The disappearance of Lyman alpha blobs: a GALEX search at z=0.8
  Lyman 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 GALEX 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 ksec, with accompanying direct
images of 3-6 ksec to assist in recognizing continuum sources. Using several
detection techniques, we find no resolved Lyman alpha emitters to a flux limit
of(1.5-9) x 10^{-15} erg/ cm^2 s, on size scales of 5-30 arcseconds. This
corresponds to line luminosities of (0.5-3) x 10^43 erg/s 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 Lyman alpha
blobs is specific to the the high-redshift Universe.
