Coherent Photons and Pomerons in Heavy Ion Collisions

Ultrarelativistic heavy ion beams carry large electromagnetic and strong absorptive fields, allowing exploration of a variety of physics. Two-photon, photon-Pomeron, and double Pomeron interactions can probe a huge variety of couplings and final states. RHIC will be the first heavy ion accelerator energetic enough to produce hadronic final states via coherent couplings. Virtual photons from the nuclear EM fields can interact in two-photon interactions, which can be exploited to study many particle spectroscopy and QCD topics. Because the photon flux scales as $Z^2$, Two-photon luminosities are large up to an energy of about \gamma\hbar c/R~ 3 GeV/c. Photon-Pomeron interactions are sensitive to how different vector mesons, including the $J/\psi$, interact with nuclear matter. $PP$ collisions rates are sensitive to the range of the Pomeron. Signals can be separated from backgrounds by using cuts on final state isolation (rapidity gaps) and $p_\perp$. We present Monte Carlo studies of different backgrounds, showing that representative signals can be extracted with good rates and signal to noise ratios.Comment: 5 pages; presented at the 6th Conference on the Intersections of Particle and Nuclear Physics, Big Sky, MO, May 27-June 2, 199

The Launching of Cold Clouds by Galaxy Outflows II: The Role of Thermal Conduction

We explore the impact of electron thermal conduction on the evolution of radiatively-cooled cold clouds embedded in flows of hot and fast material, as occur in outflowing galaxies. Performing a parameter study of three-dimensional adaptive mesh refinement hydrodynamical simulations, we show that electron thermal conduction causes cold clouds to evaporate, but it can also extend their lifetimes by compressing them into dense filaments. We distinguish between low column-density clouds, which are disrupted on very short times, and high-column density clouds with much-longer disruption times that are set by a balance between impinging thermal energy and evaporation. We provide fits to the cloud lifetimes and velocities that can be used in galaxy-scale simulations of outflows, in which the evolution of individual clouds cannot be modeled with the required resolution. Moreover, we show that the clouds are only accelerated to a small fraction of the ambient velocity because compression by evaporation causes the clouds to present a small cross-section to the ambient flow. This means that either magnetic fields must suppress thermal conduction, or that the cold clouds observed in galaxy outflows are not formed of cold material carried out from the galaxy.Comment: accepted by Ap

The Role of Turbulence in AGN Self-Regulation in Galaxy Clusters

Cool cores of galaxy clusters are thought to be heated by low-power active galactic nuclei (AGN), whose accretion is regulated by feedback. However, the interaction between the hot gas ejected by the AGN and the ambient intracluster medium is extremely difficult to simulate, as it involves a wide range of spatial scales and gas that is Rayleigh-Taylor (RT) unstable. Here we use a subgrid model for RT-driven turbulence to overcome these problems and present the first observationally-consistent hydrodynamical simulations of AGN self-regulation in galaxy clusters. For a wide range of parameter choices the cluster in our three-dimensional simulations regulates itself for at least several Gyrs years. Heating balances cooling through a string of outbreaks with a typical recurrence time of approximately 80 Myrs, a timescale that depends only on the global cluster properties.Comment: 4 pages, 1 figure, To appear in proceedings of The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters (AIP conference series

On the Formation of Molecular Clumps in QSO Outflows

We study the origin of the cold molecular clumps in quasar outflows, recently detected in CO and HCN emission. We first describe the physical properties of such radiation-driven outflows and show that a transition from a momentum- to an energy-driven flow must occur at a radial distance of R ~ 0.25 kpc. During this transition, the shell of swept up material fragments due to Rayleigh-Taylor instabilities, but these clumps contain little mass and are likely to be rapidly ablated by the hot gas in which they are immersed. We then explore an alternative scenario in which clumps form from thermal instabilities at R >~ 1 kpc, possibly containing enough dust to catalyze molecule formation. We investigate this processes with 3D two-fluid (gas+dust) numerical simulations of a kpc^3 patch of the outflow, including atomic and dust cooling, thermal conduction, dust sputtering, and photoionization from the QSO radiation field. In all cases, dust grains are rapidly destroyed in ~10,000 years; and while some cold clumps form at later times, they are present only as transient features, which disappear as cooling becomes more widespread. In fact, we only find a stable two-phase medium with dense clumps if we artificially enhance the QSO radiation field by a factor 100. This result, together with the complete destruction of dust grains, renders the interpretation of molecular outflows a very challenging problem.Comment: 17 pages, 12 figures, ApJ, In pres

What Can the Distribution of Intergalactic Metals Tell us About the History of Cosmological Enrichment?

I study the relationship between the spatial distribution of intergalactic metals and the masses and ejection energies of the sources that produced them. Over a wide range of models, metal enrichment is dominated by the smallest efficient sources, as the enriched volume scales roughly as E^{3/5} ~ M^{3/5} while the number density of sources goes as 1/M. In all cases, the earliest sources have the biggest impact, because fixed comoving distances correspond to smaller physical distances at higher redshifts. This means that most of the enriched volume is found around rare peaks, and intergalactic metals are naturally highly clustered. Furthermore, this clustering is so strong as to lead to a large overlap between individual bubbles. Thus the typical radius of enriched z ~ 3 regions should be interpreted as a constraint on groupings of sources rather than the ejection radius of a typical source. Similarly, the clustering of enriched regions should be taken as a measurement of source bias rather than mass.Comment: 10 pages, 2 figures, ApJL in pres