arXiv subjects

Geraint F. Lewis

Publications and source records attributed to Geraint F. Lewis.

At least 289 records · Page 16Linked to original sources

arXiv2007

No Way Back: Maximizing survival time below the Schwarzschild event horizon

It has long been known that once you cross the event horizon of a black hole, your destiny lies at the central singularity, irrespective of what you do. Furthermore, your demise will occur in a finite amount of proper time. In this paper, the use of rockets in extending the amount of time before the collision with the central singularity is examined. In general, the use of such rockets can increase your remaining time, but only up to a maximum value; this is at odds with the ``more you struggle, the less time you have'' statement that is sometimes discussed in relation to black holes. The derived equations are simple to solve numerically and the framework can be employed as a teaching tool for general relativity.

physics.ed-ph↗

arXiv2007

Power Spectra to 1% Accuracy between Dynamical Dark Energy Cosmologies

For dynamical dark energy cosmologies we carry out a series of N-body gravitational simulations, achieving percent level accuracy in the relative mass power spectra at any redshift. Such accuracy in the power spectrum is necessary for next generation cosmological mass probes. Our matching procedure reproduces the CMB distance to last scattering and delivers subpercent level power spectra at z=0 and z~3. We discuss the physical implications for probing dark energy with surveys of large scale structure.

astro-ph↗

arXiv2007

Resolving the Structure at the Heart of BAL Quasars Through Microlensing Induced Polarisation Variability

While amongst the most luminous objects in the universe, many details regarding the inner structure of quasars remain unknown. One such area is the mechanism promoting increased polarisation in the broad absorption line troughs of certain quasars. This study shows how microlensing can be used to differentiate between two popular models that explain such polarisation through a realistic computational analysis. By examining a statistical ensemble of correlation data between two observables (namely image brightness and polarisation of the flux coming from the quasar), it was found that through spectropolarimetric monitoring it would be possible to discern between a model with an external scattering region and a model without one.

astro-ph↗

arXiv2007

The AAT/WFI survey of the Monoceros Ring and Canis Major Dwarf galaxy: I. from l = (193 - 276)o

We present the results of an AAT wide-field camera survey of the stars in the Monoceros Ring and purported Canis Major overdensity in the Galactic longitudes of {\it l} = (193 - 276)$^\circ$. Current numerical simulations suggest that both of these structures are the result of a single on-going accretion event, although an alternative solution is that the warped and flared disc of the Galaxy can explain the origin of both of these structures. Our results show that, with regards the Monoceros Ring, the warped and flared disc is unable to reproduce the locations and strengths of the detections observed around the Galaxy. This supports a non-Galactic origin for this structure. We report 8 new detections and 2 tentative detections of the Monoceros Ring in this survey. The exact nature of the Canis Major overdensity is still unresolved although this survey provides evidence that invoking the Galactic warp is not a sufficient solution when compared with observation. Several fields in this survey are highly inconsistent with the current Galactic disc models that include a warp and flare, to such an extent that explaining their origins with these structures is problematic. (Abridged)

astro-ph↗

arXiv2006

Probing the Nature of the G1 Clump Stellar Overdensity in the Outskirts of M31

We present deep HST/ACS observations of the G1 clump, a distinct stellar overdensity lying at ~30 kpc along the south-western major axis of M31 close to the G1 globular cluster (Ferguson et al. 2002). Our well-populated colour-magnitude diagram reaches ~7 magnitudes below the red giant branch tip with 90% completeness, and allows the detection of various morphological features which can be used to derive detailed constraints on the age and metallicity of the constituent stellar population. We find that the colour-magnitude diagram is best described by a population with a large age range (>= 10 Gyr) and a relatively high mean metallicity [M/H]= -0.4. The spread in metallicity is constrained to be <=0.5 dex. The star formation rate in this region has declined over time, with the bulk of the stellar mass having formed >6 Gyr ago. Nonetheless, a non-negligible mass fraction (approximately 10%) of the population has formed in the last 2 Gyr. We discuss the nature of the G1 Clump in light of these new stellar population constraints and argue that the combination of stellar content and physical size make it unlikely that the structure is the remnant of an accreted dwarf galaxy. Instead, the strong similarity between the stellar content of the G1 Clump and that of the M31 outer disk suggests the substructure is a fragment of the outer disk, perhaps torn off from the main body during a past accretion/merger event; this interpretation is consistent with extant kinematical data. If this interpretation is correct, our analysis of the stellar content provides further evidence that the outskirts of large disk galaxies have been in place for a significant time.

astro-ph↗

arXiv2006

Topology of large-scale structure in the 2dF Galaxy Redshift Survey

We investigate the topology of the completed 2dF Galaxy Redshift Survey, drawing two flux-limited samples of the local Universe from the 2dFGRS catalogue, which contains over 220,000 galaxies at a median redshift of z = 0.11. The samples are cut at z = 0.2 and corrected for selection effects. We then use the three-dimensional genus statistic to probe the connectedness of the galaxy distribution on scales ranging from 8 to 20 Mpc, and compare these measurements with the analytical result for a Gaussian random density field, a generic prediction of inflationary models. We demonstrate consistency with inflation on the range of scales considered. We then introduce a parameterisation of the analytic genus curve formula that is sensitive to asymmetries in genus number as a function of density and use it to demonstrate that such phenomena are ruled out with 95% confidence between 8 and 16 Mpc.

