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

TL;DR

This study uses high-resolution infrared polarimetry to analyze the nuclear structure of Mrk 231, revealing hot dust scattering in the polar regions and characterizing the dust structures and starburst activity.

Contribution

It provides the first detailed high-angular resolution polarimetric imaging of Mrk 231 across multiple infrared wavelengths, elucidating the scattering mechanisms and dust geometry in the galaxy's core.

Findings

Hot dust at 731 K located 1.6 pc from the core influences polarization.

The polarized SED follows a λ^0.57 power law, indicating scattering by hot dust grains.

The central structure includes a hot dust region, a dusty torus, and a starburst component.

Abstract

We present high-angular (0.17$-$0.35 arcsec) resolution imaging polarimetric observations of Mrk 231 in the 3.1 $\mu$m filter using MMT-Pol on the 6.5-m MMT, and in the 8.7 $\mu$m, 10.3 $\mu$m, and 11.6 $\mu$m filters using CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already published observations, we compile the 1$-$12 $\mu$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 $\mu$m to 0.31 $\pm$ 0.15 per cent at 11.6 $\mu$m and follows a power-law function, $\lambda^{\sim0.57}$. The polarization angle remains constant ($\sim$108$^{\circ}$) in the 1$-$12 $\mu$m wavelength range. The dominant polarization mechanism is explained as scattering off hot dust grains in the pc-scale polar regions.