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\subsection{Structure formation in the local universe} 
                                                                %Please give 
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{\bf{
{Prof. Dr. Ulrich Klein, Dr. J\"urgen Kerp}\\
{Radioastronomisches Institut}\\
{Universit\"at Bonn}\\
{Auf dem H\"ugel 71}\\
{D-53121 BONN}\\
{Telefon: 0228/73-3674}\\
{Fax: 0228/73-1775}\\
{Email: uklein@astro.uni-bonn.de}\\}}


\subsubsection{Abstract}

Currently favoured models of Cold Dark matter (DM) are tuned to be in 
accord with an array of observationally supported cosmological parameters.
Because of their predictive power they have to be tested by confronting 
them with observations of DM on all scales. This project is to investigate 
the distribution of DM on the scale of groups and clusters of galaxies. 

The distribution of DM in groups and clusters of galaxies will be studied
using the X--ray emission of the hot gaseous cluster halos. Both, the spatial
and spectral resolution of modern X--ray telescopes such as XMM and CHANDRA
meanwhile allow to also study the kinematics of this gas, thus allowing to
disclose the dynamics of ongoing structure formation in the local (z $\le$ 0.1)
universe. Radio continuum measurements disclose the shocked interface regions 
in which pools of cooled particles are re--accelerated to highly relativistic
energies. Their synchrotron radiation exhibits characteristic properties of
the particle spectrum and the magnetic field morphology.

\subsubsection{Current status of research}

With the successful operation of COBE in 1992, a long--standing problem has
been solved, viz. the true amplitude of the anisotropy of the cosmic microwave 
background (CMB), for which prior to COBE only upper limits were available.
One of the immediate consequences of this imprint of commencing fluctuations 
onto the CMB (at the last scattering surface) is that some unknown form of
DM must dominate the universe that interacts solely by gravitation. Otherwise,
the observed level of unisotropy $\Delta$T/T$\sim 10^{-5}$ would be in flat
contradiction to the density fluctuation seen in the evolved universe (of
order $10^6$ for galaxies, $10^3$ for clusters, $>10^1$ for superclusters),
accounting for their evolution out of the initial fluctuation spectrum. 

Models of structure formation aim at producing the observed large--scale
structure as realistically as possible, but should ideally also reproduce
the hierarchy of structures, from galaxies to superclusters. Such models that
are based on Cold Dark Matter (CDM, see e.g. Jenkins et al. 1998) have been
worked out in the meantime. They appear to reproduce the observed clustering
properties of galaxies out to the currently accessible range of radial
velocities pretty well, thereby predicting the total dark mass of (e.g.)
clusters and, at the same time, the mass spectrum of galaxies within a
larger structure. 

It turns out that the models are faced with some severe problems when
compared with observations. In particular, a clear mismatch was found by Moore
et al. (1999) when confronting these hierarchical, scale--invariant model
structures with observations. While on the cluster scale (the Virgo Cluster 
in that case) the model reproduces the observed mass spectrum of member
galaxies pretty well, it overpredicts the number of satellites around 
the Milky Way by factor of $\sim$50! No one really knows what 'went wrong' 
with the Milky Way, given that the models are correct. On the other hand, 
it is difficult to make/leave (dwarf) galaxies 'dark', or make them disappear
by soft merging with the main galaxy. In the case of the Milky Way, a large
number of merged satellites would have heated its disk very strongly, which 
is not observed (Moore et al. 1999). If such a scenario  is suggested by
observations, it seems that this process produces a giant elliptical, leaving
behind a poor group (e.g. Barnes 1989; Athanassoula et al. 1997), a nice
example being NGC~1132 (see Mulchaey \& Zabludoff 1999). Given the scarce
results of recent deep HI surveys aiming at the discovery of  dwarf satellites
in galaxies and groups of galaxies (Sorar 1994; Zwaan et  al. 1997), or to
disclose intergalactic HI clouds (e.g. Disney et al. 1999), it is clear that 
a really huge effort is to be made in order to derive the HI mass function 
at the low--mass end of galaxies. On the other hand, even though the models 
may require uncomfortable changes in their parameters in order to be
reconcilable with observations, there may still be some room left for such
changes (Navarro \& Steinmetz 2000). For instance, a tilted primordial power
spectrum could do the job, i.e. suppress power on galactic and sub--galactic
scales. And finally, owing to the numerical resolution of the models,
deviations from observations are most compromised on small scales.

