1,645 research outputs found
Observations of Dense Molecular Gas in a Quasar Host Galaxy at z=6.42: Further Evidence for a Non-Linear Dense Gas - Star Formation Relation at Early Cosmic Times
We report a sensitive search for the HCN(J=2-1) emission line towards SDSS
J1148+5251 at z=6.42 with the VLA. HCN emission is a star formation indicator,
tracing dense molecular hydrogen gas (n(H2) >= 10^4 cm^-3) within star-forming
molecular clouds. No emission was detected in the deep interferometer maps of
J1148+5251. We derive a limit for the HCN line luminosity of L'(HCN) < 3.3 x
10^9 K km/s pc^2, corresponding to a HCN/CO luminosity ratio of L'(HCN)/L'(CO)
< 0.13. This limit is consistent with a fraction of dense molecular gas in
J1148+5251 within the range of nearby ultraluminous infrared galaxies (ULIRGs;
median value: L'(HCN)/L'(CO) = 0.17 {+0.05/-0.08}) and HCN-detected z>2
galaxies (0.17 {+0.09/-0.08}). The relationship between L'(HCN) and L(FIR) is
considered to be a measure for the efficiency at which stars form out of dense
gas. In the nearby universe, these quantities show a linear correlation, and
thus, a practically constant average ratio. In J1148+5251, we find
L(FIR)/L'(HCN) > 6600. This is significantly higher than the average ratios for
normal nearby spiral galaxies (L(FIR)/L'(HCN) = 580 {+510/-270}) and ULIRGs
(740 {+505/-50}), but consistent with a rising trend as indicated by other z>2
galaxies (predominantly quasars; 1525 {+1300/-475}). It is unlikely that this
rising trend can be accounted for by a contribution of AGN heating to L(FIR)
alone, and may hint at a higher median gas density and/or elevated
star-formation efficiency toward the more luminous high-redshift systems. There
is marginal evidence that the L(FIR)/L'(HCN) ratio in J1148+5251 may even
exceed the rising trend set by other z>2 galaxies; however, only future
facilities with very large collecting areas such as the SKA will offer the
sensitivity required to further investigate this question.Comment: 5 pages, 2 figures, 2 tables, to appear in ApJL (accepted October 24,
2007
A Sensitive Search for [N II]205 ÎŒm Emission in a z = 6.4 Quasar Host Galaxy
We present a sensitive search for the 3P1 â 3P0 ground-state fine structure line at 205 ÎŒm of ionized nitrogen ([N II]205ÎŒm) in one of the highest-redshift quasars (J1148+5251 at z = 6.42) using the IRAM 30 m telescope. The line is not detected at a (3Ï) depth of 0.47 Jy km s^â1, corresponding to a [N II]205ÎŒm luminosity limit of L[N II] 7) using the Atacama Large Millimeter/submillimeter Array, for which the highly excited rotational transitions of CO will be shifted outside the accessible (sub-)millimeter bands
Ontology Summit 2008 Communiqué: Towards an open ontology repository
Each annual Ontology Summit initiative makes a statement appropriate to each Summits theme as part of our general advocacy designed to bring ontology science and engineering into the mainstream. The theme this year is "Towards an Open Ontology Repository". This communiqué represents the joint position of those who were engaged in the year's summit discourse on an Open Ontology Repository (OOR) and of those who endorse below. In this discussion, we have agreed that an "ontology repository is a facility where ontologies and related information artifacts can be stored, retrieved and managed."