astro-ph↗

arXiv2006

Joining the Hubble Flow: Implications for Expanding Space

The concept of expanding space has come under fire recently as being inadequate and even misleading in describing the motion of test particles in the universe. Previous investigations have suffered from a number of shortcomings, which we seek to correct. We study the motion of test particles in the universe in detail, solving the geodesic equations of General Relativity for a number of cosmological models. In particular, we use analytic methods to examine whether particles removed from the Hubble flow asymptotically rejoin the Hubble flow, a topic that has caused confusion because of differing definitions and invalid reasoning. We conclude that particles in eternally expanding but otherwise arbitrary universes do not in general rejoin the Hubble flow.

astro-ph↗

arXiv2006

The stellar halo and outer disk of M33

We present first results from a Keck/DEIMOS spectroscopic survey of red giant branch (RGB) stars in M33. The radial velocity distributions of the stars in our fields are well described by three Gaussian components, corresponding to a candidate halo component with an uncorrected radial velocity dispersion of sigma ~ 50km/s, a candidate disk component with a dispersion sigma ~ 16km/s, and a third component offset from the disk by ~ 50km/s, but for which the dispersion is not well constrained. By comparing our data to a model of M33 based upon its HI rotation curve, we find that the stellar disk is offset in velocity by ~ 25km/s from the HI disk, consistent with the warping which exists between these components. The spectroscopic metallicity of the halo component is [Fe/H] ~ -1.5, significantly more metal-poor than the implied metallicity of the disk population ([Fe/H] ~ -0.9), which also has a broader colour dispersion than the halo population. These data represent the first detections of individual stars in the halo of M33 and, despite being ~ 10 times less massive than M31 or the Milky Way, all three of these disk galaxies have stellar halo components with a similar metallicity. The color distribution of the third component is different to the disk and the halo, but is similar to that expected for a single, coeval, stellar population, and could represent a stellar stream. More observations are required to determine the true nature of this intriguing third kinematic component in M33.

astro-ph↗

arXiv2006

The Einstein Ring 0047-2808 Revisited: A Bayesian Inversion

In a previous paper, we outlined a new Bayesian method for inferring the properties of extended gravitational lenses, given data in the form of resolved images. This method holds the most promise for optimally extracting information from the observed image, whilst providing reliable uncertainties in all parameters. Here, we apply the method to the well studied optical Einstein ring 0047-2808. Our results are in broad agreement with previous studies, showing that the density profile of the lensing galaxy is aligned within a few degrees of the light profile, and suggesting that the source galaxy (at redshift 3.6) is a binary system, although its size is only of order 1-2 kpc. We also find that the mass of the elliptical lensing galaxy enclosed by the image is (2.91$\pm$0.01)$\times10^{11}$ M$_{\sun}$. Our method is able to achieve improved resolution for the source reconstructions, although we also find that some of the uncertainties are greater than has been found in previous analyses, due to the inclusion of extra pixels and a more general lens model.

astro-ph↗

arXiv2006

On a systematic bias in SBF-based distances due to gravitational microlensing

The effect of gravitational microlensing on the determination of extragalactic distances using surface brightness fluctuations (SBF) technique is considered and a method to calculate SBF amplitudes in presence of microlensing is presented. With a simple approximation for the magnification power spectrum at low optical depth the correction to the SBF-based luminosity distance is calculated. The results suggest the effect can be safely neglected at present but may become important for SBF-based Hubble diagrams at luminosity distances of about 1 Gpc and beyond.

astro-ph↗

arXiv2006

Microlensing in phase space II: Correlations analysis

Applications of the phase space approach to the calculation of the microlensing autocorrelation function are presented. The continuous propagation equation for a random star field with a Gaussian velocity distribution is solved in the leading non-trivial approximation using the perturbation technique. It is shown that microlensing modulations can be important in the interpretation of optical and shorter-wavelength light curves of pulsars, power spectra of active galactic nuclei and coherence estimates for quasi-periodic oscillations of dwarf novae and low-mass X-ray binaries. Extra scatter in the brightness of type Ia supernovae due to gravitational microlensing is shown to be of order up to 0.2 stellar magnitudes depending on the extent of the light curves.

astro-ph↗

arXiv2006

Microlensing in phase space I: Continuous propagation of variability moments

A method to calculate the statistical properties of microlensing light curves is developed. The approach follows works by Deguchi & Watson, Seitz & Schneider and Neindorf, attempting to clarify the ideas involved and techniques used in the calculations. The method is then modified to include scattering by multiple lensing planes along the line of sight and transition to a continuous limit of this treatment for average quantities is performed leading to a Fokker-Planck type equation. The equation is solved for a particular model of the random star field and microlensing effect on the flux temporal variability is extracted. Applications in astrophysically relevant situations are discussed.

