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Geraint F. Lewis

Publications and source records attributed to Geraint F. Lewis.

At least 307 records · Page 17Linked to original sources
arXiv2005

A radial velocity survey of low Galactic latitude structures: II. The Monoceros Ring behind the Canis Major dwarf galaxy

An AAT/2dF Spectrograph Survey of low Galactic latitudes targeting the putative Canis Major dwarf galaxy, and the (possibly) associated tidal debris of stars known as the Monoceros Ring, covering Galactic coordinates 231.5$^{\circ}< ${\it l} $<$ 247.5$^{\circ}$ and -11.8$^{\circ}<${\it b}$<-3.8^{\circ}$, has revealed the presence of the Monoceros Ring in the background of the Canis Major dwarf galaxy. This detection resides at a galactocentric distance of $\sim18.9\pm$0.3\kpc (13.5$\pm$0.3\kpc heliocentric), exhibiting a velocity of $\sim132.8\pm$1.3 $\kms$ with a dispersion of $\sim22.7\pm1.7\kms$; both of these comparable with previous measurements of the Monoceros Ring in nearby fields. This detection highlights the increasing complexity of structure being revealed in recent surveys of the Milky Way thick disk and Halo.

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arXiv2005

The INT/WFC survey of the Monoceros Ring: Accretion origin or Galactic Anomaly?

We present the results of a wide-field camera survey of the stars in the Monoceros Ring, thought to be an additional structure in the Milky Way of unknown origin. Lying roughly in the plane of the Milky Way, this may represent a unique equatorial accretion event which is contributing to the Thick Disk of the Galaxy. Alternatively, the Monoceros Ring may be a natural part of the Disk formation process. With ten pointings in symmetric pairs above and below the plane of the Galaxy, this survey spans 90 degrees about the Milky Way's equator. Signatures of the stream of stars were detected in three fields, ({\it l},{\it b}) = (118,+16)$^\circ$ and (150,+15)$^\circ$ plus a more tentative detection at (150,-15)$^\circ$. Galactocentric distance estimates to these structures gave $\sim$17, $\sim$17, and $\sim$13 kpc respectively. The Monoceros Ring seems to be present on both sides of the Galactic plane, in a form different to that of the Galactic suggestive of a tidal origin with streams multiply wrapping the Galaxy. A new model of the stream has shown a strong coincidence with our results and has also provided the opportunity to make several more detections in fields in which the stream is less significant. The confirmed detection at ({\it l},{\it b}) = (123,-19)$^\circ$ at $\sim$14, kpc from the Galactic centre allows a re-examination revealing a tentative new detection with a Galactocentric distance of $\sim$21 kpc. (Abridged)

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arXiv2005

A Keck/DEIMOS Kinematic Study of Andromeda IX: dark matter on the smallest galactic scales

We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And-IX, which appears to be the lowest surface brightness and also the faintest galaxy (M_V = -8.3) found to date. Using Keck/DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion sigma_v = 6.8+3.0-2.0km/s, which coupled with the low luminosity implies a very high mass to V-band light ratio, M/L ~ 93+120-50msun/lsun (M/L > 17msun/lsun at 99% confidence). Unless strong tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter dominated system.

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arXiv2005

Internal Alignment of the Halos of Disk Galaxies in Cosmological Hydrodynamic Simulations

Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ~0.1 r_vir, is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo minor axis aligns with the disk axis. The relative orientation of these two regions of the halo are uncorrelated. The alignment of the disk and inner halo appears to take place simultaneously through their joint evolution. The disconnect between these two regions of the halo should be taken into account when modelling tidal streams in the halos of disk galaxies and when calculating intrinsic alignments of disk galaxies based on the properties of dark matter halos.

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arXiv2005

Interpreting microlensing signal in QSO 2237+0305: Stars or planets?

The multiply imaged, gravitationally lensed quasar, QSO 2237+0305, has been the subject of recent optical monitoring campaigns, with its light curves displaying uncorrelated variability attributed to gravitational microlensing by masses in the foreground galaxy. Based on these light curves, it has been claimed that the dominant microlensing population must be a population of free-floating Jupiter-like objects; such a conclusion is not new, with several similar claims in the literature. Historically, however, it has been shown that such conclusions are flawed, with an incorrect interpretation of the complex caustic network that arises at significant optical depth. This paper examines this more recent claim, illustrating that it too is flawed.

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arXiv2005

The lens and source of the optical Einstein ring gravitational lens ER 0047-2808

(Abridged) We perform a detailed analysis of the optical gravitational lens ER 0047-2808 imaged with WFPC2 on the Hubble Space Telescope. Using software specifically designed for the analysis of resolved gravitational lens systems, we focus on how the image alone can constrain the mass distribution in the lens galaxy. We find the data are of sufficient quality to strongly constrain the lens model with no a priori assumptions about the source. Using a variety of mass models, we find statistically acceptable results for elliptical isothermal-like models with an Einstein radius of 1.17''. An elliptical power-law model (Sigma \propto R^-beta) for the surface mass density favours a slope slightly steeper than isothermal with beta = 1.08 +/- 0.03. Other models including a constant M/L, pure NFW halo and (surprisingly) an isothermal sphere with external shear are ruled out by the data. We find the galaxy light profile can only be fit with a Sersic plus point source model. The resulting total M/L_B contained within the images is 4.7 h_65 +/-0.3. In addition, we find the luminous matter is aligned with the total mass distribution within a few degrees. The source, reconstructed by the software, is revealed to have two bright regions, with an unresolved component inside the caustic and a resolved component straddling a fold caustic. The angular size of the entire source is approx. 0.1'' and its (unlensed) Lyman-alpha flux is 3 x 10^-17 erg/s/cm^2.

