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\subsection{Dark Matter in our Galaxy and in the Local Group} %Please give

{\bf{
{Proff. \ K.S.\,de\,Boer, \ U. Mebold, \ P. Schneider}\\
{Sternwarte, \ Radioastr.Inst., \ Inst.f.Astrophys.Extraterr.Forsch.}\\
{Universit\"at Bonn}\\
{Auf den H\"ugel 71}\\
{D-53121 BONN}\\
{Telefon: 0228/733656 /733657/ 733676}\\
{Fax: 0228/733672}\\
{Email: \ deboer@ \ mebold@ \ peter@ \ astro.uni-bonn.de}\\}}

\subsubsection{Motivation: The nature of the problem and new possibilities}
The mass of galaxies can be determined by counting the objects of known mass 
or by searching for tracers of the overall gravitational field. 
When dealing with the Milky Way, 
we know the nature of the objects present, 
but we cannot detect them all and thus cannot `count' them,
or we look for the signature of the overall gravitational field, 
which is difficult since we live {\it within} the Milky Way. 
Astronomy has over time developed ingeneous ways to determine 
the overall mass of the Milky Way. 

Counting stars, deriving the spatial distribution from those counts,
and then the vertical force $K_z$ required to keep it all together,  
led already Oort (1932) 
%Oort, J.H., 1932, Bull. Astr. Inst. Neth. VI, 249
to suspect that there is more mass in the Milky Way disk 
than accounted for by stars.
This additional material was called {\bf dark matter}. 
The detection of the interstellar material and the measurements of the H{\sc i}
21-cm emission of the most abundant atom in the Universe by radioastronomers
since 1953 showed, however, 
that the gas fraction in the Milky Way disk is about 3\% only, 
and the mass problem remained. 

Surprisingly, radioastronomical work indicated 
that the problem is worse still. 
Studying the rotation behaviour of other galaxies, 
Bosma (1981) 
%Bosma A., 1981, AJ 86, 1825
showed that rotation curves are flat, 
do not show the expected Keplerian radial decline, 
and thus indicate the presence of much more directly un-detectable mass than
known. 
What this mass is and where it might be located is entirely unclear. 
The material might even be present in the Milky Way halo. 

New detection techniques (satellites, telescopes, detectors) 
now allow to collect much better data to better trace the presence of 
this additional ``invisible'' matter. 
Our research aims in three directions. 
These are:\\
{\bf a) Stellar dynamics in the local part of the Milky Way}\\
{\bf b) High--velocity clouds: dark dwarfs?}\\
{\bf c) Micro-lensing of xxx stars to search for `missing mass'}\\


\noindent
{\bf a) Stellar dynamics in the local part of the Milky Way}

\subsubsection{Current status of research}

Star counts performed by Bahcall \& Soneira (1980) 
%Bahcall J.N., Soneira R.M., 1980, ApJ 238, L 17
were, as all previous studies, based on photographic plates. 
These studies were very thorough, 
but lack the star classification precission needed 
to really find the spatial distribution of stars perpendicular 
to the Milky Way disk. 
>>From that data is was not possible to really quantify the gravitational field 
and thus demonstrate the presence of dark matter. 

With the {\sc Hipparcos} satellite, very precise distances and 
proper motions for stars in the solar neighbourhoud could be determined. 
The spatial distribution of A stars appears to indicate 
that there is no need for dark matter in the local galactic disk 
(Cr\'ez\'e et al., 1998, Holmberg \& Flynn 2000). 
%Cr\'ez\'e M., Chereul E., Bienaym\'e O., Pichon C., 1998, A\&A 329, 920
%Holmberg J., Flynn C., 2000, MNRAS 313, 209
On the other hand, an analysis of the velocity field by Bienaym\'e (1999) 
%Bienaym\'e O., 1999, A\&A 341, 86
showed that the galactic potential is not extremely flat and 
that a dark matter component would not confined to the galactic plane. 
However, the stars used are nearby and their number is limited, 
leaving doubt about the overall validity of the results of the 
{\sc Hipparcos} studies. 

