and Cosmology

9. The Universe at High Redshift
380
we refer to the sum of the radiation of all galaxies and
AGNs per solid angle. The interpretation of such a background
radiation depends on the sensitivity and the
angular resolution of the telescopes used. Imagine, for
instance, observing the sky with an optical camera that
has an angular resolution of only one arcminute. A relatively
isotropic radiation would then be visible at most
positions in the sky, featuring only some very bright or
very large sources. On improving the angular resolution,
more and more individual sources would become visible
– culminating in the observations of the Ultra-Deep
Fields – and the background could then be identified
as the sum of the emission of individual sources. In
analogy to this thought-experiment, one may wonder
whether the CXB or the CIB can likewise be understood
as a superposition of radiation from discrete sources.
9.3.1 The IR Background
Observations of background radiation in the infrared are
very difficult to accomplish. First, it is problematic to
measure absolute fluxes due to the thermal emission of
the detector. In addition, the emission by interplanetary
dust (and by the interstellar medium in our Milky Way)
is much more intense than the infrared flux from extragalactic
sources. For these reasons, the absolute level of
the infrared background has been determined only with
relatively large uncertainties, as displayed in Fig. 9.25.
The ISO satellite was able to resolve about 10% of
the CIB at λ = 175 μm into discrete sources. Also in
the sub-mm range (at about 850 μm) almost all of the
CIB seems to originate from discrete sources which
consist mainly of dust-rich star-formation regions (see
Sect. 9.2.3, where the source population in the sub-mm
domain was discussed).
In any case, no indication has yet been found that the
origin of the CIB is different from the emission by a population
of discrete sources, in particular of high-redshift
starburst galaxies. Further resolving the background radiation
into discrete sources will become possible by
future FIR satellites such as, for instance, Herschel.
isotropic radiation component, the CXB. Its spectrum
is a very hard (i.e., flat) power law, cut off at an energy
above ∼ 40 keV, which can roughly be described by
I ν ∝ E −0.3 exp
(− E )
, (9.3)
E 0
with E 0 ∼ 40 keV. Initially, the origin of this radiation
was unknown, since its spectral shape was different
from the spectra of sources that were known at that time.
For example, it was not possible to obtain this spectrum
by a superposition of the spectra of know AGNs.
ROSAT, with its substantially improved angular resolution
compared to earlier satellites (such as the Einstein
observatory), conducted source counts at much lower
fluxes, based on some very deep images. From this, it
was shown that at least 80% of the CXB in the energy
range between 0.5 keV and 2 keV is emitted by discrete
sources, of which the majority are AGNs. Hence
it is natural to assume that the total CXB at these low
X-ray energies originates from discrete sources, and
observations by XMM-Newton seem to confirm this.
However, the X-ray spectrum of normal AGNs is
different from (9.3), namely it is considerably steeper
(about S ν ∝ ν −0.7 ). Therefore, if these AGNs contribute
the major part of the CXB at low energies, the CXB
at higher energies cannot possibly be produced by
the same AGNs. Subtracting the spectral energy of
9.3.2 The X-Ray Background
In the 1970s, the first X-ray satellites discovered not
only a number of extragalactic X-ray sources (such as
AGNs and clusters of galaxies), but also an apparently
Fig. 9.25. Measurement of, and limits to, the CIB. Squares
denote lower limits derived from the integration of observed
source counts, diamonds are upper limits from flux measurements,
and other symbols show absolute flux measurements.
The shaded yellow range indicates the current observational
limits to the CIB