The overdense environment of a large Lyα nebula at z ≈ 2.7

Large nebulae (>= 50 kpc) emitting strongly in Ly alpha (also known as Ly alpha blobs) are likely signposts of ongoing massive galaxy formation. The relative rarity of these sources and their discovery in well-studied galaxy over-densities suggest that they may be associated with regions of high galaxy density. One of the largest Lya nebulae, discovered at a redshift of z approximate to 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 Ly alpha-emitting galaxies (LAEs) within a 30 ' x 26 ' 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 20 x 50 h(70)(-1) comoving Mpc in size, showing a factor of similar to 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 less than or similar to 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.