2,874 research outputs found
Galaxies in the distant universe: colours, redshifts and star formation
This thesis explores the properties of distant galaxies in the Universe, in particular their redshifts,
morphologies, evolutionary history and star formation processes within them.
The first part is concerned with photometric redshift estimation. I present different photo-z
methods and compare them using a sample of Luminous Red Galaxies (LRGs). Photo-z design
studies are then carried out for the upcoming Dark Energy Survey as well as the planned space-based
Euclid mission. I show the importance of adding near infra-red data to optical data in
obtaining accurate redshift estimates for both these projects and how this may prove crucial for
some of the cosmological analysis intended with them.
In Chapter 5, I present automated morphological classifications for ā¼ 1 million objects from
the Sloan Digital Sky Survey and compare them to visual classifications of the same objects obtained
as part of the Galaxy Zoo project. I find that a neural network is able to reproduce the
human classifications to an accuracy of better than 90%.
In Chapter 6 I study the evolution of the luminosity and mass functions of LRGs using spectroscopic
data. I find that these objects are mainly composed of old stars that were formed very
early in the history of the Universe and also that the most massive objects were already well assembled
at redshifts of ā¼0.8 in direct contradiction with predictions of most current models of
galaxy formation.
Chapter 7 presents an alternative means of determining the approximate nature of the stellar
initial mass function of extragalactic systems by considering timescales for low-mass star formation
in different environments. I find that a galaxyās metallicity is a key parameter in determining
the shape of its IMF and make some predictions about trends in molecular emission in different
extragalactic systems with different IMFs
Performance Evaluation of the New Connecticut Leading Employment Index Using Lead Profiles and BVAR Models.
Dua and Miller (1996) created leading and coincident employment indexes for the state of Connecticut, following Moore's (1981) work at the national level. The performance of the Dua-Miller indexes following the recession of the early 1990s fell short of expectations. This paper performs two tasks. First, it describes the process of revising the Connecticut Coincident and Leading Employment Indexes. Second, it analyzes the statistical properties and performance of the new indexes by comparing the lead profiles of the new and old indexes as well as their out-of-sample forecasting performance, using the Bayesian Vector Autoregressive (BVAR) method. The new coincident index shows improved performance in dating employment cycle chronologies. The lead profile test demonstrates that superiority in a rigorous, non-parametric statistic fashion. The mixed evidence on the BVAR forecasting experiments illustrates the truth in the Granger and Newbold (1986) caution that leading indexes properly predict cycle turning points and do not necessarily provide accurate forecasts except at turning points, a view that our results support.Business cycles, leading and coincident employment indexes, turning points, BVAR Models
A time frequency analysis of wave packet fractional revivals
We show that the time frequency analysis of the autocorrelation function is,
in many ways, a more appropriate tool to resolve fractional revivals of a wave
packet than the usual time domain analysis. This advantage is crucial in
reconstructing the initial state of the wave packet when its coherent structure
is short-lived and decays before it is fully revived. Our calculations are
based on the model example of fractional revivals in a Rydberg wave packet of
circular states. We end by providing an analytical investigation which fully
agrees with our numerical observations on the utility of time-frequency
analysis in the study of wave packet fractional revivals.Comment: 9 pages, 4 figure
Trends in Molecular Emission from Different Extragalactic Stellar Initial Mass Functions
Banerji et al. (2009) suggested that top-heavy stellar Initial Mass Functions
(IMFs) in galaxies may arise when the interstellar physical conditions inhibit
low-mass star formation, and they determined the physical conditions under
which this suppression may or may not occur. In this work, we explore the
sensitivity of the chemistry of interstellar gas under a wide range of
conditions. We use these results to predict the relative velocity-integrated
antenna temperatures of the CO rotational spectrum for several models of high
redshift active galaxies which may produce both top-heavy and unbiased IMFs. We
find that while active galaxies with solar metallicity (and top-heavy IMFs)
produce higher antenna temperatures than those with sub-solar metallicity (and
unbiased IMFs) the actual rotational distribution is similar. The high-J to
peak CO ratio however may be used to roughly infer the metallicity of a galaxy
