Simulated NGST+IFIRS Data Cubes

Distant Star Forming Galaxies

NGST will also be able to detect the very first luminous objects in the universe. The Hubble Space Telescope can find galaxies with redshifts as big as 5, but a larger telescope will be needed to find how these objects were assembled. Galaxies probably grow by the gravitation clustering and merging of sub-galactic fragments. A typical fragment contains hot young stars which ionize the gas from which they form. Consequently, the light consists of continuum starlight and nebular emission lines. These emission lines are very distintinctive in the cube that simulates the spectrum of a young star forming clump.

Prominent emission lines of known wavelengths are ideal for establishing the redshift, while features such as the hydrogen recombination lines can be used to estimate the star formation rate. In the annotated data cube emission lines familar to optical astronomers, but redshifted to infrared wavelengths, are detected. The ultraviolet Lyman alpha line at 1216 A can be seen in the highest redshift objects. The strength of the Lyman alpha line is very uncertain because it is strongly absorbed by interstellar dust. But for distant objects, the location of the Lyman break at 912 A can also be used to determine the redshift. It is very exciting that the Lyman break is clearly recorded in these spectra because this shows that IFIRS has the potential to discover the first sites of star formation in the early universe.

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