1,763 research outputs found
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
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