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\subsection{Structure of Dark Matter Halos in Galaxies} 
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{\bf{
{Prof. Dr. Ulrich Klein, Dr. Peter Kalberla}\\
{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}

One of the key issues in astrophysical research is the unresolved problem
of the nature of Dark Matter (DM). DM can be studied on various scales,
from small galaxies up to superclusters. This project is to scrutinize 
the distribution of DM in galaxies. The final goal is to deduce the density 
profile of DM structures on galaxy scales. 

In case of galaxies, density profiles can be obtained from precise rotation 
curves and optical (infrared) photometry, preferably of low--surface brightness
galaxies (known to be DM--dominated). The rotation curves must utilize both,
optical (H$\alpha$, central regions) and radio (HI, outer regions) techniques
in order to provide the high spatial and spectral resolution that is
indispensible for the anticipated accuracy. 

The derived DM density profile at large radii, together with the related 
baryonic mass dependence, will probe the cosmological processes of galaxy
formation, while at small radii, where the DM is mixed with the baryons, it
will probe the subsequent galactic evolution.


\subsubsection{Current status of research}

Since the first evidence for the existence of DM was reported by F. Zwicky 
in 1933, based on his studies of the velocity dispersion in the Coma Cluster,
its nature has remained totally undisclosed. Following the pioneering work 
by Rubin \& Ford (1980; 1982) in the optical and by Bosma (1982) in the radio
regime, numerous studies of galaxy rotation curves have been conducted. It 
became soon clear that low--mass galaxies, too, possess a significant
fraction of DM (Sargent \& Lo 1990), and may be even DM--dominated (Mateo
1998). Low--surface brightness galaxies (LSBGs) were shown to have rotation
curves similar to those of massive spirals (Stil 1999), while their DM content
may vary significantly (Swaters 1999). LSBGs are frequently DM--dominated, but
there is a large spread of their DM fraction. It was the expectation that with
this 'industry' of rotation curve analyses one should soon be able to deduce
some properties of DM halos.

During the past decade, the importance of DM in the universe became obvious
from a completely different route. It was with the successful operation of 
COBE that the amplitude of the anisotropy of the cosmic microwave background
(CMB) was measured for the first time, with $\Delta$T/T$\sim 10^{-5}$. This
rather hard number is not reconcilable with the observed density fluctuations
of the evolved universe unless the bulk of its matter is non--baryonic. Models 
that predict hierarchical structures with Cold Dark Matter (CDM, see e.g.
Jenkins et al. 1998) have been worked out in the meantime, which appear to
reproduce the observed clustering properties of galaxies out to the currently
accessible range of radial velocities pretty well (while Hot Dark Matter
theories are faced with problems). 

The so--called 'standard' $\Lambda$CDM model, which accounts for the basic
set of 'modern' cosmological parameters (critical density $\Omega_{0}$,
non--zero cosmological constant $\Lambda$ = 1 -- $\Omega_{0}$, Hubble
constant H$_{0}$, baryonic mass density $\Omega_{\rm b}$, mass fluctuation
$\sigma$), is the currently most favoured one, but nonetheless bears some
problems when juxtaposed with obsevations. Because it has its major parameters 
fixed, it is eminently falsifiable. In particular, its predictive power can
be used to check its validity on scales different from those used to tune it.
One of the major problems is that the model is at odds with the well--established 
Tully--Fisher relation (Navarro \& Steinmetz 2000). 

As far as galaxy scales are concerned, there is no reason to believe that the
current quality of observed rotation curves is sufficient to allow any firm
conclusions as to the distributions of various constituents in galaxies
(bulge, disk, halo). As pointed out by van den Bosch et al. (2000), the whole
debate that we have seen over the past years, whether or not DM halos of
galaxies possess central cusps, has not come up with any convincing conclusion.
They show that the spatial resolution of the data is not sufficient to put any
meaningful constraints on the density profiles of dark halo, or on cosmological 
parameters, in contrast to such claims in the literature. In order to overcome
the degeneracy inherent to the interpretation, high--resolution rotation
curves have to be obtained, the best targets being LSBGs or dwarf galaxies
that lack bulges and in which the contribution of the stellar disks to the
total mass is less important. 


\subsubsection{Own experience}

The study of the structure and kinematics of galaxies has long tradition at 
our institute. Investigations of halos in different types of galaxies have 
been carried out in various spectral regimes (radio continuum: Beck et al.
1994, Dumke et al. 1995, Klein et al. 1996; HI: Bajaja et al. 1994; Koribalski 
et al. 1993; cold dust and CO: Thuma et al. 2000; X--rays: Pietsch et al.
1994, 2000). 

