Yuval Birnboim

Virial shocks in galactic haloes

We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared with the infall rate, the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monatomic gas that the shock is stable when the post-shock pressure and density obey γ e f f ≡ (d In P/dt)/(d In p/dt) > 10/7. When expressed in terms of the pre-shock gas properties at radius r it reads as ρrΛ(T)/u 3 < 0.0126, where p is the gas density, u is the infall velocity and A(T) is the cooling function, with the post-shock temperature T u 2 . This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before z ∼ 2, and it never forms in haloes less massive than a few 10 1 1 M O .. In such haloes, the infalling gas is not heated to the virial temperature until it hits the disc, thus avoiding the cooling-dominated quasi-static contraction phase. The direct collapse of the cold gas into the disc should have non-trivial effects on the star formation rate and on outflows. The soft X-ray produced by the shock-heated gas in the disc is expected to ionize the dense disc environment, and the subsequent recombination would result in a high flux of La emission. This may explain both the puzzling low flux of soft X-ray background and the La emitters observed at high redshift.

Bursting and quenching in massive galaxies without major mergers or AGNs

We simulate the build-up of galaxies by spherical gas accretion through dark matter haloes, subject to the development of virial shocks. We find that a uniform cosmological accretion rate turns into a rapidly varying disc build-up rate. The generic sequence of events (Shocked-Accretion Massive Burst and Shutdown, SAMBA) consists of four distinct phases: (i) continuous cold accretion while the halo is below a threshold mass M sh ∼ 10 12 M O , (ii) tentative quenching of gas supply for ∼2 Gyr, starting abruptly once the halo is ∼M sh and growing a rapidly expanding shock, (iii) a massive burst due to the collapse of ∼10 11 M O gas in ∼0.5 Gyr, when the accumulated heated gas cools and joins new infalling gas and (iv) a long-term shutdown, enhanced by a temporary shock instability in late SAMBAs, those that quench at z ∼ 2, burst at z ∼ and end up quenched in 10 12-13 M O haloes today. The quenching and bursting occur at all redshifts in galaxies of baryonic mass ∼1011 M O and involve a substantial fraction of this mass. They arise from rather smooth accretion, or minor mergers, which, unlike major mergers, may leave the disc intact while being built in a rapid pace. The early bursts match observed maximum starbursting discs at z ≥ 2, predicted to reside in ≤10 13 M O haloes. The late bursts resemble discy luminous infrared galaxies (LIRGs) at z ≤ 1. On the other hand, the tentative quenching gives rise to a substantial population of ∼10 11 M O galaxies with a strongly suppressed star formation rate at z ∼ 1-3. The predicted long-term shutdown leads to red and dead galaxies in groups. A complete shutdown in more massive clusters requires an additional quenching mechanism, as may be provided by clumpy accretion. Alternatively, the SAMBA bursts may trigger the active galactic nucleus (AGN) activity that couples to the hot gas above M sh and helps the required quenching. The SAMBA phenomenon is predicted based on a spherical model that does not simulate star formation and feedback - it is yet to be investigated using detailed cosmological simulations.