astro-ph↗

arXiv2006

Quasar Microlensing: when compact masses mimic smooth matter

The magnification induced by gravitational microlensing is sensitive to the size of a source relative to the Einstein radius, the natural microlensing scale length. This paper investigates the effect of source size in the case where the microlensing masses are distributed with a bimodal mass function, with solar mass stars representing the normal stellar masses, and smaller masses (down to $8.5\times 10^{-5}$M$_\odot$) representing a dark matter component. It is found that there exists a critical regime where the dark matter is initially seen as individual compact masses, but with an increasing source size the compact dark matter acts as a smooth mass component. This study reveals that interpretation of microlensing light curves, especially claims of small mass dark matter lenses embedded in an overall stellar population, must consider the important influence of the size of the source.

astro-ph↗

arXiv2006

Seeing Star Formation Regions with Gravitational Microlensing

We qualitatively study the effects of gravitational microlensing on our view of unresolved extragalactic star formation regions. Using a general gravitational microlensing configuration, we perform a number of simulations that reveal that specific imprints of the star forming region are imprinted, both photometrically and spectroscopically, upon observations. Such observations have the potential to reveal the nature and size of these star forming regions, through the degree of variability observed in a monitoring campaign, and hence resolve the star formation regions in distant galaxies which are too small to be probed via more standard techniques.

astro-ph↗

arXiv2006

Gravitational microlensing of quasar Broad Line Regions: the influence of fractal structures

Recent models for the emission clouds within the Broad Line Region of quasars suggest that they are due to transient overdensities within an overall turbulent medium. If this were the case, the broad line emission would spatially appear fractal, possessing structure on a range of scales. This paper examines the influence of such fractal structure when a quasar is microlensed by a population of intervening masses. It is found that while the highest fractal levels can undergo significant microlensing magnification, when these light curves are superimposed to create an emission line profile, the resultant emission line profile remains relatively constant for physical models of the Broad Line Region. It is concluded that the detection of the possible fractal structure of Broad Line Regions via gravitational microlensing is not practical.

astro-ph↗

arXiv2006

An analytic investigation of the scatter in the integrated X-ray properties of galaxy groups and clusters

(Abridged) We revisit the scaling relationships between the dark matter mass and observed X-ray luminosity and temperature of galaxy clusters and groups in the local Universe. Specifically, we compare recent observations with analytic models of the intracluster medium in which the gas entropy distribution has been shifted by a variable amount, K0, to investigate the origin of the scatter in these scaling relations, and its influence on the luminosity and temperature functions. We find that variations in halo concentration or formation epoch (which might determine the time available for low entropy gas to cool out) are insufficient to explain the amount of scatter in the mass-luminosity relation. Instead, a range of entropy floors at a fixed halo mass, spanning approximately 50 keV sq.cm to 700 keV sq.cm, is required to match the data. This range is likely related to the variance in heating and/or cooling efficiency from halo to halo. We demonstrate that these models are consistent with the observed temperature and luminosity functions of clusters, if scatter in the mass-luminosity relation is accounted for. Finally, we present predictions for the redshift evolution of these scaling relations and luminosity/temperature functions. Comparison with recent data at z<0.7 shows reasonable agreement with a model that assumes a median entropy floor of K0=200 keV sq.cm. When observations are extended to group scales (kT < 1 keV), this evolution will have the potential to discriminate between an entropy floor that is independent of redshift (for example, in a preheating scenario) and one that depends on the cooling time of the halo.

astro-ph↗

arXiv2005

The Influence of Evolving Dark Energy on Cosmology

Observational evidence indicating that the expansion of the universe is accelerating has surprised cosmologists in recent years. Cosmological models have sought to explain this acceleration by incorporating `dark energy', of which the traditional cosmological constant is just one possible candidate. Several cosmological models involving an evolving equation of state of the dark energy have been proposed, as well as possible energy exchange to other components, such as dark matter. This paper summarises the forms of the most prominent models and discusses their implications for cosmology and astrophysics. Finally, this paper examines the current and future observational constraints on the nature of dark energy.

astro-ph↗

arXiv2005

Strong Gravitational Lens Inversion: A Bayesian Approach

If an extended source, such as a galaxy, is gravitationally lensed by a massive object in the foreground, the lensing distorts the observed image. It is straightforward to simulate what the observed image would be for a particular lens and source combination. In practice, one observes the lensed image on the sky, but blurred by atmospheric and telescopic effects and also contaminated with noise. The question that then arises is, given this incomplete data, what combinations of lens mass distribution and source surface brightness profile could plausibly have produced this image? This is a classic example of an inverse problem, and the method for solving it is given by the framework of Bayesian inference. In this paper we demonstrate the application of Bayesian inference to the problem of gravitational lens reconstruction, and illustrate the use of Markov Chain Monte Carlo simulations which can be used when the analytical calculations become too difficult. Previous methods for performing gravitational lens inversion are seen in a new light, as special cases of the general approach presented in this paper. Thus, we are able to answer, at least in principle, lingering questions about the uncertainties in the reconstructed source and lens parameters, taking into account all of the data and any prior information we may have.

astro-ph↗