This certainly does not hold on scales of galaxy clusters, where the 
modelled large--scale structure reproduces the observations pretty well.
An interesting aspect of studying clusters of galaxies is to explore their
substructure, where X--ray measurements can be utilized as very sensitive
tools (in more distant clusters this can be accomplished via gravitational
lensing). Harris et al. (1980) early on deemed the radio halos in galaxy
cliusters to represent shock zones between merging subclusters. This
picture has in the meantime matured, owing to theoretical and observational
progress. Multi--frequency measurements allow to deduce important physical
properties such as the morphology and strength of the magnetic field in such
regions, and the continuum spectrum of the synchrotron radiation. One of the
most spectacular examples is witnessed in A~2256 (R\"ottgering 1993;
Thierbach 2000). As shown by En{\ss}lin (1999; see also et En{\ss}lin  al.
1998) this yields a picture of the 3--D structure of the shock region, from
which one gains knowledge about the structure formation in the local universe. 

\subsubsection{Own experience}

The study of galaxy clusters has a long tradition in Bonn (RAIUB/MPIfR).
The emphasis of these investigations has been on the statistics and evolution
of radio sources early on, starting with the early low--frequency work by
Waldthausen (1980), and pushed towards higher frequencies (Andernach 1981), 
the focus turning to the nature of radio halos by Sievers (1988) and Thierbach
(2000). The properties of the IGM and ICM have been investigated via
multi--frequency observations of tailed and giant radio sources (Mack 1992,
Ballarati et al. 1993, Feretti et al. 1998, 1999, Zech 1995, Rottmann 1996,
Rottmann et al. 1996, Mack 1996, Fuhrmann 1999). The Sunyayev--Zeldovich
effect has been  investigated by Schallwich (1980), Klein et al. (1991), and
by Thierbach (1996). Effects of the cluster environment on the properties of
spiral galaxies in clusters have been studied by Niklas et al. (1995, Virgo 
Cluster) and by Fritz (2000, Centaurus Group).

A very important astrophysical aspect emerging from those investigations 
and relevant to the here proposed project is the interpretation of 
cluster radio halos in terms of shock--induced acceleration of pools of 
cooled particles (En{\ss}lin et al. 1998, En{\ss}lin 1999, Thierbach 2000,
En{\ss}lin et al. 2000). What we are witnessing here is the effect of 
ongoing cluster or subcluster merging, i.e. large--scale structure 
formation in the local universe. It could be shown here that this
interpretation, which had been the subject of many controversary 
discussions in the literature, nicely  explains the spectral and 
polarization properties of 'radio halos' (coined 'radio ghosts' by 
En{\ss}lin).

In order to conduct X--ray studies of galaxy clusters we shall utilize the 
new generations of excellent X--ray telescopes. X--ray studies of various 
kinds of objects have been carried out at our institute in the past, and 
still are (HVCs: Kerp et al. 1999; dwarf galaxies: Blondiau 1996, Blondiau 
et al. 1997; Walter et al. 1998; spiral galaxies: Pietsch et al. 1994,
2000; radio galaxies: Mack et al. 1997). 

\subsubsection{Goals of the project}

The amount of DM in clusters of galaxies can be deduced in three ways: 
from the velocity dispersion of the galaxies, from the variation of 
density and temperature of the hot coronal plasma (assumed to be in
hydrostatic equilibrium), and -- in case of distant clusters -- from
gravitational lensing of background objects by the cluster. The above
mentioned problems of $\Lambda$CDM models obviously necessitate further
observational tests. This requires both, the search for satellites around
external galaxies as well as a scrutiny of galaxy groups and clusters, 
with the aim to derive their mass spectrum. The existing data impressively
demonstrate the power of X--ray observations (see e.g. B\"ohringer 1995, 
for a review). New--generation X--ray telescopes will allow much more 
detailed studies of the hot cluster gas than hitherto possible.