We believe in the promise of semantic technologies based on logic, databases and the Semantic Web, a Web of exposed data and of interpretations of that data (i.e., of semantics), using common standards. Such technologies enable distinguishable, computable, reusable, and sharable meaning of Web and other artifacts, including data, documents, and services. We also believe that making that vision a reality requires additional supporting resources and these resources should be open, extensible, and provide common services over the ontologies
Active Site Mapping of Xylan-Deconstructing Enzymes with Arabinoxylan Oligosaccharides Produced by Automated Glycan Assembly
Xylan-degrading enzymes are crucial for the deconstruction of hemicellulosic biomass, making the hydrolysis products available for various industrial applications such as the production of biofuel. To determine the substrate specificities of these enzymes, we prepared a collection of complex xylan oligosaccharides by automated glycan assembly. Seven differentially protected building blocks provided the basis for the modular assembly of 2-substituted, 3-substituted, and 2-/3-substituted arabino- and glucuronoxylan oligosaccharides. Elongation of the xylan backbone relied on iterative additions of C4-fluorenylmethoxylcarbonyl (Fmoc) protected xylose building blocks to a linker-functionalized resin. Arabinofuranose and glucuronic acid residues have been selectively attached to the backbone using fully orthogonal 2-(methyl)naphthyl (Nap) and 2-(azidomethyl)benzoyl (Azmb) protecting groups at the C2 and C3 hydroxyls of the xylose building blocks. The arabinoxylan oligosaccharides are excellent tools to map the active site of glycosyl hydrolases involved in xylan deconstruction. The substrate specificities of several xylanases and arabinofuranosidases were determined by analyzing the digestion products after incubation of the oligosaccharides with glycosyl hydrolases.Fil: Senf, Deborah. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; Alemania. Freie UniversitĂ€t; AlemaniaFil: Ruprecht, Colin. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; AlemaniaFil: de Kruijff, Goswinus H. M.. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; Alemania. Freie UniversitĂ€t; Alemania. University Mainz. Institute of Institute of Organic Chemistry, Johannes Gutenberg; AlemaniaFil: Simonetti, SebastiĂĄn Osvaldo. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de QuĂmica Rosario. Universidad Nacional de Rosario. Facultad de Ciencias BioquĂmicas y FarmacĂ©uticas. Instituto de QuĂmica Rosario; Argentina. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; AlemaniaFil: Schuhmacher, Frank. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; Alemania. Freie UniversitĂ€t; AlemaniaFil: Seeberger, Peter H.. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; Alemania. Freie UniversitĂ€t; AlemaniaFil: Pfrengle, Fabian. Max Planck Institut fĂŒr Kolloid und GrenzflĂ€chenforschung; Alemania. Freie UniversitĂ€t; Alemani
Millimeter and Radio Observations of z~6 Quasars
We present millimeter and radio observations of 13 SDSS quasars at reshifts
z~6. We observed eleven of them with the Max-Planck Millimeter Bolometer Array
(MAMBO-2) at the IRAM 30m-telescope at 250 GHz and all of them with the Very
Large Array (VLA) at 1.4 GHz. Four sources are detected by MAMBO-2 and six are
detected by the VLA at >=3 sigma level. These sources, together with another 6
published in previous papers,yield a submillimeter/millimeter and radio
observed SDSS quasar sample at z~6. We use this sample to investigate the
far-infrared (FIR) andradio properties of optically bright quasars in the early
universe. We compare this sample to lower redshift samples of quasars observed
inthe submillimeter and millimeter wavelengths ((sub)mm), and find that the
distribution of the FIR to B band optical luminosity ratio (L_FIR/L_B) is
similar from z~2 to 6. We find a weak correlation between the FIR luminosity
(L_FIR) and B band optical luminosity (L_B) byincluding the (sub)mm observed
samples at all redshifts. Some strong (sub)mm detections in the z~6 sample have
radio-to-FIR ratios within the range defined by star forming galaxies, which
suggests possible co-eval star forming activity with the powerful AGN in these
sources. We calculate the rest frame radio to optical ratios (R*_1.4=L_{v,
1.4GHz}/L_{v, 4400A}) for all of the VLA observed sources in the z~6 quasar
sample. Only one radio detection in this sample, J083643.85+005453.3, has
R*_1.4~40 and can be considered radio loud. There are no strong radio sources
(R*_1.4>=100) among these SDSS quasars at z~6. These data are consistent with,
although do not set strong constraints on, a decreasing radio-loud quasar
fraction with increasing redshift.Comment: 27 pages including 6 figures. AJ accepte
Millimeter Observations of GRB 030329: Continued Evidence for a Two-Component Jet
We present the results of a dedicated campaign on the afterglow of GRB 030329
with the millimeter interferometers of the Owens Valley Radio Observatory
(OVRO), the Berkeley-Illinois-Maryland Association (BIMA), and with the MAMBO-2
bolometer array on the IRAM 30-m telescope. These observations allow us to
trace the full evolution of the afterglow of GRB 030329 at frequencies of 100
GHz and 250 GHz for the first time. The millimeter light curves exhibit two
main features: a bright, constant flux density portion and a steep power-law
decline. The absence of bright, short-lived millimeter emission is used to show
that the GRB central engine was not actively injecting energy well after the
burst. The millimeter data support a model, advocated by Berger et al., of a
two-component jet-like outflow in which a narrow angle jet is responsible for
the high energy emission and early optical afterglow, and a wide-angle jet
carrying most of the energy is powering the radio and late optical afterglow
emissionComment: Accepted to ApJ
Measuring the Spin of GRS 1915+105 with Relativistic Disk Reflection
GRS 1915+105 harbors one of the most massive known stellar black holes in the
Galaxy. In May 2007, we observed GRS 1915+105 for 117 ksec in the low/hard
state using Suzaku. We collected and analyzed the data with the HXD/PIN and XIS
cameras spanning the energy range from 2.3-55 keV. Fits to the spectra with
simple models reveal strong disk reflection through an Fe K emission line and a
Compton back-scattering hump. We report constraints on the spin parameter of
the black hole in GRS 1915+105 using relativistic disk reflection models. The
model for the soft X-ray spectrum (i.e. < 10 keV) suggests a/M = 0.56(2) and
excludes zero spin at the 4 sigma level of confidence. The model for the full
broadband spectrum suggests that the spin may be higher, a/M = 0.98(1) (1 sigma
confidence), and again excludes zero spin at the 2 sigma level of confidence.
