ar X iv : a st ro - p h / 96 05 11 4 v 1 1 8 M ay 1 99 6 IRAS FSC 15307 +

We present the highest spatial and spectral resolution near-infrared data to date of the ∼ 10 h−2 L⊙ IRAS source FSC 15307+3252 at z = 0.93, apparently the most luminous galaxy in the known Universe. Deep K-band (2.2 μm) images taken in 0. ′′4 seeing at the W. M. Keck Telescope reveal three components: (A) a bright elliptical source with a compact nucleus, (B) a resolved circular companion separated from component A by 2. ′′0 (8h−1 kpc for q0 = 0.5), and (C) a faint irregular component 1. ′′7 from A. The surface brightness profile of F15307-A is well-characterized by a de Vaucouleurs r1/4 law with re = 1. ′′4 ± 0. ′′2 (6h−1 kpc), a size comparable to local giant ellipticals. The nucleus of component A is stellar in appearance with extended structure, possibly a second nucleus ∼ 0. ′′5 away. Our 1.1–1.4 μm spectrum of the F15307 system with a resolution of 330 km s−1 shows strong emission lines of [O I] λλ6300, 6364; blended Hα + [N II] λλ6548, 6583; and [S II] λλ6716, 6731. The ∼ 900 km s−1 width of the forbidden lines and the relative strengths of the emission lines are characteristic of Seyfert 2 galaxies. The Hα line also has a broad (1900 km s−1) component. In light of the recent discovery that FSC 10214+4724, previously the most luminous known galaxy, is a gravitationally-lensed system, we explore the possibility that F15307 is also lensed. Quantitative arguments are inconclusive, but aspects of Based in part on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. Alfred P. Sloan Fellow.