We present and apply rigorous dynamical modeling with which we infer unprecedented constraints on the stellar and dark matter mass distribution within our Milky Way (MW), based on large sets of phase-space data on individual stars. Specifically, we model the dynamics of 16,269 G-type dwarfs from SEGUE, which sample 5 kpc < R GC < 12 kpc and 0.3 kpc |Z| 3 kpc. We independently fit a parameterized MW potential and a three-integral, action-based distribution function (DF) to the phase-space data of 43 separate abundance-selected sub-populations (MAPs), accounting for the complex selection effects affecting the data. We robustly measure the total surface density within 1.1 kpc of the mid-plane to 5% over 4.5 < R GC < 9 kpc. Using metal-poor MAPs with small radial scale lengths as dynamical tracers probes 4.5 R GC 7 kpc, while MAPs with longer radial scale lengths sample 7 R GC 9 kpc. We measure the mass-weighted Galactic disk scale length to be R d = 2.15 ± 0.14 kpc, in agreement with the photometrically inferred spatial distribution of stellar mass. We thereby measure dynamically the mass of the Galactic stellar disk to unprecedented accuracy: M * = 4.6 ± 0.3 + 3.0 (R 0 / kpc − 8) × 10 10 M ⊙ and a total local surface density of Σ R0 (Z = 1.1 kpc) = 68±4 M ⊙ pc −2 of which 38±4 M ⊙ pc −2 is contributed by stars and stellar remnants. By combining our surface density measurements with the terminal velocity curve, we find that the MW's disk is maximal in the sense that V c,disk /V c,total = 0.83±0.04 at R = 2.2 R d . We also constrain for the first time the radial profile of the dark halo at such small Galactocentric radii, finding that ρ DM (r; ≈ R 0 ) ∝ 1/r α with α < 1.53 at 95 % confidence. Our results show that action-based distribution-function modeling of complex stellar data sets is now a feasible approach that will be fruitful for interpreting Gaia data.

We describe a powerful technique to model and interpret the stellar line-of-sight velocity profiles of galaxies. It is based on Schwarzschild's approach to build fully general dynamical models. A representative library of orbits is calculated in a given potential, and the non-negative superposition of these orbits is determined that best fits a given set of observational constraints. Our implementation incorporates several new features: (i) we calculate velocity profiles and represent them by a Gauss-Hermite series. This allows us to constrain the orbital anisotropy in the fit. (ii) we take into account the error on each observational constraint to obtain an objective χ 2 measure for the quality-of-fit. Given the observational constraints, the technique assesses the relative likelihood of different orbit combinations in a given potential, and of models with different potentials. In our implementation only projected, observable quantities are included in the fit, aperture binning and seeing convolution of the data are properly taken into account, and smoothness of the models in phase-space can be enforced through regularization. This scheme is valid for any geometry.In a first application of this method, we focus here on spherical geometry; axisymmetric modeling is described in companion papers by Cretton et al. and van der Marel et al. We test the scheme on pseudo-data drawn from an isotropic Hernquist model, and then apply it to the issue of dark halos around elliptical galaxies. We model radially extended stellar kinematical data for the E0 galaxy NGC 2434, obtained by Carollo et al. Constant mass-to-light ratio models are clearly ruled out, regardless of the orbital anisotropy. To study the amount of dark matter needed to match the data, we considered a sequence of cosmologically motivated 'star+halo' potentials. These potentials are based on the CDM simulations by Navarro et al., but also account for the accumulation of baryonic matter; they are specified by the stellar mass-to-light ratio Υ * ,B and the characteristic halo velocity, V 200 . The star+halo models provide an excellent fit to the data, with Υ * ,B = 3.35 ± 0.25 (in B-band solar units) and V 200 = 450 ± 100 km/s. The best-fitting potential has a circular velocity V c that is constant to within ∼ 10% between 0.2-3 effective radii and is very similar to the best-fitting logarithmic potential, which has V c = 300 ± 15 km s −1 . In NGC 2434 roughly half of the mass within an effective radius is dark. Models without a dark halo overestimate the mass-to-light ratio of the stellar population by a factor of ∼ 2.

Low-ionization nuclear emission-line regions (LINERs), which exist in a large fraction of galaxies, may be the least luminous manifestation of quasar activity. As such, they may allow the study of the AGN phenomenon in the nearest galaxies. The nature of LINERs has, however, remained controversial because an AGN-like nonstellar continuum source has not been directly observed in them. We report the detection of bright ( > 2 10 16 erg s 1 cm 2 A 1 ), unresolved (FWHM < 0:1 00 ) point sources of UV ( 2300 A) emission in the nuclei of nine nearby galaxies. The galaxies were imaged using the Faint Object Camera on the Hubble Space Telescope (HST), and seven of them are from a complete sample of 110 nearby galaxies that was observed with HST. Ground-based optical spectroscopy reveals that ve of the nuclei are LINERs, three are starburst nuclei, and one is a Seyfert nucleus. The observed UV ux in each of the ve LINERs implies an ionizing ux that is su cient to account for the observed emission lines through photoionization. The detection of a strong UV continuum in the LINERs argues against shock excitation as the source of the observed emission lines, and supports the idea that photoionization excites the lines in at least some objects of this class.We have analyzed ground-based spectra for most of the Northern-hemisphere galaxies in the HST sample, and nd that 26 of them are LINERs, among which only the above ve LINERs have a detected nuclear UV source. There are no obvious di erences in the optical line intensity ratios between the UV-bright and UV-dark LINERs. If all LINERs are photoionized, then the continuum source is unobscured along our line of sight in 5=26 20% of LINERs. Alternatively, it can be argued that spectrally-similar LINERs are produced by various excitation mechanisms, and that photoionization is responsible in only about 20% of the cases. The high angular resolution allows us to set upper limits, typically several parsecs, on the physical size of the compact star-cluster or AGN-type continuum source that is emitting the UV light in these objects.