Testing hydrodynamics schemes in galaxy disc simulations

We examine how three fundamentally different numerical hydrodynamics codes follow the evolution of an isothermal galactic disc with an external spiral potential. We compare an adaptive mesh refinement code (RAMSES), a smoothed particle hydrodynamics code (SPHNG), and a volume-discretised meshless code (GIZMO). Using standard refinement criteria, we find that RAMSES produces a disc that is less vertically concentrated and does not reach such high densities as the SPHNG or gizmo runs. The gas surface density in the spiral arms increases at a lower rate for the RAMSES simulations compared to the other codes. There is also a greater degree of substructure in the SPHNG and GIZMOruns and secondary spiral arms are more pronounced. By resolving the Jeans’ length with a greater number of grid cells we achieve more similar results to the Lagrangian codes used in this study. Other alterations to the refinement scheme (adding extra levels of refinement and refining based on local density gradients) are less successful in reducing the disparity between RAMSES and SPHNG/GIZMO. Although more similar, SPHNG displays different density distributions and vertical mass profiles to all modes of gizmo (including the smoothed particle hydrodynamics version). This suggests differences also arise which are not intrinsic to the particular method but rather due to its implementation. The discrepancies between codes (in particular, the densities reached in the spiral arms) could potentially result in differences in the locations and timescales for gravitational collapse, and therefore impact star formation activity in more complex galaxy disc simulations

Valuing local perspectives on invasive species management: Moving beyond the ecosystem service-disservice dichotomy

This paper uses the concept of ecosystem disservices to explore and understand how rapid environmental change associated with an invasive plant species is framed and understood by different stakeholders. Through a focus on narratives, the paper explores how socially-differentiated populations understand the causes and consequences of a plant invasion and express preferences for often contrasting management interventions. The research design uses a workshop format to instigate a series of conversations with socially-differentiated groups of people to explore how people perceive and respond to the impact of Prosopis juliflora (a species of mesquite) in the drylands of Ethiopia. The results show that preferences for interventions differs by age, gender, location and livelihood and also by primary and secondary stakeholder. Different sets of values underpin people’s views and these contribute to the variation in the preference for different management interventions. To understand complex issues associated with alien invasive species, we find that the dichotomy between ecosystem services and disservices is artificial and call for a more dynamic and graduated view of ecosystem outputs. More practically, our research shows that P. juliflora management options need wider consideration of socially-differentiated implications and trade-offs and this requires greater efforts to engage with primary stakeholders

Evolution of starburst galaxies in the Illustris simulation

© 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. There is a consensus in the literature that starburst galaxies are triggered by interaction events. However, it remains an open question as to what extent both merging and nonmerging interactions have in triggering starbursts. In this study, we make use of the Illustris simulation to test how different triggeringmechanisms can affect starburst events.We examine the star formation rate, colour, and environment of starburst galaxies to determine if this could be why we witness a bimodality in post-starburst populations within observational studies. Further, we briefly test the extent of quenching due to active galactic nuclei feedback. From Illustris, we select 196 starburst galaxies at z = 0.15 and split them into post-merger and pre-merger/harassment-driven starburst samples. We find that 55 % of this sample have not undergone a merger in the past 2 Gyr. Both of our samples are located in low-density environments within the filament regions of the cosmic web; however, we find that premerger/ harassment-driven starbursts are in higher-density environments than post-mergerdriven starbursts. We also find that pre-merger/harassment starbursts are redder than postmerger starbursts; this could be driven by environmental effects. Both, however, produce nuclear starbursts of comparable strengths

The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations

By means of high-resolution cosmological hydrodynamical simulations of Milky Way-like disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). After undertaking a kinematic decomposition of each simulation into spheroid and disc sub-components, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally-concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the Milky Way. The fraction of low metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial Milky Way analog, we study the metallicity distribution of the stars born "in situ" relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness.Comment: 20 pages, 14 figures, MNRAS, accepte

The properties, origin and evolution of stellar clusters in galaxy simulations and observations

We investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one ‘cluster’, for the isolated galaxies we are able to model features we term ‘clusters’ with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myrs) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas onto the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback

I want what you've got: Cross platform portabiity and human-robot interaction assessment.

