New Tests of the Cluster Entropy Floor Hypothesis
Recent efforts to account for the observed X-ray luminosity - temperature relation of galaxy clusters has led to suggestions that the ICM has an apparent ``entropy floor'' at or above the level of 300 keV cm^2. Here, we propose new tests based on the thermal Sunyaev-Zeldovich effect and on the cluster gas mass - temperature trend (from X-ray data) to probe the level of excess entropy in the ICM. We show that these new tests lend further support to the case for a high entropy floor in massive clusters.

title: The impact of mergers on relaxed X‐ray clusters – III. Effects on compact cool cores
abstract: We use the simulations presented in Poole et al. to examine the effects of mergers on the properties of cool cores in X-ray clusters. Motivated by recent Chandra and XMM-Newton observations, we propose a scheme for classifying the morphology of clusters based on their surface brightness and entropy profiles. Three dominant morphologies emerge: two hosting compact cores and central temperatures which are cool [CCC (compact cool core) systems] or warm [CWC (compact warm core) systems] and one hosting extended cores which are warm [EWC (extended warm core) systems]. In the cases we have studied, CCC states are disrupted only after direct collisions with merging cluster cores. This can happen in head-on collisions or during second pericentric passage in off-axis mergers. By the time they are relaxed, our remnant cores have generally been heated to warm core (CWC or EWC) states but subsequently recover CCC states by the end of the simulation. The only case resulting in a long-lived EWC state is a slightly off-axis 3:1 merger for which the majority of shock heating occurs during the accretion of a low-entropy stream formed from the disruption of the secondary's cool core. Since t dyn « t cool for all our relaxing merger remnant cores, compression prevents their core temperatures from falling until after they relax to the compact states allowed by their remnant central entropies. This naturally explains the population of relaxed CWC systems observed in recent Chandra and XMM-Newton observations with no need to invoke active galactic nuclei feedback. The morphological segregation in the L x -T x scaling relation noted by McCarthy et al. is qualitatively reflected in the results of our mergers as well. However, none of the cases we have studied produces systems with sufficiently high central entropies to account for the most underluminous EWC systems observed. Lastly, mergers do not efficiently mix the intracluster medium in our simulations. As a result, merging systems which initially host central metallicity gradients do not yield merger remnants with flat metallicity profiles. Taken together, these results suggest that once formed, compact core systems are remarkably stable against disruption from mergers. It remains to be demonstrated exactly how the sizable observed population of extended core systems was formed.

title: The Sunyaev–Zeldovich background
abstract: The cosmic background due to the Sunyaev–Zeldovich (SZ) effect is expected to be the largest signal at mm and cm wavelengths at a resolution of a few arcminutes. We investigate some simple statistics of SZ maps and their scaling with the normalization of the matter power spectrum, σ8, as well as the effects of the unknown physics of the intracluster medium on these statistics. We show that the SZ background provides a significant background for SZ cluster searches, with the onset of confusion occurring around 1014 h−1 M⊙ in a cosmology-dependent way, where confusion is defined as typical errors in recovered flux larger than 20 per cent. The confusion limit corresponds to the mass at which there are roughly 10 clusters per square degree, with this number nearly independent of cosmology and cluster gas physics. Typical errors grow quickly as lower mass objects are included in the catalogue. 
 
 
 
We also point out that there is nothing in particular about the rms of the filtered map that makes it especially well suited for capturing aspects of the SZ effect, and other indicators of the one-point SZ probability distribution function are at least as well suited for the task. For example, the full width at half-maximum of the one-point probability distribution has a field-to-field scatter that is about 60 per cent that of the rms. 
 
 
 
The simplest statistics of SZ maps are largely unaffected by cluster physics such as preheating, although the impact of preheating is clear by eye in the maps. Studies aimed at learning about the physics of the intracluster medium will apparently require more specialized statistical indicators.

title: Revisiting the baryon fractions of galaxy clusters: a comparison with WMAP 3-yr results
abstract: The universal baryonic mass fraction (Ω b /Ω m ) can be sensitively constrained using X-ray observations of galaxy clusters. In this paper, we compare the baryonic mass fraction inferred from measurements of the cosmic microwave background with the gas mass fractions (f gas ) of a large sample of clusters taken from the recent literature. In systems cooler than 4 keV, /gas declines as the system temperature decreases. However, in higher temperature systems, f gas (r 500 ) converges to≈(0.12 ± 0.02) (h /0.72) -1.5 , where the uncertainty reflects the systematic variations between clusters at r 500· This is significantly lower than the maximum-likelihood value of the baryon fraction from the recently released Wilkinson Microwave Anisotropy Probe (WMAP) 3-yr results. We investigate possible reasons for this discrepancy, including the effects of radiative cooling and non-gravitational heating, and conclude that the most likely solution is that Ω m is higher than the best-fitting WMAP value (we find Ω m =0.36 +0.11 ¯0.08 , but consistent at the 2σ level. Degeneracies within the WMAP data require that σ 8 must also be greater than the maximum likelihood value for consistency between the data sets.