With the {\bf Double Interferometer for Visual Astrometry} 
({\sc Diva}) Satellite (R\"oser et al., 1998), 
%R\"oser S., Bastian U., de Boer K.S., H{\o}g E., R"oser H.P., Schalinski C., Schilbach E., de Vegt C., Wagner S., 1998, in IAU 23rd Gen. Ass., Highlights in Astronomy, J.Andersen (ed.), p. 583
which was recently approved by the DLR as the next German space project, 
a very homogeneous distance and spectroscopic survey will be made 
of the local Universe. 
It is expected to measure a total of 35 million stars, 
350 times the number {\sc Hipparcos} could measure. 
For stars out to well over a kpc precise {\it distances} can be determined 
with {\sc Diva} 
while for stars out to 10 kpc {\it proper motions} can be found. 
These together will be used to construct 
a kinematical model for the local part of the Milky Way. 
Using the {\it spectrophotometry} obtained with {\sc Diva}, 
precise stellar parameters can be obtained 
allowing the determination of distances to stars much farther out 
than the `parallax' stars. 
In particular that data can be used to derive the true spatial distribution 
of stars and thus the distribution of mass well into the Milky Way halo. 

\subsubsection{Own experience}

The Sternwarte has a long tradition in astrometry and was involved in 
the {\sc Hipparcos} satellite astrometric mission 
(see e.g. Geffert et al., 1997). 
%Geffert M., Klemola A.R., Hiesgen M., Schmoll J., 1997, A\&AS 124, 157
Also modern ground based measurements are carried out 
(Geffert \& Maintz 2000). 
%Geffert M., Maintz G., A\&AS 144, 227

Our studies of blue horizontal-branch (HB) stars 
(Moehler, Heber \& de Boer 1990, ...., de Boer et al., 1997) 
%Moehler S., Heber U., de Boer K.S., 1990, A\&A 239, 265
%de Boer K.S., Aguilar S\'anchez Y., Altmann M., Geffert M., Odenkirchen M., Schmidt J.H.K., Colin J., 1997, A\&A 327, 587
showed kinematical peculiarities. 
Although the HB stars are believed to be formed from the same older 
parent generation of stars, 
very blue HB stars do not seem to exist in the Milky Way halo, 
whereas redder HB stars do (Altmann \& de Boer 2000), 
%Altmann M., de Boer K.S., 2000, A\&A 353, 135
while both blue and red HB stars exist in the Milky Way disk. 
The evolution of and dynamics in the Milky Way are not yet understood!

\subsubsection{Goals of the project}

Using {\sc Diva} data, 
the spatial distribution of the stars will be determined. 
The spatial distribution itself must obey the gravitational pull of the disk 
and allows to determine the actual vertical force $K_z$ in the Milky Way, 
whose strength is based on the total mass available locally 
and elsewhere in the Milky Way. 
With the known properties of these very stars, however,  
their total mass can be calculated. 
This mass then will be compared with the mass as derived from $K_z$. 
The discrepancy between the two determinations 
will show us the amount of dark matter present in the Milky Way disk. 

Preparatory studies will have to be performed in the phase before the 
launch of {\sc Diva}, foreseen for 2004, to explore the best 
methods of finding the luminosities of the rather numerous red stars 
expected to be found in large numbers in the {\sc Diva} survey. 
It will ultimately be the luminous red giants stars, 
visible over large distances, 
which will best trace the spatial distribution of the stars in the halo. 

\subsubsection{Estimate of resources expected from DFG}

1 postdoctoral fellow and 2 doctoral students 
with experience in research of galactic structure 

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

Bereich a):\\
Prof. K.S.\,de\,Boer, 
Prof. N.N.,
AOR Dr. M.\,Geffert,
Dr. K.\,Reif