The kinematics of galaxies, in particular of low--mass ones, has been one of
the subjects of galaxy research at Bonn (Brinks \& Klein 1986; 1988; 1989;
Bajaja et al. 1994; Walter et al. 1997), in the course of which important
contributions to the question of DM in such systems were made. This is still 
one of the major topics of our on--going Graduiertenkolleg {\it The Magellanic
System, Galaxy Interaction, and the Evolution of Dwarf Galaxies}, jointly run
by the Astronomical Institutes of the Universities of Bonn and Bochum. In the 
framework of this, several dissertation and postdoc projects are and have been
devoted to the study of the properties of the ISM, with the special aspect of
widely different environments. These studies, which utilize both, the HI and
the CO lines, cover both, small and large scales. On small scales, the mutual
relation interaction star formation and the ISM are studied, with enormous
progress concerning the relevant input parameters for numerical models (Barone 
et al. 2000; Walter et al. 1998; Walter 1999; Wei{\ss} et al. 1999; Wei{\ss}
2000; Taylor et al. 1999). 

On galaxy scales, the gas kinematics is used to disclose the overall mass 
distribution, with special emphasis on the galaxies' structures and 
environments. Following thorough studies of the influence of gravitational
interactions on bulge L\"utticke 1999) and disk (Schwarzkopf 1999) structures,
the recently proposed scenario of the formation of boxy bulges via soft
merging is being investigated in a large sample of edge--on galaxies with boxy
bulges using the HI for the kinematic diganostics, but also photometry to
scrutinize the distribution oft he stellar light (Vergani, Ph.D. thesis, in
prep.). Finally, a collaboration with P. Salucci (Triest/SISSA) commenced this
year, in which the density profiles of DM halos will be subject to scrutiny,
in the way described in more detail in the next section (also Ph.D. thesis to
start this fall). The kinematics of low--mass galaxies is more difficult to
study, since their rotation velocities are naturally low. As a result, one has
a hard time to derive their mass distributions, let alone that of DM (e.g.
Kohle, 1999;  2000). In the extreme case, at the low--mass end of the mass
spectrum, the  problem of rotational support of dwarf galaxies is being
investigated (Ott et al. 2000; also on--going Ph.D.). 


\subsubsection{Goals of the project}

In the framework of this project, we plan to explore the distribution of
DM on the scale of galaxies. This will be achieved by combining high--precision 
optical (H$\alpha$) and radio (HI) rotation curves. 

In order to obtain 'clean' rotation curves, our galaxy sample will be selected 
according to the following prerequisits: Their mass distributions should not
be governed by bulges, neither by disks. It is usually a big obstacle in such
investigations if there are dominating stellar components. We shall thus cull 
a statistically significant sample from catalogues of intermediate--mass
galaxies. Genuine dwarf galaxies are not suitable either since their kinematics 
is strongly influenced by turbulence of the ISM (see Sect. x.1.3). The best
targets are therefore LSBGs as well as targets with masses like the LMC. All
of them must be carefully selected against any external influences (tidal
interaction, ram pressure in clusters).

