the first galaxies

z=7.0 Drop-Out Galaxy

This figure presents one of the most important techniques for finding galaxies at very high redshifts. This technique has been called the "Drop Out" Technique, or Lyman Break Technique. It takes advantage of the significant break that occurs in the spectrum of high-redshift galaxies due to absorption by neutral hydrogen. One possible spectrum of a star-forming galaxy at a redshift of 7 is shown in the top panel. The presence of neutral hydrogen has a rather dramatic effect on the spectrum of this galaxy -- creating a rather abrupt drop off in flux blueward of 970 nm. Astronomers often look for sources which show this abrupt drop off in flux by taking images of the sky using many different filters. Each filter has sensitivity at different wavelengths. The sensitivities of several of the more useful filters on HST are shown in the middle panel and has been used in the acquisition of data for several of the deepest HST images ever obtained (e.g., HUDF). These filters (shown from left to right) have central wavelengths of 591 nm, 776 nm, 944 nm, 1119 nm, and 1604 nm, respectively, and frequently known by the names "V", "i", "z", "J", and "H" bands, respectively. The bottom panel shows images of the redshift 7 source from the top panel, as seen through these filters. This source clearly shows up in the two longest wavelength filters "J" and "H," but completely disappears in the three bluest wavelength filters "V," "i," and "z." The presence of such a distinct break is a clear indication that we have found a galaxy at very high redshift which emitted its light at very early times.

Credit:Rychard Bouwens

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