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\subsection{The Soft X-ray Background: The Window to the Early Universe}

{\bf{
{Dr. J\"urgen Kerp and Dr. Peter M. W. Kalberla}\\
{Radioastronomisches Institut}\\
{Universit\"at Bonn}\\
{Auf dem H\"ugel 71}\\
{D-53121 BONN}\\
{Telefon: 0228/73 3667}\\
{Fax: 0228/73 3672}\\
{Email: jkerp@astro.uni-bonn.de and kalb@astro.uni-bonn.de}\\}}


\subsubsection{Motivation}
The detailed study of the formation and evolution of the early Universe
becomes feasible with observational facilities in the near future.

The formation of the first star--forming galaxies ($z \sim 20$) will become
observable in the infrared and sub-millimetre domain with {\it FIRST\/},
{\it NGST\/} and {\it ALMA\/}.
These observations will trace mainly the {\bf cold} and the weakly ionised 
phase of the baryonic matter.
This gaseous phases mark the first star--forming 
regions and the dense small--scale structures in the early Universe objects.

{\it XEUS\/} will open the X-ray window to the early Universe, while 
{\it XMM--Newton\/} already today allows to obtain an estimate of the physical
conditions of the extremely luminous high--redshift objects.
In the X-ray domain it is possible to study the {\bf hot} phase of the baryonic
matter. This hot phase settles in the potential well, hence it is {\bf the}
tracer for the large scale galactic dark matter distribution in the early
Universe. 
Here, especially the clusters of galaxies at high--redshift and active galactic
nuclei (AGN) are of interest.
The goal of future X-ray missions is to perform X-ray observations of 
high--redshift objects which will allow to study the formation of the first
gravitationally bound systems dominated by the dark matter.\\
\noindent{\bf Problems :}\\
Because of the high--redshift, most of the diagnostic X-ray emission lines are
moved into the so--called soft X-ray ($E < 1$ keV) energy range.
Here, the confusion with X-ray plasma emission belonging to the Milky Way and
its Halo as well as the strong photoelectric absorption by the galactic
interstellar medium become a severe problem.
To quantitatively study the objects of the early Universe, one has
to investigate the distribution of the Milky Way X-ray emission $E < 1$ keV
and the distribution of its X-ray attenuating interstellar matter
(absorption $\propto\,e^{-N_{\rm HI} \times~\sigma}$).
This work is necessary to build up a solid basis for further studies of the
high--redshift Universe.\\
\noindent{\bf Solutions :}\\
The large scale distribution of the soft X-ray emission of the Milky Way is
covered in the unique {\it ROSAT\/} all-sky survey.
Analysing the {\it ROSAT\/} data supplemented with {\it XMM--Newton\/} data,
it is feasible to determine the temperature and emissivity of the soft
galactic X-ray emission towards the high galactic latitude sky, where the next
generation X-ray missions will search for early Universe objects.

Initiating and performing high angular resolution all--sky surveys in the
H{\sc i} 21-cm line transition is the most important step to identify 1) the
most transparent windows to the early Universe and 2) to understand the
soft X-ray radiation transport through the X-ray absorbing interstellar
medium.
If we understand the soft X-ray radiation transfer, we can observe the
early Universe across the entire high galactic latitude sky.
This will overcome the todays limitation to ``pencil--beam'' like fields of
view to the early Universe objects.
These data have to be supplemented by an all--sky H$\alpha$ survey (Reynolds
et al. 1997), which 
provides the distribution of ionised atomic hydrogen.
Because in the soft X-ray energy range hydrogen and helium are the dominant
photoelectric absorbing atoms, the H{\sc i} and H$\alpha$ data build up a
complete data set.\\
\noindent{\bf The project :}\\
Within this project, we intend to perform a high--angular resolution all--sky
survey with 21-cm multi-feed detectors, using the world largest single dish
radio telescopes, like those at Parkes and Effelsberg or Jodrell--Bank.
We like to correlate the {\it ROSAT\/} all--sky survey and {\it XMM--Newton\/}
data with the H{\sc i} data to determine the distributions of the temperature
and emissivity of the galactic X-ray plasma on large angular scales.
The angular resolution of the H{\sc i} and X-ray data match perfect to study
the distribution of Milky Way X-ray emission.
The available H$\alpha$ all-sky survey has a much lower angular resolution
(about $1^{\rm o}$), however its distribution appears to be much smoother
than of the cold dense matter traced by H{\sc i}. Also its column density
is an order of magnitude lower than the $N_{\rm HI}$.
The combined analysis of the X-ray, H{\sc i} and H$\alpha$ all-sky surveys
will give unique insights into the physics of the Milky Way and its Halo,
but more important, it will open the window to the high--redshift objects
of the early Universe. 