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






\clearpage
\noindent
{\large \bf b) High-Velocity Clouds: Dark Dwarfs?}
\subsubsection{Current status of research}
The structure of galaxies and clusters of galaxies as individuals is well known
and adequate models are available. This is in marked contrast to our knowledge 
on groups of galaxies. Groups of galaxies are agglomerations of a few member 
galaxies with an un-sharp transition zone towards the high mass end, the 
clusters of galaxies. An important step in the understanding of cluster of 
galaxies was that the dominant fraction of baryonic matter is in form of X-ray 
emitting gas (B\"ohringer et al. 1999).
%B\"ohringer H. 1999, in Highlights in X-ray astronomy, MPE-Report 272,
%Eds. Aschenbach B., Freyberg M.J.
The total mass of a cluster of 
galaxies is subdivided in 85\% dark matter (DM), 10\% gas and 5\% in stars. 
Until the recognition of this remarkable composition, the scientific 
attraction of clusters of galaxies as well as on groups of galaxies 
of grows constantly. Of  particular interest are the physical conditions of the
gas, the total mass, the shape of the gravitational potential and the relative 
fraction of the three constituents DM, gas and stars on the total mass.

In the local group of galaxies the existence of intergalactic gas could not be 
established. A simple evaluation of the mass of the local group of galaxies 
assumes a average volume density of $n \sim 1\cdot 10^{-4}~{\rm cm^{-3}}$, 
which gives a total mass estimate of $M(gas) = 1\cdot 10^{12}~{\rm M_\odot}$. 
This mass is in the order of the total mass of the individual galaxies known to
be localized within the local group. The largest uncertainty in this estimate 
is the average volume density. This average volume density can be derived from 
interaction processes between members of the local and neighbouring galaxy 
groups.
Despite all these uncertainties, this rough estimate of the total gas mass 
indicates that this mass has to be considered for the understanding of the 
dynamics of the local group as whole.
With respect to the cosmological aspects of the local group research one has 
to compare quantitatively the composition of  total local group mass (baryonic 
to dark matter ratio) with that of clusters of galaxies.
Information on the intergalactic gas within the local group can be derived in 
principle from absorption line studies against distant objects and by the 
analysis of broad band spectroscopy of emission lines of individual members of 
the local group.
Observations of absorption lines in H$\alpha$, Na{\sc i}D  and Ca against 
sources in M31, M81 or extragalactic sources do not indicate the existence of  
gas within the local group. The reason for this non-detection may be severe 
difficulties to separate emission and absorption lines, originating within the 
Milky Way and/or its halo. The radial velocities of gas clouds within the 
Milky Way halo and of the local group are comparable. Methods to determine the 
distance of local group gas clouds independently from the radial velocity are 
affected by systematic effects, which lead to high intrinsic uncertainties.
Observations of the emission lines of local group gas clouds are faced with the
same kind of problem. The recent scientific discussion focus today on the 
so-called high-velocity clouds (HVCs). HVCs are partly localized within the 
local group of galaxies (Blitz et al. 1999).
%Blitz et al. 1999, ApJ 514, 818
The main argument for their 
localization within the local group of galaxies is based on mass estimates, 
which are quite uncertain. But assuming this working hypothesis, their masses 
have to be scaled to the square of the ratio between the old (Milky Way halo 
distance) and their new (within the local group, Mpc distance). This gives 
mass conversion factors of 100 to 1000, quite comparable to the mass of small 
dwarf galaxies.

\subsubsection{Own experience}
Our working group at the Radioastronomisches Institut is international 
identified as one of the leading groups in H{\sc i} observations of HVC.
The discovery of head-tail structures in the position-velocity frame by our 
group open the opportunity to study the interaction of  HVCs with their ambient
medium.

\subsubsection{Goals of the project}
Our working group at the Radioastronomisches Institut will improve the detailed
knowledge on groups of galaxies by studying of their gas content.
Assuming a specific interaction mechanism, these head-tail structures 
allow to evaluate physical parameters of the interacting gas clouds, in 
particular their volume densities. In addition, we anticipate to derive a rigid 
distance estimate for the HVCs based on these physical parameters. This will 
allow to identify the origin of  HVCs as objects of the Milky Way halo or as 
objects of the local group of galaxies.

\subsubsection{Estimate of resources expected from DFG}

Project a)\\
1 postdoctoral fellow and 2 doctoral students 
with experience in research of galactic structure \\
Project b)\\
Project c)\\

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

Project a)\\
Prof. K.S.\,de\,Boer, 
Prof. N.N.,
AOR Dr. M.\,Geffert,
Dr. K.\,Reif\\
Project b)\\
Prof. U. Mebold\\
Project b)\\
Prof. P. Schneider

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