Hydrostatic equilibrium

About: Hydrostatic equilibrium is a research topic. Over the lifetime, 2451 publications have been published within this topic receiving 62172 citations.

Non-thermal pressure support in X-COP galaxy clusters

Abstract: Galaxy clusters are the endpoints of structure formation and are continuously growing through the merging and accretion of smaller structures. Numerical simulations predict that a fraction of their energy content is not yet thermalized, mainly in the form of kinetic motions (turbulence, bulk motions). Measuring the level of non-thermal pressure support is necessary to understand the processes leading to the virialization of the gas within the potential well of the main halo and to calibrate the biases in hydrostatic mass estimates. We present high-quality measurements of hydrostatic masses and intracluster gas fraction out to the virial radius for a sample of 12 nearby clusters with available XMM-Newton and Planck data. We compare our hydrostatic gas fractions with the expected universal gas fraction to constrain the level of non-thermal pressure support. We find that hydrostatic masses require little correction and infer a median non-thermal pressure fraction of $\sim6\%$ and $\sim10\%$ at $R_{500}$ and $R_{200}$, respectively. Our values are lower than the expectations of hydrodynamical simulations, possibly implying a faster thermalization of the gas. If instead we use the mass calibration adopted by the Planck team, we find that the gas fraction of massive local systems implies a mass bias $1-b=0.85\pm0.05$ for SZ-derived masses, with some evidence for a mass-dependent bias. Conversely, the high bias required to match Planck CMB and cluster count cosmology is excluded by the data at high significance, unless the most massive halos are missing a substantial fraction of their baryons.

The shape of Enceladus as explained by an irregular core: Implications for gravity, libration, and survival of its subsurface ocean

Abstract: [1] Enceladus' global shape is not consistent with a simultaneously hydrostatic and fully differentiated body, but its geophysical hyperactivity strongly implies that it is differentiated. Enceladus' surface conforms to a triaxial shape, consistent with relaxation to a global geoid, but its rocky core need not be hydrostatic. A modestly irregular core, either in terms of topography or density, and dynamically aligned, would act to enhance the global geoid. Explaining the full global shape of Enceladus requires ~10 km of excess core polar flattening and ~4 km of excess core equatorial distortion, for a uniform density core. The stresses associated with this excess topography can be sustained indefinitely, but because the rocky core is not hydrostatic, Enceladus' degree-2 gravity coefficients J2 and C22 will not conform to the hydrostatic ratio of 10/3 (testable by Cassini gravity). Notably, core polar axes could be depressed by up to 4–6 km with respect to internal isobars. Such a topographic variation could help preserve polar ocean remnants. For example, if Enceladus' ocean undergoes secular freezing, it will first ground near the tidal axes. Polar seas would be the last to freeze, and survival of a south polar sea may be one reason why Enceladus' activity today is concentrated at high southern latitudes. Isostatic variations in the thickness or density of a floating ice shell may also contribute to Enceladus' nonhydrostatic shape, but a strength-supported, nonisostatic icy lithosphere is argued here to be unlikely, given Enceladus' record of regional high heat flows, faulting, and viscous relaxation.

Residual Gas Motions in the Intracluster Medium and Bias in Hydrostatic Measurements of Mass Profiles of Clusters

Abstract: We present analysis of bulk and random gas motions in the intracluster medium using high-resolution Eulerian cosmological simulations of sixteen simulated clusters, including both very relaxed and unrelaxed systems and spanning a virial mass range of 5*10^13 - 2*10^15 Msun/h. We investigate effects of the residual subsonic gas motions on the hydrostatic estimates of mass profiles and concentrations of galaxy clusters. In agreement with previous studies we find that the gas motions contribute up to ~ 5%-15% of the total pressure support in relaxed clusters with contribution increasing with cluster-centric radius. The fractional pressure support is higher in unrelaxed systems. This contribution would not be accounted for in hydrostatic estimates of the total mass profile and would lead to systematic underestimate of mass. We demonstrate that total mass can be recovered accurately if pressure due to gas motions measured in simulations is explicitly taken into account in the equation of hydrostatic equilibrium. Given that the underestimate of mass is increasing at larger radii, where gas is less relaxed and contribution of gas motions to pressure is larger, the total density profile derived from hydrostatic analysis is more concentrated than the true profile. This may at least partially explain some high values of concentrations of clusters estimated from hydrostatic analysis of X-ray data.

Reconciling optical and X‐ray mass estimates: the case of the elliptical galaxy NGC 3379

Abstract: NGC 3379 is a well-studied nearby elliptical for which optical investigations have claimed a little dark matter content, or even no dark matter. Recently, its total mass profile M(r) has been derived by exploiting Chandra observations of its extended and X-ray emitting interstellar medium, based on the hypothesis of hydrostatic equilibrium for the hot gas. The resulting total mass within the effective radius R e has been claimed to be a few times larger than that found by optical studies. Here, we show that part of the discrepancy can be due to an underestimate of the optically derived mass, and the remaining discrepancy of a factor of ∼2 can be explained by deviations from hydrostatic equilibrium of the hot gas. By using hydrodynamical simulations tailored to reproduce the observed hot gas properties of NGC 3379, and by assuming as input for the simulations the total mass profile derived optically, we show that (i) the hot gas at the present time has X-ray properties consistent with those observed only if it is outflowing over most of the galactic body, and (ii) an overestimate of M of the same size found in the recent X-ray analysis is recovered when assuming hydrostatic equilibrium. We also show that the hot gas is outflowing even for a dark matter fraction within R e as large as derived with the standard X-ray procedure based on the hydrostatic equilibrium assumption, which shows the unapplicability of the method for this galaxy. Finally, we find that the whole range of dark mass amount and distribution allowed for by optical studies is compatible with a hot gas flow with the observed X-ray properties.