We discuss these results in the context of other spin constraints and inner
disk studies in GRS 1915+105.Comment: Accepted for publication in Ap
Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
We present the ALMA survey of CO(2-1) emission from the 1/5 solar
metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0.9 arcsec ~ 2
pc) spatial resolution while covering large area: four 250 pc x 250 pc regions
that encompass ~2/3 of NGC 6822's star formation. In these regions, we resolve
~150 compact CO clumps that have small radii (~2-3 pc), narrow line width (~1
km/s), and low filling factor across the galaxy. This is consistent with other
recent studies of low metallicity galaxies, but here shown with a 15 times
larger sample. At parsec scales, CO emission correlates with 8 micron emission
better than with 24 micron emission and anti-correlates with Halpha, so that
PAH emission may be an effective tracer of molecular gas at low metallicity.
The properties of the CO clumps resemble those of similar-size structures in
Galactic clouds except of slightly lower surface brightness and CO-to-H2 ratio
~1-2 times the Galactic value. The clumps exist inside larger atomic-molecular
complexes with masses typical for giant molecular cloud. Using dust to trace H2
for the entire complex, we find CO-to-H2 to be ~20-25 times the Galactic value,
but with strong dependence on spatial scale and variations between complexes
that may track their evolutionary state. The H2-to-HI ratio is low globally and
only mildly above unity within the complexes. The SFR-to-H2 ratio is ~3-5 times
higher in the complexes than in massive disk galaxies, but after accounting for
the bias from targeting star-forming regions, we conclude that the global
molecular gas depletion time may be as long as in massive disk galaxies.Comment: Accepted for publication in The Astrophysical Journal; 22 pages, 10
figures, 7 table
First Detection of HCO+ Emission at High Redshift
We report the detection of HCO+(1-0) emission towards the Cloverleaf quasar
(z=2.56) through observations with the Very Large Array. This is the first
detection of ionized molecular gas emission at high redshift (z>2). HCO+
emission is a star formation indicator similar to HCN, tracing dense molecular
hydrogen gas (n(H_2) ~= 10^5 cm^{-3}) within star-forming molecular clouds. We
derive a lensing-corrected HCO+ line luminosity of L'(HCO+) = 3.5 x 10^9 K km/s
pc^2. Combining our new results with CO and HCN measurements from the
literature, we find a HCO+/CO luminosity ratio of 0.08 and a HCO+/HCN
luminosity ratio of 0.8. These ratios fall within the scatter of the same
relationships found for low-z star-forming galaxies. However, a HCO+/HCN
luminosity ratio close to unity would not be expected for the Cloverleaf if the
recently suggested relation between this ratio and the far-infrared luminosity
were to hold. We conclude that a ratio between HCO+ and HCN luminosity close to
1 is likely due to the fact that the emission from both lines is optically
thick and thermalized and emerges from dense regions of similar volumes. The
CO, HCN and HCO+ luminosities suggest that the Cloverleaf is a composite
AGN--starburst system, in agreement with the previous finding that about 20% of
the total infrared luminosity in this system results from dust heated by star
formation rather than heating by the AGN. We conclude that HCO+ is potentially
a good tracer for dense molecular gas at high redshift.Comment: 5 pages, 3 figures, ApJL, in press (accepted May 17, 2006
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