The Dark Matter Content of Barred Spiral Galaxies

Facilities: BIMA, 2MASS
Collaborators:Dr. M. Das, Dr. P. Teuben, Dr. S. Vogel
Status: The CO data already exist. Higher resolution data will be taken as needed. We are testing different methods of determining the gravitational potentials.

Summary: The evidence for dark matter in the outer parts of galaxies and on larger scales is compelling. However, the dark matter distribution and its contribution to the gravitational potential are poorly constrained within the luminous portions of galaxies. This severely impedes understanding of the basic structure and origin of galaxies, including the relevance of current cosmological simulations, the physical nature of the Fisher-Tully relation, and the origin of bar and spiral structure. We are using BIMA observations of CO emission in barred spirals to determine the mass to light ratio of the stellar component, and thereby enable determination of the dark matter contribution to the gravitational potential.

The luminous matter in galaxies cannot produce the extended, flat rotation curves characteristic of spiral galaxies (Rubin, Ford & Thonnard 1978, Bosma 1981), which is generally taken to indicate that the mass in the outer parts of galaxies is dominated by dark matter. However, the contribution of dark matter at smaller galactic radii is much less clear. Rotation curves trace the total dynamical mass, i.e., both the visible mass and the dark matter. A given rotation curve can be equally well fitted with a massive stellar disk and a dark matter halo which is important only in the outer disk (Maximum Disk Hypothesis; van Albada & Sancisi 1986), or alternatively with a lower mass disk and a more massive halo (Bottema 1997). As a result, the nature of spiral galaxies remains uncertain even at the most basic level.

Recently, Weiner et al. (2001a, 20001b) have shown that observations of the gas kinematics in barred spiral galaxies removes the degeneracy in the determination of the relative masses of the stellar disk and the dark matter halo. The method is powerful. Put simply, perturbations in the gas kinematics depend on the stellar mass in the bar. By comparing the hydodynamical simulations to the observed gas flow we can determine the mass of the bar. This, combined with the luminosity of the bar, gives us a direct measure for the mass to light ratio of the bar. Comparing this with the total mass of the inner galaxy determined from the rotation curve will allow us to determine the contribution of the dark matter to the overall potentials of these systems.

We are taking advantage of the new, large database of CO observations from the BIMA Survey of Nearby Galaxies (SONG) to expand the analysis of Weiner et al. to a much larger sample of galaxies. We are using 2MASS images to determine the stellar mass distribution (assuming a variety of scale heights). This mass distribution is being implemented into the hydrodynamical galaxy code (e.g. Piner, Stone & Teuben 1995) and gas flow simulations will be run for a variety of disk masses. We will also determine the relative importance of such unknowns as scale height and bulge shape to the overall values of dark matter mass.


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