Teaching Modeling to Engineers in an Undergraduate Simulation Course

A significant challenge in teaching simulation to undergraduate students is to find a way to allow them to model a real world referent system within time and student skill constraints. Several research sources highlight not only the important challenge of model development (Garcia and Ceneno, 2009, Tako, 2011) but also the increased need for model development instruction among engineers (Grasas et. al., 2013, Saltzman and Roeder, 2013). One approach to this challenge is to use a general purpose discrete event simulation software package within the course, but this presents two challenges. Teaching the package to the students takes significant time, and the package introduces limitations which may restrict their ability to model certain real-world referents, particularly in the engineering domain. A conceptual approach to solving this problem is to use a model development paradigm that abstracts away the interface to the simulation infrastructure while still allowing the students to use the full expressive nature of a programming language. Two undergraduate courses at the United States Military Academy employed this strategy via the Discrete Events Specification System – Distributed Modeling Framework (DEVS-DMF) (Kewley et. al, 2016). The DEVS abstraction allowed students to think about their model as a simple state change function with defined inputs and outputs, and DMF allowed them to program in a cloud-based Jupyter Notebook using the Python language. Students in a combat modeling course employed a variety of models to understand drone jamming, and students in an engineering capstone project employed models to account for human factors in rifle marksmanship. The effectiveness of this approach was assessed through student grades, exit-interviews, and course-end surveys. These assessments showed an increased understanding of the model development process, and students also reported greater ownership of their models. However, this experiment also highlighted some weaknesses in their understanding of underlying methodologies and programming skills

Exploring Primitives of Meaning in Support of Interoperability

Semantic mismatch between systems is due, in part, to the grouping together of terms who have defined meaning in different levels of granularity, and which are composed together into different groupings by distinct systems. It has been proposed that making use of elemental concepts (referred to here as primitives of meaning) can assist in interoperability, but seeking to define all terms at a level of granularity equal to or greater than that of all involved systems. By decomposing a system’s groups of composed terms into primitives of meaning, the building blocks that can be reassembled into the compositions required by another group (of another system, for instance) can be made apparent. While such a de-composition could serve as the basis for an interoperability enabler, having the decomposition available as a common descriptor to highlight areas of semantic misalignment should prove in itself useful. Taking doctrinal statements for US small unit infantry actions as one semantic system, we show how the elemental ideas that are grouped together into commands can be identified and isolated for reconstruction into other groupings. This is the first research step towards relying on primitives of meaning for interoperability

Utilizing Strategic Project Management Processes and the NATO Code of Best Practice to Improve Management of Experimentation Events

Systems engineering and project management are two core engineering management processes supported by core quantitative disciplines within engineering management problems. Traditional approaches to systems engineering focus on a single system being engineered and managed (i.e., project managed), while challenges addressing composition of systems of systems and the reuse of systems for new solutions require a strategic management approach that promote a process flow in which the outputs of one project (e.g., deliverables, knowledge, work documents) are captured for the benefit of other projects within and outside the project-based organization. Two other core processes of engineering management are therefore critical to be incorporated into this process flow: knowledge management and strategic management. Consequently, when applying complex simulation system or federations of simulation systems for experimentation, knowledge management and strategic management are needed. The NATO Code of Best Practice (COBP) for Command and Control Assessment is dealing with similar challenges. Within a project in support of PEO Soldier, Old Dominion University and the United States Military Academy developed new system engineering processes in support of system selection and orchestration that allow merging the knowledge and strategic management ideas with NATO\u27s recommended best practices

Structure and Feedback in 30 Doradus I: Observations

We have completed a a new optical imaging and spectrophotometric survey of a 140 x 80 pc$^2$ region of 30 Doradus centered on R136, covering key optical diagnostic emission lines including \Ha, \Hb, \Hg, [O III] $\lambda\lambda$4363, 4959, 5007, [N II] $\lambda\lambda$6548, 6584, [S II] $\lambda\lambda$6717, 6731 [S III] $\lambda$6312 and in some locations [S III] $\lambda$9069. We present maps of fluxes and intensity ratios for these lines, and catalogs of isolated ionizing stars, elephant-trunk pillars, and edge-on ionization fronts. The final science-quality spectroscopic data products are available to the public. Our analysis of the new data finds that, while stellar winds and supernovae undoubtedly produce shocks and are responsible for shaping the nebula, there are no global spectral signatures to indicate that shocks are currently an important source of ionization. We conclude that the considerable region covered by our survey is well described by photoionization from the central cluster where the ionizing continuum is dominated by the most massive O stars. We show that if 30 Dor were viewed at a cosmological distance, its integrated light would be dominated by its extensive regions of lower surface-brightness rather than by the bright, eye-catching arcs.Comment: 42 pages, 16 figures, Accepted for publication in ApJ

