Ly alpha imaging of a proto-cluster region at < z >=3.09

We present very deep narrowband observations of a volume containing a signi ficant overdensity of galaxies previously discovered in our survey for cont inuum-selected Lyman-break galaxies (LBGs) at redshifts 2.7 less than or si milar to z less than or similar to 3.4. The new observations are used in co njunction with our spectroscopic results on LBGs to compare the effectivene ss of continuum and emission-line searches for star-forming galaxies at hig h redshift, and to attempt to extend the search for members of the structur e at (z) = 3.09 to much fainter continuum luminosities. The 8'.7 x 8'.9 fie ld contains a very high surface density of emission-fine candidates, approx imately 6 times higher than in published blind narrowband searches to compa rable 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 t hat only similar to 20%-25% of all galaxies at a given UV continuum luminos ity would be flagged as narrowband-excess objects subject to the typical li mits W-lambda > 80 Angstrom in the observed frame. The remainder have lines that are too weak (<20 Angstrom rest equivalent width) to make selection b y narrowband excess effective. There is no evidence for a significantly hig her fraction of large Ly alpha fine equivalent widths at faint continuum lu minosities. We have also discovered two extremely bright, large, and diffus e Ly alpha-emitting "blobs" that are apparently associated with, but not ce ntered on, previously known Lyman-break galaxies at the redshift of the (z) = 3.09 structure. These nebulae have physical extents greater than or simi lar 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 b lobs resemble the giant Ly alpha nebulae associated with high-redshift radi o galaxies, but have <1% of the associated radio continuum flux and no obvi ous source of UV photons bright enough to excite the nebular emission. Whil e the nature of these blobs remains unclear, it is possible that they are e xcited by continuum sources that are heavily obscured along our line of sig ht, or that they are associated with cooling-flow-like phenomena. The effec tiveness of narrowband imaging for isolating large, albeit incomplete, samp les of high-redshift galaxies over a broad range of continuum luminosity ma kes the technique particularly well-suited to "mapping" known or suspected structures at high redshift. By combining the 24 spectroscopic members of t he 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.