Nicmos Polarimetry of "Polar Scattered" Seyfert 1 Galaxies

TL;DR

This study uses NICMOS 2.0 micron imaging polarimetry to investigate the polarization characteristics of six 'polar scattered' Seyfert 1 galaxies, revealing the roles of different scattering regions and their impact on observed polarization.

Contribution

It provides new high-resolution infrared polarimetric observations of Seyfert 1 galaxies, clarifying the contributions of polar and equatorial scattering regions at 2 microns.

Findings

Polarization in the circum-nuclear annulus is common among the sample.

Nuclear polarization is dominated by polar scattering in some galaxies, consistent across optical and infrared wavelengths.

Differences in polarization angles suggest distinct scattering regions and possible contamination effects.

Abstract

The nuclei of Seyfert 1 galaxies exhibit a range of optical polarization characteristics that can be understood in terms of two scattering regions producing orthogonal polarizations: an extended polar scattering region (PSR) and a compact equatorial scattering region (ESR), located within the circum-nuclear torus. Here we present NICMOS 2.0 micron imaging polarimetry of 6 "polar scattered" Seyfert 1 (S1) galaxies, in which the PSR dominates the optical polarization. The unresolved nucleus (<0.58 arcsec) is significantly polarized in only three objects, but 5 of the 6 exhibit polarization in a 0.58 to 1.5 arcsec circum-nuclear annulus. In Fairall 51 and ESO 323-G077, the polarization position angle at 2 microns (theta2m) is consistent with the average for the optical spectrum (thetav), implying that the nuclear polarization is dominated by polar scattering at both wavelengths. The same is probably true for NGC 3227. In both NGC 4593 and Mrk 766, there is a large difference between theta2m and thetav off nucleus, where polar scattering is expected to dominate. This may be due to contamination by interstellar polarization in NGC 4593, but there is no clear explanation in the case of the strongly polarized Mrk 766. Lastly, in Mrk 1239, a large change (~ 60deg) in theta2m between the nucleus and the annulus indicates that the unresolved nucleus and its immediate surroundings have different polarization states at 2 microns, which we attribute to the ESR and PSR, respectively. A further implication is that the source of the scattered 2 micron emission in the unresolved nucleus is the accretion disk, rather than torus hot dust emission.