Number density of Lyα blobs at different redshifts. Open and filled squares represent the number density from our narrowband imaging survey at z = 2.3 for the blobs with and without X-ray detections, respectively. We also show two number density estimates for the blobs in Matsuda et al. (2004), with or without correction for the over-density of the SSA22 proto-cluster region. We incorporate the same luminosity–size limits on Matsuda et al.'s result as obtained in this paper for a proper comparison. For blobs at higher redshifts, we show only upper (3 < z < 5; Saito et al. 2006) and lower limits (z = 6.595; Ouchi et al. 2008) from the Lyα blob searches in the SXDF.