Revealing components of the galaxy population through nonparametric techniques

The distributions of galaxy properties vary with environment, and are often multimodal, suggesting that the galaxy population may be a combination of multiple components. The behaviour of these components versus environment holds details about the processes of galaxy development. To release this information we apply a novel, nonparametric statistical technique, identifying four components present in the distribution of galaxy H$\alpha$ emission-line equivalent-widths. We interpret these components as passive, star-forming, and two varieties of active galactic nuclei. Independent of this interpretation, the properties of each component are remarkably constant as a function of environment. Only their relative proportions display substantial variation. The galaxy population thus appears to comprise distinct components which are individually independent of environment, with galaxies rapidly transitioning between components as they move into denser environments.Comment: 12 pages, 10 figures, accepted for publication in MNRA

An Oxygen Abundance Gradient into the Outer Disk of M81

The extended HI disk and tidal tails of M81 present an interesting environment to study the effects of galaxy interaction on star formation and chemical evolution of the outer disk of a large spiral galaxy. We present H{\alpha} imaging of the outer disk of M81 and luminosities for 40 HII regions out to about 3 times the optical radius. We have also obtained MMT spectra for 21 HII regions out to more than twice the optical radius. We derive strong line oxygen abundances for all HII regions using R_{23} based and [NII]/[OII] based calibrations and electron temperature abundances for seven regions spanning a galactocentric distance between 5.7 and 32 kpc. We also comment on the abundances of HII regions near KDG 61 and the "tidal dwarf" candidate HoIX. Our results constitute the most radially extended metallicity study for M81 to date. With this extended data set, we find an overall oxygen abundance gradient of -0.013 dex/kpc over the entire radial range. This is significantly flatter than what has been found in previous studies which were limited to the optical disk. From our temperature based abundances, we find a gradient of -0.020 dex/kpc and present the possibility of a broken gradient from these data, but note the need to obtain more temperature based abundances at intermediate galactocentric distances (~10-20 kpc) to verify whether or not this may be the case. We discuss our main result of a rather flat gradient for M81 in the context of simulations and observations of abundance gradients in other galaxies. We find that the shallow abundance gradient of M81 is likely a result of the interaction history of this galaxy.Comment: 41 pages, 11 figures, accepted for publication in MNRA

Core-Collapse Supernovae and Host Galaxy Stellar Populations

We have used images and spectra of the Sloan Digital Sky Survey to examine the host galaxies of 519 nearby supernovae. The colors at the sites of the explosions, as well as chemical abundances, and specific star formation rates of the host galaxies provide circumstantial evidence on the origin of each supernova type. We examine separately SN II, SN IIn, SN IIb, SN Ib, SN Ic, and SN Ic with broad lines (SN Ic-BL). For host galaxies that have multiple spectroscopic fibers, we select the fiber with host radial offset most similar to that of the SN. Type Ic SN explode at small host offsets, and their hosts have exceptionally strongly star-forming, metal-rich, and dusty stellar populations near their centers. The SN Ic-BL and SN IIb explode in exceptionally blue locations, and, in our sample, we find that the host spectra for SN Ic-BL show lower average oxygen abundances than those for SN Ic. SN IIb host fiber spectra are also more metal-poor than those for SN Ib, although a significant difference exists for only one of two strong-line diagnostics. SN Ic-BL host galaxy emission lines show strong central specific star formation rates. In contrast, we find no strong evidence for different environments for SN IIn compared to the sites of SN II. Because our supernova sample is constructed from a variety of sources, there is always a risk that sampling methods can produce misleading results. We have separated the supernovae discovered by targeted surveys from those discovered by galaxy-impartial searches to examine these questions and show that our results do not depend sensitively on the discovery technique.Comment: Accepted by the Astrophysical Journal (22 July 2012), conclusions not changed, extended discussion of sample construction and updated SN spectroscopic type

Large-scale shock-ionized and photo-ionized gas in M83: the impact of star formation

