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We have explored the nature of the extended emission-line region around thez=0.37 quasar 4C 37.43, using extensive ground-based and HST imaging andspectroscopy. The velocity field of the ionized gas shows gradual gradientswithin components but large jumps between components, with no obvious globalorganization. The HST [O III] image shows radial linear features on the eastside of the QSO that appear to mark the edges of an ionization cone.Concentrating on the bright emission peaks ~4\arcsec$ east of the quasar, wefind through modeling that we require at least two density regimes contributingsignificantly to the observed emission-line spectrum: one with a density of ~2cm^-3, having essentially unity filling factor, and one with a density of ~500cm^-3, having a very small (~10^-5) filling factor. Because the temperatures ofthese two components are similar, they cannot be in pressure equilibrium, andthere is no obvious source of confinement for the dense regions. We estimatethat the dense regions will dissipate on timescales <~10^4 years and thereforeneed to be continuously regenerated, most likely by shocks. Because we knowthat some QSOs, at least, begin their lives in conjunction with merger-drivenmassive starbursts in their host galaxies, an attractive interpretation is thatthe extended emission region comprises gas that has been expelled as a resultof tidal forces during the merger and is now being shocked by the galacticsuperwind from the starburst. This picture is supported by the observeddistribution of the ionized gas, the presence of velocities ranging up to ~700km s^{-1}, and the existence of at least two QSOs having similarly luminous andcomplex extended emission regions that are known to have ultra-luminous IRgalaxy hosts with current or recent starbursts.Comment: 22 pages, incl. 7 figures; to be published in The Astrophysical  Journal, 572 (June 20, 2002 issue

The Extended Emission‐Line Region of 4C 37.43