Spectropolarimetry of 'narrow-line' Seyfert 1 galaxies

Abstract

A sample of 18 "narrow-line" Seyfert 1 galaxies is studied using spectropolarimetry. Seven of these objects show polarizations higher than those expected from polarization by the interstellar medium in our own Galaxy. Of these, three (Mrk 766 = NGC 4253, Mrk 1239, and IRAS 1509-211) have polarizations high enough to allow detailed spectropolarimetric work on their emission-line profiles. Three more (Mrk 507, Mrk 957, and Mrk 1126 = NGC 7450) allow polarization analysis of their continua. In general, continuum polarizations rising to the blue and/or the lack of significant broadening of the polarized emission lines points to dust reflection as the primary polarigenic mechanism. There is some evidence that the dust is associated with the [O III]-emitting region, normally called the NLR. In the six cases in which the line polarizations can be compared it is found that Hβ is more highly polarized than the [O III] λλ4959, 5007 lines, demonstrating that these lines come from spatially distinct regions within the nucleus. The spectropolarimetry provides simultaneous spectrophotometry, which is compared to similar published data on other Seyfert 1's. These are used to study some interesting relationships, such as the correlation of FWHM of Hβ with the Hβ equivalent width. It is also found that X-ray-selected Seyfert 1's show significantly smaller Hβ equivalent widths than the sample of optically selected narrow-line 1's and Seyfert 1's.