Infrared polarimetry of Mrk 231: scattering off hot dust grains in the central core

We present high-angular (0.17−0.35 arcsec) resolution imaging polarimetric observations of Mrk 231 in the 3.1 µm filter using MMT-Pol on the 6.5-m MMT, and in the 8.7 µm, 10.3 µm, and 11.6 µm filters using CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already published observations, we compile the 1−12 µm total and polarized nuclear spectral energy distribution (SED). The total flux SED in the central 400 pc is explained as the combination of 1) a hot (731 ± 4 K) dusty structure, directly irradiated by the central engine, which is at 1.6 ± 0.1 pc away and attributed to be in the pc-scale polar region, 2) an optically-thick, smooth and disk-like dusty structure (‘torus’) with an inclination of 48 ± 23◦ surrounding the central engine, and 3) an extinguished (AV = 36 ± 5 mag) starburst component. The polarized SED decreases from 0.77 ± 0.14 per cent at 1.2 µm to 0.31 ± 0.15 per cent at 11.6 µm and follows a power-law function, λ ∼0.57. The polarization angle remains constant (∼108◦ ) in the 1−12 µm wavelength range. The dominant polarization mechanism is explained as scattering off hot dust grains in the pc-scale polar regions.