Lyα imaging of a proto-cluster region at ⟨z⟩=3.09

We present very deep narrowband observations of a volume containing a significant overdensity of galaxies previously discovered in our survey for continuum-selected Lyman-break galaxies (LBGs) at redshifts 2.7 less than or similar to z less than or similar to 3.4. The new observations are used in conjunction with our spectroscopic results on LBGs to compare the effectiveness of continuum and emission-line searches for star-forming galaxies at high redshift, and to attempt to extend the search for members of the structure at (z) = 3.09 to much fainter continuum luminosities. The 8'.7 x 8'.9 field contains a very high surface density of emission-fine candidates, approximately 6 times higher than in published blind narrowband searches to comparable depth and at similar redshift. This level of density enhancement for the (z) = 3.09 structure is consistent with that inferred from the analysis of the spectroscopic Lyman-break galaxy sample in the same region (6.0 +/- 1.2), but extends to continuum luminosities up to 2 mag fainter. We find that only similar to 20%-25% of all galaxies at a given UV continuum luminosity would be flagged as narrowband-excess objects subject to the typical limits W-lambda > 80 Angstrom in the observed frame. The remainder have lines that are too weak (<20 Angstrom rest equivalent width) to make selection by narrowband excess effective. There is no evidence for a significantly higher fraction of large Ly alpha fine equivalent widths at faint continuum luminosities. We have also discovered two extremely bright, large, and diffuse Ly alpha-emitting blobs that are apparently associated with, but not centered on, previously known Lyman-break galaxies at the redshift of the (z) = 3.09 structure. These nebulae have physical extents greater than or similar to 100 h(-1) kpc and Ly alpha line fluxes of similar to 10(-15) ergs s(-1) cm(-2), both factors of similar to 20-40 times larger than the typical line emitters at the same redshifts in the field. In many respects, these blobs resemble the giant Ly alpha nebulae associated with high-redshift radio galaxies, but have <1% of the associated radio continuum flux and no obvious source of UV photons bright enough to excite the nebular emission. While the nature of these blobs remains unclear, it is possible that they are excited by continuum sources that are heavily obscured along our line of sight, or that they are associated with cooling-flow-like phenomena. The effectiveness of narrowband imaging for isolating large, albeit incomplete, samples of high-redshift galaxies over a broad range of continuum luminosity makes the technique particularly well-suited to mapping known or suspected structures at high redshift. By combining the 24 spectroscopic members of the z = 3.09 spike with the narrowband candidates, we are able to produce a sample of 162 objects in a single relatively small field that are tither known or likely members of this large structure. A smoothed surface density map shows three regions exceeding a local overdensity of delta rho/rho = 1 on 1 h(-1) Mpc (comoving) scales; interestingly, one is centered on a z = 3.083 QSO, and another on one of the giant Ly alpha nebulae. There rs thus circumstantial evidence that the Ly alpha nebulae may, like QSOs, be linked to the sites of the largest density enhancements at high redshifts.