314 research outputs found
Cationic Ruthenium Complexes in Catalysis: The Activation of Propargylic Alcohols Through Electronically Tuned Complexes
With increasing complexity of metal complexes, it is of worthwhile interest to pursue systematic examinations of ligand modifications to study their impact on the reactivity of a catalyst. To that end, this study aimed to understand the effect electron-withdrawing ligands conveyed to catalytic activity in the etherification of propargylic alcohols and related reactions. A number of half-sandwich ruthenium complexes bearing ligands with varying electronâwithdrawing properties were synthesized and structurally characterized. Their electronic and structural properties were investigated utilizing X-ray crystallography, revealing that the series of complexes did not vary significantly in structure. The complexes were studied electronically with cyclic voltammetry, which discovered that the coordinated electron-withdrawing ligands resulted in complexes that were more difficult to oxidize and with possibly decreased electron density at the ruthenium center. All complexes showed catalytic activity in the etherification of propargylic alcohols and in the formation of oxygen-containing heterocycles from propargylic alcohols and diketones. Thermal instability offers an explanation as to why some catalyst systems do not perform very well at elevated temperatures. In a separate study, a more stable tridentate ligand was employed as an architecture for further study in this electronic fine-tuning methodology. A new ruthenium complex bearing a tridendate diacetylpyridine ligand was synthesized, characterized, and employed as catalyst in the coupling of carboxylic acids to terminal alkynes to form enol esters with good regioselectivity. Iron offers a number of advantages in transition metal catalysis, as it is inexpensive and relatively non-toxic. Based on preliminary findings from the Bauer laboratory, an in situ catalyst formed through oxidation of ferrocene boronic acid was found to be catalytically active in the etherification of propargylic acetates. Most interestingly, and opposed to all catalytic reactions performed for this study, the ferrocenium cation does not require elevated temperatures and performs well at room temperature
Keck Spectroscopy of 3<z<7 Faint Lyman Break Galaxies: The Importance of Nebular Emission in Understanding the Specific Star Formation Rate and Stellar Mass Density
The physical properties inferred from the SEDs of z>3 galaxies have been
influential in shaping our understanding of early galaxy formation and the role
galaxies may play in cosmic reionization. Of particular importance is the
stellar mass density at early times which represents the integral of earlier
star formation. An important puzzle arising from the measurements so far
reported is that the specific star formation rates (sSFR) evolve far less
rapidly than expected in most theoretical models. Yet the observations
underpinning these results remain very uncertain, owing in part to the possible
contamination of rest-optical broadband light from strong nebular emission
lines. To quantify the contribution of nebular emission to broad-band fluxes,
we investigate the SEDs of 92 spectroscopically-confirmed galaxies in the
redshift range 3.8<z<5.0 chosen because the H-alpha line lies within the
Spitzer/IRAC 3.6 um filter. We demonstrate that the 3.6 um flux is
systematically in excess of that expected from stellar continuum, which we
derive by fitting the SED with population synthesis models. No such excess is
seen in a control sample at 3.1<z<3.6 in which there is no nebular
contamination in the IRAC filters. From the distribution of our 3.6 um flux
excesses, we derive an H-alpha equivalent width (EW) distribution. The mean
rest-frame H-alpha EW we infer at 3.8<z<5.0 (270 A) indicates that nebular
emission contributes at least 30% of the 3.6 um flux. Via our
empirically-derived EW distribution we correct the available stellar mass
densities and show that the sSFR evolves more rapidly at z>4 than previously
thought, supporting up to a 5x increase between z~2 and 7. Such a trend is much
closer to theoretical expectations. Given our findings, we discuss the
prospects for verifying quantitatively the nebular emission line strengths
prior to the launch of the James Webb Space Telescope.Comment: 16 pages, 9 figures, submitted to Ap
The Baryon Content of Cosmic Structures
We make an inventory of the baryonic and gravitating mass in structures
ranging from the smallest galaxies to rich clusters of galaxies. We find that
the fraction of baryons converted to stars reaches a maximum between M500 =
1E12 and 1E13 Msun, suggesting that star formation is most efficient in bright
galaxies in groups. The fraction of baryons detected in all forms deviates
monotonically from the cosmic baryon fraction as a function of mass. On the
largest scales of clusters, most of the expected baryons are detected, while in
the smallest dwarf galaxies, fewer than 1% are detected. Where these missing
