1,356 research outputs found
Dynamical Mass Estimates for Five Young Massive Stellar Clusters
We have obtained high-dispersion spectra for four massive star clusters in
the dwarf irregular galaxies NGC 4214 and NGC 4449, using the HIRES
spectrograph on the Keck I telescope. Combining the velocity dispersions of the
clusters with structural parameters and photometry from images taken with HST,
we estimate mass-to-light ratios and compare these with simple stellar
population (SSP) models in order to constrain the stellar mass functions (MFs).
For all clusters we find mass-to-light ratios which are similar to or slightly
higher than for a Kroupa MF, and thereby rule out any MF which is deficient in
low-mass stars compared to a Kroupa-type MF. The four clusters have virial
masses ranging between 2.1E5 Msun and 1.5E6 Msun, half-light radii between 3.0
and 5.2 pc, estimated core densities in the range 2E3 Msun pc^-3 to 2E5 Msun
pc^-3 and ages between 200 Myr and 800 Myr. We also present new high-dispersion
near-infrared spectroscopy for a luminous young (about 15 Myr) cluster in the
nearby spiral galaxy NGC 6946, which we have previously observed with HIRES.
The new measurements in the infrared agree well with previous estimates of the
velocity dispersion, yielding a mass of about 1.7E6 Msun. The properties of the
clusters studied here are all consistent with the clusters being young versions
of the old globular clusters found around all major galaxies.Comment: 30 pages, including 7 figures and 9 tables. Corrected an error in
Table 2: The colors listed for N6946-1447 were not reddening corrected. This
also affected Table 9 and Fig 2, 6 and
Metal-Poor Globular Clusters and Galaxy Formation
We demonstrate a significant (> 5-sigma) correlation between the mean color
of metal-poor globular cluster (GC) systems and parent galaxy luminosity. A
Bayesian Markov Chain Monte Carlo method is introduced to find the mean color,
and is easily generalizable to quantify multimodality in other astronomical
datasets. We derive a GC color--galaxy luminosity relation of the form Z ~ L^
(0.15 +/- 0.03). When combined with evidence against a single primordial GC
metallicity--galaxy luminosity relation for protogalactic fragments, the
existence of such a correlation is evidence against both accretion and major
merger scenarios as an explanation of the entire metal-poor GC systems of
luminous galaxies. However, our relation arises naturally in an in situ picture
of GC formation, and is consistent with the truncation of metal-poor GC
formation by reionization. A further implication is that the ages of metal-poor
GCs in dwarf galaxies constrain the main epoch of galaxy formation in
hierarchical models. If the ages of old metal-poor GCs in Local Group dwarfs (>
11 Gyr) are typical of those in dwarfs elsewhere, then the bulk of galaxy
assembly (at least in clusters and groups) must have occurred at z > 2.5,
contrary to the predictions of some structure formation models.Comment: 13 pages, 1 figure, accepted for A
Probing Spectral Line Gradients Beyond One Effective Radius in NGC 3610
The outer region (0.75--1.25 r_e in the B-band) of the merger-remnant
elliptical NGC 3610 is studied using extremely high signal to noise Keck
spectra, with a supplementary spectrum of the galaxy center. Stellar population
parameters -- age, [Z/H], [/Fe] -- are measured in several apertures
along the slit. Using the multi-index simultaneous fitting method of Proctor et
al. (2004), no significant stellar population gradients are detected in the
outer parts of the galaxy. The overall gradients relative to the galaxy center
are consistent with those found in many other early-type galaxies, though the
metallicity gradient is much steeper than would be expected if NGC 3610 formed
in a major merger event. Standard analysis methods using the H index are
found to produce spurious radially variable gradients.Comment: 15 pages, 9 figures, accepted by A
Young people, crime and school exclusion: a case of some surprises
