The role of stellar radial motions in shaping galaxy surface brightness profiles

Aims. The physics driving features such as breaks observed in galaxy surface brightness (SB) profiles remains contentious. Here, we assess the importance of stellar radial motions in shaping their characteristics. Methods. We use the simulated Milky Way-mass cosmological discs from the Ramses Disc Environment Study (RaDES) to characterise the radial redistribution of stars in galaxies displaying type-I (pure exponentials), II (downbending), and III (upbending) SB profiles. We compare radial profiles of the mass fractions and the velocity dispersions of different sub-populations of stars according to their birth and current location. Results. Radial redistribution of stars is important in all galaxies regardless of their light profiles. Type-II breaks seem to be a consequence of the combined effects of outward-moving and accreted stars. The former produce shallower inner profiles (lack of stars in the inner disc) and accumulate material around the break radius and beyond, strengthening the break; the latter can weaken or even convert the break into a pure exponential. Further accretion from satellites can concentrate material in the outermost parts, leading to type-III breaks that can coexist with type-II breaks, but situated further out. Type-III galaxies would be the result of an important radial redistribution of material throughout the entire disc, as well as a concentration of accreted material in the outskirts. In addition, type-III galaxies display the most efficient radial redistribution and the largest number of accreted stars, followed by type-I and II systems, suggesting that type-I galaxies may be an intermediate case between types-II and III. In general, the velocity dispersion profiles of all galaxies tend to flatten or even increase around the locations where the breaks are found. The age and metallicity profiles are also affected, exhibiting different inner gradients depending on their SB profile, being steeper in the case of type-II systems (as found observationally). The steep type-II profiles might be inherent to their formation rather than acquired via radial redistribution

Properties of simulated Milky Way-mass galaxies in loose group and field environments

We test the validity of comparing simulated field disk galaxies with the empirical properties of systems situated within environments more comparable to loose groups, including the Milky Way's Local Group. Cosmological simulations of Milky Way-mass galaxies have been realised in two different environment samples: in the field and in environments with similar properties to the Local Group. Apart from the environments of the galaxies, the samples are kept as homogeneous as possible with equivalent ranges in last major merger time, halo mass and halo spin. Comparison of these two samples allow for systematic differences in the simulations to be identified. Metallicity gradients, disk scale lengths, colours, magnitudes and age-velocity dispersion relations are studied for each galaxy in the suite and the strength of the link between these and environment of the galaxies is studied. The bulge-to-disk ratio of the galaxies show that these galaxies are less spheroid dominated than many other simulated galaxies in literature with the majority of both samples being disk dominated. We find that secular evolution and mergers dominate the spread of morphologies and metallicity gradients with no visible differences between the two environment samples. In contrast with this consistency in the two samples there is tentative evidence for a systematic difference in the velocity dispersion-age relations of galaxies in the different environments. Loose group galaxies appear to have more discrete steps in their velocity dispersion-age relations. We conclude that at the current resolution of cosmological galaxy simulations field environment galaxies are sufficiently similar to those in loose groups to be acceptable proxies for comparison with the Milky Way provided that a similar assembly history is considered.Comment: 16 pages, 11 figures, abstract abridged for arXiv. Accepted for publication in Astronomy & Astrophysic

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

Demand-Side Threats to Power Grid Operations from IoT-Enabled Edge

The growing adoption of Internet-of-Things (IoT)-enabled energy smart appliances (ESAs) at the consumer end, such as smart heat pumps, electric vehicle chargers, etc., is seen as key to enabling demand-side response (DSR) services. However, these smart appliances are often poorly engineered from a security point of view and present a new threat to power grid operations. They may become convenient entry points for malicious parties to gain access to the system and disrupt important grid operations by abruptly changing the demand. Unlike utility-side and SCADA assets, ESAs are not monitored continuously due to their large numbers and the lack of extensive monitoring infrastructure at consumer sites. This article presents an in-depth analysis of the demand side threats to power grid operations including (i) an overview of the vulnerabilities in ESAs and the wider risk from the DSR ecosystem and (ii) key factors influencing the attack impact on power grid operations. Finally, it presents measures to improve the cyber-physical resilience of power grids, putting them in the context of ongoing efforts from the industry and regulatory bodies worldwide

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

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

Constraining sub-grid physics with high-redshift spatially-resolved metallicity distributions

Aims. We examine the role of energy feedback in shaping the distribution of metals within cosmological hydrodynamical simulations of L* disc galaxies. While negative abundance gradients today provide a boundary condition for galaxy evolution models, in support of inside-out disc growth, empirical evidence as to whether abundance gradients steepen or flatten with time remains highly contradictory. Methods. We made use of a suite of L* discs, realised with and without "enhanced" feedback. All the simulations were produced using the smoothed particle hydrodynamics code Gasoline, and their in situ gas-phase metallicity gradients traced from redshift z similar to 2 to the present-day. Present-day age-metallicity relations and metallicity distribution functions were derived for each system. Results. The "enhanced" feedback models, which have been shown to be in agreement with a broad range of empirical scaling relations, distribute energy and re-cycled ISM material over large scales and predict the existence of relatively "flat" and temporally invariant abundance gradients. Enhanced feedback schemes reduce significantly the scatter in the local stellar age-metallicity relation and, especially, the [O/Fe]-[Fe/H] relation. The local [O/Fe] distribution functions for our L* discs show clear bimodality, with peaks at [O/Fe] = -0.05 and +0.05 (for stars with [Fe/H] > -1), consistent with our earlier work on dwarf discs. Conclusions. Our results with "enhanced" feedback are inconsistent with our earlier generation of simulations realised with "conservative" feedback. We conclude that spatially-resolved metallicity distributions, particularly at high-redshift, offer a unique and under-utilised constraint on the uncertain nature of stellar feedback processes