2,721 research outputs found
The stellar mass-halo mass relation of isolated field dwarfs: a critical test of CDM at the edge of galaxy formation
We fit the rotation curves of isolated dwarf galaxies to directly measure the
stellar mass-halo mass relation () over the mass range . By accounting for cusp-core
transformations due to stellar feedback, we find a monotonic relation with
little scatter. Such monotonicity implies that abundance matching should yield
a similar if the cosmological model is correct. Using the 'field
galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the
halo mass function from the Cold Dark Matter Bolshoi simulation, we
find remarkable agreement between the two. This holds down to M, and to M if we
assume a power law extrapolation of the SDSS stellar mass function below M.
However, if instead of SDSS we use the stellar mass function of nearby galaxy
groups, then the agreement is poor. This occurs because the group stellar mass
function is shallower than that of the field below M,
recovering the familiar 'missing satellites' and 'too big to fail' problems.
Our result demonstrates that both problems are confined to group environments
and must, therefore, owe to 'galaxy formation physics' rather than exotic
cosmology.
Finally, we repeat our analysis for a Warm Dark Matter cosmology,
finding that it fails at 68% confidence for a thermal relic mass of keV, and keV if we use the power law extrapolation
of SDSS. We conclude by making a number of predictions for future surveys based
on these results.Comment: 22 pages; 2 Tables; 10 Figures. This is the version accepted for
publication in MNRAS. Key changes: (i) added substantially more information
on the surveys used to measure the stellar mass functions; (ii) added tests
of the robustness of our results. Results and conclusions unchanged from
previously. Minor typos corrected from previous versio
Valuing PSS outputs and quality changes
The Gershon review of efficiency recommended that target improvements in efficiency should be met both through financial savings and through improvements in quality of outputs. This paper reports on a pilot project designed to feed into an approach for local authorities to quantify in monetary terms quality gains in the provision of personal social services (PSS), with a specific application to the provision of home care for older people.
There are a number of practical and theoretical problems with attributing monetary values to aspects of quality. The approach described here builds on ongoing work into the measurement of PSS outputs for the purposes of National Accounts and measuring changes in productivity and efficiency more widely. This approach distinguishes what services could provide (capacity for benefit) from the quality of what is provided in practice. By attaching a financial valuation to capacity for benefit we are able to attribute a monetary valuation to changes in the quality of provision measured (in the case of home care) through service user experiences of their care.
Capacity for benefit (CfB) is defined in terms of eight domains of outcome that services address and four levels of need (no needs, all needs met, low needs, high needs) within these domains. In addition we identify whether people are living in their own homes, as a key attribute of care provision. The characteristics of the service (in terms of domains of outcome and whether living at home) and service users (in terms of level of need that need to be met) determine the CfB of a given service
Simultaneous Exoplanet Characterization and deep wide-field imaging with a diffractive pupil telescope
High-precision astrometry can identify exoplanets and measure their orbits
and masses, while coronagraphic imaging enables detailed characterization of
their physical properties and atmospheric compositions through spectroscopy. In
a previous paper, we showed that a diffractive pupil telescope (DPT) in space
can enable sub-microarcsecond accuracy astrometric measurements from wide-field
images by creating faint but sharp diffraction spikes around the bright target
star. The DPT allows simultaneous astrometric measurement and coronagraphic
imaging, and we discuss and quantify in this paper the scientific benefits of
this combination for exoplanet science investigations: identification of
exoplanets with increased sensitivity and robustness, and ability to measure
planetary masses to high accuracy. We show how using both measurements to
identify planets and measure their masses offers greater sensitivity and
provides more reliable measurements than possible with separate missions, and
therefore results in a large gain in mission efficiency. The combined
measurements reliably identify potentially habitable planets in multiple
systems with a few observations, while astrometry or imaging alone would
require many measurements over a long time baseline. In addition, the combined
measurement allows direct determination of stellar masses to percent-level
accuracy, using planets as test particles. We also show that the DPT maintains
the full sensitivity of the telescope for deep wide-field imaging, and is
therefore compatible with simultaneous scientific observations unrelated to
exoplanets. We conclude that astrometry, coronagraphy, and deep wide-field
imaging can be performed simultaneously on a single telescope without
significant negative impact on the performance of any of the three techniques.Comment: 15 pages, 6 figures. This second paper, following the paper
describing the diffractive pupil telescope (DPT) astrometric technique, shows
how simultaneous astrometry and coronagraphy observations, enabled by the DPT
