Hubble Space Telescopeand Ground‐based Observations of I Zw 1 and MRK 486 and the Variability of Polarization in Radio‐quiet Active Galactic Nuclei

Abstract

Hubble Space Telescope UV spectropolarimetry and new ground-based optical observations of the Seyfert 1/QSO I Zw 1 and highly polarized Seyfert 1 Mrk 486 are presented. I Zw 1 is found to be a polarimetric variable. Our data show that the polarization position angle has changed by ~50? compared to measurements obtained 10-15 yr ago. There is evidence that the degree of optical linear polarization may be variable as well, but for ? 4000 ?, P remained less than 1%. Optical/UV synchrotron radiation can be ruled out as the cause of the observed polarized flux and variations because the permitted lines are polarized. The optical polarization increases dramatically to the blue. In contrast, the UV polarization is roughly constant with wavelength at ~1% and falls far below the polarization level expected from ground-based observations. The overall wavelength dependence of the optical and UV polarization can be reconciled if the effect of starlight from the luminous host galaxy is taken into account. The strong increase in the optical polarization with decreasing wavelength for Mrk 486 continues into the UV and reaches nearly 8% by 2000 ?. This suggests that dust scattering may be the dominant polarizing mechanism in this object. Spectropolarimetry of H? and H? reveals complex polarization structure across the lines (mostly in the polarization position angle), leading to the conclusion that at least some of the scattering material is located within the broad-line region (BLR). These results are consistent with the BLR being composed of two distinct line-emitting regions: (1) a region of width ~1500 km s-1- (FWHM) polarized at P.A. 150?-160?, and (2) a region of width ~3000 km s-1- polarized at P.A. 100?-110?. Identification of these distinct regions in the BLR by their differing polarizations demonstrates the existence of an intermediate-line region in a low-luminosity AGN. Associated C IV absorption is detected in the UV spectrum of Mrk 486, in keeping with the general trend that high polarization in radio-quiet AGNs is somehow linked to the presence of absorbing gas. The object also shows evidence for variations in polarization. Observations acquired at two separate telescopes during 1994-1995 reveal a decrease of ~10? in polarization P.A. compared with earlier measurements. The polarization variability of these two objects, along with the deduced proximity of the scattering region to the nucleus of Mrk 486, argues that the structure of the scattering and/or emission-line regions in at least some radio-quiet AGNs evolve on relatively short time scales (a few years or less).