Strong Field-to-Field Variation of Lyman alpha Nebulae Populations at z~2.3

Abstract: Understanding the nature of distant Ly-alpha nebulae ("blobs") and connecting them to their present-day descendants requires constraining their number density, clustering, and large-scale environment. To measure these basic quantities, we conduct a deep narrowband imaging survey in four different fields, Chandra Deep Field South (CDFS), Chandra Deep Field North, and two COSMOS subfields, for a total survey area of 1.2deg^2. We discover 25 blobs at z=2.3 with Ly-alpha luminosities of 0.7-8x10^43 erg/s and isophotal areas of Aiso = 10-60 arcsec^2. The transition from compact Ly-alpha emitters (Aiso ~ a few arcsec^2) to extended blobs (Aiso > 10 arcsec^2) is continuous, suggesting a single family perhaps governed by similar emission mechanisms. Surprisingly, most blobs (16/25) are in one survey field, the CDFS. The six brightest, largest blobs with L > 1.5x10^43 erg/s and Aiso > 16 arcsec^2 lie only in the CDFS. These large, bright blobs have a field-to-field variance of sigma_v >~ 1.5 (150%) about their number density n ~ 1.0x10^-5 Mpc^-3. This variance is large, significantly higher than that of unresolved LAEs (sigma_v ~ 0.3 or 30%), and can adversely affect comparisons of blob number densities and luminosity functions among different surveys. We compare the statistics of our blobs with dark matter halos in a 1 Gpc/h cosmological N-body simulation. At z=2.3, the number density (n) implies that each bright, large blob could occupy a halo of M_halo > 10^13 Msun if most halos have detectable blobs. The predicted variance in n is consistent with that observed and corresponds to a bias of ~7. Blob halos lie at the high end of the halo mass distribution at z=2.3 and are likely to evolve into the ~10^14 Msun halos typical of galaxy clusters today. On larger scales of ~10 co-moving Mpc, blobs cluster where compact LAEs do, indicating that blobs lie in coherent, highly overdense structures.