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arXiv2005

When Darwin Met Einstein: Gravitational Lens Inversion with Genetic Algorithms

Gravitational lensing can magnify a distant source, revealing structural detail which is normally unresolvable. Recovering this detail through an inversion of the influence of gravitational lensing, however, requires optimisation of not only lens parameters, but also of the surface brightness distribution of the source. This paper outlines a new approach to this inversion, utilising genetic algorithms to reconstruct the source profile. In this initial study, the effects of image degradation due to instrumental and atmospheric effects are neglected and it is assumed that the lens model is accurately known, but the genetic algorithm approach can be incorporated into more general optimisation techniques, allowing the optimisation of both the parameters for a lensing model and the surface brightness of the source.

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arXiv2004

Limits on the Transverse Velocity of the Lensing Galaxy in Q2237+0305 from the Lack of Strong Microlensing Variability

We present a method for the determination of upper limits on the transverse velocity of the lensing galaxy in the quadruple quasar system Q2237+0305, based on the lack of strong microlensing signatures in the quasar lightcurves. The limits we derive here are based on four months of high quality monitoring data, by comparing the low amplitudes of the lightcurves of the four components with extensive numerical simulations. We make use of the absence of strong variability of the components (especially components B and D) to infer that a "flat" time interval of such a length is only compatible with an effective transverse velocity of the lensing galaxy of v_bulk <=630 km/s for typical microlenses masses of M_microlens=0.1 M_solar (or v_bulk <=2160 km/s for M_microlens=1.0 M_solar) at the 90% confidence level. This method may be applicable in the future to other systems.

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arXiv2004

Modeling the evolution of infrared luminous galaxies: the influence of the Luminosity-Temperature distribution

The evolution of the luminous infrared galaxy population is explored using a pure luminosity evolution model which incorporates the locally observed luminosity-temperature distribution for IRAS galaxies. Pure luminosity evolution models in a fixed $Λ$CDM cosmology are fitted to submillimeter (submm) and infrared counts, and backgrounds. It is found that the differences between the locally determined bivariate model and the single variable luminosity function (LF) do not manifest themselves in the observed counts, but rather are primarily apparent in the dust temperatures of sources in flux limited surveys. Statistically significant differences in the redshift distributions are also observed. The bivariate model is used to predict the counts, redshifts and temperature distributions of galaxies detectable by {\it Spitzer}. The best fitting model is compared to the high-redshift submm galaxy population, revealing a median redshift for the total submm population of $z=1.8^{+0.9}_{-0.4}$, in good agreement with recent spectroscopic studies of submillimeter galaxies. The temperature distribution for the submm galaxies is modeled to predict the radio/submm indices of the submm galaxies, revealing that submm galaxies exhibit a broader spread in spectral energy distributions than seen in the local IRAS galaxies.

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arXiv2004

Probing Sub-parsec Structure in the Lyman Alpha Forest with Gravitational Microlensing

We present the results of microlens ray-tracing simulations showing the effect of absorbing material between a source quasar and a lensing galaxy in a gravitational lens system. We find that, in addition to brightness fluctuations due to microlensing, the strength of the absorption line relative to the continuum varies with time, with the properties of the variations depending on the structure of the absorbing material. We conclude that such variations will be measurable via UV spectroscopy of image A of the gravitationally lensed quasar Q2237+0305 if the Lyman Alpha clouds between the quasar and the lensing galaxy possess structure on scales smaller than $\sim 0.1$ pc. The time scale for the variations is on the order of order years to decades, although very short term variability can occur. While the Lyman alpha lines may not be accessible at all wavelengths, this approach is applicable to any absorption system, including metal lines.

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arXiv2004

The Canis Major Dwarf Galaxy

Recent observational evidence suggests that the Sagittarius dwarf galaxy represents the only major ongoing accretion event in the Galactic halo, accounting for the majority of stellar debris identified there. This paper summarizes the recent discovery of another potential Milky Way accretion event, the Canis Major dwarf galaxy. This dwarf satellite galaxy is found to lie just below the Galactic plane and appears to be on an equatorial orbit. Unlike Sagittarius, which is contributing to the Galactic halo, the location and eventual demise of Canis Major suggests that it represents a building block of the thick disk.

