Squeaky Wheel Optimization

We describe a general approach to optimization which we term `Squeaky Wheel' Optimization (SWO). In SWO, a greedy algorithm is used to construct a solution which is then analyzed to find the trouble spots, i.e., those elements, that, if improved, are likely to improve the objective function score. The results of the analysis are used to generate new priorities that determine the order in which the greedy algorithm constructs the next solution. This Construct/Analyze/Prioritize cycle continues until some limit is reached, or an acceptable solution is found. SWO can be viewed as operating on two search spaces: solutions and prioritizations. Successive solutions are only indirectly related, via the re-prioritization that results from analyzing the prior solution. Similarly, successive prioritizations are generated by constructing and analyzing solutions. This `coupled search' has some interesting properties, which we discuss. We report encouraging experimental results on two domains, scheduling problems that arise in fiber-optic cable manufacturing, and graph coloring problems. The fact that these domains are very different supports our claim that SWO is a general technique for optimization

EFOSC2 Spectroscopy of SWIRE-CDFS Galaxies

We present the optical spectra of a sample of 34 SWIRE-CDFS sources observed with EFOSC2 on the ESO 3.6m Telescope. We have used the spectra and spectroscopic redshifts to validate our photometric redshift codes and SED template fitting methods. 12 of our sources are Infrared Luminous Galaxies. Of these, five belong to the class of ULIRGs and one to the class of HLIRGs with evidence of both an AGN and starburst component contributing to their extreme infrared luminosity for 3, starburst contributing for 1 and AGN contributing for 2 of them.Comment: 14 pages, 11 figures, 4 tables, accepted for publication in MNRA

Ground-based detection of a cloud of methanol from Enceladus: When is a biomarker not a biomarker?

Saturn's moon Enceladus has vents emerging from a sub-surface ocean, offering unique probes into the liquid environment. These vents drain into the larger neutral torus in orbit around Saturn. We present a methanol (CH3OH) detection observed with IRAM 30-m from 2008 along the line-of-sight through Saturn's E-ring. Additionally, we also present supporting observations from the Herschel public archive of water (ortho-H2O; 1669.9 GHz) from 2012 at a similar elongation and line-of-sight. The CH3OH 5(1,1)-4(1,1) transition was detected at 5.9 sigma confidence. The line has 0.43 km/s width and is offset by +8.1 km/s in the moon's reference frame. Radiative transfer models allow for gas cloud dimensions from 1750 km up to the telescope beam diameter ~73000 km. Taking into account the CH3OH lifetime against solar photodissociation and the redshifted line velocity, there are two possible explanations for the CH3OH emission: methanol is primarily a secondary product of chemical interactions within the neutral torus that (1) spreads outward throughout the E-ring or (2) originates from a compact, confined gas cloud lagging Enceladus by several km/s. We find either scenario to be consistent with significant redshifted H2O emission (4 sigma) measured from the Herschel public archive. The measured CH3OH:H2O abundance (> 0.5 per cent) significantly exceeds the observed abundance in the direct vicinity of the vents (~0.01 per cent), suggesting CH3OH is likely chemically processed within the gas cloud with methane (CH4) as its parent species.Comment: 16 pages, 4 figures, accepted for publication in the International Journal of Astrobiology (IJA

Diffusion Monte Carlo study of two-dimensional liquid 4^4He

The ground-state properties of two-dimensional liquid $^4$He at zero temperature are studied by means of a quadratic diffusion Monte Carlo method. As interatomic potential we use a revised version of the HFDHE2 Aziz potential which is expected to give a better description of the interaction between helium atoms. The equation of state is determined with great accuracy over a wide range of densities in the liquid phase from the spinodal point up to the freezing density. The spinodal decomposition density is estimated and other properties of the liquid, such as radial distribution function, static form factor, momentum distribution and density dependence of the condensate fraction are all presented.Comment: 19 pages, RevTex 3.0, 7 figures available upon reques

Chandra Observations of Extended X-ray Emission in Arp 220

We resolve the extended X-ray emission from the prototypical ultraluminous infrared galaxy Arp 220. Extended, faint edge-brightened, soft X-ray lobes outside the optical galaxy are observed to a distance of 10 to 15 kpc on each side of the nuclear region. Bright plumes inside the optical isophotes coincide with the optical line emission and extend 11 kpc from end to end across the nucleus. The data for the plumes cannot be fit by a single temperature plasma, and display a range of temperatures from 0.2 to 1 keV. The plumes emerge from bright, diffuse circumnuclear emission in the inner 3 kpc centered on the Halpha peak, which is displaced from the radio nuclei. There is a close morphological correspondence between the Halpha and soft X-ray emission on all spatial scales. We interpret the plumes as a starburst-driven superwind, and discuss two interpretations of the emission from the lobes in the context of simulations of the merger dynamics of Arp 220.Comment: Accepted for publication in ApJ; see also astro-ph/0208477 (Paper 1

The Double Quasar Q2138-431: Lensing by a Dark Galaxy?

We report the discovery of a new gravitational lens candidate Q2138-431AB, comprising two quasar images at a redshift of 1.641 separated by 4.5 arcsecs. The spectra of the two images are very similar, and the redshifts agree to better than 115 km.sec$^{-1}$. The two images have magnitudes $B_J = 19.8$ and $B_J = 21.0$, and in spite of a deep search and image subtraction procedure, no lensing galaxy has been found with $R < 23.8$. Modelling of the system configuration implies that the mass-to-light ratio of any lensing galaxy is likely to be around $1000 M_{\odot}/L_{\odot}$, with an absolute lower limit of $200 M_{\odot}/L_{\odot}$ for an Einstein-de Sitter universe. We conclude that the most likely explanation of the observations is gravitational lensing by a dark galaxy, although it is possible we are seeing a binary quasar.Comment: 17 pages (Latex), 8 postscript figures included, accepted by MNRA

Observation of Single Transits in Supercooled Monatomic Liquids

A transit is the motion of a system from one many-particle potential energy valley to another. We report the observation of transits in molecular dynamics (MD) calculations of supercooled liquid argon and sodium. Each transit is a correlated simultaneous shift in the equilibrium positions of a small local group of particles, as revealed in the fluctuating graphs of the particle coordinates versus time. This is the first reported direct observation of transit motion in a monatomic liquid in thermal equilibrium. We found transits involving 2 to 11 particles, having mean shift in equilibrium position on the order of 0.4 R_1 in argon and 0.25 R_1 in sodium, where R_1 is the nearest neighbor distance. The time it takes for a transit to occur is approximately one mean vibrational period, confirming that transits are fast.Comment: 19 pages, 8 figure