Lyα Imaging of a Proto–Cluster Region at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \…

We present very deep imaging observations, through a narrow-band filter tunedto Lyman alpha at , of a volume containing a significant over-densityof galaxies at this redshift previously discovered in our survey forcontinuum-selected Lyman break galaxies (LBGs). The new observations are usedin conjunction with our spectroscopic results on LBGs to compare theeffectiveness of continuum and emission line searches for star forming galaxiesat high redshift, and to extend the search for members of the structure at =3.09 to much fainter continuum luminosities. We find that only 20-25 percent ofall galaxies at a given UV continuum luminosity would be flagged as narrow-bandexcess objects subject to the typical limits W_Lya > 80 Angstroms in theobserved frame. The density enhancement of strong Lyman alpha emitters in thisfield is consistent with that inferred from the analysis of the spectroscopicLyman break galaxy sample in the same region (6+/-1), but extends to continuumluminosities up to 2 magnitudes fainter. There is no evidence for asignificantly higher fraction of large Lyman alpha line equivalent widths atfaint continuum luminosities. By combining the 24 spectroscopic members of thez=3.09 ``spike'' with the narrow-band candidates, we are able to produce asample of 162 objects which are either known or likely members of this largestructure. We have also discovered two extremely large and diffuse Lyman alphaemitting ``blobs'', which have physical extents >100/h kpc and Lyman alpha linefluxes 20-40 times larger than the typical line emitters at the same redshiftsin the field. The possible nature of the ``blobs'' is discussed. (abstractabridged)Comment: To appear in the ApJ, 31 pages, aaste