Non-radiative Hydrodynamical Simulations Research Articles

The effect of hydrodynamics alone on the subhalo population in a ΛCDM rich cluster sized dark matter halo

We perform a set of non-radiative hydro-dynamical (NHD) simulations of a rich cluster sized dark matter halo from the Phoenix Project with three different numerical resolutions, to investigate the effect of hydrodynamics alone on the subhalo population in the halo. Compared to dark matter only (DMO) simulations of the same halo, subhaloes are less abundant for relatively massive subhaloes (M sub > 2.5 × 109 h −1 M ⊙, or V max > 70 km s−1) but more abundant for less massive subhaloes in the NHD simulations. This results in different shapes in the subhalo mass/V max function in two different sets of simulations. At given subhalo mass, the subhaloes less massive than 1010 h −1 M ⊙ have larger V max in the NHD than DMO simulations, while V max is similar for the subhaloes more massive than the mass value. This is mainly because the progenitors of present day low mass subhaloes have larger concentration parameters in the NHD than DMO simulations. The survival number fraction of the accreted low mass progenitors of the main halo at redshift 2 is about 50 percent higher in the NHD than DMO simulations.

The segregation of baryons and dark matter during halo assembly

The standard galaxy formation theory assumes that baryons and dark matter are initially well mixed before becoming segregated due to radiative cooling. We use non-radiative hydrodynamical simulations to explicitly examine this assumption and find that baryons and dark matter can also be segregated due to different characteristics of gas and dark matter during the buildup of the halo. As a result, baryons in many haloes do not originate from the same Lagrangian region as the dark matter. When using the fraction of corresponding dark matter and gas particles in the initial conditions (the ‘paired fraction’) as a proxy of the dark matter and gas segregation strength of a halo, on average about 25 per cent of the baryonic and dark matter of the final halo are segregated in the initial conditions. This is at odds with the assumption of the standard galaxy formation model. A consequence of this effect is that the baryons and dark matter of the same halo initially experience different tidal torques and thus their angular momentum vectors are often misaligned. The degree of the misalignment is largely preserved during later halo assembly and can be understood with the tidal torque theory. The result challenges the precision of some semi-analytical approaches that utilize dark matter halo merger trees to infer properties of gas associated with dark matter haloes.

Accurate method of modeling cluster scaling relations in modified gravity

We propose a new method to model cluster scaling relations in modified gravity. Using a suite of non-radiative hydrodynamical simulations, we show that the scaling relations of cumulative gas quantities, such as the Sunyaev Zel'dovich effect (Compton-y parameter) and the x-ray Compton-Y parameter, can be accurately predicted using the known results in the $\Lambda$CDM model with a precision of $\sim3\%$. This method provides a reliable way to analyze the gas physics in modified gravity using the less-demanding and much more efficient pure cold dark matter simulations. Our results therefore have important theoretical and practical implications in constraining gravity using cluster surveys.

ON THE ORIGIN OF THE ANGULAR MOMENTUM PROPERTIES OF GAS AND DARK MATTER IN GALACTIC HALOS AND ITS IMPLICATIONS

We perform a set of non-radiative hydrodynamical simulations of merging spherical halos in order to understand the angular momentum (AM) properties of the galactic halos seen in cosmological simulations. The universal shape of AM distributions seen in simulations is found to be generically produced as a result of mergers. The universal shape is such that it has an excess of low AM material and hence cannot explain the exponential structure of disk galaxies. A resolution to this is suggested by the spatial distribution of low AM material which is found to be in the centre and a conical region close to the axis of rotation. A mechanism that preferentially discards the material in the centre and prevents the material along the poles from falling onto the disc is proposed as a solution. We implement a simple geometric criteria for selective removal of low AM material and show that in order for 90% of halos to host exponential discs one has to reject at least 40% of material. Next, we explore the physical mechanisms responsible for distributing the AM within the halo during a merger. For dark matter there is an inside-out transfer of AM, whereas for gas there is an outside-in transfer, which is due to differences between collisionless and gas dynamics. We also explain the apparent high spin of dark matter halos undergoing mergers and show that a criteria stricter than what is currently used, would be required to detect such unrelaxed halos. Finally, we demonstrate that the misalignment of AM between gas and dark matter only occurs when the intrinsic spins of the merging halos are not aligned with the orbital AM of the system. The self-misalignment (orientation of AM when measured in radial shells not being constant), which could be the cause of warps and anomalous rotation in disks galaxies, also occurs under similar conditions.

