377 research outputs found
Ni-Catalyzed Carboxylation of C(sp²)–S Bonds with CO₂: Evidence for the Multifaceted Role of Zn
Nickel-catalyzed reductive carboxylation reactions of aryl electrophiles typically require the use of metallic reducing agents. At present, the prevailing perception is that these serve as both a source of electrons and as a source of Lewis acids that may aid CO₂ insertion into the Ni–C bond. Herein, we provide evidence for the in situ formation of organometallic species from the metallic reductant, a step that has either been ruled out or has been unexplored in catalytic carboxylation reactions with metal powder reductants. Specifically, we demonstrate that Zn(0) acts as a reductant and that Zn(II) generates arylzinc species that might play a role in the C(sp²)–S carboxylation of arylsulfonium salts. Overall, the reductive Ni-catalyzed C(sp²)–S carboxylation reaction proceeds under mild conditions in a non-amide solvent, displays a wide substrate scope, and can be applied to the formal para C–H carboxylation of arenes
A Bayesian approach to the semi-analytic model of galaxy formation: methodology
We believe that a wide range of physical processes conspire to shape the
observed galaxy population but we remain unsure of their detailed interactions.
The semi-analytic model (SAM) of galaxy formation uses multi-dimensional
parameterisations of the physical processes of galaxy formation and provides a
tool to constrain these underlying physical interactions. Because of the high
dimensionality, the parametric problem of galaxy formation may be profitably
tackled with a Bayesian-inference based approach, which allows one to constrain
theory with data in a statistically rigorous way. In this paper we develop a
SAM in the framework of Bayesian inference. We show that, with a parallel
implementation of an advanced Markov-Chain Monte-Carlo algorithm, it is now
possible to rigorously sample the posterior distribution of the
high-dimensional parameter space of typical SAMs. As an example, we
characterise galaxy formation in the current CDM cosmology using the
stellar mass function of galaxies as an observational constraint. We find that
the posterior probability distribution is both topologically complex and
degenerate in some important model parameters, suggesting that thorough
explorations of the parameter space are needed to understand the models. We
also demonstrate that because of the model degeneracy, adopting a narrow prior
strongly restricts the model. Therefore, the inferences based on SAMs are
conditional to the model adopted. Using synthetic data to mimic systematic
errors in the stellar mass function, we demonstrate that an accurate
observational error model is essential to meaningful inference.Comment: revised version to match published article published in MNRA
Bringing dogs onto campus : inclusions and exclusions of animal bodies in organisations
Since the early years of the 20th century, work organizations have largely been places where animal bodies are absent or invisible. Recently, US and UK universities have facilitated therapy dog visits to improve students' wellbeing. In this article we analyse data on therapy dog visits to a UK university library as a starting point for thinking about other than human animals in organizations and the gendered dimensions of their inclusion and exclusion. Rather than focusing solely on the benefits of these encounters for students, we put the experiences of the dogs and their guardians centre stage, along with those of the library staff and the students. Drawing on observations of visits to a UK university library in 2015–2016, and a total of 16 interviews with library staff, guardians and students, we explore the instrumental rationale for the programme and the efforts to control any potential disruption of normative organizational expectations
The Arecibo Legacy Fast ALFA Survey: VI. Second HI Source Catalog of the Virgo Cluster Region
We present the third installment of HI sources extracted from the Arecibo
Legacy Fast ALFA extragalactic survey. This dataset continues the work of the
Virgo ALFALFA catalog. The catalogs and spectra published here consist of data
obtained during the 2005 and 2006 observing sessions of the survey. The catalog
consists of 578 HI detections within the range 11h 36m < R.A.(J2000) < 13h 52m
and +08 deg < Dec.(J2000) < +12 deg, and cz_sun < 18000 km/s. The catalog
entries are identified with optical counterparts where possible through the
examination of digitized optical images. The catalog detections can be
classified into three categories: (a) detections of high reliability with S/N >
6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c)
signals of lower S/N which coincide spatially with an optical object and known
redshift. 75% of the sources are newly published HI detections. Of particular
note is a complex of HI clouds projected between M87 and M49 that do not
coincide with any optical counterparts. Candidate objects without optical
counterparts are few. The median redshift for this sample is 6500 km/s and the
cz distribution exhibits the local large scale structure consisting of Virgo
and the background void and the A1367-Coma supercluster regime at cz_sun ~7000
km/s. Position corrections for telescope pointing errors are applied to the
dataset by comparing ALFALFA continuum centroid with those cataloged in the
NRAO VLA Sky Survey. The uncorrected positional accuracy averages
27 arcsec ~(21 arcsec ~median) for all sources with S/N > 6.5 and is of order
~21 arcsec ~(16 arcsec ~median) for signals with S/N > 12. Uncertainties in
distances toward the Virgo cluster can affect the calculated HI mass
distribution.Comment: 25 pages, 1 Table, 8 figures, Accepted by the Astronomical Journa
Linear/circular spectropolarimetry of diffuse interstellar bands
Context. The identification of the carriers of diffuse interstellar bands
(DIBs) remains one of the long-standing mysteries in astronomy. The detection
of a polarisation signal in a DIB profile can be used to distinguish between a
dust or gas-phase carrier. The polarisation profile can give additional
information on the grain or molecular properties of the absorber. In order to
detect and measure the linear and circular polarisation of the DIBs we observed
reddened lines of sight showing continuum polarisation. For this study we
selected two stars HD 197770 and HD 194279. We used high-resolution (R~64.000)
spectropolarimetry in the wavelength range from 3700 to 10480 Angstrom with the
ESPaDOnS echelle spectrograph mounted at the CFHT.
