and Cosmology

9. The Universe at High Redshift
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to their old stellar populations (manifested in clusters of
galaxies in form of the red cluster sequence), so that the
redshift of this type of galaxy can be estimated very accurately
from multicolor information. However, this is
only the case if the 4000-Å break is located in between
two of the applied filters. For z 1 this is no longer the
case in the optical range of the spectrum. Other types
of galaxies show larger variations in their spectral energy
distribution, depending, e.g., on the star-formation
history.
Photometric redshifts are particularly useful for statistical
purposes, for instance in situations in which the
exact redshift of each individual galaxy in a sample is
of little relevance. However, by using a sufficient number
of filters a redshift accuracy of Δz ∼ 0.03(1 + z) is
achievable, as demonstrated in Fig. 9.8 by a comparison
of photometric redshifts with redshifts determined
spectroscopically for galaxies in the field of the
HDF-North.
9.1.3 Hubble Deep Field(s)
The HDF-N. In 1995, an unprecedented observing program
was conducted with the HST. A deep image in four
filters (U 300 ,B 450 ,V 606 ,andI 814 ) was observed with
the Wide Field/Planetary Camera 2 (WFPC2) on-board
HST, covering a field of ∼ 5.3 arcmin 2 , with a total
exposure time of about 10 days. This resulted in the
deepest sky image of that time, displayed in Fig. 9.9.
The observed field was carefully selected such that it
did not contain any bright sources. Furthermore, the position
of the field was chosen such that the HST was
able to continually point into this direction, a criterion
excluding all but two relatively small regions on the
sky, due to the low HST orbit around the Earth. Another
special feature of this program was that the data
became public immediately after reduction, less than
a month after the final exposures had been taken. Astronomers
worldwide immediately had the opportunity
Fig. 9.8. Photometric redshift versus the spectroscopic redshift
for galaxies in the HDF-North. Photometric data in four
optical and two NIR bands have been used here. We see how
accurate photometric redshifts can be – their quality depends
on the photometric accuracy in the individual filters, the number
of filters used, the redshift and the type of the galaxy, and
also on details of the applied analysis method
Fig. 9.9. The Hubble Deep Field (North), at its time by far the
deepest image of the sky. In December 1995, the HST was
pointed to this field for about 10 days, and observations were
conducted in four different filters. The raw and reduced data
were made publicly available worldwide as early as Jan. 15,
1996. In this image, which spans about 5 square arcminutes,
about 3000 galaxies are visible, extending over a wide range
in redshift