1999ApJ...524..746X

We present an optical spectrum for Mrk 273x, the X-ray source 1'.3 to the northeast of Mrk 273. The new spectrum indicates that the object is at a much higher redshift (0.458) than the value previously reported by us (0.0376). All the detected emission lines show properties of Seyfert 2 galaxies. Mrk 273x has one of the highest X-ray and radio luminosities (L_X~1.1x10⁴⁴ ergs.s^–1, L_1.37GHz~2.0x10⁴⁰ ergs.s^–1) among Seyfert 2 galaxies, yet it has a low neutral hydrogen column density, N_H~4.4x10²⁰ cm^–2. These properties seem difficult to explain in the unified scheme of active galactic nuclei. The spectrum that we previously used to identify the redshift for Mrk 273x turns out to belong to diffuse clumps in the northeast tail/plume about 20" (∼20 kpc in projected distance) from the nuclear region of Mrk 273. A new spectrum for this region was obtained; this spectrum is essentially the same as the previous one. These observations indicate that these clumps have the same redshift as Mrk 273 and are physically associated with the merger. The spectrum is dominated by strong emission from the [O III] doublet, with Seyfert 2-like line ratios, which indicate that these northeast clumps are probably excited by the shock+precursor mechanism during the merging process. This mechanism may be operating in other ultraluminous IRAS galaxies as well.