We report the results of [O III] λ5007 surveys for planetary nebulae (PNe) in six galaxies: NGC 2403, NGC 3115, NGC 3351, NGC 3627, NGC 4258, and NGC 5866. Using on-band/off-band [O III] λ5007 images, as well as images taken in Hα, we identify samples of PNe in these galaxies and derive their distances using the planetary nebula luminosity function (PNLF). We then combine these measurements with previous data to compare the PNLF, Cepheid, and surface brightness fluctuation (SBF) distance scales. We use a sample of 13 galaxies to show that the absolute magnitude of the PNLF cutoff is fainter in small, low-metallicity systems, but the trend is well modeled by the theoretical relation of Dopita, Jacoby, & Vassiliadis (1992). When this metallicity dependence is removed, the scatter between the Cepheid and PNLF distances becomes consistent with the internal errors of the methods and independent of any obvious galaxy parameter. We then use these data to recalibrate the zero point of the PNLF distance scale. We use a sample of 28 galaxies to show that the scatter between the PNLF and SBF distance measurements agrees with that predicted from the techniques' internal errors, and that there is no systematic trend between the distance residuals and stellar population. However, we also find that the PNLF and SBF methods have a significant scale offset: Cepheid-calibrated PNLF distances are, on average, ∼ 0.3 mag smaller than Cepheid-calibrated SBF distances. We discuss the possible causes of this offset, and suggest that internal extinction in the bulges of the SBF calibration galaxies is the principle cause of the discrepancy. If this hypothesis is correct, then the SBF-based Hubble Constant must be increased by ∼ 7%. We also use our distance to NGC 4258 to argue that the short distance scale to the LMC is correct, and and that the global Hubble Constant inferred from the HST Key Project should be increased by 8 ± 3% to H 0 = 78 ± 7 km s −1 Mpc −1 .

The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos and the properties of the Milky Way's dwarf galaxy satellites. We review the current observational and theoretical status of these "small-scale controversies." Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: Recent numerical simulations and analytical models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies, where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low-mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.dark matter | cosmology | galaxy formation T he cold dark matter (CDM) hypothesis-that dark matter consists of a weakly interacting particle whose velocity dispersion in the early universe was too small to erase structure on a galactic or subgalactic scale-emerged in the early 1980s and quickly became a central element of the theory of cosmic structure formation. Influential early papers include Peebles' calculation of cosmic microwave background (CMB) anisotropies and the matter power spectrum (1), discussions of galaxy formation with particle dark matter by Bond et al. (2) and Blumenthal et al. (3,4), and Davis et al.'s (5) numerical simulations of galaxy clustering. By the mid-1990s, the simplest CDM model with scale-invariant primordial fluctuations and a critical matter density of ðΩ m = 1Þ had run afoul of multiple lines of observational evidence, including the shape of the galaxy power spectrum, estimates of the mean matter density from galaxy clusters and galaxy motions, the age of the universe inferred from estimates of the Hubble constant, and the amplitude of matter clustering extrapolated forward from the fluctuations measured in the CMB. Many variants on "canonical" CDM were proposed to address these challenges, and by the turn of the century, the combination of supernova evidence for cosmic acceleration and CMB evidence for ...

We present high-resolution optical velocity fields from DensePak integral field spectroscopy, along with derived rotation curves, for a sample of low surface brightness galaxies. In the limit of no baryons, we fit the NFW and pseudoisothermal halo models to the data and find the rotation curve shapes and halo central densities to be better described by the isothermal halo. For those galaxies with photometry, we present halo fits for three assumptions of the stellar mass-to-light ratio, Υ * . We find that the velocity contribution from the baryons is significant enough in the maximum disk case that maximum disk and the NFW halo are mutually exclusive. We find a substantial cusp mass excess at the centers of the galaxies, with at least two times more mass expected in the cuspy CDM halo than is allowed by the data. We also find that to reconcile the data with ΛCDM, ∼20 km s −1 noncircular motions are needed and/or the power spectrum has a lower amplitude on the scales we probe.