arXiv subjects

Geraint F. Lewis

Publications and source records attributed to Geraint F. Lewis.

At least 217 records · Page 12Linked to original sources

arXiv2015

NGC 147, NGC 185 and CassII: a genetic approach to orbital properties, star formation and tidal debris

NGC147, NGC185 and CassiopeiaII (CassII) have similar positions in the sky, distances and measured line of sight velocities. This proximity in phase space suggests that these three satellites of M31 form a subgroup within the Local Group. Nevertheless, the differences in their star formation history and interstellar medium, and the recent discovery of a stellar stream in NGC~147, combined with the lack of tidal features in the other two satellites, are all indications of complex and diverse interactions between M31 and these three satellites. We use a genetic algorithm to explore the different orbits that these satellites can have and select six sets of orbits that could best explain the observational features of the NGC147, NGC185 and CassII satellites. The parameters of these orbits are then used as a starting point for N-body simulations. We present models for which NGC147, NGC185 and CassII are a bound group for a total time of at least one Gyr but still undergo different interactions with M31 and as a result NGC147 has a clear stellar stream whereas the other two satellites have no significant tidal features. This result shows that it is possible to find solutions that reproduce the contrasting properties of the satellites and for which NGC147-NGC185-CassII have been gravitationally bound.

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arXiv2015

Trans-Dimensional Bayesian Inference for Gravitational Lens Substructures

We introduce a Bayesian solution to the problem of inferring the density profile of strong gravitational lenses when the lens galaxy may contain multiple dark or faint substructures. The source and lens models are based on a superposition of an unknown number of non-negative basis functions (or "blobs") whose form was chosen with speed as a primary criterion. The prior distribution for the blobs' properties is specified hierarchically, so the mass function of substructures is a natural output of the method. We use reversible jump Markov Chain Monte Carlo (MCMC) within Diffusive Nested Sampling (DNS) to sample the posterior distribution and evaluate the marginal likelihood of the model, including the summation over the unknown number of blobs in the source and the lens. We demonstrate the method on two simulated data sets: one with a single substructure, and one with ten. We also apply the method to the g-band image of the "Cosmic Horseshoe" system, and find evidence for more than zero substructures. However, these have large spatial extent and probably only point to misspecifications in the model (such as the shape of the smooth lens component or the point spread function), which are difficult to guard against in full generality.

astro-ph.IM↗

arXiv2015

The Spatially-Resolved Star Formation History of the M31 Outer Disc

We present deep Hubble Space Telescope Advanced Camera for Surveys observations of the stellar populations in two fields lying at 20 and 23 kpc from the centre of M31 along the south-west semi-major axis. These data enable the construction of colour-magnitude diagrams reaching the oldest main-sequence turn-offs (~13 Gyr) which, when combined with another field at 25 kpc from our previous work, we use to derive the first precision constraints on the spatially-resolved star formation history of the M31 disc. The star formation rates exhibit temporal as well as field-to-field variations, but are generally always within a factor of two of their time average. There is no evidence of inside-out growth over the radial range probed. We find a median age of ~7.5 Gyr, indicating that roughly half of the stellar mass in the M31 outer disc was formed before z ~ 1. We also find that the age-metallicity relations (AMRs) are smoothly increasing from [Fe/H]~-0.4 to solar metallicity between 10 and 3 Gyr ago, contrary to the flat AMR of the Milky Way disc at a similar number of scale lengths. Our findings provide insight on the roles of stellar feedback and radial migration in the formation and evolution of large disc galaxies.