%It is becoming increasingly apparent, that
%supermassive black holes --located within the very centres of AGNs-- are
%integral features of the formation of galaxie groups and clusters.
%Accordingly, the search for supermassive black holes at high redshift $z > 5$
%is of high importance for the understanding of galaxy formation.
%Already with {\it XMM--Newton\/} the search for very luminous AGNs at high
%redshift is feasible, i. e.
%$L_{\rm X-ray}({\rm AGN})~>~3\cdot 10^{45}~{\rm erg~s^{-1}}$
%at a redshift of about $z~\simeq~5$ is detectable today. 
%
%These observations can only identify the brightest
%objects in the Universe, which may not be representative for the
%in the Universe at all.
%The {\it XEUS\/} mission will observe the ``normal'' population of objects
%in the early Universe.
%The spectral capabilities of the present and future X-ray detectors will allow
%to study the chemical composition of the matter accreted onto the supermassive
%black holes in the AGNs and of the intracluster gas of clusters of galaxies.
%For example, the intracluster gas is characterised by the X-ray bremsstrahlung e
%mission
%and the Fe{\sc xxv} emission line. X-ray lines like the Fe{\sc xxv},
%O{\sc vii} and Si{\sc xiii} are important tracers for the temperature,
%electron density and chemical composition of the intracluster gas.
%Studies of high redshift clusters of galaxies will allow unique clues on the
%star formation history of the Universe. Stars are considered to be
%the ``chemical factories'' for the production of the metals detectable in the
%X-ray lines of the intercluster gas. 

\subsubsection{Current status of research}
>>From the {\it ROSAT\/} deep fields, we know that
by far the dominant fraction of the X-ray background radiation (XRB) is
produced by the superposed emission of AGNs and clusters of galaxies (Hasinger et al. 1998).
The contribution of X-ray emitting individual galaxies and stars to the XRB is
negligible.
Only towards the tiny most transparent windows through the galactic
interstellar medium (minima of X-ray absorbing galactic interstellar medium)
the high--redshift X-ray sky is studied at present ({\it ROSAT\/}, 
{\it XMM--Newton\/}, {\it Chandra\/}).
This is mainly because the redshift moves the X-ray spectrum of the high $z$
objects into the soft X-ray energy regime.
\begin{equation}
E(z) = \frac{1}{z+1}E(z = 0)
\end{equation} 
For example, the important Fe{\sc xxv} line at $E(z = 0) =$ 6.7 keV which
traces intercluster gas will be detectable at $z = 10$ at $E(z = 10)~=$ 0.61 
keV.
In this soft X-ray energy regime below $E~<$ 1 keV the photoelectric 
absorption of the galactic interstellar medium becomes one of the most severe 
problems, the absorption cross section of matter is
$\sigma~\propto~E^{-3}$ (Wilms et al. 2000).
The mean free path length of a 0.6 keV photon through the galactic
interstellar medium is very short compared with the thickness of the galactic
gas layers (Kalberla \& Kerp 1998).

The X-ray emission and absorption in the soft X-ray energy window 
($E\,<\,1$ keV) is subject of a very controversial debate.
Whether the soft XRB is build up by the superposition of
individual supernova remnants (Snowden et al. 1998) or
it is produced by the X-ray emission of the local hot bubble and the Milky Way
Halo plasma (Pietz et al. 1998).
If the first working--hypothesis is correct, the soft galactic X-ray emission
will vary in all relevant physical parameters on arcmin scale.
These variations of the soft X-ray emission will introduce large uncertainties
in the analysis of high--redshift objects by disentangling their emission
from that of the galactic foreground.
If the second working--hypothesis is correct, the situation is much less
complicated, but we need to determine the distribution of the X-ray absorbing
interstellar medium as accurate as possible.
Accordingly, we have now the unique opportunity to constrain and to 
disclose the origin of the galactic soft X-ray emission and to determine
the distribution of the X-ray absorbing interstellar medium to mitigate
or to overcome severe data analysis problems in future while studying the
high--redshift Universe. 

In addition to the photoelectric absorption, the confusion of
the high--redshifted X-ray lines with those of the X-ray plasma located 
within the Milky Way halo and the close neighbourhood of the Sun becomes a 
severe problem.
Both plasma have temperatures of about $T~\sim~1\cdot 10^6$ K (Pietz et al.
1998).
Their X-ray spectra are expected to be dominated by a wealth of emission lines
of the partly ionised heavy elements.
Accordingly, one has to separate the faint X-ray lines of the high $z$ object
from the bright diffuse X-ray emission of the galactic plasma.

