The galaxy stellar mass-star formation rate relation: Evidence for an evolving stellar initial mass function?

The evolution of the galaxy stellar mass--star formation rate relationship (M*-SFR) provides key constraints on the stellar mass assembly histories of galaxies. For star-forming galaxies, M*-SFR is observed to be fairly tight with a slope close to unity from z~0-2. Simulations of galaxy formation reproduce these trends owing to the generic dominance of smooth and steady cold accretion in these systems. In contrast, the amplitude of the M*-SFR relation evolves markedly differently than in models. Stated in terms of a star formation activity parameter alpha=(M*/SFR)/(t_H-1 Gyr), models predict a constant alpha~1 out to redshifts z=4+, while the observed M*-SFR relation indicates that alpha increases by X3 from z~2 until today. The low alpha at high-z not only conflicts with models, but is also difficult to reconcile with other observations of high-z galaxies. Systematic biases could significantly affect measurements of M* and SFR, but detailed considerations suggest that none are obvious candidates to reconcile the discrepancy. A speculative solution is considered in which the stellar initial mass function (IMF) evolves towards more high-mass star formation at earlier epochs. Following Larson, a model is investigated in which the characteristic mass Mhat where the IMF turns over increases with redshift. The observed and predicted M*-SFR evolution may be brought into agreement if Mhat=0.5(1+z)^2 Mo out to z~2. Such evolution broadly matches recent observations of cosmic stellar mass growth, and the resulting z=0 cumulative IMF is similar to the paunchy IMF favored by Fardal et al to reconcile the observed cosmic star formation history with present-day fossil light measures. [abridged]Comment: 14 pages, MNRAS, accepted version. Significant expansion of discussion; includes comparisons to new observation

The Growth of Red Sequence Galaxies in a Cosmological Hydrodynamic Simulation

We examine the cosmic growth of the red sequence in a cosmological hydrodynamic simulation that includes a heuristic prescription for quenching star formation that yields a realistic passive galaxy population today. In this prescription, halos dominated by hot gas are continually heated to prevent their coronae from fueling new star formation. Hot coronae primarily form in halos above \sim10^12 M\odot, so that galaxies with stellar masses \sim10^10.5 M\odot are the first to be quenched and move onto the red sequence at z > 2. The red sequence is concurrently populated at low masses by satellite galaxies in large halos that are starved of new fuel, resulting in a dip in passive galaxy number densities around \sim10^10 M\odot. Stellar mass growth continues for galaxies even after joining the red sequence, primarily through minor mergers with a typical mass ratio \sim1:5. For the most massive systems, the size growth implied by the distribution of merger mass ratios is typically \sim2\times the corresponding mass growth, consistent with observations. This model reproduces mass-density and colour-density trends in the local universe, with essentially no evolution to z = 1, with the hint that such relations may be washed out by z \sim 2. Simulated galaxies are increasingly likely to be red at high masses or high local overdensities. In our model, the presence of surrounding hot gas drives the trends with both mass and environment.Comment: 15 pages, 8 figures. MNRAS accepte

Factory modelling: data guidance for analysing production, utility and building architecture systems

Work on energy and resource reduction in factories is dependent on the availability of data. Typically, available sources are incomplete or inappropriate for direct use and manipulation is required. Identifying new improvement opportunities through simulation across factory production, utility and building architecture domains requires analysis of model feasibility, particularly in terms of system data composition, input resolution and simulation result fidelity. This paper reviews literature on developing appropriate model data for assessing energy and material flows at factory level. Gaps are found in guidance for analysis and integration of resource-flows across system boundaries. The process for how data was prepared, input and iteratively developed alongside conceptual and simulation models is described. The case of a large-scale UK manufacturer is presented alongside discussions on challenges associated with factory level modelling, and the insights gained from understanding the effect of data clarity on system performance