baryons reside is unclear.Comment: ApJ Letters, in pres
New Constraints on Cosmic Reionization from the 2012 Hubble Ultra Deep Field Campaign
Understanding cosmic reionization requires the identification and
characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble
Ultra Deep Field (UDF12) campaign has acquired the deepest infrared images with
the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time,
systematically explored the galaxy population deep into the era when cosmic
microwave background (CMB) data indicates reionization was underway. The UDF12
campaign thus provides the best constraints to date on the abundance,
luminosity distribution, and spectral properties of early star-forming
galaxies. We synthesize the new UDF12 results with the most recent constraints
from CMB observations to infer redshift-dependent ultraviolet (UV) luminosity
densities, reionization histories, and electron scattering optical depth
evolution consistent with the available data. Under reasonable assumptions
about the escape fraction of hydrogen ionizing photons and the intergalactic
medium clumping factor, we find that to fully reionize the universe by redshift
z~6 the population of star-forming galaxies at redshifts z~7-9 likely must
extend in luminosity below the UDF12 limits to absolute UV magnitudes of
M_UV\sim -13 or fainter. Moreover, low levels of star formation extending to
redshifts z~15-25, as suggested by the normal UV colors of z\simeq7-8 galaxies
and the smooth decline in abundance with redshift observed by UDF12 to
z\simeq10, are additionally likely required to reproduce the optical depth to
electron scattering inferred from CMB observations.Comment: Version accepted by ApJ (originally submitted Jan 5, 2013). The UDF12
website can be found at http://udf12.arizona.ed
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Paediatric extracranial germ-cell tumours.
Management of paediatric extracranial germ-cell tumours carries a unique set of challenges. Germ-cell tumours are a heterogeneous group of neoplasms that present across a wide age range and vary in site, histology, and clinical behaviour. Patients with germ-cell tumours are managed by a diverse array of specialists. Thus, staging, risk stratification, and treatment approaches for germ-cell tumours have evolved disparately along several trajectories. Paediatric germ-cell tumours differ from the adolescent and adult disease in many ways, leading to complexities in applying age-appropriate, evidence-based care. Suboptimal outcomes remain for several groups of patients, including adolescents, and patients with extragonadal tumours, high tumour markers at diagnosis, or platinum-resistant disease. Survivors have significant long-term toxicities. The challenge moving forward will be to translate new insights from molecular studies and collaborative clinical data into improved patient outcomes. Future trials will be characterised by improved risk-stratification systems, biomarkers for response and toxic effects, rational reduction of therapy for low-risk patients and novel approaches for poor-risk patients, and improved international collaboration across paediatric and adult cooperative research groups.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/S1470-2045(15)00545-
Expanding the search for galaxies at z ~7-10 with new NICMOS Parallel Fields
We have carried out a search for galaxies at z ~ 7-10 in ~14.4 sq. arcmin of
new NICMOS parallel imaging taken in the Great Observatories Origins Deep
Survey (GOODS, 5.9 sq. arcmin), the Cosmic Origins Survey (COSMOS, 7.2 sq.
arcmin), and SSA22 (1.3 sq. arcmin). These images reach 5 sigma sensitivities
of J110 = 26.0-27.5 (AB), and combined they increase the amount of deep
near-infrared data by more than 60% in fields where the investment in deep
optical data has already been made. We find no z>7 candidates in our survey
area, consistent with the Bouwens et al. (2008) measurements at z~7 and 9 (over
23 sq. arcmin), which predict 0.7 galaxies at z~7 and <0.03 galaxies at z~9. We
estimate that 10-20% of z>7 galaxies are missed by this survey, due to
incompleteness from foreground contamination by faint sources. For the case of
luminosity evolution, assuming a Schecter parameterization with a typical phi*
= 10^-3 Mpc^-3, we find M* > -20.0 for z~7 and M* > -20.7 for z~9 (68%
confidence). This suggests that the downward luminosity evolution of LBGs
continues to z~7, although our result is marginally consistent with the z~6 LF
of Bouwens et al.(2006, 2007). In addition we present newly-acquired deep
MMT/Megacam imaging of the z~9 candidate JD2325+1433, first presented in Henry
et al. (2008). The resulting weak but significant detection at i' indicates
that this galaxy is most likely an interloper at z~2.7.Comment: Accepted to ApJ. Replacement includes updated discussion of
incompleteness from foreground contaminatio
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Comparison of carboplatin versus cisplatin in the treatment of paediatric extracranial malignant germ cell tumours: A report of the Malignant Germ Cell International Consortium.
