Simeon Bird

Properties of galaxies reproduced by a hydrodynamic simulation

Previous simulations of the growth of cosmic structures have broadly reproduced the ‘cosmic web’ of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies, because of numerical inaccuracies and incomplete physical models. Moreover, they were unable to track the small-scale evolution of gas and stars to the present epoch within a representative portion of the Universe. Here we report a simulation that starts 12 million years after the Big Bang, and traces 13 billion years of cosmic evolution with 12 billion resolution elements in a cube of 106.5 megaparsecs a side. It yields a reasonable population of ellipticals and spirals, reproduces the observed distribution of galaxies in clusters and characteristics of hydrogen on large scales, and at the same time matches the ‘metal’ and hydrogen content of galaxies on small scales.

Did LIGO detect dark matter

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 20M_{⊙}≲M_{bh}≲100M_{⊙} where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will rapidly spiral inward due to the emission of gravitational radiation and ultimately will merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2-53  Gpc^{-3} yr^{-1} rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have neither optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next-generation experiments will be invaluable in performing these tests.

Damped Lyman α absorbers as a probe of stellar feedback

We examine the abundance, clustering and metallicity of Damped Lyman- Absorbers (DLAs) in a suite of hydrodynamic cosmological simulations using the moving mesh code AREPO . We incorporate models of supernova and AGN feedback, as well as molecular hydrogen formation. We compare our simulations to the column density distribution function at z = 3, the total DLA abundance at z = 2 4, the measured DLA bias at z = 2:3 and the DLA metallicity distribution at z = 2 4. Our preferred models produce populations of DLAs in good agreement with most of these observations. The exception is the DLA abundance at z < 3, which we show requires stronger feedback in 10 11 12 h 1 M mass halos. While the DLA population probes a wide range of halo masses, we nd the cross-section is dominated by halos of mass 10 10 10 11 h 1 M and virial velocities 50 100 km s 1 . The simulated DLA population has a linear theory bias of 1:7, whereas the observations require 2:17 0:2. We show that non-linear growth increases the bias in our simulations to 2:3 at k = 1hMpc 1 , the smallest scale observed. The scale-dependence of the bias is, however, very different in the simulations compared against the observations. We show that, of the observations we consider, the DLA abundance and column density function provide the strongest constraints on the feedback model.

The impact of galactic feedback on the circumgalactic medium

Galactic feedback strongly affects the way galactic environments are enriched. We examine this connection by performing a suite of cosmological hydrodynamic simulations, exploring a range of parameters based on the galaxy formation model developed in Vogelsberger et al. We examine the effects of AGN feedback, wind mass loading, wind specific energy, and wind metal-loading on the properties of the circumgalactic medium (CGM) of galaxies with M_(halo) > 10^(11) M⊙. Note that while the V13 model was tuned to match observations including the stellar mass function, no explicit tuning was done for the CGM. The wind energy per unit outflow mass has the most significant effect on the CGM enrichment. High-energy winds launch metals far beyond the virial radius. AGN feedback also has a significant effect, but only at z < 3. We compare to high-redshift HI and CIV observations. All our simulations produce the observed number of Damped Lyman α Absorbers. At lower column density, several of our simulations produce enough Lyman Limit Systems (LLS) 100 kpc from the galaxy, but in all cases the LLS abundance falls off with distance faster than observations, with too few LLS at 200 kpc. Further, in all models the CIV abundance drops off too sharply with distance, with too little CIV 100–200 kpc from the galaxy. Higher energy wind models produce more extended CIV but also produce less stars, in tension with star formation rate density observations. This highlights the fact that circumgalactic observations are a strong constraint on galactic feedback models.