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arXiv2015

Hidden from view: Coupled Dark Sector Physics and Small Scales

We study cluster mass dark matter haloes, their progenitors and surroundings in an coupled Dark Matter-Dark Energy model and compare it to quintessence and $Λ$CDM models with adiabatic zoom simulations. When comparing cosmologies with different expansions histories, growth functions & power spectra, care must be taken to identify unambiguous signatures of alternative cosmologies. Shared cosmological parameters, such as $σ_8$, need not be the same for optimal fits to observational data. We choose to set our parameters to $Λ$CDM $z=0$ values. We find that in coupled models, where DM decays into DE, haloes appear remarkably similar to $Λ$CDM haloes despite DM experiencing an additional frictional force. Density profiles are not systematically different and the subhalo populations have similar mass, spin, and spatial distributions, although (sub)haloes are less concentrated on average in coupled cosmologies. However, given the scatter in related observables ($V_{\rm max},R_{V_{\rm max}}$), this difference is unlikely to distinguish between coupled and uncoupled DM. Observations of satellites of MW and M31 indicate a significant subpopulation reside in a plane. Coupled models do produce planar arrangements of satellites of higher statistical significance than $Λ$CDM models, however, in all models these planes are dynamically unstable. In general, the nonlinear dynamics within and near large haloes masks the effects of a coupled dark sector. The sole environmental signature we find is that small haloes residing in the outskirts are more deficient in baryons than their $Λ$CDM counterparts. The lack of a pronounced signal for a coupled dark sector strongly suggests that such a phenomena would be effectively hidden from view.

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arXiv2015

Eppur si muove: Positional and kinematic correlations of satellite pairs in the low Z universe

We have recently shown (Ibata et al. 2014) that pairs of satellite galaxies located diametrically opposite each other around their host possess predominantly anti-correlated velocities. This is consistent with a scenario in which $\sim 50$% of satellite galaxies belong to kinematically-coherent rotating planar structures, similar to those detected around the giant galaxies of the Local Group. Here we extend this analysis, examining the incidence of satellites of giant galaxies drawn from an SDSS photometric redshift catalog. We find that there is a $\sim 17$% overabundance ($> 3 σ$ significance) of candidate satellites at positions diametrically opposite a spectroscopically confirmed satellite. We show that cosmological simulations do not possess this property when the contamination is included, and that there are in fact, after subtracting contamination, 2 to 3 times more satellites diametrically opposed to a spectroscopically confirmed satellite than at $90°$ from it. We also examine the correlation between the satellite pair positions and the orientation of the host galaxy major axis. We find that those satellite pairs with anti-correlated velocities have a strong preference ($\sim 3:1$) to align with the major axis of the host whereas those with correlated velocities display the opposite behavior. This correlation of the satellite alignments appears to be stronger than the well-documented preference of satellites to be located close to the major axis of their host. We finally show that repeating a similar analysis to Ibata et al. (2014) with same-side satellites is generally hard to interpret, but is not inconsistent with our previous results when strong quality-cuts are applied on the sample. All these unexpected correlations strongly suggest that a substantial fraction of satellite galaxies are causally-linked in their formation and evolution.

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arXiv2014

Comparing the observable properties of dwarf galaxies on and off the Andromeda plane

The thin, extended planes of satellite galaxies detected around both the Milky Way and Andromeda are not a natural prediction of the LCDM paradigm. Galaxies in these distinct planes may have formed and evolved in a different way (e.g., tidally) to their off-plane neighbours. If this were the case, one would expect the on- and off-plane dwarf galaxies in Andromeda to have experienced different evolutionary histories, which should be reflected by the chemistries, dynamics, and star formation histories of the two populations. In this work, we present new, robust kinematic observations for 2 on-plane M31 dSphs (And XVI and XVII) and compile and compare all available observational metrics for the on- and off-plane dwarfs to search for a signal that would corroborate such a hypothesis. We find that, barring their spatial alignment, the on- and off-plane Andromeda dwarf galaxies are indistinguishable from one another, arguing against vastly different formative and evolutionary histories for these two populations.

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arXiv2014

The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories

While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep HST/ACS data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour-magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of 'disc-like' fields indicate that most of their mass has formed since z~1, with one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed within the last 5 Gyr. These fields are also characterised by an age--metallicity relation showing rapid chemical enrichment to solar metallicity by z=1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from through disc heating in the recent past.

