Ultraviolet Spectropolarimetry of MRK 231

Details Ultraviolet Spectropolarimetry of MRK 231 Ultraviolet Spectropolarimetry of MRK 231

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...184.6405s&link_type=abstract

American Astronomical Society, 184th AAS Meeting, #64.05; Bulletin of the American Astronomical Society, Vol. 26, p.967

Astronomy and Astrophysics

Astronomy

Scientific paper

We present Hubble Space Telescope observations of the peculiar Seyfert 1 galaxy Mrk 231. Ultraviolet (1600--3300 Angstroms ) linear spectropolarimetric measurements of the object's nuclear region were obtained using the Faint Object Spectrograph. New ground-based, high signal-to-noise ratio optical spectropolarimetry is also presented. Both the degree and position angle of the UV continuum polarization redward of the Mg II emission line are comparable to the values observed at the shortest wavelengths accessible to ground-based instruments (P ~ 15%; theta ~ 90(deg) ). However, from the peak of the redshifted Mg II emission line through its P-Cygni-like absorption feature, the polarization is depressed to half that level (similar to the decrease in P seen in the blue wing of Hα ). By ~ 2700 Angstroms (observed frame) the polarization recovers to about 17% but then falls rapidly at shorter wavelengths. In fact, for lambda < 1800 Angstroms the object is essentially unpolarized. The complex wavelength dependence of the UV polarization is evidence that light from a young stellar population (presumably located around the active nucleus) dilutes the polarization from the highly reddened AGN. Unreddened starlight that dominates the flux at wavelengths shorter than about 2400 Angstroms also explains the abrupt change in the spectral slope seen in this part of the spectrum. Decomposition of the AGN and stellar components to the total flux indicates that the intrinsic polarization of the AGN light exceeds 20% in the UV. In addition, evidence from optical polarimetry that there is more than one scattering cloud producing the observed polarization in Mrk 231 is supported by the fact that theta rotates from 90(deg) to 130(deg) between 3300 and 1800 Angstroms . This research is supported by NASA grant NAG 5--1630 and NSF grant AST 91--14087.

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