3,650 research outputs found
Warm molecular hydrogen in outflows from Ultraluminous Infrared Galaxies
Ultraluminous infrared galaxies (ULIRGs) show on average three times more
emission in the rotational transitions of molecular hydrogen than expected
based on their star formation rates. Using Spitzer archival data we investigate
the origin of excess warm H_2 emission in 115 ULIRGs of the IRAS 1 Jy sample.
We find a strong correlation between H_2 and [FeII] line luminosities,
suggesting that excess H_2 is produced in shocks propagating within neutral or
partially ionized medium. This view is supported by the correlations between
H_2 and optical line ratios diagnostic of such shocks. The galaxies powered by
star formation and those powered by active nuclei follow the same relationship
between H_2 and [FeII], with emission line width being the major difference
between these classes (about 500 and 1000 km/sec, respectively). We conclude
that excess H_2 emission is produced as the supernovae and active nuclei drive
outflows into the neutral interstellar medium of the ULIRGs. A weak positive
correlation between H_2 and the length of the tidal tails indicates that these
outflows are more likely to be encountered in more advanced mergers, but there
is no evidence for excess H_2 produced as a result of the collision shocks
during the final coalescence.Comment: 16 pages, MNRAS accepted. Complete data tables and Spitzer spectra
are available at http://www.pha.jhu.edu/~zakamska/ULIRGS
PNNL Results from 2010 CALIBAN Criticality Accident Dosimeter Intercomparison Exercise
This document reports the results of the Hanford personnel nuclear accident dosimeter (PNAD) and fixed nuclear accident dosimeter (FNAD) during a criticality accident dosimeter intercomparison exercise at the CEA Valduc Center on September 20-23, 2010. Pacific Northwest National Laboratory (PNNL) participated in a criticality accident dosimeter intercomparison exercise at the Commissariat a Energie Atomique (CEA) Valduc Center near Dijon, France on September 20-23, 2010. The intercomparison exercise was funded by the U.S. Department of Energy, Nuclear Criticality Safety Program, with Lawrence Livermore National Laboratory as the lead Laboratory. PNNL was one of six invited DOE Laboratory participants. The other participating Laboratories were: Lawrence Livermore National Laboratory (LLNL), Los Alamos National Laboratory (LANL), Savannah River Site (SRS), the Y-12 National Security Complex at Oak Ridge, and Sandia National Laboratory (SNL). The goals of PNNL's participation in the intercomparison exercise were to test and validate the procedures and algorithm currently used for the Hanford personnel nuclear accident dosimeters (PNADs) on the metallic reactor, CALIBAN, to test exposures to PNADs from the side and from behind a phantom, and to test PNADs that were taken from a historical batch of Hanford PNADs that had varying degrees of degradation of the bare indium foil. Similar testing of the PNADs was done on the Valduc SILENE test reactor in 2009 (Hill and Conrady, 2010). The CALIBAN results are reported here
Defect-dependent colossal negative thermal expansion in UiO-66(Hf) metal-organic framework
Thermally-densified hafnium terephthalate UiO-66(Hf) is shown to exhibit the
strongest isotropic negative thermal expansion (NTE) effect yet reported for a
metal-organic framework (MOF). Incorporation of correlated vacancy defects
within the framework affects both the extent of thermal densification and the
magnitude of NTE observed in the densified product. We thus demonstrate that
defect inclusion can be used to tune systematically the physical behaviour of a
MOF.Comment: 8 pages, 4 figures, revise
Autopodial development is selectively impaired by misexpression of chordin-like 1 in the chick limb
AbstractChordin-like 1 (CHRDL1) is a secreted bone morphogenetic protein (BMP) antagonist expressed in mesenchymal tissues whose function in development of the skeleton has not been examined in detail. Here we show Chrdl1 is dynamically expressed in the early distal limb bud mesenchyme, with expression becoming downregulated as development proceeds. Chrdl1 expression is largely excluded from the critical signaling center of the posterior limb bud, the Zone of Polarizing Activity (ZPA), as has been described for the BMP antagonist Gremlin (GREM1) (Scherz et al., 2004, Science, 305, 396–399). Unlike Grem1, Chrdl1 is expressed in the hindlimb by a small subset of ZPA cells and their descendants suggesting divergent regulation and function between the various BMP antagonists. Ectopic expression of Chrdl1 throughout the avian limb bud using viral misexpression resulted in an oligodactyly phenotype with loss of digits from the anterior limb, although the development of more proximal elements of the zeugopod and stylopod were unaffected. Overgrowths of soft tissue and syndactyly were also observed, resulting from impaired apoptosis and failure of the anterior mesenchyme to undergo SOX9-dependent chondrogenesis, instead persisting as an interdigital-like soft tissue phenotype. Sonic hedgehog (SHH) and fibroblast growth factor (FGF) signaling were upregulated and persisted later in development, however these changes were only detected late in limb development at timepoints when endogenous Grem1 would normally be downregulated and increasing BMP signaling would cause termination of Shh and Fgf expression. Our results suggest that the early stages of the GREM1–SHH–FGF signaling network are resistant to Chrdl1-overexpression, leading to normal formation of proximal limb structures, but that later Bmp expression, impaired by ectopic CHRDL1, is essential for formation of the correct complement of digits