We investigate the ionization structure of the nebular gas in M83 using the line diagnostic diagram, [O III](5007 \degA)/H{\beta} vs. [S II](6716 \deg A+6731 \deg A)/H{\alpha} with the newly available narrowband images from the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST). We produce the diagnostic diagram on a pixel-by-pixel (0.2" x 0.2") basis and compare it with several photo- and shock-ionization models. For the photo-ionized gas, we observe a gradual increase of the log([O III]/H{\beta}) ratios from the center to the spiral arm, consistent with the metallicity gradient, as the H II regions go from super solar abundance to roughly solar abundance from the center out. Using the diagnostic diagram, we separate the photo-ionized from the shock-ionized component of the gas. We find that the shock-ionized H{\alpha} emission ranges from ~2% to about 15-33% of the total, depending on the separation criteria used. An interesting feature in the diagnostic diagram is an horizontal distribution around log([O III]/H{\beta}) ~ 0. This feature is well fit by a shock-ionization model with 2.0 Z\odot metallicity and shock velocities in the range of 250 km/s to 350 km/s. A low velocity shock component, < 200 km/s, is also detected, and is spatially located at the boundary between the outer ring and the spiral arm. The low velocity shock component can be due to : 1) supernova remnants located nearby, 2) dynamical interaction between the outer ring and the spiral arm, 3) abnormal line ratios from extreme local dust extinction. The current data do not enable us to distinguish among those three possible interpretations. Our main conclusion is that, even at the HST resolution, the shocked gas represents a small fraction of the total ionized gas emission at less than 33% of the total. However, it accounts for virtually all of the mechanical energy produced by the central starburst in M83.Comment: Accepted for publication in ApJ. aastex preprint 12pt, 21 pages, 13 figure

The relation between metallicity, stellar mass and star formation in galaxies: an analysis of observational and model data

We study relations between stellar mass, star formation and gas-phase metallicity in a sample of 177,071 unique emission line galaxies from the SDSS-DR7, as well as in a sample of 43,767 star forming galaxies at z=0 from the cosmological semi-analytic model L-GALAXIES. We demonstrate that metallicity is dependent on star formation rate at fixed mass, but that the trend is opposite for low and for high mass galaxies. Low-mass galaxies that are actively forming stars are more metal-poor than quiescent low-mass galaxies. High-mass galaxies, on the other hand, have lower gas-phase metallicities if their star formation rates are small. Remarkably, the same trends are found for our sample of model galaxies. We find that massive model galaxies with low gas-phase metallicities have undergone a gas-rich merger in the past, inducing a starburst which exhausted their cold gas reservoirs and shut down star formation. This led to a gradual dilution in the gas-phase metallicities of these systems via accretion of gas. These model galaxies have lower-than-average gas-to-stellar mass ratios and higher-than-average central black hole masses. We confirm that massive galaxies with low gas-phase metallicities in our observational sample also have very massive black holes. We propose that accretion may therefore play a significant role in regulating the gas-phase metallicities of present-day massive galaxies.Comment: 18 pages, 18 figures, 2 tables. Submitted to MNRA

A Survey of MgII Absorption at 2 < z < 6 with Magellan / FIRE: I: Sample and Evolution of the MgII Frequency

We present initial results from the first systematic survey for MgII quasar absorption lines at z > 2.5. Using infrared spectra of 46 high-redshift quasars, we discovered 111 MgII systems over a path covering 1.9 < z < 6.3. Five systems have z > 5, with a maximum of z = 5.33 - the most distant MgII system now known. The comoving MgII line density for weaker systems (Wr < 1.0A) is statistically consistent with no evolution from z = 0.4 to z = 5.5, while that for stronger systems increases three-fold until z \sim 3 before declining again towards higher redshifts. The equivalent width distribution, which fits an exponential, reflects this evolution by flattening as z approaches 3 before steepening again. The rise and fall of the strong absorbers suggests a connection to the star formation rate density, as though they trace galactic outflows or other byproducts of star formation. The weaker systems' lack of evolution does not fit within this interpretation, but may be reproduced by extrapolating low redshift scaling relations between host galaxy luminosity and absorbing halo radius to earlier epochs. For the weak systems, luminosity-scaled models match the evolution better than similar models based on MgII occupation of evolving CDM halo masses, which greatly underpredict dN/dz at early times unless the absorption efficiency of small haloes is significantly larger in the early universe. Taken together, these observations suggest that the general structure of MgII-bearing haloes was put into place early in the process of galaxy assembly. Except for a transient appearance of stronger systems near the peak epoch of cosmic star formation, the basic properties of MgII absorbers have evolved fairly little even as the (presumably) associated galaxy population grew substantially in stellar mass and half light radius.Comment: Submitted to ApJ. 22 pages, 21 figure