The Cusp/Core Problem in Galactic Halos: Long-Slit Spectra for a Large Dwarf Galaxy Sample

Abstract

We derive inner dark matter halo density profiles for a sample of 165 low-mass galaxies using rotation curves obtained from high-quality, long-slit optical spectra assuming minimal disks and spherical symmetry. For ρ(r)~r-α near the galaxy center, we measure median inner slopes ranging from αm=0.22+/-0.08 to 0.28+/-0.06 for various subsamples of the data. This is similar to values found by other authors and in stark contrast to the intrinsic cusps (αint~1) predicted by simulations of halo assembly in cold dark matter (CDM) cosmologies. To elucidate the relationship between αm and αint in our data, we simulate long-slit observations of model galaxies with halo shapes broadly consistent with the CDM paradigm. Simulations with αint=1/2 and 1 recover both the observed distribution of αm and correlations between αm and primary observational parameters such as distance and disk inclination, whereas those with αint=5/4 are marginally consistent with the data. Conversely, the hypothesis that low-mass galaxies have αint=3/2 is rejected. While the simulations do not imply that the data favor intrinsic cusps over cores, they demonstrate that the discrepancy between αm and αint~1 for our sample does not necessarily imply a genuine conflict between our results and CDM predictions; rather, the apparent cusp/core problem may be reconciled by considering the impact of observing and data-processing techniques on rotation curves derived from long-slit spectra.