The present research on the early Universe is focused on the X-ray energy
window above $E > $ 1 keV,
to avoid the confusion present below $E < $ 1 keV with the Milky Way X-ray 
plasma and the strong photoelectric absorption.
To explore really the high--redshift Universe beyond $z > 5$, one has to
overcome both severe and not independent problems:
\begin{itemize}
\item To evaluate the distribution, the X-ray spectrum and chemical
composition of the X-ray plasma in the local environment of the Sun and of
that of the Milky Way halo.
\item To evaluate the distribution on arcmin scale and chemical composition
of the photoelectric absorbing interstellar medium.
\end{itemize}

\subsubsection{Own experience}
Our group is involved
since more than a decade in the analysis of the soft X-ray background below
$E~<~$ 1 keV, just where the early Universe becomes visible.
Based on the {\it ROSAT\/} all--sky survey data, our group developed for
the first time a self consistent model of the soft X-ray background radiation
(Pietz et al. 1998, Kalberla \& Kerp 1998).
This model is based on an empirical approach combining X-ray and neutral
atomic hydrogen data in a unique way.
Using this approach it is possible to model the observed soft X-ray intensity
distribution of the
X-ray sky down to the accuracy limits of the {\it ROSAT\/} all--sky survey.
We have now a rigid first estimate of the temperatures and the emissivity 
distributions of both X-ray plasma as well as of the absorbing interstellar
medium.

\subsubsection{Goals of the project}
The project aim is to determine the distribution of the soft X-ray emission
of the Milky Way and the X-ray absorbing interstellar medium.
The X-ray emission is available in form of the {\it ROSAT\/} all-sky survey
with an angular resolution of 12 arcmin (Snowden et al. 1997).
The best H{\sc i} surveys offer at least a three times 
lower angular resolution (Hartmann \& Burton 1996).
The available multi-feed technology at the Parkes telescope for 
instance makes it possible to survey the entire sky within a few months
with the world largest single dish radio telescopes.
This data will match perfect to the {\it ROSAT\/} all-sky survey.
H{\sc i} data with even higher angular resolution will also be necessary for
follow--up studies of specific objects.
These data can be obtained today using the {\it ATCA\/} and {\it VLA\/} and
in future ideally the {\it Square Kilometer Array \/}.
However, we need to determine very accurately the total H{\sc i} flux,
undetectable with radio interferometers, with the highest possible angular
resolution using the world largest single dish radio telescope.
Accordingly, we plan to survey the southern sky with the Parkes telescope,
while the northern sky will be ideally observed with the Effelsberg radio
dish.
The combined analysis of X-ray, H{\sc i} and H$\alpha$ data give us the unique
opportunity to model the soft X-ray emission of the entire Milky Way, 
especially towards high galactic latitudes.
This work will free us from the constraint to observe the high--redshift
Universe through the tiny ``Lockman'' or {\it Chandra\/}--window.
It will be possible to evaluate the X-ray radiation transport of the 
high--redshifted X-ray emission through the attenuating interstellar medium.
For these aims, we have to investigate the following points:
\begin{itemize}
\item To search for temperature gradients on small-- and large--angular scales
across the X-ray sky ({\it ROSAT\/}, H{\sc i} and H$\alpha$ data).
\item To evaluate the chemical composition of the local and Milky Way halo
plasma ({\it XMM--Newton\/} data).
\item To determine the distribution of the absorbing interstellar medium on
angular scales of a few arcmin (multi-feed H{\sc i} all--sky surveys).
\end{itemize}   

\noindent{\bf References:}\\
Hasinger G., Burg R., Giacconi R., et al., 1998, A\&A 329, 483\\
Kalberla P.M.W., Kerp J., 1998, A\&A 339, 745\\
Pietz J., Kerp J., Kalberla P.M.W., et al., 1998, A\&A 332, 55\\
Reynolds R., Tufte S.L., Haffner M., et al., 1998, Publ. Astron. Soc. Aust
 15, 14\\
Snowden S.L., Egger R., Freyberg M.J., et al., 1997, ApJ 485, 125\\
Snowden S.L., Egger R., Finkbeiner D.P., et al., 1998, ApJ 493, 715\\
Wilms J., Allen A., McCray R., 2000, ApJ in press (astro-ph/0008425)\\

\subsubsection{Estimate of resources expected from DFG}
Two Ph. D. students. Financial support to build a 21--cm multi-feed system
for the Effelsberg telescope. 

\subsubsection{Members of research group relevant for the project}
Dr. J\"urgen Kerp and Dr. Peter M. W. Kalberla

\subsubsection{Connections to other projects of current SFB proposal}


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