Results. High S/N and high resolution Stokes V (circular), Q and U (linear)
spectra were obtained. We constrained upper limits by a factor of 10 for
previously observed DIBs. Furthermore, we analysed ~30 additional DIBs for
which no spectropolarimetry data has been obtained before. This included the
9577 A DIB and the 8621 A DIB.
Conclusions. The lack of polarisation in 45 DIB profiles suggests that none
of the absorption lines is induced by a grain-type carrier. The strict upper
limits, less than ~0.01%, derived for the observed lines-of-sight imply that if
DIBs are due to gas-phase molecules these carriers have polarisation
efficiencies which are at least 6 times, and up to 300 times, smaller than
those predicted for grain-related carriers.Comment: 6 pages + 13 pages online material, submitted to A&
Dark matter dominated dwarf disc galaxy Segue 1
Several observations reveal that dwarf galaxy Segue 1 has a dark matter (DM)
halo at least ~ 200 times more massive than its visible baryon mass of only ~
103 solar masses. The baryon mass is dominated by stars with perhaps an
interstellar gas mass of < 13 solar masses. Regarding Segue 1 as a dwarf disc
galaxy by its morphological appearance of long stretch, we invoke the dynamic
model of Xiang-Gruess, Lou & Duschl (XLD) to estimate its physical parameters
for possible equilibria with and without an isopedically magnetized gas disc.
We estimate the range of DM mass and compare it with available observational
inferences. Due to the relatively high stellar velocity dispersion compared to
the stellar surface mass density, we find that a massive DM halo would be
necessary to sustain disc equilibria. The required DM halo mass agrees grossly
with observational inferences so far. For an isopedic magnetic field in a gas
disc, the ratio f between the DM and baryon potentials depends strongly on the
magnetic field strength. Therefore, a massive DM halo is needed to counteract
either the strong stellar velocity dispersion and rotation of the stellar disc
or the magnetic Lorentz force in the gas disc. By the radial force balances,
the DM halo mass increases for faster disc rotation.Comment: 5 pages, 2 figures, accepted for publication in MNRA
Star Formation in AEGIS Field Galaxies since z=1.1 : The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-Forming Galaxies
We analyze star formation (SF) as a function of stellar mass (M*) and
redshift z in the All Wavelength Extended Groth Strip International Survey
(AEGIS). For 2905 field galaxies, complete to 10^10(10^10.8) Msun at z<0.7(1),
with Keck spectroscopic redshifts out to z=1.1, we compile SF rates (SFR) from
emission lines, GALEX, and Spitzer MIPS 24 micron photometry, optical-NIR M*
measurements, and HST morphologies. Galaxies with reliable signs of SF form a
distinct "main sequence (MS)", with a limited range of SFR at a given M* and z
(1 sigma < +-0.3 dex), and log(SFR) approximately proportional to log(M*). The
range of log(SFR) remains constant to z>1, while the MS as a whole moves to
higher SFR as z increases. The range of SFR along the MS constrains the
amplitude of episodic variations of SF, and the effect of mergers on SFR.
