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. 
M. K. M. Prescott (1), N. Kashikawa (2), A. Dey (3), Y. Matsuda (4)   ((1) University of Arizona, (2) NAOJ, (3) NOAO, (4) Ky...