A Quasar-Galaxy Mixing Diagram: Quasar Spectral Energy Distribution Shapes in the Optical to Near-Infrared

We define the quasar-galaxy mixing diagram by the slopes of the spectral energy distribution (SED) of quasars from $1\mu m$ to 3000 \AA\ and from $1\mu m$ to 3 $\mu m$ in the rest frame. The mixing diagram can easily distinguish among quasar-dominated, galaxy-dominated and reddening-dominated SED shapes. By studying the position of the 413 XMM selected Type 1 AGN in the wide-field "Cosmic Evolution Survey" (COSMOS) in the mixing diagram, we find that a combination of the Elvis et al. (1994, hereafter E94) quasar SED with various contributions from galaxy emission and some dust reddening is remarkably effective in describing the SED shape near $1\mu m$ for large ranges of redshift, luminosity, black hole mass and Eddington ratio of type 1 AGN. In particular, the location in the mixing diagram of the highest luminosity AGN is very close (within 1$\sigma$) to that expected on the basis of the E94 SED. The mixing diagram can also be used to estimate the host galaxy fraction and reddening in the SED. We also show examples of some outliers which might be AGN in different evolutionary stages compared to the majority of AGN in the quasar-host galaxy co-evolution cycle.

 Comments: 16 pages, 13 figures, 2 tables, submitted to MNRAS Subjects: Galaxy Astrophysics (astro-ph.GA) Cite as: arXiv:1210.3044 [astro-ph.GA] (or arXiv:1210.3044v1 [astro-ph.GA] for this version)