Typical galaxies spend ~67(95)% of their lifetime since z=1 within a factor of
<~ 2(4) of their average SFR at a given M* and z. The dominant mode of the
evolution of SF since z~1 is apparently a gradual decline of the average SFR in
most individual galaxies, not a decreasing frequency of starburst episodes, or
a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z~1 seem
to mostly reflect the high SFR typical for massive galaxies at that epoch. The
smooth MS may reflect that the same set of few physical processes governs star
formation prior to additional quenching processes. A gradual process like gas
exhaustion may play a dominant role.Comment: 5 pages, 1 figure, emulateapj; ApJ Letters, accepted; AEGIS special
issue; proof-level corrections added; title change
A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII
We present the first spectroscopic analysis of the faint M31 satellite
galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data
for two further faint companions, And IX and AndXII. By combining data obtained
using the DEIMOS spectrograph mounted on the Keck II telescope with deep
photometry from the Suprime-Cam instrument on Subaru, we have calculated global
properties for the dwarfs, such as systemic velocities, metallicites and
half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both
photometrically and spectroscopically, from their stacked spectrum), and as
such, they continue to follow the luminosity-metallicity relationship
established with brighter dwarfs. We are unable to resolve a dispersion for And
XI due to small sample size and low S/N, but we set a one sigma upper limit of
sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity
dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and
derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4
(+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss
each satellite in the context of the Mateo relations for dwarf spheroidal
galaxies, and the Universal halo profiles established for Milky Way dwarfs
(Walker et al. 2009). For both galaxies, this sees them fall below the
Universal halo profiles of Walker et al. (2009). When combined with the
findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites
are twice as extended (in terms of both half-light and tidal radii) as their
Milky Way counterparts, these results suggest that the satellite population of
the Andromeda system could inhabit halos that are significantly different from
those of the Milky Way in terms of their central densities (abridged).Comment: 26 pages, 18 figures, MNRAS submitte
Mid-Infrared Galaxy Luminosity Functions from the AGN and Galaxy Evolution Survey
We present galaxy luminosity functions at 3.6, 4.5, 5.8, and 8.0 micron
measured by combining photometry from the IRAC Shallow Survey with redshifts
from the AGN and Galaxy Evolution Survey of the NOAO Deep Wide-Field Survey
Bootes field. The well-defined IRAC samples contain 3800-5800 galaxies for the
3.6-8.0 micron bands with spectroscopic redshifts and z < 0.6. We obtained
relatively complete luminosity functions in the local redshift bin of z < 0.2
for all four IRAC channels that are well fit by Schechter functions. We found
significant evolution in the luminosity functions for all four IRAC channels
that can be fit as an evolution in M* with redshift, \Delta M* = Qz. While we
measured Q=1.2\pm0.4 and 1.1\pm0.4 in the 3.6 and 4.5 micron bands consistent
with the predictions from a passively evolving population, we obtained
Q=1.8\pm1.1 in the 8.0 micron band consistent with other evolving star
formation rate estimates. We compared our LFs with the predictions of
semi-analytical galaxy formation and found the best agreement at 3.6 and 4.5
micron, rough agreement at 8.0 micron, and a large mismatch at 5.8 micron.
These models also predicted a comparable Q value to our luminosity functions at
8.0 micron, but predicted smaller values at 3.6 and 4.5 micron. We also
measured the luminosity functions separately for early and late-type galaxies.
While the luminosity functions of late-type galaxies resemble those for the
total population, the luminosity functions of early-type galaxies in the 3.6
and 4.5 micron bands indicate deviations from the passive evolution model,
especially from the measured flat luminosity density evolution. Combining our
estimates with other measurements in the literature, we found (53\pm18)% of the
present stellar mass of early-type galaxies has been assembled at z=0.7.Comment: 39 pages, 15 figures, submitted to ApJ (revised following the referee
report
The role of mergers in driving morphological transformation over cosmic time
Accepted for publication in MNRASUnderstanding the processes that trigger morphological transformation is central to understanding how and why the Universe transitions from being disc-dominated at early epochs to having the morphological mix that is observed today. We use Horizon-AGN, a cosmological hydrodynamical simulation, to perform a comprehensive study of the processes that drive morphological change in massive (M*/M ⊙ > 10 10) galaxies over cosmic time. We show that (1) essentially all the morphological evolution in galaxies that are spheroids at z = 0 is driven by mergers with mass ratios greater than 1: 10; (2) major mergers alone cannot produce today's spheroid population - minor mergers are responsible for a third of all morphological transformation over cosmic time and are its dominant driver after z ~ 1; (3) prograde mergers trigger milder morphological transformation than retrograde mergers - while both types of event produce similar morphological changes at z > 2, the average change due to retrograde mergers is around twice that due to their prograde counterparts at z ~ 0; (4) remnant morphology depends strongly on the gas fraction of a merger, with gas-rich mergers routinely re-growing discs; and (5) at a given stellar mass, discs do not exhibit drastically different merger histories from spheroids - disc survival in mergers is driven by acquisition of cold gas (via cosmological accretion and gas-rich interactions) and a preponderance of prograde mergers in their merger histories.Peer reviewedFinal Accepted Versio
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