provided we know whether it is active or quiescent. The metallicity strongly
influences the shape of the IMF. High order CO transitions are also found to
provide a good diagnostic for high far-UV intensity and low metallicity
counterparts of Milky Way type systems both of which show some evidence for
having top-heavy IMFs. We also compute the relative abundances of molecules
known to be effective tracers of high density gas in these galaxy models. We
find that the molecules CO and CS may be used to distinguish between solar and
sub-solar metallicity in active galaxies at high redshift whereas HCN, HNC and
CN are found to be relatively insensitive to the IMF shape at the large visual
magnitudes typically associated with extragalactic sources.Comment: 26 Pages, 8 Figures, Accepted for publication in Ap
Recommended from our members
Development of low cost packaged fibre optic sensors for use in reinforced concrete structures
There is an ongoing need to measure strains in reinforced concrete structures more reliably and under a range of circumstances e.g. long term durability (such as effects of cracking and reinforcement corrosion), response to normal working loads and response under abnormal load conditions. Fibre optic sensors have considerable potential for this purpose and have the additional advantages, including of immunity to electromagnetic interference and light weight (Grattan et al., 2000). This is important in railway scenarios and particularly so when the lines are electrified. Their small size allows for easy installation. However, their use as commercial āpackagedā devices (traditionally seen as necessary to achieve adequate robustness) is limited by their high cost relative to other sensor devices such as encapsulated electric resistance strain gauges. This paper describes preliminary work to produce a cost-effective and easy-to-use technique for encapsulating fibre optic sensors in resin using 3D printing techniques to produce a robust, inexpensive āpackagedā sensor system suitable for use with concrete structures. The work done to date has shown this to be a convenient and economical way of producing multiple sensors which were suitable for both surface mounting and embedment in reinforced concrete structures. The proof-of-concept testing to which the trial packages were subjected is described in the paper and the results indicate that 3D printed packages have considerable potential for further development and use in a variety of civil engineering applications, competing well with more conventional sensor systems
Mesoscopic superposition and sub-Planck-scale structure in molecular wave packets
We demonstrate the possibility of realizing sub-Planck-scale structures in
the mesoscopic superposition of molecular wave packets involving vibrational
levels. The time evolution of the wave packet, taken here as the SU(2) coherent
state of the Morse potential describing hydrogen iodide molecules, produces
macroscopicquantum- superposition-like states, responsible for the above
phenomenon. We investigate the phase-space dynamics of the coherent state
through the Wigner function approach and identify the interference phenomena
behind the sub-Planck-scale structures. The optimal parameter ranges are
specified for observing these features.Comment: 4 pages, 3 figure
Entanglement by linear SU(2) transformations: generation and evolution of quantum vortex states
We consider the evolution of a two-mode system of bosons under the action of
a Hamiltonian that generates linear SU(2) transformations. The Hamiltonian is
generic in that it represents a host of entanglement mechanisms, which can thus
be treated in a unified way. We start by solving the quantum dynamics
analytically when the system is initially in a Fock state. We show how the two
modes get entangled by evolution to produce a coherent superposition of vortex
states in general, and a single vortex state under certain conditions. The
degree of entanglement between the modes is measured by finding the explicit
analytical dependence of the Von Neumann entropy on the system parameters. The
reduced state of each mode is analyzed by means of its correlation function and
spatial coherence function. Remarkably, our analysis is shown to be equally as
valid for a variety of initial states that can be prepared from a two-mode Fock
state via a unitary transformation and for which the results can be obtained by
mere inspection of the corresponding results for an initial Fock state. As an
example, we consider a quantum vortex as the initial state and also find
conditions for its revival and charge conjugation. While studying the evolution
of the initial vortex state, we have encountered and explained an interesting
situation in which the entropy of the system does not evolve whereas its wave
function does. Although the modal concept has been used throughout the paper,
it is important to note that the theory is equally applicable for a
two-particle system in which each particle is represented by its bosonic
creation and annihilation operators.Comment: 6 figure
- ā¦