The Overdense Environment of a Large Lyman-alpha Nebula at z~2.7

Large nebulae (>50 kpc) emitting strongly in Lya (also known as Lya ``blobs'') are likely signposts of ongoing massive galaxy formation. The relative rarity of these sources and their discovery in well-studied galaxy overdensities suggest that they may be associated with regions of high galaxy density. One of the largest Lya nebulae, discovered at a redshift of z~2.7 via its strong mid-infrared emission, provides an unbiased test of this association. We have carried out a deep intermediate-band imaging survey for Lya-emitting galaxies (LAEs) within a 30'x26' field of view around this Lya nebula. This is the first study of the environment of a Lya nebula found without a priori knowledge of its surroundings. We find that the nebula is located in an overdense region, at least 20x50 1/h_70 comoving Mpc in size, showing a factor of ~3 LAE number density enhancement relative to the edge of the field. Given the predicted number of such overdensities, we rule out the possibility of a chance coincidence at the <1% level. This study, in conjunction with previous work, provides strong confirmation of the association between the largest Lya nebulae and overdense regions of the Universe.