arXiv subjects

Brad K. Gibson

Publications and source records attributed to Brad K. Gibson.

At least 91 records · Page 5Linked to original sources

arXiv2008

Simulating the Mass-Metallicity Relation from z=1

We use 112 N-body/hydrodynamical simulations in the standard Cold Dark Matter universe, to follow the formation of galaxy-sized halos and investigate the chemical enrichment of both the stellar component and the interstellar medium of galaxies, with stellar masses larger than 1e9 Msun. The resulting chemical properties of the simulated galaxies are broadly consistent with the observations. The predicted relationship between the mean metallicity and the galaxy stellar mass for both the stellar and the gaseous components at z=0 are in agreement with the relationships observed locally. The predicted scatter about these relationships, which is traced to the differing merging histories amongst the simulated galaxies with similar final masses, is similar to that observed. The predicted correlations between the total mass and the stellar mass of galaxies in our simulated sample from the present epoch up to z=1 agree with observed ones. The stellar mass versus metallicity relation and its associated scatter are reproduced by the simulations as consequences of the increasing efficiency of the conversion of gas into stars with stellar mass, and the differing merging histories amongst the galaxies with similar masses. The old ages of simulated low mass galaxies at z=0, and the weak level of chemical evolution for massive galaxies suggest however that our modeling of the supernova feedback may be incomplete, or that other feedback processes have been neglected. (Abridged)

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arXiv2007

Self-consistent massive disks in triaxial dark matter halos

Galactic disks in triaxial dark matter halos become deformed by the elliptical potential in the plane of the disk in such a way as to counteract the halo ellipticity. We develop a technique to calculate the equilibrium configuration of such a disk in the combined disk-halo potential, which is based on the method of Jog (2000) but accounts for the radial variation in both the halo potential and the disk ellipticity. This crucial ingredient results in qualitatively different behavior of the disk: the disk circularizes the potential at small radii, even for a reasonably low disk mass. This effect has important implications for proposals to reconcile cuspy halo density profiles with low surface brightness galaxy rotation curves using halo triaxiality. The disk ellipticities in our models are consistent with observational estimates based on two-dimensional velocity fields and isophotal axis ratios.

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arXiv2007

The Chemistry of the Local Group

Simulations of the chemical enrichment histories of ten Local Group (LG) dwarf galaxies are presented, employing empirically-derived star formation histories (SFHs), a rich network of isotopic and elemental nucleosynthetic yields, and a range of prescriptions for supernova (SN)-driven outflows. Our main conclusions are that (i) neutron-capture element patterns (particularly that of Ba/Y) suggest a strong contribution from low- and intermediate-mass stars, (ii) neutron star mergers may play a relatively larger role in the nucleosynthesis of dwarfs, (iii) SN feedback alone can explain the observed gas fraction in dwarf irregulars (dIrrs), but dwarf spheroidals (dSphs) require almost all their gas to be removed via ram pressure and/or tidal stripping, (iv) the predicted heavy Mg isotope enhancements in the interstellar medium of dwarfs may provide an alternate solution to claims of a varying fine structure (v) the gas lost from dwarfs have O,Si/C abundances in broad agreement with intergalactic medium abundances at redshifts 2<z<4, and (vi) the chemical properties of dSphs are well-matched by preventing galactic winds from re-accreting, whilst those of dIrrs are better-matched by incorporating metallicity-dependent cooling and re-accretion of hot winds. Finally, doubts are cast upon a claimed association between LG dSph UMaII and High-Velocity Cloud Complex A.

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arXiv2006

Galactic Sodium from AGB Stars

Galactic chemical evolution models which include sodium from type II supernovae alone underestimate the abundance of sodium in the interstellar medium by a factor of two to three over about 3 dex in metallicity and predict a flat behaviour in the evolution of [Na/Fe] at super-solar metallicities. Conversely, recent observations of stars with [Fe/H]~+0.4 suggest that [Na/Fe] increases at high metallicity. We have combined stellar evolution models of asymptotic giant branch and Wolf-Rayet stars with the latest supernova yields in an attempt to resolve these problems ... and have created many more.

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arXiv2006

The Spatial Distribution of the Galactic First Stars II: SPH Approach

We use cosmological, chemo-dynamical, smoothed particle hydrodynamical simulations of Milky-Way-analogue galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z~4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z=0 (< 0.8 Msol), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z=0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I, and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.

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arXiv2006

The Spatial Distribution of the Galactic First Stars I: High-Resolution N-body Approach

We study the spatial distribution of Galactic metal-free stars by combining an extremely high-resolution (7.8 X 10^5 solar masses per particle) Cold Dark Matter N-body simulation of the Milky-Way with a semi-analytic model of metal enrichment. This approach allows us to resolve halos with virial temperatures down to the 10^4K atomic cooling limit, and it is sufficiently flexible to make a number of robust conclusions, despite the extremely uncertain properties of the first stars. Galactic metal-free stars are formed over a large redshift range, which peaks at z~10, but continues down to z~5, contributing stars at wide range of Galactocentric radii. Stars containing only metals generated by primordial stars are similarly widespread. Neither changing the efficiency of metal dispersal by two orders of magnitude, nor drastically changing the approximations in our semi-analytical model can affect these result. Thus, if they have sufficiently long lifetimes, a significant number of stars formed in initially primordial star clusters should be found in the nearby Galactic halo regardless of the specifics of metal-free star formation. Observations of metal abundances in Galactic halo stars should be taken as directly constraining the properties of primordial stars, and the lack of metal-free halo stars today should be taken as strongly suggesting a 0.8 solar mass lower limit on the primordial initial mass function.

