A Lensed Arc in the Low Redshift Cluster Abell 2124

We report the discovery of an arc-like object 27" from the center of the cD galaxy in the redshift $z=0.066$ cluster A2124. Observations with the Keck II telescope reveal that the object is a background galaxy at $z=0.573$, apparently lensed into an arc of length \sim 8 \farcs5 and total R magnitude $m_R = 20.86\pm0.07$. The width of the arc is resolved; we estimate it to be $\sim$0\farcs6 after correcting for seeing. A lens model of the A2124 core mass distribution consistent with the cluster galaxy velocity dispersion reproduces the observed arc geometry and indicates a magnification factor \gta 9. With this magnification, the strength of the [OII] \lambda 3727 line implies a star-formation rate of SFR \sim 0.4 h^{-2}\msun yr^{-1}$. A2124 thus appears to be the lowest redshift cluster known to exhibit strong lensing of a distant background galaxy.Comment: 6 pages using emulateapj.sty; 4 Postscript figures; Figure 4 uses color. Accepted for publication, but ApJ Letters' new policy of counting data images makes the manuscript too long; will appear in main journal. This final version has minor correction

Private Governance Responses to Climate Change: The Case of Global Civil Aviation

This Article explores how private governance can reduce the climate effects of global civil aviation. The civil aviation sector is a major contributor to climate change, accounting for emissions comparable to a top ten emitting country. National and international governmental bodies have taken important steps to address civil aviation, but the measures adopted to date are widely acknowledged to be inadequate. Civil aviation poses particularly difficult challenges for government climate mitigation efforts. Many civil aviation firms operate globally, emissions often occur outside of national boundaries, nations differ on their respective responsibilities, and demand is growing rapidly. Although promising new technologies are emerging, they will take time to develop and adopt. This Article argues that private initiatives can overcome many of these barriers. Private initiatives can motivate civil aviation firms to act absent government pressure at the national level and can create pressure for mitigation that transcends national boundaries. The Article argues that it is time to develop a private climate governance agenda for civil aviation and identifies examples of the types of existing and new initiatives that could be included in the effort. If public and private policymakers can overcome the tendency to focus almost exclusively on public governance, private initiatives can yield large and prompt emissions reductions from global civil aviation, buy time for more comprehensive government measures, and complement the government measures when they occur

The Merger of Small and Large Black Holes

We present simulations of binary black holes mergers in which, after the common outer horizon has formed, the marginally outer trapped surfaces (MOTSs) corresponding to the individual black holes continue to approach and eventually penetrate each other. This has very interesting consequences according to recent results in the theory of MOTSs. Uniqueness and stability theorems imply that two MOTSs which touch with a common outer normal must be identical. This suggests a possible dramatic consequence of the collision between a small and large black hole. If the penetration were to continue to completion then the two MOTSs would have to coalesce, by some combination of the small one growing and the big one shrinking. Here we explore the relationship between theory and numerical simulations, in which a small black hole has halfway penetrated a large one.Comment: 17 pages, 11 figure