A Cluster or Filament of Galaxies at Redshift z = 2.5?

We report the discovery of 56 new Lyα-emitting candidates (LECs) at redshift z ≈ 2.5 in a field of 8' × 14' around two previously known weak radio QSOs and a cosmic microwave background decrement (CMBD) that is plausibly due to the Sunyaev-Zeldovich effect. Broadband and medium-band imaging at the redshifted Lyα wavelength have allowed us to identify the LECs at the redshift of the QSOs. Three of the brightest LECs have been confirmed spectroscopically, with redshifts between z = 2.501 and z = 2.557; one of them is another QSO. Excluding the third QSO, the four spectroscopically confirmed objects form a 3' filament with a rest-frame velocity dispersion of 1000 km s-1 lying adjacent to the CMBD, and there is a significant concentration of LECs at the northwest end of the filament around the brightest QSO. If confirmed, a velocity dispersion ~1000 km s-1 on a proper scale of ~1 Mpc at redshift z = 2.5 would, in and of itself, constrain the cosmological model to low Ω.