Intensity Mapping with the 21-cm and Lyman Alpha Lines
  The 21-cm and Lyman Alpha lines are the dominant line-emission spectral features at opposite ends of the spectrum of hydrogen. Each line can be used to create three dimensional intensity maps of large scale structure. The sky brightness at low redshift due to Lyman Alpha emission is estimated to be 0.4 Jy/Steradian, which is brighter than the zodiacal light foreground. 
J. B. Peterson and Enrique Suarez
astro-ph.CO