Extended Lyα nebulae at z ≃ 2.3: An extremely rare and strongly clustered population?

To obtain an unbiased sample of bright Lyα blobs (LLyα≳ 10⁴³ erg s^-1), we have undertaken a blind, wide-field, narrowband imaging survey in the National Optical Astronomy Observatory Deep Wide Field Survey Boötes field with the Steward Bok 2.3 m telescope. After searching over 4.82 deg² at z = 2.3, we discover four Lyα blobs with LLyα = 1.6-5.3×10⁴³ erg s^-1, isophotal areas of 28-57 □″, and broad Lyα line profiles (Δν = 900-1250 km s^-1). In contrast with the extended Lyα halos associated with high-z radio galaxies, none of our four blobs are radio-loud. The X-ray luminosities and optical spectra of these blobs are diverse. Two blobs (3 and 4) are X-ray detected with LX(2-7 keV) = 2-4 ×10⁴⁴ erg s^-1 and have broad optical emission lines (C IV) characteristic of active galactic nucleus (AGN), implying that 50% of our sample blobs are associated with a strong AGN. The other 50% of the blobs (1 and 2) are not X-ray or optically detected as AGN down to similar limits. The number density of the four blobs is ∼ 3 ×10^-6 h₇₀³ Mpc ^-3, comparable to that of galaxy clusters at similar redshifts and 3×lower than that found in the SSA22 proto-cluster at z = 3.1, even after accounting for the over-density of that region. The two X-ray undetected blobs are separated by only 70″ (550 kpc) and have almost identical redshifts (≲ 360 kpc along the line of sight), suggesting that they are part of the same system. Given the rarity of the blobs and our discovery of a close pair, we speculate that blobs occupy the highest density regions and thus may be precursors of today's rich-cluster galaxies.