We present high-resolution Hα rotation curves for a sample of 15 dwarf and low surface brightness galaxies. From these, we derive limits on the slopes of the central mass distributions, using both a direct inversion of the rotation curves as well as detailed mass models. Assuming the density distributions of dark matter halos follow a power-law at small radii, ρ(r) ∝ r −α , we find inner slopes in the range 0 < ∼ α < ∼ 1 for most galaxies. Thus, even with the relatively high spatial resolution of the Hα rotation curves presented here the inner slopes are poorly constrained. In general, halos with constant density cores (α = 0) provide somewhat better fits, but the majority of our galaxies (∼ 75 percent) are also consistent with α = 1, as long as the R-band stellar massto-light ratios are smaller than about 2. Halos with α = 1.5, however, are ruled out in virtually every case. In order to investigate the robustness of these results we discuss and model several possible causes of systematic errors including non-circular motions, galaxy inclination, slit width, seeing, and slit alignment errors. Taking the associated uncertainties into account, we conclude that even for the ∼ 25 percent of the cases where α = 1 seems inconsistent with the rotation curves, we cannot rule out cusp slopes this steep. Inclusion of literature samples similar to the one presented here leads to the same conclusion when the possibility of systematic errors is taken into account. In the ongoing debate on whether the rotation curves of dwarf and low surface brightness galaxies are consistent with predictions for a cold dark matter universe, we argue that our sample and the literature samples discussed in this paper provide insufficient evidence to rule out halos with α = 1. At the same time, we note that none of the galaxies in these samples require halos with steep cusps, as most are equally well or better explained by halos with constant density cores.
We present new constraints on the density profiles of dark matter (DM) halos in seven nearby dwarf galaxies from measurements of their integrated stellar light and gas kinematics. The gas kinematics of low mass galaxies frequently suggest that they contain constant density DM cores, while N-body simulations instead predict a cuspy profile. We present a data set of high resolution integral field spectroscopy on seven galaxies and measure the stellar and gas kinematics simultaneously. Using Jeans modeling on our full sample, we examine whether gas kinematics in general produce shallower density profiles than are derived from the stars. Although 2/7 galaxies show some localized differences in their rotation curves between the two tracers, estimates of the central logarithmic slope of the DM density profile, γ, are generally robust. The mean and standard deviation of the logarithmic slope for the population are γ = 0.67 ± 0.10 when measured in the stars and γ = 0.58 ± 0.24 when measured in the gas. We also find that the halos are not under-concentrated at the radii of half their maximum velocities. Finally, we search for correlations of the DM density profile with stellar velocity anisotropy and other baryonic properties. Two popular mechanisms to explain cored DM halos are an exotic DM component or feedback models that strongly couple the energy of supernovae into repeatedly driving out gas and dynamically heating the DM halos. While such models do not yet have falsifiable predictions that we can measure, we investigate correlations that may eventually be used to test models. We do not find a secondary parameter that strongly correlates with the central DM density slope, but we do find some weak correlations. The central DM density slope weakly correlates with the abundance of α elements in the stellar population, anti-correlates with HI fraction, and anti-correlates with vertical orbital anisotropy. We expect, if anything, the opposite of these three trends for feedback models. Determining the importance of these correlations will require further model developments and larger observational samples.
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