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arXiv2006

Chemical Evolution of the Galactic Bulge

Adopting a single-zone framework, with accretion of primordial gas on a free-fall timescale, the chemical evolution of the Galactic bulge is calculated, assuming (i) a corresponding rapid timescale for star formation, and (ii) an initial mass function biased towards massive stars. We emphasise here the uncertainties associated with the underlying physics (specifically, stellar nucleosynthesis) and how those uncertainties are manifest in the predicted abundance ratio patterns in the resulting present-day Galactic bulge stellar populations.

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arXiv2006

Two Disk Components from a Gas Rich Disk-Disk Merger

We employ N-body, smoothed particle hydrodynamical simulations, including detailed treatment of chemical enrichment, to follow a gas-rich merger which results in a galaxy with disk morphology. We trace the kinematic, structural and chemical properties of stars formed before, during, and after the merger. We show that such a merger produces two exponential disk components, with the older, hotter component having a scale-length 20% larger than the later-forming, cold disk. Rapid star formation during the merger quickly enriches the protogalactic gas reservoir, resulting in high metallicities of the forming stars. These stars form from gas largely polluted by Type II supernovae, which form rapidly in the merger-induced starburst. After the merger, a thin disk forms from gas which has had time to be polluted by Type Ia supernovae. Abundance trends are plotted, and we examine the proposal that increased star formation during gas-rich mergers may explain the high alpha-to-iron abundance ratios which exist in the relatively high-metallicity thick disk component of the Milky Way.

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arXiv2006

Chemodynamical Simulations of Elliptical Galaxies

We review recent developments in the field of chemodynamical simulations of elliptical galaxies, highlighting (in an admittedly biased fashion) the work conducted with our cosmological N-body/SPH code GCD+. We have demonstrated previously the recovery of several primary integrated early-type system scaling relations (e.g. colour-magnitude relation, L_X-T_X-[Fe/H]_X) when employing a phenomenological AGN heating scheme in conjunction with a self-consistent treatment of star formation, supernovae feedback, radiative cooling, chemical enrichment, and stellar/X-ray population synthesis. Here we emphasise characteristics derived from the full spatial information contained within the simulated dataset, including stellar and coronal morphologies, metallicity distribution functions, and abundance gradients.

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arXiv2006

On the origin of anomalous velocity clouds in the Milky Way

We report that neutral hydrogen (HI) gas clouds, resembling High Velocity Clouds (HVCs) observed in the Milky Way (MW), appear in MW-sized disk galaxies formed in high-resolution Lambda Cold Dark Matter (LCDM) cosmological simulations which include gas-dynamics, radiative cooling, star formation, supernova feedback, and metal enrichment. Two such disk galaxies are analyzed, and HI column density and velocity distributions in all-sky Aitoff projections are constructed. The simulations demonstrate that LCDM is able to create galaxies with sufficient numbers of anomalous velocity gas clouds consistent with the HVCs observed within the MW, and that they are found within a galactocentric radius of 150 kpc. We also find that one of the galaxies has a polar gas ring, with radius 30 kpc, which appears as a large structure of HVCs in the Aitoff projection. Such large structures may share an origin similar to extended HVCs observed in the MW, such as Complex C.

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arXiv2006

N-body simulations of the Magellanic Stream

A suite of high-resolution N-body simulations of the Magellanic Clouds -- Milky Way system are presented and compared directly with newly available data from the HI Parkes All-Sky Survey (HIPASS). We show that the interaction between Small and Large Magellanic Clouds results in both a spatial and kinematical bifurcation of both the Stream and the Leading Arm. The spatial bifurcation of the Stream is readily apparent in the HIPASS data, and the kinematical bifurcation is also tentatively identified. This bifurcation provides strong support for the tidal disruption origin for the Magellanic Stream. A fiducial model for the Magellanic Clouds is presented upon completion of an extensive parameter survey of the potential orbital configurations of the Magellanic Clouds and the viable initial boundary conditions for the disc of the Small Magellanic Cloud. The impact of the choice of these critical parameters upon the final configurations of the Stream and Leading Arm is detailed.

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arXiv2006

Are Red Tidal Features Unequivocal Signatures of Major Dry Mergers?

We use a cosmological numerical simulation to study the tidal features produced by a minor merger with an elliptical galaxy. We find that the simulated tidal features are quantitatively similar to the red tidal features, i.e., dry tidal features, recently found in deep images of elliptical galaxies at intermediate redshifts. The minor merger in our simulation does not trigger star formation due to active galactic nuclei heating. Therefore, both the tidal features and the host galaxy are red, i.e. a dry minor merger. The stellar mass of the infalling satellite galaxy is about 10^10 Msun, and the tidal debris reach the surface brightness of mu_R~27 mag arcsec^-2. Thus, we conclude that tidal debris from minor mergers can explain the observed dry tidal features in ellipticals at intermediate redshifts, although other mechanisms (such as major dry mergers) may also be important.

