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Register a new userThe baryon fraction of LambdaCDM haloes
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Robert A. Crain
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We investigate the baryon fraction in dark matter haloes formed in non-radiative gas-dynamical simulations of the LambdaCDM cosmogony. By combining a realisation of the Millennium Simulation (Springel et al.) with a simulation of a smaller volume focussing on dwarf haloes, our study spans five decades in halo mass, from 10^10 Msun/h to 10^15 Msun/h. We find that the baryon fraction within the halo virial radius is typically 90% of the cosmic mean, with an rms scatter of 6%, independently of redshift and of halo mass down to the smallest resolved haloes. Our results show that, contrary to the proposal of Mo et al. (2005), pre-virialisation gravitational heating is unable to prevent the collapse of gas within galactic and proto-galactic haloes, and confirm the need for non-gravitational feedback in order to reduce the efficiency of gas cooling and star formation in dwarf galaxy haloes. Simulations including a simple photoheating model (where a gas temperature floor of T_{floor} = 2x10^4 K is imposed from z=11) confirm earlier suggestions that photoheating can only prevent the collapse of baryons in systems with virial temperatures T_{200} < ~2.2 T_{floor} ~ 4.4x10^4 K (corresponding to a virial mass of M_{200} ~ 10^10 Msun/h and a circular velocity of V_{200} ~ 35 km/s). Photoheating may thus help regulate the formation of dwarf spheroidals and other galaxies at the extreme faint-end of the luminosity function, but it cannot, on its own, reconcile the abundance of sub-L* galaxies with the vast number of dwarf haloes expected in the LambdaCDM cosmogony. The lack of evolution or mass dependence seen in the baryon fraction augurs well for X-ray cluster studies that assume a universal and non-evolving baryon fraction to place constraints on cosmological parameters.
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Baryons and cold dark matter (CDM) did not comove prior to recombination. This leads to differences in the local baryon and CDM densities, the so-called baryon-CDM isocurvature perturbations $delta_{bc}$. These perturbations are usually neglected in the analysis of Large-Scale Structure data but taking them into account might become important in the era of high precision cosmology. Using gravity-only 2-fluid simulations we assess the impact of such perturbations on the dark matter halos distribution. In particular, we focus on the baryon fraction in halos as a function of mass and large-scale $delta_{bc}$, which also allows us to study details of the nontrivial numerical setup required for such simulations. We further measure the cross-power spectrum between the halo field and $delta_{bc}$ over a wide range of mass. This cross-correlation is nonzero and negative which shows that halo formation is impacted by $delta_{bc}$. We measure the associated bias parameter $b_{delta_{bc}}$ and compare it to recent results, finding good agreement. Finally we quantify the impact of such perturbations on the halo-halo power spectrum and show that this effect can be degenerate with the one of massive neutrinos for surveys like DESI.
The MareNostrum Universe is one of the largest cosmological SPH simulation done so far. It consists of $1024^3$ dark and $1024^3$ gas particles in a box of 500 $h^{-1}$ Mpc on a side. Here we study the shapes and spins of the dark matter and gas components of the 10,000 most massive objects extracted from the simulation as well as the gas fraction in those objects. We find that the shapes of objects tend to be prolate both in the dark matter and gas. There is a clear dependence of shape on halo mass, the more massive ones being less spherical than the less massive objects. The gas distribution is nevertheless much more spherical than the dark matter, although the triaxiality parameters of gas and dark matter differ only by a few percent and it increases with cluster mass. The spin parameters of gas and dark matter can be well fitted by a lognormal distribution function. On average, the spin of gas is 1.4 larger than the spin of dark matter. We find a similar behavior for the spins at higher redshifts, with a slightly decrease of the spin ratios to 1.16 at $z=1.$ The cosmic normalized baryon fraction in the entire cluster sample ranges from $Y_b = 0.94$, at $z=1$ to $Y_b = 0.92$ at $z=0$. At both redshifts we find a slightly, but statistically significant decrease of $Y_b$ with cluster mass.
The cross-correlation of Sunyaev-Zeldovich effect (SZ) and weak-lensing imaging surveys can be used to test how well hot baryons trace dark matter in clusters of galaxies. We examine this concept using mock SZ and weak-lensing surveys based on the forthcoming AMiBA experiment and generated from a pre-heated cosmological N-body/hydrodynamic simulation. A cross-correlation diagram between matched lensing convergence and Compton y maps exhibits butterfly-wing-like structures, corresponding to individual clusters, that encode rich information about the distributions of hot gas and dark matter. When the cluster redshift and temperature are available the slope of a wing reveals the cluster gas fraction and the width of the wing indicates how badly the hot gas traces dark matter. On the basis of simulated data we discuss systematic errors in the projected gas fraction estimates that would be obtained from such survey comparisons.
The puzzling correlation between the spin parameter lambda of galactic disks and the disk-to-halo mass fraction fdisk is investigated. We show that such a correlation arises naturally from uncertainties in determining the virial masses of dark matter halos. This result leads to the conclusion that the halo properties derived from fits to observed rotation curves are still very uncertain which might explain part of the disagreements between cosmological models and observations. We analyse lambda and fdisk as function of the adopted halo virial mass. Reasonable halo concentrations require fdisk=0.01-0.07 which is significantly smaller than the universal baryon fraction. Most of the available gas either never settled into the galactic disks or was ejected subsequently. In both cases it is not very surprising that the specific angular momentum distribution of galactic disks does not agree with the cosmological predictions which neglect these effects.
We combine ASCA and ROSAT X-ray data to constrain the radial dark matter distribution in the primary cluster of A2256, free from the isothermality assumption. Both instruments indicate that the temperature declines with radius. The region including the central galaxy has a multicomponent spectrum, which results in a wide range of allowed central temperatures. We find that the secondary subcluster has a temperature and luminosity typical of a rich cluster; however, the ASCA temperature map shows no signs of an advanced merger. It is therefore assumed that the primary cluster is in hydrostatic equilibrium. The data then require dark matter density profiles steeper than rho ~ r^-2.5 in its outer part. Acceptable models have a total mass within r=1.5 Mpc (the virial radius) of 6.0+-1.5 10^14 Msun at the 90% confidence, about 1.6 times smaller than the mass derived assuming isothermality. Near the center, dark matter profiles with and without central cusps are consistent with the data. Total mass inside the X-ray core (r=0.26 Mpc) is 1.28+-0.08 10^14 Msun, which exceeds the isothermal value by a factor of 1.4. Although the confidence intervals above may be underestimates since they do not include possible asymmetry and departures from hydrostatic equilibrium, the behavior of the mass distribution, if applicable to other clusters, can bring into better agreement X-ray and lensing mass estimates, but aggravate the ``baryon catastrophe. The observed considerable increase in the gas content with radius, not anticipated by simulations, may imply that a significant fraction of thermal gas energy comes from sources other than gravity and merger shocks.
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