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arXiv2014

Another coincidence problem for $Λ$CDM?

Over the last nine years of cosmic microwave background observations, the Wilkinson Microwave Anisotropy Probe ($WMAP$) results were consistent with a $Λ$CDM cosmological model in which the age of the Universe is one Hubble time, and the time-averaged value of the deceleration parameter is consistent with zero. This curious observation has been put forward as a new coincidence problem for the $Λ$CDM concordance cosmology, which is in fact a `greater' coincidence than the near equality of the density parameters of matter and the cosmological constant. At the moment of writing these conference proceedings, the Planck Collaboration has released its first cosmological data, which revealed a small shift in the $Λ$CDM cosmological parameters when compared to $WMAP$. We show that under the assumption of a spatially flat $Λ$CDM cosmology, Planck's results remove this coincidence problem for $Λ$CDM at greater than 99\% confidence level.

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arXiv2014

Probing spatial homogeneity with LTB models: a detailed discussion

Do current observational data confirm the assumptions of the cosmological principle, or is there statistical evidence for deviations from spatial homogeneity on large scales? To address these questions, we developed a flexible framework based on spherically symmetric, but radially inhomogeneous Lemaitre-Tolman-Bondi (LTB) models with synchronous Big Bang. We expanded the (local) matter density profile in terms of flexible interpolation schemes and orthonormal polynomials. A Monte Carlo technique in combination with recent observational data was used to systematically vary the shape of these profiles. In the first part of this article, we reconsider giant LTB voids without dark energy to investigate whether extremely fine-tuned mass profiles can reconcile these models with current data. While the local Hubble rate and supernovae can easily be fitted without dark energy, however, model-independent constraints from the Planck 2013 data require an unrealistically low local Hubble rate, which is strongly inconsistent with the observed value; this result agrees well with previous studies. In the second part, we explain why it seems natural to extend our framework by a non-zero cosmological constant, which then allows us to perform general tests of the cosmological principle. Moreover, these extended models facilitate explorating whether fluctuations in the local matter density profile might potentially alleviate the tension between local and global measurements of the Hubble rate, as derived from Cepheid-calibrated type Ia supernovae and CMB experiments, respectively. We show that current data provide no evidence for deviations from spatial homogeneity on large scales. More accurate constraints are required to ultimately confirm the validity of the cosmological principle, however.

astro-ph.CO↗

arXiv2014

On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution

Halo stars orbit within the potential of the Milky Way and hence their kinematics can be used to understand the underlying mass distribution. However, the inferred mass distribution depends sensitively upon assumptions made on the density and the velocity anisotropy profiles of the tracers. Also, there is a degeneracy between the parameters of the halo and that of the disk or bulge. Here, we decompose the Galaxy into bulge, disk and dark matter halo and then model the kinematic data of the halo BHB and K-giants from the SEGUE. Additionally, we use the gas terminal velocity curve and the Sgr A$^*$ proper motion. With $R_\odot = 8.5$kpc, our study reveals that the density of the stellar halo has a break at $17.2^{+1.1}_{-1.0}$ kpc, and an exponential cut-off in the outer parts starting at $97.7^{+15.6}_{-15.8}$kpc. Also, we find the velocity anisotropy is radially biased with $β_s= 0.4\pm{0.2}$ in the outer halo. We measure halo virial mass $M_{\text{vir}} = 0.80^{+0.31}_{-0.16} \times 10^{12} M_{\odot}$, concentration $c=21.1^{+14.8}_{-8.3}$, disk mass of $0.95^{+0.24}_{-0.30}\times10^{11} M_{\odot}$, disk scale length of $4.9^{+0.4}_{-0.4}$ kpc and bulge mass of $0.91^{+0.31}_{-0.38} \times 10^{10} M_{\odot}$. The mass of halo is found to be small and this has important consequences. The giant stars reveal that the outermost halo stars have low velocity dispersion interestingly suggesting a truncation of the stellar halo density rather than a small overall mass of the Galaxy. Our estimates of local escape velocity $v_{\rm esc} = 550.9^{+32.4}_{-22.1}$ kms$^{-1}$ and dark matter density $ρ^{\rm DM}_{\odot} = 0.0088^{+0.0024}_{-0.0018} M_{\odot} {\rm pc^{-3}} $ ($0.35^{+0.08}_{-0.07}$ GeV cm$^{-3}$) are in good agreement with recent estimates. Some of the above estimates are depended on the adopted value of $R_\odot$ and of outer power-law index of the tracer number density.