During the 1990s the number of young people being permanently excluded from schools in England and Wales increased dramatically from 2,910 (1990/91) to a peak of 12,700 (1996/97). Coinciding with this rise was a resurgence of the debate centring on lawless and delinquent youth. With the publication of Young People and Crime (Graham and Bowling 1995) and Misspent Youth (Audit Commission 1996) the 'common sense assumption' that exclusion from school inexorably promoted crime received wide support, with the school excludee portrayed as another latter day 'folk devil'. This article explores the link between school exclusion and juvenile crime, and offers some key findings from a research study undertaken with 56 young people who had experience of being excluded from school. Self-report interview questions reveal that whilst 40 of the young people had offended, 90% (36) reported that the onset of their offending commenced prior to their first exclusion. Moreover, 50 (89.2% of the total number of young people in the sample), stated that they were no more likely to offend subsequent to being excluded and 31 (55.4%) stated that they were less likely to offend during their exclusion period. Often, this was because on being excluded, they were 'grounded' by their parents
Scattering in Anti-de Sitter Space and Operator Product Expansion
We develop a formalism to evaluate generic scalar exchange diagrams in
AdS_{d+1} relevant for the calculation of four-point functions in AdS/CFT
correspondence. The result may be written as an infinite power series of
functions of cross-ratios. Logarithmic singularities appear in all orders
whenever the dimensions of involved operators satisfy certain relations. We
show that the AdS_{d+1} amplitude can be written in a form recognisable as the
conformal partial wave expansion of a four-point function in CFT_{d} and
identify the spectrum of intermediate operators. We find that, in addition to
the contribution of the scalar operator associated with the exchanged field in
the AdS diagram, there are also contributions of some other operators which may
possibly be identified with two-particle bound states in AdS. The CFT
interpretation also provides a useful way to ``regularize'' the logarithms
appearing in AdS amplitude.Comment: 39 pages, using harvmac and epsf, eight figures; discussion in
coinciding pole cases expanded, references added, misprints correcte
D-brane Instantons as Gauge Instantons in Orientifolds of Chiral Quiver Theories
Systems of D3-branes at orientifold singularities can receive
non-perturbative D-brane instanton corrections, inducing field theory operators
in the 4d effective theory. In certain non-chiral examples, these systems have
been realized as the infrared endpoint of a Seiberg duality cascade, in which
the D-brane instanton effects arise from strong gauge theory dynamics. We
present the first UV duality cascade completion of chiral D3-brane theories, in
which the D-brane instantons arise from gauge theory dynamics. Chiral examples
are interesting because the instanton fermion zero mode sector is topologically
protected, and therefore lead to more robust setups. As an application of our
results, we provide a UV completion of certain D-brane orientifold systems
recently claimed to produce conformal field theories with conformal invariance
broken only by D-brane instantons.Comment: 50 pages, 32 figures. v2: version published in JHEP with references
adde
Brane inflation and the WMAP data: a Bayesian analysis
The Wilkinson Microwave Anisotropy Probe (WMAP) constraints on string
inspired ''brane inflation'' are investigated. Here, the inflaton field is
interpreted as the distance between two branes placed in a flux-enriched
background geometry and has a Dirac-Born-Infeld (DBI) kinetic term. Our method
relies on an exact numerical integration of the inflationary power spectra
coupled to a Markov-Chain Monte-Carlo exploration of the parameter space. This
analysis is valid for any perturbative value of the string coupling constant
and of the string length, and includes a phenomenological modelling of the
reheating era to describe the post-inflationary evolution. It is found that the
data favour a scenario where inflation stops by violation of the slow-roll
conditions well before brane annihilation, rather than by tachyonic
instability. Concerning the background geometry, it is established that log(v)
> -10 at 95% confidence level (CL), where "v" is the dimensionless ratio of the
five-dimensional sub-manifold at the base of the six-dimensional warped
conifold geometry to the volume of the unit five-sphere. The reheating energy
scale remains poorly constrained, Treh > 20 GeV at 95% CL, for an extreme
equation of state (wreh ~ -1/3) only. Assuming the string length is known, the
favoured values of the string coupling and of the Ramond-Ramond total
background charge appear to be correlated. Finally, the stochastic regime
(without and with volume effects) is studied using a perturbative treatment of
