With so many variables, it is not surprising that AGN SEDs are so diverse, it may be more surprising that we see such similarities from source to source! This leads to the question of selection. Are they mostly similar because we select them to be? Would we recognise an AGN that was, for example, so heavily absorbed that few if any of the general characteristics we associate with an AGN are present? Motivated by the large number of apparently normal galaxies being found by the Chandra Deep Surveys, Moran et al. (2002) show how host galaxy light may dominate optical AGN signatures in Sy2s and thus hide a significant fraction of the population.
Thus, perhaps the most critical open question is "What is an AGN?". New AGN are being found in radio (Webster et al. 1995), IR (Cutri et al. 2001) nd deeper optical surveys, 2DF and SDSS (Richards et al. 2003). Models for the Cosmic X-ray Background (CXRB) require a new population of X-ray absorbed AGN (Gilli et al. 2001). Chandra and XMM-Newton find X-ray sources sufficiently luminous to be AGN but with no optical AGN characteristics (Figure 6, Green et al. 2004 (ChaMP), Norman et al. 2002). Similarly, bright radio galaxies with strong, unresolved cores show no optical AGN signatures. Are these all AGN? If so how do we relate them to the more standard BEL AGN? What are a minimum set of properties which define an AGN? How can we observe the intrinsic population?
 |
Figure 6. Candidate buried AGN, ChaMP survey, Green et al. (2004). |
Since AGN are multiwavelength emitters, we need a view in multiple wavebands. For example the red AGN found in the 2MASS survey are hard, weak X-ray sources (Wilkes et al. 2002) and may contribute ~ 30% of the CXRB depending on their evolution (Figure 7, Wilkes et al. 2003). But X-ray surveys must cover more area to deeper flux levels to find the same kinds of sources. The next generation of multi-wavelength surveys will provide both deep, small areas (GOODS) and shallower, wider areas (SWIRE; ChaMP, Silverman, this volume) and sample the same/overlapping pieces of the intrinsic population. The combination of sensitive IR and X-ray, provided by Chandra, XMM-Newton and SIRTF, is particularly powerful as the far-IR sees all luminous IR sources while the X-rays select the AGN from amongst the predominant IR galaxy population. These are exciting times!
 |
Figure 7. Contribution of 2MASS red AGN to the CXRB for two different evolutionary models, Wilkes et al. 2003. |