The radio observations will have to be performed with the ATCA (southern
galaxies) and the VLA (northern ones). The WSRT will not be considered in 
this project because of its lower angular resolution. In the framework of 
the WHISP programme (Westerbork observations of neutral Hydrogen in
Irregular and SPiral galaxies) Swaters (1999) studied a sample of about 80 
dwarf galaxies. He showed that the spatial resolution obtained with the WSRT
at $\lambda$21~cm is insufficient for galaxies that small. Optical (I--band)
photometry of the target galaxies will serve to derive the mass distribution
of the stellar constituent. Slit spectroscopy in the H$\alpha$ line will
complement the HI data. This complementary approach also ensures that in
(the frequent) case of a central HI deficiency the kinematic information
is still gained (via H$\alpha$). We shall thus obtain high--precision
rotation curves over the whole body of each galaxy. The derived DM density 
profile at large radii, together with the related baryonic mass dependence,
will probe the cosmological processes of galaxy formation, while at small
radii, where the DM is mixed with the baryons, it will probe the subsequent
galactic evolution. Optical rotation curves alone are not sufficient, since
there is evidence for large DM core radii that even exceed the radius of the
optical disk (Salucci \& Burkert 2000).
\\\\
References
\begin{description}
\item Bajaja, E., Huchtmeier, W., Klein, U.: 1994, A\&A 285, 385 
\item Barone, L.T., Heithausen, A., H\"uttemeister, S., Fritz, T., Klein, U.: 
        2000, MNRAS, in press 
\item Beck, R., Carilli, C.L., Holdaway, M.A., Klein, U.: 1994, A\&A 292, 409
\item Boriello, A., Salucci, P.: 2000, MNRAS, in press
\item Bosma, A.: 1981, Ph.D: thesis, Univ. Groningen
\item van den Bosch, F.C., Robertson, B.E., Dalcanton, J.J., de Block, W.J.G.: 
        2000, AJ 119, 1579 
\item Brinks, E., Klein, U.: 1986, in {\it Star-Forming Galaxies and Related 
        Objects}, D. Kunth, T.X. Thuan and J. Tranh Than Van (eds.), Edition
        Fronti\`eres, Paris, p. 281 
\item Brinks, E., Klein, U.: 1988, MNRAS 231, 63P 
\item Brinks, E., Klein, U.: 1989, Astroph. Sp. Sc. 156, 183   
\item Dumke, M., Wielebinski, R., Krause, M., Klein, U.: 1995, A\&A 302, 691 
\item Fritz, T., Mebold, U., Klein, U., C\^ot\'e, S.: 1998, in {\it The
        Magellanic Clouds and Other Dwarf Galaxies}, R.J. Braun and T. Richtler, 
        (eds.), Shaker--Verlag, P. 247 
\item Fritz, T.: 2000, Ph.D. thesis, Univ. Bonn
\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 Klein, U., Hummel, E., Bomans, D.J., Hopp, U.: 1996, A\&A 313, 396 
\item Kohle, S.: 1999, Ph. D. thesis, Univ. Bonn
\item Kohle, S., Henkel, C., Klein, U.: 2000, A\&A, subm.
\item Koribalski, B., Dahlem, M., Mebold, U., Brinks, E.: 1993, A\&A 268, 14 
\item L\"utticke, R.: 1999, Ph.D. thesis, Univ. Bochum
\item Mateo, M.: 1998, in {\it The Magellanic Clouds and Other Dwarf Galaxies},
        Workshop of the Bonn/Bochum Graduiertenkolleg, T. Richtler \& J.M. 
        Braun (eds.), Shaker--Verlag, p. 53 
\item Navarro, J.F., Frank, C.S., White, S.D.M.: 1996, Ap.J. 462, 563
\item Navarro, J.F., Steinmetz, M.: 2000, Ap.J. 528, 607
\item Ott, J., Walter, F., Brinks, E., Klein, U.: 2000, A.J., subm. (2000)
\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 Rubin, V.C., Ford, W.K., Thonnard, N.: 1980, Ap.J. 238, 471
\item Rubin, V.C., Ford, W.K., Thonnard, N.: 1982, Ap.J. 261, 43
\item Salucci, P., Burkert, A.: 2000, Ap.J. 537, L9
\item Sargent, W.L.W., Lo, K--Y.: 1990, in {\it Windows on Galaxies}, G. Fabbiano, 
        J.S. Gallagher and A. Renzini (eds.), Kluwer Acad. Publ., Dordrecht,
        p. 253
\item Schwarzkopf, U.: 1999, Ph.D. thesis, Univ. Bochum
\item Stil, J.: 1999, Ph.D. thesis, Univ. Leiden
\item Swaters, R.: 1999, Ph.D. thesis, Univ. Groningen
\item Taylor, C.L., Greve, A., H\"uttemeister, S., Klein, U.: 1999, A\&A 349,
        424 
\item Thuma, G., Neininger, N., Klein, U., Wielebinski, R.: 2000, A\&A 358, 65 
\item Walter, F., Brinks, E., Duric, N., Klein, U.: 1997, A.J. 113, 2031   
\item Walter,  Kerp, J., Duric, N., F.Brinks, E., Klein, U.: 1998, Ap.J. 502, 
        L143  
\item Walter, F.: 1999, Ph.D. thesis, Univ. Bonn
\item Wei{\ss}, A., Walter, F., Neininger, N., Klein, U.: 1999, A\&A 345, L23
\item Wei{\ss}, A.: 2000, Ph.D. thesis, Univ. Bonn

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\subsubsection{Estimate of resources expected from DFG}

We have already embarked on a first project to investigate the rotation 
curves of a fair sample of LSBGs. This project is embedded in our 
Graduiertenkolleg, as it requires a Ph.D. student for successful completion.
It is, however, foreseeable that a thorough and systematic investigation in
this research topic requires more efforts, both, in terms of man power and 
as far as resources are concerned. In order to plan and carry out the
observations, reduce the data, interprete and publish them, we require 
an experienced postdoctoral fellow, i.e. a full position (BATIIa). The 
postdoctoral fellow is expected to plan and carry out the relevant
observations. He is to publish them in refereed journals, and at the same
time present results on pertinent conferences. He is furthermore required
to organize the collaboration with other groups (with P. Salucci in Triest).
This necessitates travel funds to attend two conferences per year, and two
visits to the collaborating insitutes. The HI data cubes obtained in the 
course of this project requires a dedicated state--of--the--art computer
with periphery. This foreseeably requires about 5~kDM in the first year, and
about 2~kDM in the subsequent ones.


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

Priv. Doz. Dr. A. Heithausen (staff member); Dr. P. Kalberla (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 x.x.x. In particular, our proposed
search for dwarf satellites around massive galaxies perfectly complements
the search for, and study of, HVCs in the Milky Way and the Andromeda 
galaxy. The proposed scrutiny of galaxy rotation curves provides the 
obvious link to project x.x.x that investigates the nature of DM.  

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