The BAHAMAS project: Calibrated hydrodynamical simulations for large-scale structure cosmology

The evolution of the large-scale distribution of matter is sensitive to a variety of fundamental parameters that characterise the dark matter, dark energy, and other aspects of our cosmological framework. Since the majority of the mass density is in the form of dark matter that cannot be directly observed, to do cosmology with large-scale structure one must use observable (baryonic) quantities that trace the underlying matter distribution in a (hopefully) predictable way. However, recent numerical studies have demonstrated that the mapping between observable and total mass, as well as the total mass itself, are sensitive to unresolved feedback processes associated with galaxy formation, motivating explicit calibration of the feedback efficiencies. Here we construct a new suite of large-volume cosmological hydrodynamical simulations (called BAHAMAS, for BAryons and HAloes of MAssive Systems) where subgrid models of stellar and Active Galactic Nucleus (AGN) feedback have been calibrated to reproduce the present-day galaxy stellar mass function and the hot gas mass fractions of groups and clusters in order to ensure the effects of feedback on the overall matter distribution are broadly correct. We show that the calibrated simulations reproduce an unprecedentedly wide range of properties of massive systems, including the various observed mappings between galaxies, hot gas, total mass, and black holes, and represent a significant advance in our ability to mitigate the primary systematic uncertainty in most present large-scale structure tests.

Reproducing the kinematics of damped Lyman α systems

We examine the kinematic structure of Damped Lyman- Systems (DLAs) in a series of cosmological hydrodynamic simulations using the AREPO code. We are able to match the distribution of velocity widths of associated low ionisation metal absorbers substantially better than earlier work. Our simulations produce a population of DLAs dominated by halos with virial velocities around 70 km s 1 , consistent with a picture of relatively small, faint objects. In addition, we reproduce the observed correlation between velocity width and metallicity and the equivalent width distribution of SiII. Some discrepancies of moderate statistical signicance remain; too many of our spectra show absorption concentrated at the edge of the prole and there are slight dierences in the exact shape of the velocity width distribution. We show that the improvement over previous work is mostly due to our strong feedback from star formation and our detailed modelling of the metal ionisation state.

Moving-mesh cosmology: properties of neutral hydrogen in absorption

We examine the distribution of neutral hydrogen in cosmological simulations carried out with the new moving-mesh code AREPO and compare it with the corresponding GADGET simulations based on the smoothed particle hydrodynamics (SPH) technique. The two codes use identical gravity solvers and baryonic physics implementations, but very different methods for solving the Euler equations, allowing us to assess how numerical effects associated with the hydro-solver impact the results of simulations. Here we focus on an analysis of the neutral gas, as detected in quasar absorption lines. We find that the high column density regime probed by Damped Lyman-alpha (DLA) and Lyman Limit Systems (LLS) exhibits significant differences between the codes. GADGET produces spurious artefacts in large halos in the form of gaseous clumps, boosting the LLS cross-section. Furthermore, it forms halos with denser central baryonic cores than AREPO, which leads to a substantially greater DLA cross-section from smaller halos. AREPO thus produces a significantly lower cumulative abundance of DLAs, which is intriguingly in much closer agreement with observations. The column density function, however, is not altered enough to significantly reduce the discrepancy with the observed value. For the low column density gas probed by the Lyman-alpha forest, the codes differ only at the level of a few percent, suggesting that this regime is quite well described by both methods, a fact that is reassuring for the many Lyman-alpha studies carried out with SPH thus far. While the residual differences are smaller than the errors on current Lyman-alpha forest data, we note that this will likely change for future precision experiments.

Minimally parametric power spectrum reconstruction from the Lyman α forest

Current results from the Lyman α forest assume that the primordial power spectrum of density perturbations follows a simple power-law form. We present the first analysis of Lyman α data to study the effect of relaxing this strong assumption on primordial and astrophysical constraints. We perform a large suite of numerical simulations, using them to calibrate a minimally parametric framework for describing the power spectrum. Combined with cross-validation, a statistical technique which prevents overfitting of the data, this framework allows us to reconstruct the power spectrum shape without strong prior assumptions. We find no evidence for deviation from scale-invariance; our analysis also shows that current Lyman α data do not have sufficient statistical power to robustly probe the shape of the power spectrum at these scales. In contrast, the ongoing Baryon Oscillation Sky Survey will be able to do so with high precision. Furthermore, this near-future data will be able to break degeneracies between the power spectrum shape and astrophysical parameters.

The BlueTides simulation: first galaxies and reionization

We introduce the BlueTides simulation and report initial results for the luminosity functions of the first galaxies and AGN, and their contribution to reionization. BlueTides was run on the BlueWaters cluster at NCSA from

An efficient implementation of massive neutrinos in non-linear structure formation simulations

z=99