The Enrichment History of Baryons in the Universe

We present predictions for the cosmic metal budget in various phases of baryons from redshift z=6-0, taken from a cosmological hydrodynamic simulation that includes a well-constrained model for enriched galactic outflows. We find that substantial amounts of metals are found in every baryonic phase at all epochs, with diffuse intergalactic gas dominating the metal budget at early epochs and stars and halo gas dominating at recent epochs. We provide a full accounting of metals in the context of the missing metals problem at z~2.5, showing that ~40% of the metals are in galaxies, and the remainder is divided between diffuse IGM gas and shocked gas in halos and filamentary structures. Comparisons with available observations of metallicity and metal mass fraction evolution show broad agreement. We predict stars have a mean metallicity of one-tenth solar already at z=6, which increases slowly to one-half solar today, while stars just forming today have typically solar metallicity. Our HI column density-weighted mean metallicity (comparable to Damped Ly-alpha system metallicities) slowly increases from one-tenth to one-third solar from z=6-1, then falls to one-quarter solar at z=0. The global mean metallicity of the universe tracks ~50% higher than that of the diffuse phase down to z~1, and by z=0 it has a value around one-tenth solar. Metals move towards higher densities and temperatures with time, peaking around the mean cosmic density at z=2 and an overdensity of 100 at z=0. We study how carbon and oxygen ions trace the path of metals in phase space, and show that OIII-OVII lines provide the most practical option for constraining intergalactic medium metals at z<2.Comment: 10 pages, MNRAS accepted. Minor changes, Figure 1c fixe

Frand v. Compulsory Licensing: The Lesser of the Two Evils

This paper focuses on two types of licenses that can best be described as outliers—FRAND and compulsory licenses. Overall, these two specific forms of licenses share the objective of producing a fair and reasonable license of a technology protected by intellectual property. The comparable objective notwithstanding, each type of license achieves this end using different mechanisms. The FRAND license emphasizes providing the licensee with reasonable terms, e.g., by preventing a standard patent holder from extracting unreasonably high royalty rates. By contrast, compulsory licenses emphasize the public benefit that flows from enabling access to an otherwise inaccessible invention. Ultimately, both forms of license attempt to create a value for the licensed product that can be remarkably different from the product’s true market value. Nevertheless, both forms ultimately benefit the end-consumer who pays less to access a product subject to either of these forms of license. In comparing these two forms of licenses, the paper hopes to determine whether one form is better than the other, and if so, from whose perspective—the consumer, the licensor or the licensee. In doing so, this paper compares the different prevailing efforts to embrace such licenses as well as the impact of such licenses on the industry

The Simulated HI Sky at low redshift

Observations of intergalactic neutral hydrogen can provide a wealth of information about structure and galaxy formation, potentially tracing accretion and feedback processes on Mpc scales. Below a column density of NHI ~ 10^19 cm-2, the "edge" or typical observational limit for HI emission from galaxies, simulations predict a cosmic web of extended emission and filamentary structures. We study the distribution of neutral hydrogen and its 21cm emission properties in a cosmological hydrodynamic simulation, to gain more insights into the distribution of HI below NHI ~ 10^19 cm-2. Such Lyman Limit systems are expected to trace out the cosmic web, and are relatively unexplored. Beginning with a 32 h^-1 Mpc simulation, we extract the neutral hydrogen component by determining the neutral fraction, including a post-processed correction for self-shielding based on the thermal pressure. We take into account molecular hydrogen, assuming an average density ratio Omega_H2 / Omega_HI = 0.3 at z = 0. The statistical properties of the HI emission are compared with observations, to assess the reliability of the simulation. The simulated HI distribution robustly describes the full column density range between NHI ~ 10^14 and NHI ~ 10^21 cm-2 and agrees very well with available measurements from observations. Furthermore there is good correspondence in the statistics when looking at the two-point correlation function and the HI mass function. The reconstructed maps are used to simulate observations of existing and future telescopes by adding noise and taking account of the sensitivity of the telescopes. The general agreement in statistical properties of HI suggests that neutral hydrogen as modeled in this hydrodynamic simulation is a fair representation of that in the Universe. (abridged)Comment: 20 pages, 17 figures, Accepted for publication in A&A, figures compressed to low resolution; high-resolution version available at: http://www.astro.rug.nl/~popping/simulated_HI_sky.pd