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arXiv2014

A Genetic Approach to the History of the Magellanic Clouds

The history of the Magellanic Clouds is investigated using N-body hydrodynamic simulations where the initial conditions are set by a genetic algorithm. This technique allows us to identify possible orbits for the Magellanic Clouds around the Milky Way, by directly comparing the simulations with observational constraints. We explore the parameter space of the interaction between the Magellanic Clouds and the Milky Way, considering as free parameters the proper motions of the Magellanic Clouds, the virial mass and the concentration parameter ($c$) of the Galactic dark matter halo. The best orbital scenarios presented here are considered with two different sets of parameters for the Milky Way disc and bulge components. The total circular velocity at the Sun's position ($\rm{R_{\odot}}=8.5\,\rm {kpc}$) is directly calculated from the rotation curve of the corresponding Galactic mass model. Our analysis suggests that the Magellanic Clouds have orbited inside the virial radius of the Milky Way for at least $3\,\rm{Gyr}$, even for low mass haloes. However, this is possible only with high values for the concentration parameter ($c\ge20$). In both orbital models presented here, the mutual interaction between the Magellanic Clouds is able to reproduce the observed features of the Magellanic System.

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arXiv2014

Velocity anti-correlation of diametrically opposed galaxy satellites in the low redshift universe

Recent work has shown that both the Milky Way and the Andromeda galaxies possess the unexpected property that their dwarf satellite galaxies are aligned in thin and kinematically coherent planar structures. It is now important to evaluate the incidence of such planar structures in the larger galactic population, since the Local Group may not be a sufficiently representative environment. Here we report that the measurement of the velocity of pairs of diametrically opposed galaxy satellites provides a means to determine statistically the prevalence of kinematically coherent planar alignments. In the local universe (redshift $z<0.05$), we find that such satellite pairs out to a galactocentric distance of $150$ kpc are preferentially anti-correlated in their velocities (99.994% confidence level), and that the distribution of galaxies in the larger scale environment (beyond 150 kpc and up to $\approx 2$ Mpc) is strongly elongated along the axis joining the inner satellite pair ($>7σ$ confidence). Our finding may indicate that co-rotating planes of satellites, similar to that seen around the Andromeda galaxy, are ubiquitous in nature, while their coherent motion also suggests that they are a significant repository of angular momentum on $\sim 100$ kpc scales.

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arXiv2014

Stochastic Star Formation & Feedback: Mapping Low-Mass Galaxies to Dark Matter Haloes

Comparison of observed satellite galaxies of the Milky Way (hereafter MW) with dark matter subhaloes in cosmological $N$-body simulations of MW-mass haloes suggest that such subhaloes, if they exist, are occupied by satellites in a stochastic fashion. We examine how inefficient massive star formation and associated supernova feedback in high-redshift progenitors of present-day low-mass subhaloes might contribute to this stochasticity. Using a Monte Carlo approach to follow the assembly histories of present-day low-mass haloes with $10^7 \lesssim M \leq 10^{10}$ ${\rm M}_{\odot}$, we identify when cooling and star formation is likely to proceed, and observe that haloes with present-day masses $\lesssim 10^9 {\rm M}_{\odot}$ never grow sufficiently massive to support atomic hydrogen line cooling. Noting that the star formation timescale decreases sharply with stellar mass as $t_{\rm PMS} \propto m_{\ast}^{-2.5}$, we argue that, should the conditions for high mass star formation arise in low-mass haloes, the ensuing supernovae are likely to disrupt ongoing lower-mass star formation and unbind gas within the halo. This potentially star-forming gas is unlikely to be replenished in lower mass haloes because of, e.g. cosmological reionization, and so we expect galaxy formation to be stymied in a manner that depends on host halo assembly history and the efficiency and timing of star formation in proto-galaxies, which we illustrate using a Monte Carlo model. Based on these simple physical arguments, we assert that stochasticity of star formation and feedback is an essential but overlooked ingredient in modelling galaxy formation on the smallest scales.