In the framework of this project, we plan to observationally explore the
structures of visible matter and DM on scales of groups and clusters of
galaxies, with emphasis on their (still forming) substructures. The primary
tool to accomplish this is provided by X--ray measurements of the pervasive 
hot plasma surrounding the central galaxies. Numerous examples for this
possibility have been delivered by the ROSAT  mission, and Schuecker \& 
B\"ohringer (1999) have identified a substantial fraction of clusters
possessing radio halos or relics out of the ROSAT cluster sample. X--ray
observations probe both, the temperature and density of the ionized gas.
Imprints of the collision zone are particularly distinctly visible in the
temperature profiles across such regions, a first prominent example reported
by (Kikuchi et al. 2000). 

While it is mandatory to use modern X--ray telescopes at their best, much 
can be gained by 'data mining'. The ROSAT archive still contains a host of 
individual pointed observations - as yet unreduced. Likewise, the All Sky 
Survey can be regarded as a treasure box. These data will serve as an 
important tool for our candidate search and sample definition as far as 
galaxy clusters are concerned. For the derivation of detailed properties 
of the hot gas (distribution of density and temperature) it is mandatory to
utilize modern X--ray telescopes in the framework of this project. Besides
their greatly improved sensitivity and their much improved angular resolution,
one important advantage is that their spectral resolution allows to even
investigate relative motions of the hot gas via spectroscopy of (mainly) the
iron line (several 100~km~s$^{-1}$. 

