A ug 2 00 2 Evidence for Extremely High Dust Polarization Efficiency in NGC 3184

Recent studies have found the Type II-plateau supernova (SN) 1999gi to be highly polarized (pmax = 5.8%, where pmax is the highest degree of polarization measured in the optical bandpass; Leonard & Filippenko 2001) and minimally reddened (E[B−V ] = 0.21±0.09 mag; Leonard et al. 2002). From multiple lines of evidence, including the convincing fit of a “Serkowski” interstellar polarization (ISP) curve to the continuum polarization shape, we conclude that the bulk of the observed polarization is likely due to dust along the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new spectropolarimetric observations of four distant Galactic stars close to the l-o-s to SN 1999gi (two are within 0.02), and find that all are null to within 0.2%, effectively eliminating Galactic dust as the cause of the high polarization. The high ISP coupled with the low reddening implies an extraordinarily high polarization efficiency for the dust along this lo-s in NGC 3184: ISP/E[B−V ] = 31 −9 % mag . This is inconsistent with the empirical Galactic limit (ISP/E[B−V ] < 9% mag), and represents the highest polarization efficiency yet confirmed for a single sight line in either the Milky Way or an external galaxy. Subject headings: galaxies: individual (NGC 3184) — galaxies: ISM — supernovae: individual (SN 1999gi) — techniques: polarimetric