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

Abstract

We present high-angular (0.17$-$0.35 arcsec) resolution imaging polarimetric observations of Mrk 231 in the 3.1 $\hat{I}¼$m filter using MMT-Pol on the 6.5-m MMT, and in the 8.7 $\hat{I}¼$m, 10.3 $\hat{I}¼$m, and 11.6 $\hat{I}¼$m filters using CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already published observations, we compile the 1$-$12 $\hat{I}¼$m 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 $\pm$ 4 K) dusty structure, directly irradiated by the central engine, which is at 1.6 $\pm$ 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 $\pm$ 23${}^{\circ }$ surrounding the central engine, and 3) an extinguished (A$_{\mbox{V}} =$ 36 $\pm$ 5 mag) starburst component. The polarized SED decreases from 0.77 $\pm$ 0.14 per cent at 1.2 $\hat{I}¼$m to 0.31 $\pm$ 0.15 per cent at 11.6 $\hat{I}¼$m and follows a power-law function, $\hat{I}{»}^{\sim 0.57}$. The polarization angle remains constant ($\sim$108${}^{\circ }$) in the 1$-$12 $\hat{I}¼$m wavelength range. The dominant polarization mechanism is explained as scattering off hot dust grains in the pc-scale polar regions.

9 pages, 2 figures. Accepted for publication in MNRAS