Dual positive and negative regulation of GPCR signaling by GTP hydrolysis
G protein-coupled receptors (GPCRs) regulate a variety of intracellular pathways through their ability to promote the binding of GTP to heterotrimeric G proteins. Regulator of G protein signaling (RGS) proteins increase the intrinsic GTPase activity of G-subunits and are widely regarded as
negative regulators of G protein signaling. Using yeast we demonstrate that GTP hydrolysis is not only required for desensitization, but is essential for achieving a high maximal (saturated level) response. Thus RGS-mediated GTP hydrolysis acts as both a negative (low stimulation) and
positive (high stimulation) regulator of signaling. To account for this we generated a new kinetic model of the G protein cycle where GGTP enters an inactive GTP-bound state following effector activation. Furthermore, in vivo and in silico experimentation demonstrates that maximum signaling output first increases and then decreases with RGS concentration. This unimodal, non-monotone
dependence on RGS concentration is novel. Analysis of the kinetic model has revealed a dynamic network motif that shows precisely how inclusion of the inactive GTP-bound state for the G produces this unimodal relationship
The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
We present an analysis of the distribution of H-alpha emission measures for
the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin
H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic
latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude
classical H II regions, leaving only sightlines that sample the diffuse WIM.
The distribution of EM sin |b| for the full sample is poorly characterized by a
single normal distribution, but is extraordinarily well fit by a lognormal
distribution, with = 0.146 +/- 0.001 and standard deviation
0.190 +/- 0.001. drops from 0.260 +/- 0.002 at Galactic
latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The
distribution may widen slightly at low Galactic latitude. We compare the
observed EM distribution function to the predictions of three-dimensional
magnetohydrodynamic simulations of isothermal turbulence within a
non-stratified interstellar medium. We find that the distribution of EM sin |b|
is well described by models of mildy supersonic turbulence with a sonic Mach
number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field
strength. The model also successfully predicts the distribution of dispersion
measures of pulsars and H-alpha line profiles. In the best fitting model, the
turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8
kpc scale height of the layer. The WIM gas has a lognormal distribution of
densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also
discuss the implications of these results for interpreting the filling factor,
the power requirement, and the magnetic field of the WIM.Comment: 16 pages, 13 figures, ApJ in press. Replacement reflects version
accepted for publicatio
Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium
We investigate models for the photoionization of the widespread diffuse
ionized gas in galaxies. In particular we address the long standing question of
the penetration of Lyman continuum photons from sources close to the galactic
midplane to large heights in the galactic halo. We find that recent
hydrodynamical simulations of a supernova-driven interstellar medium have low
density paths and voids that allow for ionizing photons from midplane OB stars
to reach and ionize gas many kiloparsecs above the midplane. We find ionizing
fluxes throughout our simulation grids are larger than predicted by one
dimensional slab models, thus allowing for photoionization by O stars of low
altitude neutral clouds in the Galaxy that are also detected in Halpha. In
previous studies of such clouds the photoionization scenario had been rejected
and the Halpha had been attributed to enhanced cosmic ray ionization or
scattered light from midplane H II regions. We do find that the emission
measure distributions in our simulations are wider than those derived from
Halpha observations in the Milky Way. In addition, the horizontally averaged
height dependence of the gas density in the hydrodynamical models is lower than
inferred in the Galaxy. These discrepancies are likely due to the absence of
magnetic fields in the hydrodynamic simulations and we discuss how
magnetohydrodynamic effects may reconcile models and observations.
Nevertheless, we anticipate that the inclusion of magnetic fields in the
dynamical simulations will not alter our primary finding that midplane OB stars
are capable of producing high altitude diffuse ionized gas in a realistic
three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure
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