A Filamentary Structure of Massive Star-forming Galaxies Associated with an X-Ray-absorbed QSO at z=1.8

The genesis of spheroids is central to our understanding of galaxy formation: they are relatively simple systems, containing about half the stellar mass of the universe. A major subset of spheroids, massive elliptical galaxies, are preferentially found in clusters where they exhibit old coeval stellar populations suggesting that they formed synchronously at early epochs. Here we report Submillimeter Common-User Bolometric Array submillimeter imaging of a region around a z=1.8 X-ray-selected QSO. The image reveals a remarkable ~400 kpc long chain of galaxies, each with an obscured star formation rate sufficiently high to build a massive spheroid in less than 1 Gyr. The large overdensity of these galaxies relative to expectations for a random field implies that they probably reside in a structure associated with the QSO. We suggest that this star formation is associated with galaxy mergers or encounters within the filament, such as those predicted by the popular hierarchical model of galaxy formation. Our observations suggest that strong absorption in the X-ray spectra of QSOs at high redshifts may result from a veil of gas thrown up by a merger or merger-induced activity, rather than an orientation-dependent obscuring torus. It is argued that these systems are the precursors of elliptical galaxies found today in the core regions of all rich galaxy clusters.