Large-scale mass distribution in the Illustris simulation

P Torrey,M Vogelsberger,D Steinhauser,M Haider,L Hernquist,V Springel,S Genel

doi:10.1093/mnras/stw077

Abstract

Observations at low redshifts thus far fail to account for all of the baryons expected in the Universe according to cosmological constraints. A large fraction of the baryons presumably resides in a thin and warm-hot medium between the galaxies, where they are difficult to observe due to their low densities and high temperatures. Cosmological simulations of structure formation can be used to verify this picture and provide quantitative predictions for the distribution of mass in different large-scale structure components. Here we study the distribution of baryons and dark matter at different epochs using data from the Illustris simulation. We identify regions of different dark matter density with the primary constituents of large-scale structure, allowing us to measure mass and volume of haloes, filaments and voids. At redshift zero, we find that 49 % of the dark matter and 23 % of the baryons are within haloes more massive than the resolution limit of $2\times 10^8$ M$_\odot$. The filaments of the cosmic web host a further 45 % of the dark matter and 46 % of the baryons. The remaining 31 % of the baryons reside in voids. The majority of these baryons have been transported there through active galactic nuclei feedback. We note that the feedback model of Illustris is too strong for heavy haloes, therefore it is likely that we are overestimating this amount. Categorizing the baryons according to their density and temperature, we find that 17.8 % of them are in a condensed state, 21.6 % are present as cold, diffuse gas, and 53.9 % are found in the state of a warm-hot intergalactic medium.

Highlights

The large-scale structure of the Universe is determined by the dominant dark matter, which makes up 26 % of today’s mass–energy content of the Universe
We should note that the mass found in haloes depends on the resolution of a simulation, Table 1 only gives the fraction of mass in haloes which can be resolved in the Illustris simulation
The resolution limit in terms of mass is primarily given by the mass of a single dark matter particle which is 6.26 × 106 M

Summary

Introduction

The large-scale structure of the Universe is determined by the dominant dark matter, which makes up 26 % of today’s mass–energy content of the Universe. As dark matter is not detectable directly, the large-scale structure must be inferred from the baryons. Recent analysis (Planck Collaboration 2015) of the cosmic microwave background fluctuations by the Planck mission found that baryons amount to Ωb = 4.8 % of the present-day critical density of the Universe This corresponds to a mean baryon density of ρb = 4.1 × 10−31g cm−3. While X-ray observations of galaxy clusters do find baryon contents close to the primordial value (Simionescu et al 2011), and Ovii absorption line measurements at X-ray energies hint at large reservoirs of hot gas around spiral galaxies (Gupta et al 2012), approximately 30 % of the baryons are still not accounted for in the local Universe (Shull et al 2012). The baryons are missing on cosmological scales, but the baryon-to-dark matter ratio in galaxies falls short of the primordial ratio (Bell et al 2003; McGaugh et al 2010)

Results

Discussion

Conclusion