arXiv subjects

Brad K. Gibson

Publications and source records attributed to Brad K. Gibson.

At least 127 records · Page 7Linked to original sources

arXiv2004

High-resolution N-body Simulations of Galactic Cannibalism: The Magellanic Stream

Hierarchical clustering represents the favoured paradigm for galaxy formation throughout the Universe; due to its proximity, the Magellanic system offers one of the few opportunities for astrophysicists to decompose the full six-dimensional phase-space history of a satellite in the midst of being cannibalised by its host galaxy. The availability of improved observational data for the Magellanic Stream and parallel advances in computational power has led us to revisit the canonical tidal model describing the disruption of the Small Magellanic Cloud and the consequent formation of the Stream. We suggest improvements to the tidal model in light of these recent advances.

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arXiv2004

Constraints on Early Nucleosynthesis from the Abundance Pattern of a Damped Ly-alpha System at z = 2.626

We have investigated chemical evolution in the young universe by analysing the detailed chemical enrichment pattern of a metal-rich galaxy at high redshift. The recent detection of over 20 elements in the gas-phase of a damped Lyman-alpha absorber (DLA) at z = 2.626 represents an exciting new avenue for exploring early nucleosynthesis. Given a strict upper age of ~2.5 Gyr and a gas-phase metallicity about one third solar, we have shown the DLA abundance pattern to be consistent with the predictions of a chemical evolution model in which the interstellar enrichment is dominated by massive stars with a small contribution from Type Ia supernovae. Discrepancies between the empirical data and the models are used to highlight outstanding issues in nucleosynthesis theory, including a tendency for Type II supernovae models to overestimate the magnitude of the "odd-even" effect at subsolar metallicities. Our results suggest a possible need for supplemental sources of magnesium and zinc, beyond that provided by massive stars.

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arXiv2004

The Chemo-Dynamical Evolution of Elliptical Galaxies: Pre-heating and AGN heating

We study the chemodynamical evolution of elliptical galaxies and their X-ray and optical properties using high-resolution cosmological simulations. Our Tree N-body/SPH code includes a self-consistent treatment of radiative cooling, star formation, supernovae feedback, and chemical enrichment. We present a series of LCDM cosmological simulations which trace the spatial and temporal evolution of abundances of heavy elements in both the stellar and gas components of galaxies. A giant elliptical galaxy formed in one of the simulations is quantitatively compared with the observational data in both the X-ray and optical regime. We implement a treatment of both pre-heating and AGN heating in this simulation, and examine the effect of these processes on elliptical galaxy formation. We find that the adopted pre-heating (T=10^7 K at z=4) is not strong enough to explain the observed X-ray or optical properties. On the other hand, our AGN heating model in which the gas inflow induces the AGN heating is consistent with both the X-ray and optical properties.

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arXiv2004

The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way

We modeled the evolution of the Milky Way to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun.

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arXiv2003

Galactic Chemical Evolution

The primary present-day observables upon which theories of galaxy evolution are based are a system's morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.

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arXiv2003

Stellar Halo Constraints on Simulated Late Type Galaxies

How do late type spiral galaxies form within the context of a CDM cosmology? We contrast N-body, smoothed particle hydrodynamical simulations of galaxy formation which employ two different supernova feedback mechanisms. Observed mass and metallicity distributions of the stellar halos of the Milky Way and M31 provide constraints on these models. A strong feedback model, incorporating an adiabatic phase in star burst regions, better reproduces these observational properties than our comparative thermal feedback model. This demonstrates the importance of energy feedback in regulating star formation in small systems, which collapse at early epochs, in the evolution of late type disk galaxies.

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arXiv2003

Satellite galaxies in cosmological dark matter halos

We present preliminary results from a series of high-resolution N-body simulations that focus on 8 dark matter halos each of order a million particles within the virial radius. We follow the time evolution of hundreds of individually tracked satellite galaxies and relate their physical properties to the differing halo environmental conditions. Our main science driver is to understand how satellite galaxies lose their mass and react to tidal stripping. Unlike previous work our results are performed in a fully self-consistent cosmological context. The preliminary results demonstrate that while environment may vary considerably with respects to formation time and richness of substructure, the satellites evolve similarly.