Ram pressure profiles in galaxy groups and clusters

Using a hybrid method which combines non-radiative hydrodynamical simulations with a semi-analytic model of galaxy formation, we determine the ram pressure as a function of halocentric distance experienced by galaxies in haloes with virial masses 12.5 <= log (M_200 h/M_Sun) < 15.35, for redshifts 0 <= z <= 3. The ram pressure is calculated with a self-consistent method which uses the simulation gas particles to obtain the properties of the intergalactic medium. The ram pressure profiles obtained can be well described by beta profile models, with parameters that depend on redshift and halo virial mass in a simple fashion. The fitting formulae provided here will prove useful to include ram pressure effects into semi-analytic models based on methods which lack gas physics, such as dark matter-only simulations or the Press-Schechter formalism.

Ram pressure stripping in a galaxy formation model - I. A novel numerical approach

We develop a new numerical approach to describe the action of ram pressure stripping (RPS) within a semi-analytic model of galaxy formation and evolution which works in combination with non-radiative hydrodynamical simulations of galaxy clusters. The new feature in our method is the use of the gas particles to obtain the kinematical and thermodynamical properties of the intragroup and intracluster medium (ICM). This allows a self-consistent estimation of the RPS experienced by satellite galaxies. We find that the ram pressure (RP) in the central regions of clusters increases approximately one order of magnitude between z = 1 and 0, consistent with the increase in the density of the ICM. The mean RP experienced by galaxies within the virial radius increases with decreasing redshift. In clusters with virial masses M_vir ~10^15 h^-1 M_Sun, over 50 per cent of satellite galaxies have experienced RP ~10^(-11) h^2 dyn cm^-2 or higher for z <= 0.5. In smaller clusters (M_vir ~10^14 h^-1 M_Sun) the mean RP are approximately one order of magnitude lower at all redshifts. RPS has a strong effect on the cold gas content of galaxies for all cluster masses. At z = 0, over 70 per cent of satellite galaxies within the virial radius are completely depleted of cold gas. For the more massive clusters the fraction of depleted galaxies is already established at z ~ 1, whereas for the smaller clusters this fraction increases appreciably between z = 1 and 0. This indicates that the rate at which the cold gas is stripped depends on the virial mass of the host cluster. Compared to our new approach, the use of an analytic profile to describe the ICM results in an overestimation of the RP larger than 50 per cent for z >= 0.5.

EXPLORING THE ENERGETICS OF INTRACLUSTER GAS WITH A SIMPLE AND ACCURATE MODEL

The state of the hot gas in clusters of galaxies is investigated with a set of model clusters, created by assuming a polytropic equation of state (Gamma=1.2) and hydrostatic equilibrium inside gravitational potential wells drawn from a dark matter simulation. Star formation, energy input, and nonthermal pressure support are included. To match the gas fractions seen in non-radiative hydrodynamical simulations, roughly 5% of the binding energy of the dark matter must be transferred to the gas during cluster formation; the presence of nonthermal pressure support increases this value. In order to match X-ray observations, scale-free behavior must be broken. This can be due to either variation of the efficiency of star formation with cluster mass M_500, or the input of additional energy proportional to the formed stellar mass M_F. These two processes have similar effects on X-ray scalings. If 9% of the gas is converted into stars, independent of cluster mass, then feedback energy input of 1.2e-5*M_Fc^2 (or ~1.0 keV per particle) is required to match observed clusters. Alternatively, if the stellar mass fraction varies as M_500^-0.26 then a lower feedback of 4e-6*M_Fc^2 is needed, and if the stellar fraction varies as steeply as M_500^-0.49 then no additional feedback is necessary. The model clusters reproduce the observed trends of gas temperature and gas mass fraction with cluster mass, as well as observed entropy and pressure profiles; thus they provide a calibrated basis with which to interpret upcoming SZ surveys. One consequence of the increased gas energy is that the baryon fraction inside the virial radius is less than roughly 90% of the cosmic mean, even for the most massive clusters.