A New Approach to Characterizing the SEDs of AGN from Deep Multi-wavelength Observations

We use data of the CDFN 2Ms survey and related multi-wavelength observations to investigate the nature of the intrinsic SEDs of AGN, how they vary with redshift and luminosity, and how the data are affected by obscuration and possible contributions from starbursts. We believe that shifting observational data to the emitted frame provides a direct way to visualize effects of obscuration and changes in the underlying SED from object to object. 1 Shifting from the Observed to the Emitted Frame The 2 Ms Chandra Deep Field-North survey is the deepest X-ray observation of the universe in the 0.5-8.0 keV band. For 270 of 503 point sources in the catalogue of [1], spectroscopic redshifts could be obtained. This enables us to transform observed fluxes directly into emitted luminosities : Lem(νem) = Lν,obs(νobs(1 + z)) (1) where Lν,obs = 4πdLfν,obs(νobs) with z = source redshift, fν,obs(νobs) = observed flux, and dL = dL(z) = luminosity distance. Photometric data in the HK’, z’, I, R, V, B and U bands are from [2]. Fig. 1 shows a selection of SEDs constructed by this method. X-ray data are plotted as power-law with photon index Γeff and cover a 0.5-8.0 keV band shifted into the emitted frame. The redshift of each source is denoted to the right. Also plotted is the standard SED [3] for low-redshift, radio-quiet quasars. Throughout our analysis we use Ωλ =0.73, Ωm =0.27 and h=0.71. Plotting SEDs in the emitted frames provides a powerful tool to visualise often dramatic changes from object to object. Immediately apparent are the • wide ranges in luminosities (up to 6 orders of magnitude) • different wavelength/energy regimes probed in sources of different redshifts – the observed NIR/opt. bands shift into the UV for the emitted frame – the 0.5-8.0 keV observed X-ray regime transforms into a ∼ 3 − 50 keV emitted interval for sources with the highest redshift of z ∼ 5.0 • strong variations in X-ray slopes • strong variations in optical extinction and reddening. ESO Symposia: Growing Black Holes, pp. 463–465, 2005. © Springer-Verlag Berlin Heidelberg 2005 464 S. Frank and P. Osmer Classifying the Sources The 2Msec Chandra exposure probes very faint X-ray sources hitherto not studied at cosmological distances. Norman et al. [5] classify 136 of them as normal starforming/starburst galaxies with little/no AGN activity. An example of such a galaxy’s SED is presented in Fig. 1. The remaining AGN can be grouped by the visual appearance of their broadband SEDs into three main categories. • Luminous, unabsorbed AGN – X-ray power-law slope close to the canonical Γeff ∼ 1.9 of the Elvis-SED. – The optical/UV data show a rise or stay flat beyond 2500 A. – The optical-to-X-ray flux ratio is similar to the Elvis et al. ratio. • Red and X-ray flat AGN – X-ray power-law slope close to Γeff ∼ 1.9. – The optical/UV data show a sharp decline beyond 2500 A. • Red and X-ray steep AGN – The X-ray power-law slope is much less than Γeff ∼ 1.9, indicative of either an intrinsically hard spectrum or absorption. – The optical/UV data show a sharp decline beyond 2500 A. The redshift distribution of the four object types are presented in table 1. The low-z regime is dominated by galaxies, whereas sources with higher z appear to be predominantly unabsorbed AGN. Both redshift and luminosity effects can cause this schism. 2 First Results • Transforming observed fluxes into broadband SEDs in the emitted frame allows us to study luminosity and redshift effects simultaneously. Fig. 1. Broadband SEDs of select CDF-N 2Msec sources representing the four different categories of objects. For further information see text. A New Approach to Characterizing the SEDs of AGN 465 Table 1. Redshift distribution of the four classes of objects. Note that by default Norman et al.(2004) only analysed sources at z 1.2 Normal Galaxies 129 0 Unobscured AGN 14 27 Red and X-ray steep/flat AGN 47/33 8/14 • Apparent dust obscuration and reddening in the UV/optical can, but need not be, accompanied by steep X-ray spectra (Γeff < 1.0). • As both the 2500 A and 2keV rest-frame regions can be subject to heavy obscuration, we suggest that an optical-Xray index αox based on the (emitted) R-band and 8 keV fluxes would be a better indicator of the intrinsic source properties. • Be aware of galaxies and starbursts at low z ! • Our approach yields new insights about the population of absorbed AGN that seem to dominate at low luminosity and redshift.