Human-robot interaction is a subtle, yet critical aspect of design that must be assessed during the development of both the human-robot interface and robot behaviors if the human-robot team is to effectively meet the complexities of the task environment. Testing not only ensures that the system can successfully achieve the tasks for which it was designed, but more importantly, usability testing allows the designers to understand how humans and robots can, will, and should work together to optimize workload distribution. A lack of human-centered robot interface design, the rigidity of sensor configuration, and the platform-specific nature of research robot development environments are a few factors preventing robotic solutions from reaching functional utility in real word environments. Often the difficult engineering challenge of implementing adroit reactive behavior, reliable communication, trustworthy autonomy that combines with system transparency and usable interfaces is overlooked in favor of other research aims. The result is that many robotic systems never reach a level of functional utility necessary even to evaluate the efficacy of the basic system, much less result in a system that can be used in a critical, real-world environment. Further, because control architectures and interfaces are often platform specific, it is difficult or even impossible to make usability comparisons between them. This paper discusses the challenges inherent to the conduct of human factors testing of variable autonomy control architectures and across platforms within a complex, real-world environment. It discusses the need to compare behaviors, architectures, and interfaces within a structured environment that contains challenging real-world tasks, and the implications for system acceptance and trust of autonomous robotic systems for how humans and robots interact in true interactive teams

Changing social contracts in climate-change adaptation

Risks from extreme weather events are mediated through state, civil society and individual action 1 , 2 . We propose evolving social contracts as a primary mechanism by which adaptation to climate change proceeds. We use a natural experiment of policy and social contexts of the UK and Ireland affected by the same meteorological event and resultant flooding in November 2009. We analyse data from policy documents and from household surveys of 356 residents in western Ireland and northwest England. We find significant differences between perceptions of individual responsibility for protection across the jurisdictions and between perceptions of future risk from populations directly affected by flooding events. These explain differences in stated willingness to take individual adaptive actions when state support retrenches. We therefore show that expectations for state protection are critical in mediating impacts and promoting longer-term adaptation. We argue that making social contracts explicit may smooth pathways to effective and legitimate adaptation

Culture as a mediator of climate change adaptation: Neither static nor unidirectional

Though there is increasing recognition of the cultural dimensions that shape climate change adaptation, our experience from working with actors engaged in adaptation policy and practice suggests that the role of culture still tends to be conceived in overly narrow and fixed terms. This is exemplified in portrayals of conservative cultural norms as stifling positive change. A growing body of research across the world indicates that the reality is seldom as simple as this – culture works in complex and variable ways, and, most importantly, is inherently dynamic. Drawing especially from research work on vulnerability and adaptation conducted in semi-arid regions, we illustrate this argument by briefly exploring three themes - multiple knowledge systems for farming in Botswana, the dynamics of pastoralist values and livelihoods in Kenya, and the interplay of caste and livelihood choices in India. Understanding how different facets of culture such as these operate in context helps move away from viewing culture statically as a barrier or enabler, and toward a more plural and dynamic appreciation of the role of culture in adaptation. This includes recognising the potential for factors that may be construed as barriers to become enablers. Critical, balanced engagement with cultural dimensions in both research and practice, understanding and working with these dynamic social structures, is essential if adaptation is to create meaningful and lasting change for those who need it most

Lenticular galaxies with UV-rings

By using the public UV imaging data obtained by the GALEX (Galaxy Ultraviolet Explorer) for nearby galaxies, we have compiled a list of lenticular galaxies possessing ultraviolet rings - starforming regions tightly confined to particular radial distances from galactic centers. We have studied large-scale structure of these galaxies in the optical bands by using the data of the SDSS (Sloan Digital Sky Survey): we have decomposed the galactic images into large-scale disks and bulges, have measured the ring optical colours from the residual images after subtracting model disks and bulges, and have compared the sizes of the rings in the optical light and in the UV-band. The probable origin of the outer starforming ring appearances in unbarred galaxies demonstrating otherwise the regular structure and homogeneously old stellar population beyond the rings is discussed.Comment: 9 pages plus one big colour figure in the Appendix; the slightly expanded version of the paper accepted to Astronomy Letter

The cold gas content of bulgeless dwarf galaxies

We present an analysis of the neutral hydrogen (H i) properties of a fully cosmological hydrodynamical dwarf galaxy, run with varying simulation parameters. As reported by Governato et al., the high-resolution, high star formation density threshold version of this galaxy is the first simulation to result in the successful reproduction of a (dwarf) spiral galaxy without any associated stellar bulge. We have set out to compare in detail the H i distribution and kinematics of this simulated bulgeless disc with what is observed in a sample of nearby dwarfs. To do so, we extracted the radial gas density profiles, velocity dispersion (e.g. velocity ellipsoid and turbulence) and the power spectrum of structure within the cold interstellar medium (ISM) from the simulations. The highest resolution dwarf, when using a high-density star formation threshold comparable to densities of giant molecular clouds, possesses bulk characteristics consistent with those observed in nature, though the cold gas is not as radially extended as that observed in nearby dwarfs, resulting in somewhat excessive surface densities. The lines-of-sight velocity dispersion radial profiles have values that are in good agreement with the observed dwarf galaxies, but due to the fact that only the streaming velocities of particles are tracked, a correction to include the thermal velocities can lead to profiles that are quite flat. The ISM power spectra of the simulations appear to possess more power on smaller spatial scales than that of the Small Magellanic Cloud. We conclude that unavoidable limitations remain due to the unresolved physics of star formation and feedback within parsec-scale molecular cloud