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arXiv2014

Warm Dark Haloes Accretion Histories and their Gravitational Signatures

We study clusters in Warm Dark Matter (WDM) models of a thermally produced dark matter particle $0.5$ keV in mass. We show that, despite clusters in WDM cosmologies having similar density profiles as their Cold Dark Matter (CDM) counterparts, the internal properties, such as the amount of substructure, shows marked differences. This result is surprising as clusters are at mass scales that are {\em a thousand times greater} than that at which structure formation is suppressed. WDM clusters gain significantly more mass via smooth accretion and contain fewer substructures than their CDM brethren. The higher smooth mass accretion results in subhaloes which are physically more extended and less dense. These fine-scale differences can be probed by strong gravitational lensing. We find, unexpectedly, that WDM clusters have {\em higher} lensing efficiencies than those in CDM cosmologies, contrary to the naive expectation that WDM clusters should be less efficient due to the fewer substructures they contain. Despite being less dense, the larger WDM subhaloes are more likely to have larger lensing cross-sections than CDM ones. Additionally, WDM subhaloes typically reside at larger distances, which radially stretches the critical lines associated with strong gravitational lensing, resulting in excess in the number of clusters with large radial cross-sections at the $\sim2σ$ level. Though lensing profile for an individual cluster vary significantly with the line-of-sight, the radial arc distribution based on a sample of $\gtrsim100$ clusters may prove to be the crucial test for the presence of WDM.

astro-ph.CO↗

arXiv2014

Resolving the mass--anisotropy degeneracy of the spherically symmetric Jeans equation I: theoretical foundation

A widely employed method for estimating the mass of stellar systems with apparent spherical symmetry is dynamical modelling using the spherically symmetric Jeans equation. Unfortunately this approach suffers from a degeneracy between the assumed mass density and the second order velocity moments. This degeneracy can lead to significantly different predictions for the mass content of the system under investigation, and thus poses a barrier for accurate estimates of the dark matter content of astrophysical systems. In a series of papers we describe an algorithm that removes this degeneracy and allows for unbiased mass estimates of systems of constant or variable mass-to-light ratio. The present contribution sets the theoretical foundation of the method that reconstructs a unique kinematic profile for some assumed free functional form of the mass density. The essence of our method lies in using flexible B-spline functions for the representation of the radial velocity dispersion in the spherically symmetric Jeans equation. We demonstrate our algorithm through an application to synthetic data for the case of an isotropic King model with fixed mass-to-light ratio, recovering excellent fits of theoretical functions to observables and a unique solution. The mass-anisotropy degeneracy is removed to the extent that, for an assumed functional form of the potential and mass density pair $(Φ,ρ)$, and a given set of line-of-sight velocity dispersion $σ_{los}^2$ observables, we recover a unique profile for $σ_{rr}^2$ and $σ_{tt}^2$. Our algorithm is simple, easy to apply and provides an efficient means to reconstruct the kinematic profile.

