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.