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arXiv2004

Gravitational Microlensing of Fractal Sources

Gravitational microlensing has proven to be a powerful tool in the study of quasars, providing some of the strongest limits on the scales of structure in the central engine. Typically sources are considered to be smoothly varying on some particular scale; such simple sources result in recognisable time scales in microlensing light curves from which the size of the source can be determined. Various emission processes, however, result in sources with a fractal appearance, possessing structure on a range of scales. Here, the gravitational microlensing of such fractal sources at the heart of quasars is considered. It is shown that the resulting light curves reflect the fractal nature of the sources, possessing pronounced structure at various scales, markedly different to the case with the random distribution of emission clouds that are typically considered. Hence, the determination of a characteristic scale of variability in a microlensing light curve may not necessarily reveal the size of the individual emission clouds, the key value that is required to determine the physical state of the emission region, rather it may correspond to a particular hierarchy in a fractal structure. Current X-ray satellites can detect such fractal structure via the monitoring of gravitationally lensed quasars during a microlensing event, providing a test of high energy emission processes in quasars.

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arXiv2004

Measuring transverse velocities in gravitationally lensed extragalactic systems using an annual parallax effect

A parallax method to determine transverse velocity in a gravitationally lensed system is described. Using the annual motion of the Earth around the Sun allows us to probe the local structure of the magnification map that, under certain assumptions, can be used to infer the effective transverse velocity. The method is applied to OGLE data for QSO2237+0305 and the velocity value is estimated to be about (15 +/- 10) km/s if attributed to the lensing galaxy or about (420 +/- 300) km/s if attributed to the quasar. We find this estimate unreasonably small and conclude that we have not measured a parallax effect. We give a short list of properties that a system should possess to allow a successful implementation of this method.

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arXiv2003

Anisotropy in the Distribution of Satellite Galaxy Orbits

Nearby clusters such as Virgo and Coma possess galaxy distributions which tend to be aligned with the principal axis of the cluster itself. This has also been confirmed by a recent statistical analysis of some 300 Abell clusters where the effect has been linked to the dynamical state of the cluster. Moreover, the orbits of satellite galaxies in galactic systems like our own Milky Way also demonstrate a high degree of anisotropy - the so-called Holmberg effect, the origin of which has been the subject of debate for more than 30 years. This study presents the analysis of cosmological simulations focusing on the orbits of satellite galaxies within dark matter halos. The apocentres of the orbits of these satellites are preferentially found within a cone of opening angle ~40 around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established as both our oldest dark matter halos do show a clear anisotropy signal. Further analysis connects this distribution to the infall pattern of satellites along the filaments: the orbits are determined rather by the environment of the host halo than some "dynamical selection" during their life within the host's virial radius.

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arXiv2003

Gravitational microlensing of quasar broad line regions at large optical depths

Recent estimates of the scale of structures at the heart of quasars suggest that the region responsible for the broad line emission are smaller than previously thought. With this revision of scale, the broad line region is amenable to the influence of gravitational microlensing. This study investigates the influence on microlensing at high optical depth on a number of current models of the Broad Line Region (BLR). It is found that the BLR can be significantly magnified by the action of microlensing, although the degree of magnification is dependent upon spatial and kinematic structure of the BLR. Furthermore, while there is a correlation between the microlensing fluctuations of the continuum source and the BLR, there is substantial scatter about this relation, revealing that broadband photometric monitoring is not necessarily a guide to microlensing of the BLR. The results of this study demonstrate that the spatial and kinematic structure within the BLR may be determined via spectroscopic monitoring of microlensed quasars.

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arXiv2003

The Sizes and Kinematic Structure of Absorption Systems Towards the Lensed Quasar APM08279+5255

We have obtained spatially resolved spectra of the z=3.911 triply imaged QSO APM08279+5255 using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We study the line of sight equivalent width (EW) differences and velocity shear of high and low ionization absorbers (including a damped Lyman alpha [DLA] system identified in a spatially unresolved ground based spectrum) in the three lines of sight. We find that high ionization systems (primarily CIV absorbers) do not exhibit strong EW variations on scales <0.4 kpc; their fractional EW differences are typically less than 30%. When combined with previous work on other QSO pairs, we find that the fractional variation increases steadily with separation out to at least ~100 kpc. Conversely, low ionization systems (primarily MgII absorbers) show strong variations (often > 80%) over kpc scales. A minimum radius for strong (EW > 0.3 A) MgII systems of > 1.4 kpc is inferred from absorption coincidences in all lines of sight. For weak MgII absorbers (EW < 0.3 A), a maximum likelihood analysis indicates a most probable coherence scale of 2.0 kpc for a uniform spherical geometry, with 95% confidence limits ranging between 1.5 and 4.4 kpc. Finally, for systems with weak absorption that can be confidently converted to column densities, we find constant N(CIV)/N(SiIV) across the three lines of sight. Similarly, the [Al/Fe] ratios in the z = 2.974 DLA are consistent with solar relative abundances over a transverse distance of \~0.35 kpc. (abrdiged)

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arXiv2003

The Structure of High Redshift Galactic Halos

Observations of multiple or lensed QSOs at high redshift can be used to probe transverse structure in intervening absorption systems. Here we present results obtained from STIS spectroscopy of the unique triply imaged QSO APM08279+5255 at z = 3.9 and study the coherence scales of intervening low and high ionization absorbers.

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