PURPOSE: To compare the outcomes of paediatric and adolescent extracranial malignant germ cell tumour (GCT) patients treated with either carboplatin or cisplatin on clinical trials conducted by the Children's Oncology Group (COG) and the Children's Cancer and Leukaemia Group (CCLG). METHODS: The Malignant Germ Cell International Consortium (MaGIC) has created a database of the GCT clinical trials conducted since 1983 by COG (United States, Canada and Australia), which used cisplatin-based regimens, and by CCLG (United Kingdom), which used carboplatin-based regimens. Using the parametric cure model, this study compared the overall 4-year event-free survival (EFS), stratified by age, stage, site and the a-priori defined MaGIC 'risk' groups: standard risk ((SR) 1 (EFS >80%; age 80%, age ℠11y) and poor risk (PR) (EFS †70%, age ℠11y). RESULTS: Cisplatin-based therapy was used in 620 patients; carboplatin was used in 163 patients. In the overall multivariate cure model, the two regimens did not differ significantly (cisplatin: 4-year EFS 86%; 95% confidence interval (CI) 83-89% versus carboplatin 4-year EFS 86%; 95% CI 79-90%; p = 0.87). No significant differences were noted in stratified analyses by site, stage, age and MaGIC risk groups: SR1 (p = 0.20), SR2 (p = 0.55) or PR (p = 0.72) patients. CONCLUSIONS: In these trials conducted contemporaneously, there is no significant difference in outcome observed overall, or any subset of patients, who were treated with regimens containing cisplatin versus carboplatin These results suggested sufficient equipoise to justify a randomised trial to evaluate the effectiveness of carboplatin versus cisplatin in the treatment of children, adolescents and young adults with standard risk GCT, which is currently underway
A Lyman Break Galaxy Candidate at z~9
We report the discovery of a z~9 Lyman Break Galaxy (LBG) candidate, selected
from the NICMOS Parallel Imaging Survey as a J-dropout with J110 - H160 = 1.7.
Spitzer/IRAC photometry reveals that the galaxy has a blue H160 - 3.6 um color,
and a spectral break between 3.6 and 4.5 um. We interpret this break as the
Balmer break, and derive a best-fit photometric redshift of z~9. We use Monte
Carlo simulations to test the significance of this photometric redshift, and
show a 96% probability of z>7. We estimate a lower limit to the comoving number
density of such galaxies at z~9 of phi > 3.8 x 10^{-6} Mpc^{-3}. If the high
redshift of this galaxy is confirmed, this will indicate that the luminous end
of the rest-frame UV luminosity function has not evolved substantially from z~
9 to z~3. Still, some small degeneracy remains between this z~9 model and
models at z~2-3; deep optical imaging (reaching I ~ 29 AB) can rule out the
lower-z models.Comment: Accepted to ApJ Letter
High star formation rates as the origin of turbulence in early and modern disk galaxies
High spatial and spectral resolution observations of star formation and
kinematics in early galaxies have shown that two-thirds are massive rotating
disk galaxies with the remainder being less massive non-rotating objects. The
line of sight averaged velocity dispersions are typically five times higher
than in today's disk galaxies. This has suggested that
gravitationally-unstable, gas-rich disks in the early Universe are fuelled by
cold, dense accreting gas flowing along cosmic filaments and penetrating hot
galactic gas halos. However these accreting flows have not been observed, and
cosmic accretion cannot power the observed level of turbulence. Here we report
on a new sample of rare high-velocity-dispersion disk galaxies we have
discovered in the nearby Universe where cold accretion is unlikely to drive
their high star-formation rates. We find that the velocity dispersion is most
fundamentally correlated with their star-formation rates, and not their mass
nor gas fraction, which leads to a new picture where star formation itself is
the energetic driver of galaxy disk turbulence at all cosmic epochs.Comment: 9 pages, 2 figures, Supplimentary Info available at:
http://pulsar.swin.edu.au/~agreen/nature/sigma_mean_arXiv.pdf. Accepted for
publication in Natur
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