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.


[Figure 1: http://www.subaru.naoj.org/Science/press_release/9909/SDF.html]
The “Subaru Deep Field” studied here. Almost all objects in this image are
galaxies. About 350 galaxies are detected in this field.

[Figure 2: http://www.subaru.naoj.org/Science/press_release/2001/04/Fig2_e.gif]
Relation between brightness of galaxy and the contribution to extragalactic
background light in near infrared. The study was extended to faint galaxies
by the present observation and analysis.

[Figure 3: http://www.subaru.naoj.org/Science/press_release/2001/04/Fig3_e.gif]
A schematic view of extragalactic background

[Figure 4: http://www.subaru.naoj.org/Science/press_release/2001/04/Fig4_e.gif]
The types of objects contributing to extragalactic background lights.