Blazar Demographics from Multiwavelength Surveys

Blazars, which are active galactic nuclei with relativistic jets aligned with the line of sight, provide valuable insight into how black holes produce jets, how the jets are collimated, and how much power jets carry into the interstellar and intergalactic media. They exhibit a wide range of radio through gamma-ray spectral energy distributions (SEDs), leading to strong selection effects in any particular survey. This has contributed to a long-standing controversy about the numbers of blazars as a function of their luminosity. The recent Fermi catalog contains a higher fraction of BL Lac objects than earlier EGRET catalog (which had a higher flux limit), and therefore supports the view that low luminosity blazars are more numerous than high luminosity blazars (contrary to Giommi & Padovani papers in 1994, 1995). We show that associating SED shape with luminosity explains quantitatively the different SED types found in existing radio, X-ray, and gamma-ray blazar samples. In particular, we use simulations to produce mock samples selected at arbitrary wavelengths and flux limits, then compare these predictions to existing blazar samples. In this way we rule out the possibility that radio-luminous, EGRET-detected blazars are the dominant sub-group. Furthermore, we show that the strong negative evolution reported in X-ray-selected blazars, compared to positively evolving radio-selected blazars, is caused by uneven selection (due to SED shape) from a uniformly evolving, underlying population.