A new code for astrophysical magnetohydrodynamics (MHD) is described. The code has been designed to be easily extensible for use with static and adaptive mesh refinement. It combines higher-order Godunov methods with the constrained transport (CT) technique to enforce the divergence-free constraint on the magnetic field. Discretization is based on cell-centered volume-averages for mass, momentum, and energy, and face-centered area-averages for the magnetic field. Novel features of the algorithm include (1) a consistent framework for computing the time- and edge-averaged electric fields used by CT to evolve the magnetic field from the time- and area-averaged Godunov fluxes, (2) the extension to MHD of spatial reconstruction schemes that involve a dimensionally-split time advance, and (3) the extension to MHD of two different dimensionally-unsplit integration methods. Implementation of the algorithm in both C and Fortran95 is detailed, including strategies for parallelization using domain decomposition. Results from a test suite which includes problems in one-, two-, and three-dimensions for both hydrodynamics and MHD are given, not only to demonstrate the fidelity of the algorithms, but also to enable comparisons to other methods. The source code is freely available for download on the web.Comment: 61 pages, 36 figures. accepted by ApJ

The Astropy Project supports and fosters the development of open-source and openly developed Python packages that provide commonly needed functionality to the astronomical community. A key element of the Astropy Project is the core package astropy, which serves as the foundation for more specialized projects and packages. In this article, we provide an overview of the organization of the Astropy project and summarize key features in the core package, as of the recent major release, version 2.0. We then describe the project infrastructure designed to facilitate and support development for a broader ecosystem of interoperable packages. We conclude with a future outlook of planned new features and directions for the broader Astropy Project.

We suggest that the high-velocity clouds (HVCs) are large clouds, with typical diameters of 25 kpc, containing 3 ] 107 of neutral gas and 3 ] 108 of dark matter, falling onto the Local Group ; M _ M _ altogether the HVCs contain 1010 of neutral gas. Our reexamination of the Local Group hypothesis M _ for the HVCs connects their properties to the hierarchical structure formation scenario and to the gas seen in absorption toward quasars. We show that at least one HVC complex (besides the Magellanic Stream) must be extragalactic at a distance of more than 40 kpc from the Galactic center, with a diameter greater than 20 kpc and a mass of more than 108We discuss a number of other clouds that M _ . are positionally associated with the Local Group galaxies, and we show that the entire ensemble of HVCs is inconsistent with a Galactic origin. The observed kinematics imply rather that the HVCs are falling toward the Local Group barycenter. We simulate the dynamical evolution of the Local Group and Ðnd that material falling onto the Local Group reproduces the location of two of the three most signiÐcant groupings of clouds and the kinematics of the entire cloud ensemble (excluding the Magellanic Stream). We interpret the third grouping (the A, C, and M complexes) as the nearest HVC. It is tidally unstable and is falling onto the Galactic disk. We interpret the more distant HVCs as gas contained within dark matter "" minihalos ÏÏ moving along Ðlaments toward the Local Group. Most poor galaxy groups should contain similar H I clouds bound to the group at large distances from the individual galaxies. We suggest that the HVCs are local analogs of the Lyman limit absorbing clouds observed against distant quasars. Our picture implies that the chemical evolution of the Galactic disk is governed by episodic infall of metal-poor HVC gas that only slowly mixes with the rest of the interstellar medium.We argue that there is a Galactic fountain in the Milky Way, but that the fountain does not explain the origin of the HVCs. Our analysis of the H I data leads to the detection of a vertical infall of lowvelocity gas toward the plane and implies that the H I disk is not in hydrostatic equilibrium. We suggest that the fountain is manifested mainly by relatively local neutral gas with characteristic velocities of 6 km s~1 rather than 100 km s~1.The Local Group infall hypothesis makes a number of testable predictions. The HVCs should have subsolar metallicities. Their Ha emission should be less than that seen from the Magellanic Stream. The clouds should not be seen in absorption against nearby stars. The clouds should be detectable in both emission and absorption around other galaxy groups. We show that current observations are consistent with these predictions and discuss future tests.

We present an analysis of the molecular gas distributions in the 29 barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of Nearby Galaxies (SONG). For galaxies that are bright in CO, we confirm the conclusion by Sakamoto et al. (1999b) that barred spirals have higher molecular gas concentrations in the central kiloparsec. The SONG sample also includes 27 galaxies below the CO brightness limit used by Sakamoto et al. Even in these less CO-bright galaxies we show that high central gas concentrations are more common in barred galaxies, consistent with radial inflow driven by the bar. However, there is a significant population of early-type (Sa-Sbc) barred spirals (6 of 19) that have no molecular gas detected in the nuclear region, and have very little out to the bar co-rotation radius. This suggests that in barred galaxies with gas-deficient nuclear regions the bar has already driven most of the gas within the bar co-rotation radius to the nuclear region, where it has been consumed by star formation. The median mass of nuclear molecular gas is over four times higher in early type bars than in late type (Sc-Sdm) bars. Since previous work has shown that the gas consumption rate is an order of magnitude higher in early type bars, this implies that the early types have significantly higher bar-driven inflows. The lower accretion rates in late type bars can probably be attributed to the known differences in -2bar structure between early and late types. Despite the evidence for bar-driven inflows in both early and late Hubble type spirals, the data indicate that it is highly unlikely for a late type galaxy to evolve into an early type via barinduced gas inflow. Nonetheless, secular evolutionary processes are undoubtedly present, and pseudo-bulges are inevitable; evidence for pseudo-bulges is likely to be clearest in early-type galaxies because of their high gas inflow rates and higher star formation activity.

We present interferometric CO observations made with the Combined Array for Millimeter-wave Astronomy (CARMA) of galaxies from the Extragalactic Database for Galaxy Evolution survey (EDGE). These galaxies are selected from the Calar Alto Legacy Integral Field Area (CALIFA) sample, mapped with optical integral field spectroscopy. EDGE provides good quality CO data (3σ sensitivitity Σ mol ∼ 11 M pc −2 before inclination correction, resolution ∼ 1.4 kpc) for 126 galaxies, constituting the largest interferometric CO survey of galaxies in the nearby universe. We describe the survey, the data characteristics, the data products, and present initial science results. We find that the exponential scale-lengths of the molecular, stellar, and star-forming disks are approximately equal, and galaxies that are more compact in molecular gas than in stars tend to show signs of interaction. We characterize the molecular to stellar ratio as a function of Hubble type and stellar mass, present preliminary results on the resolved relations between the molecular gas, stars, and star formation rate, and discuss the dependence of the resolved molecular depletion time on stellar surface density, nebular extinction, and gas metallicity. EDGE provides a key dataset to address outstanding topics regarding gas and its role in star formation and galaxy evolution, which will be publicly available on completion of the quality assessment.