Clustered Supernovae Drive Powerful Galactic Winds After Super-Bubble Breakout

We use three-dimensional hydrodynamic simulations of vertically stratified patches of galactic discs to study how the spatio-temporal clustering of supernovae (SNe) enhances the power of galactic winds. SNe that are randomly distributed throughout a galactic disc drive inefficient galactic winds because most supernova remnants lose their energy radiatively before breaking out of the disc. Accounting for the fact that most star formation is clustered alleviates this problem. Super-bubbles driven by the combined effects of clustered SNe propagate rapidly enough to break out of galactic discs well before the clusters' SNe stop going off. The radiative losses post-breakout are reduced dramatically and a large fraction ($\gtrsim 0.2$) of the energy released by SNe vents into the halo powering a strong galactic wind. These energetic winds are capable of providing strong preventative feedback and eject substantial mass from the galaxy with outflow rates on the order of the star formation rate. The momentum flux in the wind is only of order that injected by the SNe, because the hot gas vents before doing significant work on the surroundings. We show that our conclusions hold for a range of galaxy properties, both in the local Universe (e.g., M82) and at high redshift (e.g., $z \sim 2$ star forming galaxies). We further show that if the efficiency of forming star clusters increases with increasing gas surface density, as suggested by theoretical arguments, the condition for star cluster-driven super-bubbles to break out of galactic discs corresponds to a threshold star formation rate surface density for the onset of galactic winds $\sim 0.03$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$, of order that observed.Comment: 19 pages, 12 figures, and 3 page appendix with 6 figures. Movies available at http://w.astro.berkeley.edu/~dfielding/#SNeDrivenWinds

The biasing of baryons on the cluster mass function and cosmological parameter estimation

We study the effect of baryonic processes on the halo mass function in the galaxy cluster mass range using a catalogue of 153 high resolution cosmological hydrodynamical simulations performed with the AMR code ramses. We use the results of our simulations within a simple analytical model to gauge the effects of baryon physics on the halo mass function. Neglect of AGN feedback leads to a significant boost in the cluster mass function similar to that reported by other authors. However, including AGN feedback not only gives rise to systems that are similar to observed galaxy clusters, but they also reverse the global baryonic effects on the clusters. The resulting mass function is closer to the unmodified dark matter halo mass function but still contains a mass dependent bias at the 5-10% level. These effects bias measurements of the cosmological parameters, such as $\sigma_8$ and $\Omega_m$. For current cluster surveys baryonic effects are within the noise for current survey volumes, but forthcoming and planned large SZ, X-ray and multi-wavelength surveys will be biased at the percent level by these processes. The predictions for the halo mass function including baryonic effects need to be carefully studied with larger and improved simulations. However, simulations of full cosmological boxes with the resolution we achieve and including AGN feedback are still computationally challenging.Comment: 12 pages, 3 tables, 6 figures, accepted for publication in MNRA

Brightest Cluster Galaxies in Cosmological Simulations with Adaptive Mesh Refinement: Successes and Failures

A large sample of cosmological hydrodynamical zoom-in simulations with Adaptive Mesh Refinement (AMR) is analysed to study the properties of simulated Brightest Cluster Galaxies (BCGs). Following the formation and evolution of BCGs requires modeling an entire galaxy cluster, because the BCG properties are largely influenced by the state of the gas in the cluster and by interactions and mergers with satellites. BCG evolution is also deeply influenced by the presence of gas heating sources such as Active Galactic Nuclei (AGNs) that prevent catastrophic cooling of large amounts of gas. We show that AGN feedback is one of the most important mechanisms in shaping the properties of BCGs at low redshift by analysing our statistical sample of simulations with and without AGN feedback. When AGN feedback is included BCG masses, sizes, star formation rates and kinematic properties are closer to those of the observed systems. Some small discrepancies are observed only for the most massive BCGs and in the fraction of star-forming BCGs, effects that might be due to physical processes that are not included in our model.Comment: 11 pages, 2 tables, 8 figures. Accepted for publication in MNRA