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arXiv2014

Resolving the mass--anisotropy degeneracy of the spherically symmetric Jeans equation II: optimum smoothing and model validation

The spherical Jeans equation is widely used to estimate the mass content of a stellar systems with apparent spherical symmetry. However, this method suffers from a degeneracy between the assumed mass density and the kinematic anisotropy profile, $β(r)$. In a previous work, we laid the theoretical foundations for an algorithm that combines smoothing B-splines with equations from dynamics to remove this degeneracy. Specifically, our method reconstructs a unique kinematic profile of $σ_{rr}^2$ and $σ_{tt}^2$ for an assumed free functional form of the potential and mass density $(Φ,ρ)$ and given a set of observed line-of-sight velocity dispersion measurements, $σ_{los}^2$. In Paper I (submitted to MNRAS: MN-14-0101-MJ) we demonstrated the efficiency of our algorithm with a very simple example and we commented on the need for optimum smoothing of the B-spline representation; this is in order to avoid unphysical variational behaviour when we have large uncertainty in our data. In the current contribution we present a process of finding the optimum smoothing for a given data set by using information of the behaviour from known ideal theoretical models. Markov Chain Monte Carlo methods are used to explore the degeneracy in the dynamical modelling process. We validate our model through applications to synthetic data for systems with constant or variable mass-to-light ratio $Υ$. In all cases we recover excellent fits of theoretical functions to observables and unique solutions. Our algorithm is a robust method for the removal of the mass-anisotropy degeneracy of the spherically symmetric Jeans equation for an assumed functional form of the mass density.

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arXiv2014

The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum

We reveal the highly structured nature of the Milky Way stellar halo within the footprint of the PAndAS photometric survey from blue main sequence and main sequence turn-off stars. We map no fewer than five stellar structures within a heliocentric range of ~5 to 30 kpc. Some of these are known (the Monoceros Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three well-defined stellar structures that could be, at least partly, responsible for the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In particular, we trace a new faint stellar stream located at a heliocentric distance of ~17 kpc. With a surface brightness of Σ_V ~ 32-32.5 mag/arcsec^2, it follows an orbit that is almost parallel to the Galactic plane north of M31 and has so far eluded surveys of the Milky Way halo as these tend to steer away from regions dominated by the Galactic disk. Investigating our follow-up spectroscopic observations of PAndAS, we serendipitously uncover a radial velocity signature from stars that have colors and magnitudes compatible with the stream. From the velocity of eight likely member stars, we show that this stellar structure is dynamically cold, with an unresolved velocity dispersion that is lower than 7.1 km/s at the 90-percent confidence level. Along with the width of the stream (300-650 pc), its dynamics points to a dwarf-galaxy-accretion origin. The numerous stellar structures we can map in the Milky Way stellar halo between 5 and 30 kpc and their varying morphology is a testament to the complex nature of the stellar halo at these intermediate distances.

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arXiv2014

A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation

In a recent contribution, Bahl \& Baumgardt investigated the incidence of planar alignments of satellite galaxies in the Millennium-II simulation, and concluded that vast thin planes of dwarf galaxies, similar to that observed in the Andromeda galaxy (M31), occur frequently by chance in $Λ$-Cold Dark Matter cosmology. However, their analysis did not capture the essential fact that the observed alignment is simultaneously radially extended, yet thin, and kinematically unusual. With the caveat that the Millennium-II simulation may not have sufficient mass resolution to identify confidently simulacra of low-luminosity dwarf galaxies, we re-examine that simulation for planar structures, using the same method as employed by Ibata et al. (2013) on the real M31 satellites. We find that 0.04\% of host galaxies display satellite alignments that are at least as extreme as the observations, when we consider their extent, thickness and number of members rotating in the same sense. We further investigate the angular momentum properties of the co-planar satellites, and find that the median of the specific angular momentum derived from the line of sight velocities in the real M31 structure ($1.3\times10^4$ km/s kpc) is very high compared to systems drawn from the simulations. This analysis confirms that it is highly unlikely that the observed structure around the Andromeda galaxy is due to a chance occurrence. Interestingly, the few extreme systems that are similar to M31 arise from the accretion of a massive sub-halo with its own spatially-concentrated entourage of orphan satellites.

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