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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

An HI census of Loose Groups of Galaxies

We present results from our Parkes Multibeam HI survey of 3 loose groups of galaxies that are analogous to the Local Group. This is a survey of groups containing only spiral galaxies with mean separations of a few hundred kpc, and total areas of approximately 1 sq. Mpc; groups similar to our own Local Group. We present a census of the HI-rich objects in these groups down to an M(HI), 1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these detections from follow-up Compact Array observations. We found 7 new HI-rich members in the 3 groups, all of which have stellar counterparts and are, therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich galaxies in these groups is less than in the Local Group meaning that the ``missing satellite'' problem is not unique. No high-velocity cloud analogs were found in any of the groups. If HVCs in these groups are the same as in the Local Group, this implies that HVCs must be located within ~300-400 kpc of the Milky Way.

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arXiv2003

Searching for Variability in the Globular Cluster Messier 4

Time-series data taken with the Hubble Space Telescope of a field six core radii (~5') from the center of the globular cluster Messier 4, and covering a period of about 10 weeks in early 2001, have been analyzed in search of variable objects. Various criteria were employed to select candidate variable stars. Period searches were performed on the selected candidates using phase dispersion minimization (PDM). The reliability of the PDM search results was tested using synthetic light curves of eclipsing binary stars and sinusoidal light curves of different periods. Results from this analysis showed that there are probably no eclipsing binary stars or periodic variables in our field with periods on the order of a few hours to a few days, down to limiting magnitudes of V~25 and I~24, which is consistent with the absence of contact binaries such as W Ursae Majoris systems. However, one candidate variable star does show an increase in brightness of ~0.1 magnitudes in both bandpasses, which seems to last for a few days. Possible explanations concerning the nature of this object include a binary system with a white dwarf primary and a low-mass main sequence secondary, or a BY Draconis variable star. We are able to set an upper limit to the observed fraction of photometric variability in this dataset of 0.05%.

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arXiv2003

The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro

We establish an extragalactic, zero-motion frame of reference within the deepest optical image of a globular star cluster, an HST 123-orbit exposure of M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster based on their proper motions over the 6-year baseline between these and previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are also found on the same sight line using image-morphology techniques. This fixed reference frame allows us to independently determine the fundamental Galactic constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly, the galaxies allow a direct measurement of M4's absolute proper motion, mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies. The clear separation of galaxies from stars in these deep data also allow us to search for inner-halo white dwarfs. We model the conventional Galactic contributions of white dwarfs along our line of sight and predict 7.9 (thin disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at which we can clearly delineate stars from galaxies (V = 29). An additional 2.5 objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo objects, 70% of which are of the DA type. After considering the kinematics and morphology of the objects in our data set, we find the number of white dwarfs to be consistent with the predictions for each of the conventional populations. However, we do not find any evidence for dark halo white dwarfs.

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arXiv2003

Galactic Halos in MONDian Cosmological Simulations

In this paper a series of high-resolution n-body simulations is presented in which the equations of motion have been changed to account for MOdified Newtonian Dynamics (MOND). It is shown that a low-Omega0 MONDian model with an appropriate choice for the normalisation sigma8 can lead to similar clustering properties at redshift z=0 as the commonly accepted (standard) LCDM model. However, such a model shows no significant structures at high redshift with only very few objects present beyond z>3 that can be readily ascribed to the low Omega0 value adopted. The agreement with LCDM at redshift z=0 is driven by the more rapid structure evolution in MOND. Moreover, galaxy formation appears to be more strongly biased in MONDian cosmologies. Within the current implementation of MOND density profiles of gravitationally bound objects at z=0 can still be fitted by the universal NFW profile but MOND halos are less clumpy.

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arXiv2003

The Metallicity of Pre-Galactic Globular Clusters: Observational consequences of the first stars

We explore a scenario where metal-poor globular clusters (GCs) are enriched by the first supernovae in the Universe. If the first stars in a 10^7 Msun dark halo were very massive (>180 Msun), then a pair instability supernova from a single massive star can produce sufficient iron to enrich 10^6 Msun of pristine, primordial gas to [Fe/H] ~ -2. In such a scenario, where a single massive star acts as a seed for halo GCs, the accurate abundance analysis of GC stars would allow a direct measurement of the Population III initial mass. Using the latest theoretical yields for zero metallicity stars in the mass range 140-260 Msun, we find that the metals expelled from a ~230 Msun star are consistent with [Si/Fe] and [Ca/Fe] observed in GC stars. However, no single star in this mass range can simultaneously explain all halo GC heavy-element abundance ratios, such as [V/Fe], [Ti/Fe] and [Ni/Fe]. These require a combination masses for the Population III stellar progenitors. The various observational consequences of this scenario are discussed.