Gravitational quenching in massive galaxies and clusters by clumpy accretion

We consider a simple gravitational heating mechanism for the long-term quenching of cooling flows and star formation in massive dark matter haloes hosting elliptical galaxies and clusters. We showed earlier that the virial shock heating in haloes ≥10 12 M ⊙ triggers natural quenching in 10 12 -10 13 M ⊙ haloes. Our present analytic estimates and simple simulations argue that the long-term quenching in haloes ≥M min ∼ 7 x 1012 M ⊙ could be due to the gravitational energy of cosmological accretion delivered to the inner halo hot gas by cold flows via ram-pressure drag and local shocks. M min is obtained by comparing the gravitational power of infall into the potential well with the overall radiative cooling rate. The heating wins if the gas inner density cusp is not steeper than r -0.5 and if the masses in the cold and hot phases are comparable. The effect is stronger at higher redshifts, making the maintenance easier also at later times. Particular energy carriers into the halo core are cold gas clumps of ∼10 5 -10 8 M ⊙ . Clumps ≥10 5 M ⊙ penetrate to the inner halo with sufficient kinetic energy before they disintegrate, but they have to be ≤10 8 M ⊙ for the drag to do enough work in a Hubble time. Pressure-confined ∼10 4 K clumps are stable against their own gravity and remain gaseous once below the Bonnor-Ebert mass ∼10 8 M ⊙ . Such clumps are also immune to tidal disruption. Clumps in the desired mass range could emerge by thermal instability in the outer halo or in the filaments that feed it if the conductivity is not too high. Alternatively, such clumps may be embedded in dark matter subhaloes if the ionizing flux is ineffective, but they separate from their subhaloes by ram pressure before entering the inner halo. Heating by dynamical friction becomes dominant for massive satellites, which can contribute up to one-third of the total gravitational heating. We conclude that gravitational heating by cosmological accretion is a viable alternative to active galactic nucleus feedback as a long-term quenching mechanism.

Dark Halo Cusp: Asymptotic Convergence

We propose a model for how the buildup of dark halos by merging satellites produces a characteristic inner cusp, with a density profile ρ ∝ r, where αin → αas 1, as seen in cosmological N-body simulations of hierarchical clustering scenarios. Dekel, Devor, & Hetzroni argue that a flat core of αin 1. Using merger N-body simulations, we learn that this cusp is stable under a sequence of mergers and derive a practical tidal mass transfer recipe in regions where the local slope of the halo profile is α > 1. According to this recipe, the ratio of mean densities of the halo and initial satellite within the tidal radius equals a given function ψ(α), which is significantly smaller than unity (compared to being ~1 according to crude resonance criteria) and is a decreasing function of α. This decrease makes the tidal mass transfer relatively more efficient at larger α, which means steepening when α is small and flattening when α is large, thus causing convergence to a stable solution. Given this mass transfer recipe, linear perturbation analysis, supported by toy simulations, shows that a sequence of cosmological mergers with homologous satellites slowly leads to a fixed-point cusp with an asymptotic slope αas > 1. The slope depends only weakly on the fluctuation power spectrum, in agreement with cosmological simulations. During a long interim period the profile has an NFW-like shape, with a cusp of 1 < αin < αas. Thus, a cusp is enforced if enough compact satellite remnants make it intact into the inner halo. In order to maintain a flat core, satellites must be disrupted outside the core, possibly as a result of a modest puffing up due to baryonic feedback.

DYNAMICS AND MAGNETIZATION IN GALAXY CLUSTER CORES TRACED BY X-RAY COLD FRONTS

Cold fronts (CFs)—density and temperature plasma discontinuities—are ubiquitous in cool cores of galaxy clusters, where they appear as X-ray brightness edges in the intracluster medium, nearly concentric with the cluster center. We analyze the thermodynamic profiles deprojected across core CFs found in the literature. While the pressure appears continuous across these CFs, we find that all of them require significant centripetal acceleration beneath the front. This is naturally explained by a tangential, nearly sonic bulk flow just below the CF, and a tangential shear flow involving a fair fraction of the plasma beneath the front. Such shear should generate near-equipartition magnetic fields on scales 50pc from the front and could magnetize the entire core. Such fields would explain the apparent stability of cool core CFs and the recently reported CF-radio minihalo association.

Gravitational quenching by clumpy accretion in cool-core clusters: convective dynamical response to overheating

Many galaxy clusters pose a ‘cooling-flow problem’, where the observed X-ray emission from their cores is not accompanied by enough cold gas or star formation. A continuous energy source is required to balance the cooling rate over the whole core volume. We address the feasibility of a gravitational heating mechanism, utilizing the gravitational energy released by the gas that streams into the potential well of the cluster dark matter halo. We focus here on a specific form of gravitational heating in which the energy is transferred to the medium thorough the drag exerted on inflowing gas clumps. Using spherisymmetric hydro simulations with a subgrid representation of these clumps, we confirm our earlier estimates that in haloes

Cold fronts by merging of shocks

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of merging between two shocks propagating in the same direction. Such shock mergers typically result in a quasi-spherical, factor � 1:4− 2:7 discontinuity in density and in temperature. These CFs may be found as far out as the virial shock, unlike what is expected in other CF formation models. As a demonstration of this effect, we use one dimensional simulations of clusters and show that shock induced cold fronts form when perturbations such as explosions or mergers occur near the cluster’s centre. Perturbations at a cluster’s core induce periodic merging between the virial shock and outgoing secondary shocks. These collisions yield a distinctive, concentric, geometric sequence of CFs which trace the expansion of the virial shock.

