THE DISAPPEARANCE OF Lyα BLOBS: A GALEX SEARCH AT z=0.8

Ly alpha blobs-luminous, spatially extended emission-line nebulae, often lacking bright continuum counterparts-are common in dense environments at high redshift. Until recently, atmospheric absorption and filter technology have limited our knowledge of any similar objects at z <= 2. We use Galaxy Evolution Explorer slitless spectroscopy to search for similar objects in the rich environments of two known cluster and supercluster fields at z = 0.8, where the instrumental sensitivity peaks. The regions around Cl 1054-0321 and Cl 0023+0423 were each observed in slitless-spectrum mode for 10-19 ks, with accompanying direct images of 3-6 ks to assist in recognizing continuum sources. Using several detection techniques, we find no resolved Ly alpha emitters to a flux limit of (1.5-9) x 10(-15) erg cm(-2) s(-1), on size scales of 5-30 arcsec. This corresponds to line luminosities of (0.5-3) x 10(43) erg s(-1) for linear scales 35-200 kpc. Comparison with both blind and targeted surveys at higher redshifts indicates that the population must have evolved in comoving density at least as strongly as (1 + z)(3). These results suggest that the population of Ly alpha blobs is specific to the high-redshift universe.