Scott T. Miller

Extraplanar Emission-Line Gas in Edge-On Spiral Galaxies. I. Deep Emission-Line Imaging

The extraplanar diffuse ionized gas (eDIG) in 17 nearby, edge-on disk galaxies is studied using deep Taurus Tunable Filter H? and [N II] ?6583 images and conventional interference filter H?+[N II] ??6548, 6583 images that reach flux levels generally below ~1 ? 10-17 ergs s-1 cm-2 arcsec-2. [N II] ?6583/H? excitation maps are available for 10 of these objects. All but one galaxy in the sample exhibit eDIG. The contribution of the eDIG to the total H? luminosity is relatively constant, on the order of 12% ? 4%. The H? scale height of the eDIG derived from a two-exponential fit to the vertical emission profile ranges from 0.4 to 17.9 kpc, with an average of 4.3 kpc. This average value is noticeably larger than the eDIG scale height measured in our Galaxy and other galaxies. This difference in scale height is probably due in part to the lower flux limits of our observations. The ionized mass of the extraplanar component inferred by assuming a constant filling factor of 0.2 and a constant path length through the disk of 5 kpc ranges from 1.4 ? 107 to 2.4 ? 108 M?, with an average value of 1.2 ? 108 M?. Under these same assumptions, the recombination rate required to keep the eDIG ionized ranges from 0.44 ? 106 to 13 ? 106 s-1 cm-2 of the disk, or about 10%-325% of the Galactic value. A quantitative analysis of the topology of the eDIG confirms that several galaxies in the sample have a highly structured eDIG morphology. The distribution of the eDIG emission is often correlated with the locations of the H II regions in the disk, supporting the hypothesis that the predominant source of ionization of the eDIG is photoionization from OB stars located in the H II regions. A strong correlation is found between the IR (or far-IR) luminosities per unit disk area (basically a measure of the star formation rate per unit disk area) and the extraplanar ionized mass, further providing support for a strong connection between the disk and eDIG components in these galaxies. The excitation maps confirm that the [N II]/H? ratios are systematically higher in the eDIG than in the disk. Although photoionization by disk OB stars is generally able to explain these elevated [N II]/H? ratios, a secondary source of ionization appears to be needed when one also takes into account other line ratios; more detail is given in a companion paper (our Paper II).

Extraplanar Emission-Line Gas in Edge-on Spiral Galaxies. II. Optical Spectroscopy

The results from deep long-slit spectroscopy of nine edge-on spiral galaxies with known extraplanar line emission are reported. Emission from Hα, [N II] λλ6548, 6583, and [S II] λλ6716, 6731 is detected out to heights of a few kiloparsecs in all of these galaxies. Several other fainter diagnostic lines such as [O I] λ6300, [O III] λλ4959, 5007, and He I λ5876 are also detected over a smaller scale. The relative strengths, centroids, and widths of the various emission lines provide constraints on the electron density, temperature, reddening, source(s) of ionization, and kinematics of the extraplanar gas. In all but one galaxy, photoionization by massive OB stars alone has difficulties explaining all of the line ratios in the extraplanar gas. Hybrid models that combine photoionization by OB stars and another source of ionization such as photoionization by turbulent mixing layers or shocks provide a better fit to the data. The (upper limits on the) velocity gradients measured in these galaxies are consistent with the predictions of the galactic fountain model to within the accuracy of the measurements.

A hybridizable discontinuous Galerkin method for modeling fluid-structure interaction

Abstract This work presents a novel application of the hybridizable discontinuous Galerkin (HDG) finite element method to the multi-physics simulation of coupled fluid–structure interaction (FSI) problems. Recent applications of the HDG method have primarily been for single-physics problems including both solids and fluids, which are necessary building blocks for FSI modeling. Utilizing these established models, HDG formulations for linear elastostatics, a nonlinear elastodynamic model, and arbitrary Lagrangian–Eulerian Navier–Stokes are derived. The elasticity formulations are written in a Lagrangian reference frame, with the nonlinear formulation restricted to hyperelastic materials. With these individual solid and fluid formulations, the remaining challenge in FSI modeling is coupling together their disparate mathematics on the fluid–solid interface. This coupling is presented, along with the resultant HDG FSI formulation. Verification of the component models, through the method of manufactured solutions, is performed and each model is shown to converge at the expected rate. The individual components, along with the complete FSI model, are then compared to the benchmark problems proposed by Turek and Hron [1] . The solutions from the HDG formulation presented in this work trend towards the benchmark as the spatial polynomial order and the temporal order of integration are increased.

Numerical investigation of coupling schemes for structural acoustics

Loosely coupled schemes for structural-acoustic coupling are examined that obtain the same order of accuracy as the monolithic scheme. The coupling algorithms are implemented in Sierra-SD, a massively parallel finite element application for structural dynamics and acoustics. By adapting the predictor-corrector scheme of Farhat et al. (2006), second order time accuracy is achieved with the loosely coupled approach. Node-to-face mappings allow arbitrary discretizations of the structural-acoustic interface. Convergence rates are verified with a one dimensional piston problem with known solution. Numerical results are compared to a shock induced plate experimental benchmark. Computational times for loose and strong coupling are compared for a sphere scattering problem. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energys Nat...

A spacetime finite element method for coupled acoustic fluid-structure interactions

The spacetime discontinuous Galerkin (SDG) finite element method is applied to problems of acoustic propagation. The spacetime discontinuous Galerkin are a family of discontinuous finite element methods for hyperbolic systems of equations. They utilize an advancing front mesh generation procedure that allows local “patches” of elements to become decoupled from the global solution domain through causality. Causal space-time meshes enable a locally implicit solution procedure with provably linear computational complexity. Mesh adaptivity is shown to effectively resolve propagating waves and minimize dispersion error. Numerical results demonstrate the effectiveness of the proposed solution method, including achieving optimal convergence rates. Time domain simulations of several canonical acoustic problems are presented.