Lyman Alpha Imaging of a Proto-Cluster Region at <z>=3.09

We present very deep imaging observations, through a narrow-band filter tuned to Lyman alpha at <z = 3.09>, of a volume containing a significant over-density of galaxies at this redshift previously discovered in our survey for continuum-selected Lyman break galaxies (LBGs). 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 extend the search for members of the structure at <z>= 3.09 to much fainter continuum luminosities. We find that only 20-25 percent of all galaxies at a given UV continuum luminosity would be flagged as narrow-band excess objects subject to the typical limits W_Lya > 80 Angstroms in the observed frame. The density enhancement of strong Lyman alpha emitters in this field is consistent with that inferred from the analysis of the spectroscopic Lyman break galaxy sample in the same region (6+/-1), but extends to continuum luminosities up to 2 magnitudes fainter. There is no evidence for a significantly higher fraction of large Lyman alpha line equivalent widths at faint continuum luminosities. By combining the 24 spectroscopic members of the z=3.09 ``spike'' with the narrow-band candidates, we are able to produce a sample of 162 objects which are either known or likely members of this large structure. We have also discovered two extremely large and diffuse Lyman alpha emitting ``blobs'', which have physical extents >100/h kpc and Lyman alpha line fluxes 20-40 times larger than the typical line emitters at the same redshifts in the field. The possible nature of the ``blobs'' is discussed. (abstract abridged)