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 ≲ z ≲ 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 87 × 89 field contains a very high surface density of emission-line 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 ~20%-25% of all galaxies at a given UV continuum luminosity would be flagged as narrowband-excess objects subject to the typical limits W_λ > 80 Å in the observed frame. The remainder have lines that are too weak (<20 Å rest equivalent width) to make selection by narrowband excess effective. There is no evidence for a significantly higher fraction of large Lyα line equivalent widths at faint continuum luminosities. We have also discovered two extremely bright, large, and diffuse Lyα-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 ≳100 h^-1 kpc and Lyα line fluxes of ~10^-15 ergs s^-1 cm^-2, both factors of ~20-40 times larger than the typical line emitters at the same redshifts in the field. In many respects, these blobs resemble the giant Lyα 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 either known or likely members of this large structure. A smoothed surface density map shows three regions exceeding a local overdensity of δρ/ρ = 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 Lyα nebulae. There is thus circumstantial evidence that the Lyα nebulae may, like QSOs, be linked to the sites of the largest density enhancements at high redshifts.