Strong field-to-field variation of Lyα nebulae populations at z ≃ 2.3

Understanding the nature of distant Lyα nebulae, aka "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 (CDFN), and two COSMOS subfields, for a total survey area of 1.2 deg ². We discover 25 blobs at z = 2.3 with Lyα luminosities of L_Lyα= (0.7-8) x 10⁴³ erg s^-1 and isophotal areas of A_iso = 10-60 □′′. The transition from compact Lyα emitters (LAEs; A^iso ∼ a few □′′) to extended Lyα blobs (A_iso > 10 □′′) 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 _{ly α} ≳ 1.5 x 10⁴³ erg s^-1 and A _iso > 16 □′′ lie only in the CDFS. These large, bright blobs have a field-to-field variance of σ_υ ≳ 1.5 (150%) about their number density n ∼1.0^+1.8_0.6 x 10^-5 Mpc^-3. This variance is large, significantly higher than that of unresolved LAEs (σ_υ ∼ 0.3 or 30%), and can adversely affect comparisons of blob number densities and luminosity functions (LFs) among different surveys. Our deep, blind survey allows us to construct a reliable blob LF We compare the statistics of our blobs with dark matter halos in a 1 h^-1 Gpc cosmological N-body simulation. At z = 2.3, n implies that each bright, large blob could occupy a halo of M _halo ≳10¹³ M_⊙ 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 M_⊙ halos typical of galaxy clusters today. On larger scales of ∼10 comoving Mpc, blobs cluster where compact LAEs cluster, indicating that blobs lie in coherent, highly overdense structures.