concept, constrain the orbital parameters and mass of exoplanet
The Photoeccentric Effect and Proto-Hot Jupiters I. Measuring photometric eccentricities of individual transiting planets
Exoplanet orbital eccentricities offer valuable clues about the history of
planetary systems. Eccentric, Jupiter-sized planets are particularly
interesting: they may link the "cold" Jupiters beyond the ice line to close-in
hot Jupiters, which are unlikely to have formed in situ. To date,
eccentricities of individual transiting planets primarily come from radial
velocity measurements. Kepler has discovered hundreds of transiting Jupiters
spanning a range of periods, but the faintness of the host stars precludes
radial velocity follow-up of most. Here we demonstrate a Bayesian method of
measuring an individual planet's eccentricity solely from its transit light
curve using prior knowledge of its host star's density. We show that eccentric
Jupiters are readily identified by their short ingress/egress/total transit
durations -- part of the "photoeccentric" light curve signature of a planet's
eccentricity --- even with long-cadence Kepler photometry and
loosely-constrained stellar parameters. A Markov Chain Monte Carlo exploration
of parameter posteriors naturally marginalizes over the periapse angle and
automatically accounts for the transit probability. To demonstrate, we use
three published transit light curves of HD 17156 b to measure an eccentricity
of e = 0.71 +0.16/-0.09, in good agreement with the discovery value e =
0.67+/-0.08 based on 33 radial-velocity measurements. We present two additional
tests using actual Kepler data. In each case the technique proves to be a
viable method of measuring exoplanet eccentricities and their confidence
intervals. Finally, we argue that this method is the most efficient, effective
means of identifying the extremely eccentric, proto hot Jupiters predicted by
Socrates et al. (2012).Comment: ApJ, 756, 122. Received 2012 April 5; accepted 2012 July 9; published
2012 August 2
Education’s not black and white, it’s vibrant grey
This paper offers a learner’s eye-view of a journey through education, written in an auto-narrative style. Sarah’s story spans from Secondary School to College and through University finishing at the point of Graduation. Revealed in this open and honest account is an insight to coping with home life whilst navigating the trials of the Education system. Sarah makes criticisms of her peers at every level reminding us how competitive some students have to be to get on and achieve success in their education. Critical judgements are also made about Sarah’s School teachers and University lecturers, which are at times as rewarding as they are uncomfortable, but always truthful. Consequently, there is much to learn by both academics and students from this sensitive and vulnerable personal revelation. Sarah’s evidence in turn points to some fundamental questions about the genuine outcomes of the Educational system, e.g. what are we actually teaching young people to be like? And do we like the product in terms of their values, beliefs and motives? A concluding message from Sarah’s perspective is that greater independence in learning, freedom in thinking and equipping people to reason, judge and make decisions in whatever realm, may be defining steps towards becoming educated
Application of parameter estimation to aircraft stability and control: The output-error approach
The practical application of parameter estimation methodology to the problem of estimating aircraft stability and control derivatives from flight test data is examined. The primary purpose of the document is to present a comprehensive and unified picture of the entire parameter estimation process and its integration into a flight test program. The document concentrates on the output-error method to provide a focus for detailed examination and to allow us to give specific examples of situations that have arisen. The document first derives the aircraft equations of motion in a form suitable for application to estimation of stability and control derivatives. It then discusses the issues that arise in adapting the equations to the limitations of analysis programs, using a specific program for an example. The roles and issues relating to mass distribution data, preflight predictions, maneuver design, flight scheduling, instrumentation sensors, data acquisition systems, and data processing are then addressed. Finally, the document discusses evaluation and the use of the analysis results
Kramers-Kronig constrained variational analysis of optical spectra
A universal method of extraction of the complex dielectric function
from
experimentally accessible optical quantities is developed. The central idea is
that is parameterized independently at each node of a
properly chosen anchor frequency mesh, while is
dynamically coupled to by the Kramers-Kronig (KK)
transformation. This approach can be regarded as a limiting case of the
multi-oscillator fitting of spectra, when the number of oscillators is of the
order of the number of experimental points. In the case of the normal-incidence
reflectivity from a semi-infinite isotropic sample the new method gives
essentially the same result as the conventional KK transformation of
reflectivity. In contrast to the conventional approaches, the proposed
technique is applicable, without readaptation, to virtually all types of
linear-response optical measurements, or arbitrary combinations of
measurements, such as reflectivity, transmission, ellipsometry {\it etc.}, done
on different types of samples, including thin films and anisotropic crystals.Comment: 10 pages, 7 figure
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