Simulations of Jet Heating in Galaxy Clusters: Successes and Challenges

We study how jets driven by active galactic nuclei influence the cooling flow in Perseus-like galaxy cluster cores with idealised, non-relativistic, hydrodynamical simulations performed with the Eulerian code ATHENA using high-resolution Godunov methods with low numerical diffusion. We use novel analysis methods to measure the cooling rate, the heating rate associated to multiple mechanisms, and the power associated with adiabatic compression/expansion. A significant reduction of the cooling rate and cooling flow within 20 kpc from the centre can be achieved with kinetic jets. However, at larger scales and away from the jet axis, the system relaxes to a cooling flow configuration. Jet feedback is anisotropic and is mostly distributed along the jet axis, where the cooling rate is reduced and a significant fraction of the jet power is converted into kinetic power of heated outflowing gas. Away from the jet axis weak shock heating represents the dominant heating source. Turbulent heating is significant only near the cluster centre, but it becomes inefficient at 50 kpc scales where it only represents a few percent of the total heating rate. Several details of the simulations depend on the choice made for the hydro solver, a consequence of the difficulty of achieving proper numerical convergence for this problem: current physics implementations and resolutions do not properly capture multi-phase gas that develops as a consequence of thermal instability. These processes happen at the grid scale and leave numerical solutions sensitive to the properties of the chosen hydro solver.Comment: Accepted for publication on MNRA

Infall near clusters of galaxies: comparing gas and dark matter velocity profiles

We consider the dynamics in and near galaxy clusters. Gas, dark matter and galaxies are presently falling into the clusters between approximately 1 and 5 virial radii. At very large distances, beyond 10 virial radii, all matter is following the Hubble flow, and inside the virial radius the matter particles have on average zero radial velocity. The cosmological parameters are imprinted on the infall profile of the gas, however, no method exists, which allows a measurement of it. We consider the results of two cosmological simulations (using the numerical codes RAMSES and Gadget) and find that the gas and dark matter radial velocities are very similar. We derive the relevant dynamical equations, in particular the generalized hydrostatic equilibrium equation, including both the expansion of the Universe and the cosmological background. This generalized gas equation is the main new contribution of this paper. We combine these generalized equations with the results of the numerical simulations to estimate the contribution to the measured cluster masses from the radial velocity: inside the virial radius it is negligible, and inside two virial radii the effect is below 40%, in agreement the earlier analyses for DM. We point out how the infall velocity in principle may be observable, by measuring the gas properties to distance of about two virial radii, however, this is practically not possible today.Comment: 7 pages, 3 figures, to appear in MNRA

The formation of the brightest cluster galaxies in cosmological simulations: the case for active galactic nucleus feedback

We use 500 pc resolution cosmological simulations of a Virgo-like galaxy cluster to study the properties of the brightest cluster galaxy (BCG) that forms at the centre of the halo. We compared two simulations; one incorporating only supernova feedback and a second that also includes prescriptions for black hole growth and the resulting active galactic nucleus (AGN) feedback from gas accretion. As previous work has shown, with supernova feedback alone we are unable to reproduce any of the observed properties of massive cluster ellipticals. The resulting BCG rotates quickly, has a high Sérsic index, a strong mass excess in the centre and a total central density profile falling more steeply than isothermal. Furthermore, it is far too efficient at converting most of the available baryons into stars which is strongly constrained by abundance matching. With a treatment of black hole dynamics and AGN feedback the BCG properties are in good agreement with data: they rotate slowly, have a cored surface density profile, a flat or rising velocity dispersion profile and a low stellar mass fraction. The AGN provides a new mechanism to create cores in luminous elliptical galaxies; the core expands due to the combined effects of heating from dynamical friction of sinking massive black holes and AGN feedback that ejects gaseous material from the central region

Rhapsody-G simulations: galaxy clusters as baryonic closed boxes and the covariance between hot gas and galaxies

Within a sufficiently large cosmic volume, conservation of baryons implies a simple `closed box' view in which the sum of the baryonic components must equal a constant fraction of the total enclosed mass. We present evidence from Rhapsody-G hydrodynamic simulations of massive galaxy clusters that the closed-box expectation may hold to a surprising degree within the interior, non-linear regions of haloes. At a fixed halo mass, we find a significant anti-correlation between hot gas mass fraction and galaxy mass fraction (cold gas + stars), with a rank correlation coefficient of -0.69 within $R_{500c}$. Because of this anti-correlation, the total baryon mass serves as a low-scatter proxy for total cluster mass. The fractional scatter of total baryon fraction scales approximately as $0.02 (\Delta_c/100)^{0.6}$, while the scatter of either gas mass or stellar mass is larger in magnitude and declines more slowly with increasing radius. We discuss potential observational tests using cluster samples selected by optical and hot gas properties; the simulations suggest that joint selection on stellar and hot gas has potential to achieve 5% scatter in total halo mass.Comment: 10 pages, 6 figures, 3 tables; replaced to match published versio