THE HYDRODYNAMIC STABILITY OF GASEOUS COSMIC FILAMENTS

Virial shocks at the edges of cosmic-web structures are a clear prediction of standard structure formation theories. We derive a criterion for the stability of the post-shock gas and of the virial shock itself in spherical, filamentary, and planar infall geometries. When gas cooling is important, we find that shocks become unstable, and gas flows uninterrupted toward the center of the respective halo, filament, or sheet. For filaments, we impose this criterion on self-similar infall solutions. We find that instability is expected for filament masses between 1011 and 1013 Mpc−1. Using a simplified toy model, we then show that these filaments will likely feed halos with 1010 M ⊙ M halo 1013 M ⊙ at redshift z = 3, as well as 1012 M ⊙ M halo 1015 M ⊙ at z = 0. The instability will affect the survivability of the filaments as they penetrate gaseous halos in a non-trivial way. Additionally, smaller halos accreting onto non-stable filaments will not be subject to ram pressure inside the filaments. The instreaming gas will continue toward the center and stop either once its angular momentum balances the gravitational attraction, or when its density becomes so high that it becomes self-shielded to radiation.

The role of penetrating gas streams in setting the dynamical state of galaxy clusters

We utilize cosmological simulations of 16 galaxy clusters at redshifts \zeq{0} and \zeq{0.6} to study the effect of inflowing streams on the properties of the X-ray emitting intracluster medium. We find that the mass accretion occurs predominantly along streams that originate from the cosmic web and consist of heated gas. Clusters that are unrelaxed in terms of their X-ray morphology are characterized by higher mass inflow rates and deeper penetration of the streams, typically into the inner third of the virial radius. The penetrating streams generate elevated random motions, bulk flows and cold fronts. The degree of penetration of the streams may change over time such that clusters can switch from being unrelaxed to relaxed over a time-scale of several giga years.

Instability of Supersonic Cold Streams Feeding Galaxies I: Linear Kelvin-Helmholtz Instability with Body Modes

Massive galaxies at high redshift are predicted to be fed from the cosmic web by narrow, dense, cold streams. These streams penetrate supersonically through the hot medium encompassed by a stable shock near the virial radius of the dark-matter halo. Our long-term goal is to explore the heating and dissipation rate of the streams and their fragmentation and possible breakup, in order to understand how galaxies are fed, and how this affects their star-formation rate and morphology. We present here the first step, where we analyze the linear Kelvin-Helmholtz instability (KHI) of a cold, dense slab or cylinder flowing through a hot, dilute medium in the transonic regime. The current analysis is limited to the adiabatic case with no gravity and assuming equal pressure in the stream and the medium. By analytically solving the linear dispersion relation, we find a transition from a dominance of the familiar rapidly growing surface modes in the subsonic regime to more slowly growing body modes in the supersonic regime. The system is parameterized by three parameters: the density contrast between the stream and the medium, the Mach number of stream velocity with respect to the medium, and the stream width with respect to the halo virial radius. We find that a realistic choice for these parameters places the streams near the mode transition, with the KHI exponential-growth time in the range 0.01-10 virial crossing times for a perturbation wavelength comparable to the stream width. We confirm our analytic predictions with idealized hydrodynamical simulations. Our linear-KHI estimates thus indicate that KHI may in principle be effective in the evolution of streams by the time they reach the galaxy. More definite conclusions await the extension of the analysis to the nonlinear regime and the inclusion of cooling, thermal conduction, the halo potential well, self-gravity and magnetic fields.