Research: Reddened QSOs


Optical, near-, mid- and far-IR images of a reddened QSO identified by combining the Sloan Digital Sky Survey (SDSS) at optical and the Two Micron All Sky Survey (2MASS) survey at near-IR and selecting sources with very red optical/near-IR colours (R-K>5, J-K>2). Study of the panchromatic SED of this class of sources provides unique insights on their nature.

An interesting recent development in studies of AGN has been the discovery of reddened QSOs. These systems have broad optical emission lines, but unlike the traditional QSO population, have red UV/optical continua, suggesting moderate amounts of dust extinction. There has been increasing interest in the nature of these objects, as it is suggested that they may represent an early stage of AGN evolution. In this picture AGN are born in a cocoon of dust and gas clouds, which eventually will be blown away by the energy output from the central engine, allowing it to shine unobscured for short period of time. This process will also terminate the star-formation in the nuclear regions of the host galaxy, as the gas, the raw material to form stars, is swept away. Reddened QSOs are systems shortly before or during the blow-out stage in the scenario above.

Constraining the level of star-formation and exploring evidence for AGN driven outflows in reddened QSOs are key tests to the evolutionary picture above. We have compiled a sample of reddened QSOs (Georgakakis et al. in 2009) with multi-wavelength photometric observations at UV/optical (SDSS), near-IR (2MASS), mid- and far-IR (Spitzer) as well as radio (FIRST, NVSS). Modeling the Spectral Energy Distribution of these sources is a powerful diagnostic of their nature. There is evidence for a higher level of star-formation (measured by the far-IR) for a given AGN power (approximated by the mid-IR) compared to UV/optically selected QSOs. There is also evidence for a higher fraction of radio sources among reddened QSOs suggesting outflows. Both these findings are consistent with the young AGN scenario.