The optical polarization of near-infrared selected quasi-stellar objects

Optical broadband polarimetry is presented for near-infrared color-selected active galactic nuclei (AGNs) classified as quasi-stellar objects (QSOs) based on their K-s-band luminosity. More than 10% of a sample of 70 QSOs discovered in the Two Micron All Sky Survey (2MASS) with J-K-s > 2 and M-Ks < 23 show high broadband linear polarization ( P > 3%), and values range to a maximum of P similar to 11%. High polarization tends to be associated with the most luminous objects at K-s and with QSOs having the highest near-IR-to-optical flux ratios. The 2MASS QSO sample includes objects possessing a wide range of optical spectral types. High polarization is seen in two of 22 broad emission-line (type 1) objects, but similar to 1/4 of the QSOs of intermediate spectral type ( type 1.5-1.9) are highly polarized. None of the nine QSOs classified as type 2 exhibit P > 3%. It is likely that the unavoidable inclusion of unpolarized starlight from the host galaxy within the observation aperture results in reduced polarization for the narrow emission-line objects. The high polarization of 2MASS-discovered QSOs supports the conclusion inferred from their near-IR and optical colors: that the nuclei of many of these objects are obscured to some degree by dust. Correlations between optical polarization and near-IR luminosity and color imply that the dominant polarizing mechanism in the sample is scattering of AGN light into our line of sight by material located in relatively unobscured regions near the nucleus. The broadband polarization properties of the 2MASS QSO sample are compared to those of other, predominantly radio-quiet, QSOs and are found to be consistent with the idea that the orientation of AGNs to the line of sight plays a major role in determining their observed properties.