Measuring Stellar and Dark Mass Fractions in Spiral Galaxies
We explore the relative importance of the stellar mass density as compared to the inner dark halo, for the observed gas kinematics thoughout the disks of spiral galaxies. We perform hydrodynamical simulations of the gas flow in a sequence of potentials wi
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Abstract. We explore the relative importance of the stellar mass density as compared to the inner dark halo, for the observed gas kinematics thoughout the disks of spiral galaxies. We perform hydrodynamical simulations of the gas flow in a sequence of potentials with varying the stellar contribution to the total potential. The stellar portion of the potential was derived empirically from K-band photometry. The output of the simulations - namely the gas density and the gas velocity field - are then compared to the observed spiral arm morphology and the Ha gas kinematics. We solve for the best matching spiral pattern speed and draw conclusions on how massive the stellar disk can be at most. For the case of the galaxy NGC 4254 (Messier 99) we demonstrate that the prominent spiral arms of the stellar component would overpredict the non-circular gas motions unless an axisymmetric dark halo component adds significantly in the radial range Rexp < R < 3 Rexp .
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Introduction
In almost all galaxy formation scenarios, non-baryonic dark matter plays an important role. Today's numerical simulations of cosmological structure evolution quite successfully reproduce the observed galaxy distribution in the universe [3]. While galaxies form and evolve inside dark halos their physical appearance depends strongly on the local star formation and merging history. At the same time the halos evolve and merge as well. According to the simulations, we expect that the dark matter is important in the inner parts of galaxies [5],[6] and that it thus has a considerable influence on the kinematics. These predictions are in contrast to some studies which indicate that galactic stellar disks - at least of barred spiral galaxies - alone dominate the kinematics of the inner regions [lJ. Apparently this is also the case in our own Milky Way [2J. Determining individual mass fractions of the luminous and dark matter is not a straightforward task. The rotation curve of a disk galaxy is only sensitive to the total amount of gravitating matter, but does not allow the distinction between the two mass density profiles. For a detailed analysis it is necessary to adopt more refined methods to separate out the different profiles. Previous investigations used for example knowledge of the kinematics of rotating bars [8] or the geometry of gravitational lens systems [4J. Here we would like to exploit the fact, that the stellar mass in disk galaxies is often organized in spiral arms, i.e. in coherent non-axisymmetric structures. In most proposed scenarios, the dark matter, however, is collisionless and dominated by random motions. Therefore it is not susceptible to spiral structures. If the stellar mass dominates, H. V. Klapdor-Kleingrothaus (ed.), Dark Matter in Astro- and Particle Physics © Springer-Verlag Berlin Heidelberg 2001
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Fig. 1. Near infrared K'-band (2.1Ilm) image of NGC 4254 with a total exposure time of 20 minutes at the Calar Alto 3.5 m telescope. Bright foreground star
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