Hi I have a question which is about the lyman break drop-out technique for identifying high red shift galaxies. I am basically trying to understand how the technique works. My current understanding is that the drop-out technique makes use of the fact that very high red shift galaxies illuminate intervening neutral clouds of gas which produce absorption lines in the UV spectrum. There is strong absorption when the observed red shifted wave length is shorter the Lyman α line, and there is even stronger absorption when the wave length is shorter than the red shifted observed Lyman limit of 91.2 nm. Photons with shorter wavelengths than this are completely absorbed by the intervening hydrogen. The galaxy is observed used two broad band filters, such as B and U for a z = 4 galaxy. The galaxy is visible in the B filter but not in the U filter – hence the term 'drop-out' technique. The Lyman α line is used to gain greater precision in the measurements. Is this correct? Or have I go this all upside down. To be honest I have not yet fully understood what the technique is being used for. Is it being used to find the red shift of these very distant galaxies. Why do they use broadband filters? Thanks in advance for any replies.