Before leveraging the output of the simulation to better understand the different types of galaxies that form, and the evolutionary paths they take towards their present day state, we need to ensure that the simulation is a faithful representation of reality. Due to the complexity of the problem, numerical calculations can never fully capture the answer across all scales of space and time.
A finite resolution the size of the smallest details which we can include means that some processes, such as the birth of individual stars, cannot be directly captured in a cosmological simulation. As a result, we implement many physical approximations, and the burden then arises to ensure that the particular problems of galaxy formation we wish to study are well represented under these approximations.
Therefore, we introduce some of the first insights from the Illustris simulation, by first describing this essential comparison against observations:. Many areas of the simulation remain unexplored as researchers associated with the Illustris project continue to investigate interesting and unexpected results. The ultimate goal in each case is a deepening of our understanding of the processes by which the observed population of galaxies formed and evolved over cosmic time. Understanding galaxy formation, however, has broader implications for scales both smaller and larger: galaxies are the sites where individual stars, as well as the planets and the possible tracers of life they harbor, form.
At the same time, galaxies are the visible tracers of the large-scale structure of the universe itself, and therefore inexorably linked to the Big Bang, inflation, and the mysterious nature of both dark matter and dark energy. Moving forward, particular attention to specific discrepancies between simulation and observation will allow us to better understand our galaxy formation model.
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By addressing these issues, and improving in both physical realism and numerical methods, we can start to think about the next generation of cosmological hydrodynamic simulations after Illustris. We describe the key science goals of the project, relate them to open questions in our theoretical understanding of galaxy formation and evolution, and describe more technical details of the simulation below. Since the advent of modern observational surveys, the sheer volume of available data on the properties of galaxies, their distribution within the large-scale structure of the universe, and their evolution in time has exploded.
Numerical calculations are required to probe past the linear regime of early time, and efforts modeling only the gravitational interaction of dark matter and dark energy, neglecting the role of gas and baryonic physics, have led to significant physical insight. Nevertheless, such DM-only simulations do not directly predict anything about the galaxies themselves, requiring an extra step in order to bridge the gap with observations. The latter approach enables the complex interaction of the different baryonic components gas, stars, black holes to be treated at a much smaller scale, ideally yielding a self-consistent and powerfully predictive calculation.
They have been harnessed to study the effects of different models or model variations on a particular problem, without explicitly aiming to reproduce the large number of observational constraints available. Only in the past few years have several groups started projects with similar approaches and objectives as the Illustris simulation. These simulations have been steadily improving in three distinct ways: 1 by increasing in size, both in terms of volume and the number of resolution elements employed, 2 by improving the scope, complexity, and physical fidelity of the sub-grid models required to provide a complete and accurate description of the many processes that govern galaxy formation, and 3 by developing more accurate and efficient numerical methods.
Below we detail three areas of investigation which have strongly motivated Illustris. A long-standing problem in galaxy formation simulations has been that simulated galaxies are overly compact compared to the observed population, indicating that the angular momentum distribution between the galaxies and their surroundings was incorrect.
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Our preliminary findings using the AREPO code indicate that the moving-mesh approach may help solve this problem. However, for the sake of comparison, these simulations did not include feedback processes that are believed to be responsible for removing a large fraction of baryons from galaxies. The Illustris simulation volume is 27 times larger, allowing us to obtain a statistical representation of the cosmological galaxy population. By comparing the morphological distribution of this population to locally observed galaxies we can calibrate uncertainties in our modeling of star formation and feedback.
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We can study the evolution of the gas and metal content of galaxies over time, and compare against the observed galaxy mass-metallicity relation. We can investigate the properties and nature of damped Lyman-alpha absorbers. Unlike in DM-only simulations, we can make a prediction for the galaxy-galaxy merger rate as a function of mass ratio, redshift, and mass. These results are important in understanding the growth of galaxies through mergers, and for calibrating the prescriptions used to quantify merger rates in semi-analytical models.tr.enesubazejyz.cf
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Our previous simulations have already demonstrated that exponential disks are a natural consequence of the hierarchical galaxy assembly process. We can now use Illustris to understand in detail how these profiles arise, by tracing the evolutionary history of the simulated galaxies back to higher redshift, and following the acquisition of angular momentum in the baryon component.
Our numerical method has demonstrated significant advantages in the hydrodynamic treatment, particularly important in this regime. Additionally, the Monte Carlo tracer particle scheme and the inclusion of many such tracers in the main Illustris simulation allow us to reconstruct the time evolution of gas which is accreting into halos and galaxies. Observations have revealed that galaxies drive high velocity outflows into their surroundings.
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This material can shock heat, adding hot gas to the halo, while denser parts of the outflows can drive small shocks and possibly survive to large distances. Outflows can compress halo gas, increasing its density and radiative cooling rate, which can cause it to 'rain back' onto the galaxy. Outflows will also interact with cosmological inflow, possibly modulating the nature or rates of this supply of new gas for the central galaxy.
We can address the consequences and detectability of both the infalling and outflowing material in galactic halos, and compare to observational signatures of gas in halos and metals in the CGM as probed by methods such as quasar absorption line spectroscopy. By accounting for chemical enrichment processes - and the energetic feedback which redistributes both gas and metals - we can make accurate synthetic observations of the circumgalactic material in the simulations.
In the past few decades, a compelling picture has emerged indicating that black holes are common inhabitants of the vast majority of galaxies.
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The observed scaling relationships between BH masses and their host galaxy properties imply that we need to understand how BHs grow and affect their surroundings. We can explore the evolution of the black hole accretion rate as a function of time, and the link to the host galaxy. We can also explore the impact of AGN feedback on the dichotomy between blue and red galaxies, the link to the quenching of star-formation, and how this picture changes with redshift.
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It is commonly assumed that active SMBHs at high redshift reside in rare, massive halos embedded in gas-rich, over-dense environments, while recent observational efforts are still working to unambiguously demonstrate that this is the case. For instance, in analogy to the downsizing of galaxies, it is observed that at high redshifts more massive BHs are powering bright AGN, while at lower redshifts the peak of quasar activity shifts towards smaller mass BHs. With a representative sample of simulated BHs, whose accretion rates are measured directly from the surrounding gas, it will be possible to probe the downsizing of the AGN population as a whole, and why and in which hosts SMBHs enter radiatively inefficient regimes.
In this approach, an unstructured Voronoi tessellation of the simulation volume allows for dynamic and adaptive spatial discretization, where a set of mesh generating points are moved along with the gas flow. This mesh is used to solve the equations of ideal hydrodynamics using a second order, finite volume, directionally un-split Godunov-type scheme, with an exact Riemann solver. The gravitational force is calculated with a split Tree-PM approach, where long-range forces are calculated from a particle-mesh method, and short-range forces are calculated with a hierarchical octree algorithm.
Our galaxy formation model is based on the inclusion of several additional astrophysical processes:. A series of simulations are run at different resolutions, and a second set is run with only dark matter. Such payments continue to be your responsibility. Free shipping for domestic orders will last until December 31st, Free shipping for international orders is a limited time only special promotion and may be terminated by Souq at any time without prior notice, at our sole discretion.
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