Once the interface zones of merging (sub)clusters have been identified, 
these have to be observed in the radio continuum. Multi--frequency radio 
continuum observations will be used to derive properties such as the 
morphology and strength of the magnetic field within these radio halos, 
and the continuum spectrum of the synchrotron radiation. This requires 
both, interferometers (at low frequencies) as well as the Effelsberg 
100--m telescope (at high frequencies), in order to fix the break in 
the spectrum, hence the time elapsed since the particles were
(re--)accelerated. 
\\\\ 
References
\begin{description}
\item Andernach, H.: 1981, Ph.D. thesis, Univ. Bonn
\item Athanassoula, E., Makino, J., Bosma, A.: 1997, MNRAS 286, 825
\item Ballarati, B., Feretti, L., Giovannini, G., Andernach, H., Klein, U.:
        1993, in {\it Clustering in the Universe}, D. Ferbal and A. Mazure
        (eds.), Editions Fronti\`eres, Gif-sur-Yvette, p. 229
\item Barnes, J.E.: 1989, Nature 338, 123
\item Blondiau, M.: 1996, Ph.D. thesis, Univ. Bonn
\item Blondiau, M., Kerp, J., Klein, U., Mebold, U.: 1997, A\&A 323, 585 
\item B\"ohringer, H.: 1995, Ann. New York Acad. Sciences 759, 67
\item Enn{\ss}lin, T., Biermann, P.L., Klein, U., Kohle, S.: 1998, A\&A 
        332, 395
\item Enn{\ss}lin, T.: 1999, Ph.D. thesis, Univ. Bonn
\item Enn{\ss}lin, T., Simon, P., Biermann, P.L., Klein, U., Kohle, S., 
        Kronberg, P.P.: 2000, Ap.J., subm.
\item Feretti, L., Dallacasa, D., Govoni, F., Giovannini, G., Taylor, G.B.,
        Klein, U.: 1998, 344, 472 
\item Feretti, L., Giovannini, G., Klein, U., Mack, K.--H., Sijbring, L.G., 
        Zech, G.: 1999, A\&A 331, 475
\item Fritz, T.: 2000, Ph.D. thesis, Univ. Bonn
\item Fuhrmann, L.: 1999, Diploma thesis, Univ. Bonn 
\item Harris, D.E.: 1980, 
\item Jenkins, A., Frenk, C. S., Pearce, F. R., Thomas, P. A., Colberg, J. M.,
        White, S. D. M., Couchman, H. M. P., Peacock, J. A., Efstathiou, G.,
        Nelson, A. H.: 1998, Ap.J. 499, 20
\item Kikuchi, K., Itoh, C., Furusho, T., et al.: 2000, Ap.J. 531, L95
\item Kerp, J., Burton, W.B., Egger, R., Freyberg, M.J., Hartmann, D.,
        Kalberla, P.M.W., Mebold, U., Pietz, J.: 1999, A\&A 342, 213  
\item Klein, U., Rephaeli, Y., Schlickeiser, R., Wielebinski, R.: 1991, 
        A\&A 244, 43  
\item Mack, K.--H.: 1992, Diploma thesis, Univ. Bonn
\item Mack, K.--H., Feretti, L., Giovannini, G., Klein, U.: 1993, A\&A 280,
        63
\item Mack, K.--H.: 1996, Ph.D. thesis, Univ. Bonn
\item Mack, K.--H., Kerp, J., Klein, U.: 1997, A\&A 324, 870 
\item Moore, B., Ghigna, S., Governato, F., Lake, G., Quinn, T., Stadel, J.: 
        1999, Ap.J. 514, 133
\item Mulchaey, J.S., Zabludoff, A.I.: 1999, Ap.J. 514, 133
\item Navarro, J.F., Steinmetz, M.: 2000, Ap.J. 528, 607
\item Niklas, S., Klein, U., Wielebinski, R.: 1995, A\&A 293, 56 
\item Pietsch, W., Vogler, A., Kahabka, P., Jain, A.,  Klein, U.: 1994, A\&A
        284, 386 
\item Pietsch, W., Vogler, A., Klein, U., Zinnecker, H.: 2000, A\&A 360, 24 
\item R\"ottgering,
 H.: 1993, Ph.D. thesis, Univ. Leiden
\item Rottmann, H.: 1996, Diploma thesis, Univ. Bonn
\item Rottmann, H., Mack, K.--H., Klein, U., Wielebinski, R.: 1996, A\&A 
        309, L19 
\item Schallwich, D.: 1981, Ph.D. thesis, Univ. Bonn
\item Schuecker, P., B\"ohringer, H.: 1999, in {\it Diffuse Thermal and
        Relativistic Plasma in Galaxy Clusters}, MPE-Report 271, p. 43
\item Sievers, A.: 1988, Ph.D. thesis, Univ. Bonn
\item Sorar, E.: 1994, Ph.D. thesis, Univ. Pittsburgh
\item Thierbach, M.: 1996, Diploma thesis, Univ. Bonn
\item Thierbach, M.: 2000, Ph.D. thesis, Univ. Bonn
\item Waldthausen, H.: 1980, Ph.D. thesis, Univ. Bonn
\item Walter, F., Kerp, J., Duric, N., F.Brinks, E., Klein, U.: 1998,
        Ap.J. 502, L143 
\item Zech, G.: 1995, Diploma thesis, Univ. Bonn
\item Zwaan, M.A., Briggs, F.H., Sprayberry, D., Sorar, E.: 1997, Ap.J. 
        490, 173
\end{description}


\subsubsection{Estimate of resources expected from DFG}

Manpower: The programme outlined above obviously requires experts who are
familar with observational techniques in two rather different spectral
regimes. The load of work makes a full position (BATIIa) for each spectral
domain indispensible. The postdoctoral fellows are expected to plan and carry
out the relevant observations. They are to publish them in refereed journals,
and at the same time present results on pertinent conferences. They are
also required to organize the collaboration with other groups (with L.
Feretti at CNR in Bologna and H. B\"ohringer at the MPIE in Garching).
This necessitates travel funds to attend two conferences per year, and
two visits to the collaborating insitutes. In order for the postdocs to
efficiently cope with the data analysis they have to be provided with
powerful state--of--the--art computers and some periphery. This foreseeably
requires some 10~kDM in the first year, and about 2~kDM in the subsequent
ones.


\subsubsection{Members of research group relevant for the project}

Dr. J. Kerp (staff member); Prof. Dr. U. Klein (project leader);
Prof. Dr. U. Mebold (managing director).


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

This project is intimately related to project x.x.x. The proposed scrutiny 
of DM in groups and clusters is linked to project x.x.x that investigates 
the structure of DM on galaxy scales, as well as to project x.x.x that aims
at disclosing the nature of DM. 

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