SPACE SCIENCECounting All The Light In Deep Space
A team of astronomers from the National Astronomical Observatory of Japan, the University of Tokyo, and Kyoto University has completed a careful analysis of a very deep image taken at near-infrared wavelengths.
The "Subaru Deep Field" (SDF) was observed soon after the first light of Subaru Telescope, and subsequent study has revealed that the galaxies detected in the image account for more than 90% of all the galactic light in the Universe.
This is a higher fraction than that of the optical Hubble Deep Field images, and the SDF is therefore the deepest image of the Universe ever taken.
The SDF was imaged at a wavelength of 2.1 microns (Figure 1), and detected some of the faintest galaxies ever observed, down to a magnitude of 24.5.
The team used their models of galaxy evolution to predict how many faint galaxies would be missed in deep images, and discovered that the galaxies they detected in the SDF image accounted for more than 90% of the total near-infrared light from all the galaxies in the Universe along this line of sight (Figure 2). Subaru is now seeing almost to the edge of the Universe and very little extra light from fainter galaxies would be seen using more sensitive observations.
Although the Subaru observations can account for almost all of the light emitted by galaxies in the Universe, measurements from satellites have revealed that the total amount of extragalactic background light (Figure 3) is 3 times larger.
It was previously believed that all the near-infrared extragalactic light came from discrete galaxies (Figure 4); but these latest observations reveal that there is a great deal of light unaccounted for, which cannot be due to normal galaxies. Resolving this discrepancy will be an important challenge for future astronomy.
These results are published in the April 1 issue of the Astrophysical Journal Letters.
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