title: Testing cold dark matter with the hierarchical build-up of stellar light
abstract: The hierarchical growth of mass in the Universe is a pillar of all cold dark matter (CDM) models. In this paper we demonstrate that this principle leads to a robust, falsifiable prediction of the stellar content of groups and clusters, which is testable with current observations and is relatively insensitive to the details of baryonic physics or cosmological parameters. Since it is difficult to preferentially remove stars from dark matter dominated systems, when these systems merge the fraction of total mass in stars can only increase (via star formation) or remain constant, relative to the fraction in the combined systems prior to the merger. Therefore, hierarchical models can put strong constraints on the observed correlation between stellar fraction,f*, and total system mass, M 500 . In particular, if this relation is fixed and does not evolve with redshift, CDM models predict b = d logf*/d logM 500 ≥-0.3. This constraint can be weakened if the f*-M 500 relation evolves strongly, but this implies more stars must be formed in situ in groups at low redshift. Conservatively requiring that at least half the stars in groups were formed by z = 1, the constraint from evolution models is b ≥ -0.35. Since the most massive clusters (M ∼ 10 15 M ⊙ ) are observed to have f * ∼ 0.01, this means that groups with M = 5 x 10 13 M ⊙ must havef*≤ 0.03. Recent observations by Gonzalez, Zaritsky & Zabludoff indicate a much steeper relation, withf* > 0.04 in groups leading to b ≈-0.64. If confirmed, this would rule out hierarchical structure formation models: today's clusters could not have been built from today's groups, or even from the higher redshift progenitors of those groups. We perform a careful analysis of these and other data to identify the most important systematic uncertainties in their measurements. Although correlated uncertainties on stellar and total masses might explain the steep observed relation, the data are only consistent with theory if the observed group masses are systematically underestimated.

title: Destruction of stellar disks by photoevaporation
abstract: Photoevaporation may provide an explanation for the short lifetimes of disks around young stars. With the exception of neutral oxygen lines, the observed low-velocity forbidden line emission from T Tauri stars can be reproduced by photoevaporating models. The natural formation of a gap in the disk at several AU due to photoevaporation and viscous spreading provides a possible halting mechanism for migrating planets and an explanation for the abundance of observed planets at these radii.

title: Optical Model in the Interior of the Nucleus
abstract: It is shown how phase relationships between partial waves in the optical model result in focusing effects which are largest for surface partial waves. In the case of very low incident energies where the wavelength of a partial wave is large compared to the nuclear surface thickness, it is shown how the phase relationships may enable surfaceand volume-reaction mechanisms for a direct reaction to be distinguished by looking at gross features of the argular distribution. Thus, low-energy direct interactions may be used as a probe for the nuclear interior.

title: Continuum in the atomic optical model
abstract: The optical potential in principle gives a complete description of the effect of the target continuum on an electron-scattering problem truncated to a discrete set of channels. A momentum-space form for the optical potential has been derived and computed for the continuum in various approximations for the case of hydrogen. The total ionization cross section is excellently reproduced in the weak-coupling approximation above 100 eV by the most detailed model, and is tolerable as low as 50 eV.

title: Theory of (p,2p) reactions at medium energies
abstract: A new and effective means of calculating matrix elements relevant to direct interactions is introduced and applied to a distorted-wave t matrix approximation analysis of the /sup 12/C(p,2p9/sup 11/B and /sup 16/O(p,2p)/sup 15/N reactions at 45-50 MeV incident energy. A realistic separable two-body t matrix has been employed and core polarization, whereby the final state is generated after excitation of giant resonance target states, has been included. The integrals involved in the matrix elements were calculated analytically by making a parametrization of the distorted scattering wave functions, the nucleon-nucleon interaction, and the bound state wave function, the parameters being chosen in accordance with elastic (p,p) and inelastic (p,p') data and realistic shell model wave functions. The results indicate the inadequacy of a distorted-wave t-matrix approximation analysis at these energies.

title: Electron-atom collisions
abstract: Preface 1.  Experimental techniques for cross-section measurements 3. Background quantum mechanics in the atomic context 4. One-electron problems 5. Theory of atomic bound states 6. Formal scattering theory 7. Calculation of scattering amplitudes 8. Spin-independent scattering observables 9. Spin-dependent scattering observables 10.  Electron momentum spectroscopy References Index.

title: Analytic distorted waves for intermediate-energy alpha particles