermal) IR correlates with the total non-thermal (radio to X-ray) luminosity, so that the IR output is an accurate indicator of the AGN activity. • The ratio of the IR to non-thermal output should therefore be a measure of the degree of beaming of the non-thermal radiation, though the geometry of the dust near the AGN is unclear (ALMA imaging will help). Figure 1. IR spectral distributions for ten representative objects from the 3CRR low-z sample, with the flux densities scaled as indicated. Note the progression of spectral shapes from FR1 (top) to FR2 (bottom) sources. Figure 4. 3C 442A at 3.6 m (left) and 8 m (right). The three galaxies in this group share a common envelope. The middle galaxy is associated with the radio source and shows the strongest dust emission (Fig. 1; Birkinshaw et al. 2006b).