the Langevin equation. The validity of such an approximate scheme is discussed
and shown to be too limited for a full characterisation of the quantum effects.Comment: 65 pages, 15 figures, uses iopart. Shortened version, updated
references. Matches publication up to appendix B kept on the arXi
Noncommutative geometry, Quantum effects and DBI-scaling in the collapse of D0-D2 bound states
We study fluctuations of time-dependent fuzzy two-sphere solutions of the
non-abelian DBI action of D0-branes, describing a bound state of a spherical
D2-brane with N D0-branes. The quadratic action for small fluctuations is shown
to be identical to that obtained from the dual abelian D2-brane DBI action,
using the non-commutative geometry of the fuzzy two-sphere. For some of the
fields, the linearized equations take the form of solvable Lam\'e equations. We
define a large-N DBI-scaling limit, with vanishing string coupling and string
length, and where the gauge theory coupling remains finite. In this limit, the
non-linearities of the DBI action survive in both the classical and the quantum
context, while massive open string modes and closed strings decouple. We
describe a critical radius where strong gauge coupling effects become
important. The size of the bound quantum ground state of multiple D0-branes
makes an intriguing appearance as the radius of the fuzzy sphere, where the
maximal angular momentum quanta become strongly coupled.Comment: 34 pages, Latex; v2: Minor correction in conformal transformation of
couplings, references adde
Normal Globular Cluster Systems in Massive Low Surface Brightness Galaxies
We present the results of a study of the globular cluster systems of 6
massive spiral galaxies, originally cataloged as low surface brightness
galaxies but here shown to span a wide range of central surface brightness
values, including two intermediate to low surface brightness galaxies. We used
the Advanced Camera for Surveys on board HST to obtain photometry in the F475W
and F775W bands and select sources with photometric and morphological
properties consistent with those of globular clusters. A total of 206
candidates were identified in our target galaxies. From a direct comparison
with the Galactic globular cluster system we derive specific frequency values
for each galaxy that are in the expected range for late-type galaxies. We show
that the globular cluster candidates in all galaxies have properties consistent
with globular cluster systems of previously studied galaxies in terms of
luminosity, sizes and color. We establish the presence of globular clusters in
the two intermediate to low surface brightness galaxies in our sample and show
that their properties do not have any significant deviation from the behavior
observed in the other sample galaxies. Our results are broadly consistent with
a scenario in which low surface brightness galaxies follow roughly the same
evolutionary history as normal (i.e. high surface) brightness galaxies except
at a much lower rate, but require the presence of an initial period of star
formation intense enough to allow the formation of massive star clusters.Comment: 14 pages, 6 figures. AJ accepte
Co-designing marine science beyond good intentions: support stakeholders’ empowerment in transformative pathways
Calls for science to innovate by including stakeholders' in the creation of marine knowledge have been rising, to create impact beyond laboratories and to contribute to the empowerment of local communities when interacting with marine and coastal ecosystems. As a transdisciplinary group of scientists working on co-designing research projects, this paper draws upon our experiences to further define the concept and seek to improve the process of co-design. We highlight the key barriers for co-design processes to contribute to increasing stakeholders' capacity to produce intended effects on marine policy. We suggest that stakeholder engagement requires overcoming the resistance to non-scientific knowledge sources and considering power asymmetries in the governance and management of the ocean. We argue that power and politics must be placed at the very heart of the production of a co-designed marine science and must be an aspect of the facilitation itself. In this paper, we aim to provide insights to navigate throughout the journey of stakeholder engagement, with the critical perspective necessary to make this process socially and environmentally effective
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