The Density Profile of the Dark Matter Halo of NGC 4605

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Abstract
We have obtained ~100 pc resolution CO and ~60 pc resolution Halpha observations of the dwarf spiral galaxy NGC 4605. We use them to derive a high resolution rotation curve and study the central density profile of NGC 4605's dark matter halo. We find that these observations do not agree with the predictions of most high resolution Cold Dark Matter calculations. We investigate two extreme cases: 1) NGC 4605 has a maximal exponential disk, that we model using K-band observations and remove to study the structure of its dark matter halo, and 2) NGC 4605 is dark matter dominated and its disk is dynamically negligible. Because the mass-to-light ratio of the maximal disk is already very low we favor the first solution, which indicates the halo has one component with a density profile rho ~ r^{-0.6} out to R~2.8 kpc. In the second case, the rotation curve requires the presence of two components: a small (~600 pc) core surrounded by a much steeper rho ~ r^{-1.1} halo. Removal of intermediate (submaximal) disks does not ameliorate the discrepancy between the predictions and the observations.Comment: 8 pages, 5 figures. Revised version (minor changes), accepted for publication in The Astrophysical Journal. Also available at http://astron.berkeley.edu/~bolatto