Star Formation Across the Taffy Bridge: UGC 12914/15

We present BIMA two-field mosaic CO(1-0) images of the Taffy galaxies (UGC 12914/15), which show the distinct taffy-like radio continuum emission bridging the two spiral disks. Large amounts of molecular gas (1.4 x 10^{10} Msun, using the standard Galactic CO-to-H$_2$ conversion applicable to Galactic disk giant molecular clouds [GMCs]) were clearly detected throughout the taffy bridge between the two galaxies, which, as in the more extreme case of HI, presumably results from a head-on collision between the two galaxies. The highest CO concentration between the two galaxies corresponds to the H_alpha source in the taffy bridge near the intruder galaxy UGC 12915. This HII region is also associated with the strongest source of radio continuum in the bridge, and shows both morphological and kinematic connections to UGC 12915. The overall CO distribution of the entire system agrees well with that of the radio continuum emission, particularly in the taffy bridge. This argues for the star formation origin of a significant portion of the radio continuum emission. Compared to the HI morphology and kinematics, which are strongly distorted owing to the high-speed collision, CO better defines the orbital geometry and impact parameter of the interaction, as well as the disk properties (e.g., rotation, orientation) of the progenitor galaxies. Based on the 20cm-to-CO ratio maps, we conclude that the starburst sites are primarily located in UGC 12915 and the H_alpha source in the bridge and show that the molecular gas in the taffy bridge is forming into stars with star formation efficiency comparable to that of the target galaxy UGC 12914 and similar to that in the Galactic disk.Comment: Minor typo/style corrections to match with the published version (AJ, Nov. issue). A single .ps.gz file of the entire paper can be downloaded from http://spider.ipac.caltech.edu/staff/gao/Taffy/all.ps.g

Studies of single top production at the Tevatron

I present the newest measurements of the production cross sections of single top quarks in pp collisions at √s = 1.96TeV in a data sample corresponding to an integrated luminosity of 5.4 fb−1 collected by the D0 detector at the Fermilab Tevatron Collider. The data is also used to extract limits on the CKM Matrix element |Vtb|

Exciton states in monolayer MoSe2: impact on interband transitions

We combine linear and non-linear optical spectroscopy at 4K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In 1-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we detect for the A- and B-exciton the 2p state 180meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than 2 orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our post-Density Functional Theory calculations show in the conduction band along the $K-\Gamma$ direction a local minimum that is energetically and in k-space close to the global minimum at the K-point. This has a potentially strong impact on the polarization and energy of the excitonic states that govern the interband transitions and marks an important difference to MoS2 and WSe2 monolayers.Comment: 8 pages, 3 figure

The microstructure of plasmodesmata in internodal stem tissue of the Saccharum hybrid var. NCo376 : evidence for an apoplasmic loading pathway

The distribution, structure and functional state of plasmodesmata were investigated to gain a clearer understanding of the sucrose transport pathway to the storage parenchyma cells in stem tissue in Saccharum officinarum var. NCo376. Evidence from structural studies on sugarcane stems by electron microscopy indicated that there are numerous plasmodesmata from the vascular bundles through to the storage parenchyma cells in mature stem tissue. Our studies, supported by fluorescence microscopy and iontophoresis, indicate that there are functional plasmodesmata in the phloemunloading pathway from transport phloem tissue to the bundle sheath in Saccharum, which could support symplasmic transport; plasmodesmata outside of the sheath cells in the storage parenchyma appear to be constricted by sphincter-like structures within their neck regions. Staining with Aniline Blue revealed evidence of large callose deposits, which co-localized with plasmodesmatal aggregates in the walls of the storage parenchyma cells. This suggests that the sucrose transport into, and accumulation by, storage parenchyma of mature stem tissue is under apoplasmic control

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

Correlation between the Extraordinary Hall Effect and Resistivity

We study the contribution of different types of scattering sources to the extraordinary Hall effect. Scattering by magnetic nano-particles embedded in normal-metal matrix, insulating impurities in magnetic matrix, surface scattering and temperature dependent scattering are experimentally tested. Our new data, as well as previously published results on a variety of materials, are fairly interpreted by a simple modification of the skew scattering model

Backward Reachability of Array-based Systems by SMT solving: Termination and Invariant Synthesis

The safety of infinite state systems can be checked by a backward reachability procedure. For certain classes of systems, it is possible to prove the termination of the procedure and hence conclude the decidability of the safety problem. Although backward reachability is property-directed, it can unnecessarily explore (large) portions of the state space of a system which are not required to verify the safety property under consideration. To avoid this, invariants can be used to dramatically prune the search space. Indeed, the problem is to guess such appropriate invariants. In this paper, we present a fully declarative and symbolic approach to the mechanization of backward reachability of infinite state systems manipulating arrays by Satisfiability Modulo Theories solving. Theories are used to specify the topology and the data manipulated by the system. We identify sufficient conditions on the theories to ensure the termination of backward reachability and we show the completeness of a method for invariant synthesis (obtained as the dual of backward reachability), again, under suitable hypotheses on the theories. We also present a pragmatic approach to interleave invariant synthesis and backward reachability so that a fix-point for the set of backward reachable states is more easily obtained. Finally, we discuss heuristics that allow us to derive an implementation of the techniques in the model checker MCMT, showing remarkable speed-ups on a significant set of safety problems extracted from a variety of sources.Comment: Accepted for publication in Logical Methods in Computer Scienc

Chandra Observations of the Interacting NGC 4410 Galaxy Group

We present high resolution X-ray imaging data from the ACIS-S instrument on the Chandra telescope of the nearby interacting galaxy group NGC 4410. Four galaxies in the inner portion of this group are clearly detected by Chandra, including the peculiar low luminosity radio galaxy NGC 4410A. In addition to a nuclear point source, NGC 4410A contains diffuse X-ray emission, including an X-ray ridge extending out to about 12" (6 kpc) to the northwest of the nucleus. This ridge is coincident with an arc of optical emission-line gas, which has previously been shown to have optical line ratios consistent with shock ionization. This structure may be due to an expanding superbubble of hot gas caused by supernovae and stellar winds or by the active nucleus. The Chandra observations also show four or five possible compact ultra-luminous X-ray (ULX) sources (L(x) >= 10^39 erg/s) associated with NGC 4410A. At least one of these candidate ULXs appears to have a radio counterpart, suggesting that it may be due to an X-ray binary with a stellar-mass black hole, rather than an intermediate mass black hole. In addition, a faint diffuse intragroup X-ray component has been detected between the galaxies (L(x) ~ 10^41 erg/s). This supports the hypothesis that the NGC 4410 group is in the process of evolving via mergers from a spiral-dominated group (which typically have no X-ray-emitting intragroup gas) to an elliptical-dominated group (which often have a substantial intragroup medium).Comment: 27 pages, 14 figures; Accepted by Astronomical Journal; color images at http://www.etsu.edu/physics/bsmith/research/n4410.htm

Vortex lattice studies in CeCoIn5 with H perpendicular to c

We present small angle neutron scattering studies of the vortex lattice (VL) in CeCoIn5 with magnetic fields applied parallel (H) to the antinodal [100] and nodal [110] directions. For H || [100], a single VL orientation is observed, while a 90 degree reorientation transition is found for H || [110]. For both field orientations and VL configurations we find a distorted hexagonal VL with an anisotropy, Gamma = 2.0 +/- 0.05. The VL form factor shows strong Pauli paramagnetic effects similar to what have previously been reported for H || [001]. At high fields, above which the upper critical field (Hc2) becomes a first-order transition, an increased disordering of the VL is observed.Comment: 5 pages, 4 figure