THE OVERDENSE ENVIRONMENT OF A LARGE Lyα NEBULA AT z≈2.7

Large nebulae (&50 kpc) emitting strongly in Lyα (also known as Lyα “blobs”) are likely signposts of ongoing massive galaxy formation. The relative rarity of these sources and their discovery in wellstudied galaxy overdensities suggest that they may be associated with regions of high galaxy density. One of the largest Lyα 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 Lyα-emitting galaxies (LAEs) within a 30×26 field of view around this Lyα nebula. This is the first study of the environment of a Lyα nebula found without a priori knowledge of its surroundings. We find that the nebula is located in an overdense region, at least 20×50 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 Lyα nebulae and overdense regions of the Universe. Subject headings: galaxies: formation — galaxies: high-redshift — large-scale structure of universe