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arXiv2006

Cosmological implications of dwarf spheroidal chemical evolution

The chemical properties of dwarf spheroidals in the local group are shown to be inconsistent with star formation being truncated after the reionization epoch (z~8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like Sculptor indicate strong s-process production from low-mass stars whose lifetimes are comparable with the duration of the pre-reionization epoch. The chemical evolution of Sculptor is followed using a model with SNeII and SNeIa feedback and mass- and metallicity-dependent nucleosynthetic yields for elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the interstellar medium with s-elements. This robust result challenges the suggestion that most of the local group dwarf spheroidals are fossils of reionization and supports the case for large initial dark matter halos.

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arXiv2006

Origin of Two Distinct Populations in Dwarf Spheroidal Galaxies

We study the chemical and kinematic properties of the first galaxies which formed at a high redshift, using high resolution cosmological numerical simulations, and compared them with the recent observational results for the Sculptor dwarf spheroidal galaxy by Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient, within a single populations, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate mass stars, compared to Type II supernovae.

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arXiv2006

MACHOs in dark matter haloes

Using eight dark matter haloes extracted from fully-self consistent cosmological N-body simulations, we perform microlensing experiments. A hypothetical observer is placed at a distance of 8.5 kpc from the centre of the halo measuring optical depths, event durations and event rates towards the direction of the Large Magellanic Cloud. We simulate 1600 microlensing experiments for each halo. Assuming that the whole halo consists of MACHOs, f = 1.0, and a single MACHO mass is m_M = 1.0 Msun, the simulations yield mean values of tau = 4.7^{+5.0}_{-2.2} 10^{-7} and Gamma = 1.6^{+1.3}_{-0.6} 10^{-6} events/star/year. We find that triaxiality and substructure can have major effects on the measured values so that tau and Gamma values of up to three times the mean can be found. If we fit our values of tau and Gamma to the MACHO collaboration observations (Alcock et al. 2000), we find f = 0.23^{+0.15}_{-0.13} and m_M = 0.44^{+0.24}_{-0.16}. Five out of the eight haloes under investigation produce f and m_M values mainly concentrated within these bounds.

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arXiv2006

The Importance of Interactions for Mass Loss from Satellite Galaxies in Cold Dark Matter Haloes

We investigate the importance of interactions between dark matter substructures for the mass loss they suffer whilst orbiting within a sample of high resolution galaxy cluster mass Cold Dark Matter haloes formed in cosmological N-body simulations. We have defined a quantitative measure that gauges the degree to which interactions are responsible for mass loss from substructures. This measure indicates that interactions are more prominent in younger systems when compared to older more relaxed systems. We show that this is due to the increased number of encounters a satellite experiences and a higher mass fraction in satellites. This is in spite of the uniformity in the distributions of relative distances and velocities of encounters between substructures within the different host systems in our sample. Using a simple model to relate the net force felt by a single satellite to the mass loss it suffers, we show that interactions with other satellites account for ~30% of the total mass loss experienced over its lifetime. The relation between the age of the host and the importance of interactions increases the scatter about this mean value from ~25% for the oldest to ~45% for the youngest system we have studied. We conclude that satellite interactions play a vital role in the evolution of substructure in dark matter halos and that a significant fraction of the tidally stripped material can be attributed to these interactions.

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arXiv2005

Gas Rich Mergers in Disk Formation

In order to explain disk galaxy formation within the hierarchical structure formation, it seems that gas rich mergers must play an important role. We review here our previous studies which have shown the importance of mergers at high redshift being gas rich, in the formation of both the stellar halo and thick disk components of disk galaxies. Regulation of star formation in the building blocks of our galaxy is required to form a low mass low metallicity stellar halo. This regulation results in high redshift, gas rich mergers during which the thick disk forms. In these proceedings, we categorise stars from our simulated disk galaxy into thin and thick disk components by using the Toomre diagram.

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arXiv2005

The Stellar Halo Metallicity - Luminosity Relationship for Spiral Galaxies

The stellar halos of spiral galaxies bear important chemo-dynamical signatures of galaxy formation. We present here the analysis of 89 semi-cosmological spiral galaxy simulations, spanning ~ 4 magnitudes in total galactic luminosity. These simulations sample a wide variety of merging histories and show significant dispersion in halo metallicity at a given total luminosity - more than a factor of ten in metallicity. Our preliminary analysis suggests that galaxies with a more extended merging history possess halos which have younger and more metal rich stellar populations than the stellar halos associated with galaxies with a more abbreviated assembly. A correlation between halo metallicity and its surface brightness has also been found, reflecting the correlation between halo metallicity and its stellar mass. Our simulations are compared with recent Hubble Space Telescope observations of resolved stellar halos in nearby spirals.

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