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arXiv2003

Multi-wavelength Cosmological Simulations of Elliptical Galaxies

We study the chemodynamical evolution of elliptical galaxies and their X-ray and optical properties using high-resolution cosmological simulations. Our Tree N-body/SPH code includes a self-consistent treatment of radiative cooling, star formation, supernovae feedback, and chemical enrichment. We present a series of LCDM cosmological simulations which trace the spatial and temporal evolution of abundances of heavy element in both the stellar and gas components of galaxies. A giant elliptical galaxy formed in the simulations is quantitatively compared with the observational data in both the X-ray and optical regime. X-ray spectra of the hot gas are constructed via the use of the vmekal plasma model, and analysed using XSPEC with the XMM EPN response function. Optical properties are derived by the population synthesis for the stellar component. We find that radiative cooling is important to interpret the observed X-ray luminosity, temperature, and metallicity of the hot gas of elliptical galaxies. However, this cooled gas also leads to excessive star formation at low redshift, and therefore results in underlying galactic stellar populations which are too blue with respect to observations. Time variation and radial dependence of X-ray properties and abundance ratios, such as [O/Fe] and [Si/Fe], of the X-ray emitting hot gas are also discussed.

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arXiv2003

Is High-Velocity Cloud Complex C Associated with the Galactic Warp?

We test the hypothesis that High-Velocity gas cloud Complex C is actually a high-latitude spiral arm extension in the direction of the Galactic warp, as opposed to the standard interpretation - that of a once extragalactic, but now infalling, gas cloud. A parallel Tree N-body code was employed to simulate the tidal interaction of a satellite perturber with the Milky Way. We find that a model incorporating a perturber of the mass of the Large Magellanic Cloud on a south-to-north polar orbit, crossing the disk at ~15 kpc, does yield a high- velocity, high-latitude, extension consistent with the spatial, kinematical and column density properties of Complex C. Unless this massive satellite remains undiscovered because of either a fortuitous alignment with the Galactic bulge (feasible within the framework of the model), or the lack of any associated baryonic component, we conclude that this alternative interpretation appears unlikely.

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arXiv2003

Simulating a White Dwarf-dominated Galactic Halo

Observational evidence has suggested the possibility of a Galactic halo which is dominated by white dwarfs (WDs). While debate continues concerning the interpretation of this evidence, it is clear that an initial mass function (IMF) biased heavily toward WD precursors (1 < m/Msol < 8), at least in the early Universe, would be necessary in generating such a halo. Within the framework of homogeneous, closed-box models of Galaxy formation, such biased IMFs lead to an unavoidable overproduction of carbon and nitrogen relative to oxygen (as measured against the abundance patterns in the oldest stars of the Milky Way). Using a three-dimensional Tree N-body smoothed particle hydrodynamics code, we study the dynamics and chemical evolution of a galaxy with different IMFs. Both invariant and metallicity-dependent IMFs are considered. Our variable IMF model invokes a WD-precursor-dominated IMF for metallicities less than 5% solar (primarily the Galactic halo), and the canonical Salpeter IMF otherwise (primarily the disk). Halo WD density distributions and C,N/O abundance patterns are presented. While Galactic haloes comprised of ~5% (by mass) of WDs are not supported by our simulations, mass fractions of ~1-2% cannot be ruled out. This conclusion is consistent with the present-day observational constraints.

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arXiv2003

Deriving the Metallicity Distribution Function of Galactic Systems

The chemical evolution of the Milky Way is investigated using a dual-phase metal-enriched infall model in which primordial gas fuels the earliest epoch of star formation, followed by the ongoing formation of stars from newly accreted gas. The latest metallicity distribution of local K-dwarfs is reproduced by this model, which allows the Galactic thin disk to form from slightly metal-enriched gas with alpha-element enhancement. Our model predicts ages for the stellar halo and thin disk of 12.5 and 7.4 Gyr, respectively, in agreement with empirically determined values. The model presented in this paper is compared with a similar dual-phase infall model from Chiappini et al. (2001). We discuss a degeneracy that enables both models to recover the K-dwarf metallicity distribution while yielding different star formation histories. The metallicity distribution function (MDF) of K-dwarfs is proposed to be more directly comparable to chemical evolution model results than the G-dwarf distribution because lower mass K-dwarfs are less susceptible to stellar evolutionary effects. The K-dwarf MDF should consequently be a better probe of star formation history and provide a stronger constraint to chemical evolution models than the widely used G-dwarf MDF. The corrections that should be applied to a G-dwarf MDF are quantified for the case of the outer halo of NGC 5128.

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arXiv2003

The cosmic production of Helium

We estimate the cosmic production rate of helium relative to metals ($ΔY/ΔZ$) using K dwarf stars in the Hipparcos catalog with accurate spectroscopic metallicities. The best fitting value is $ΔY/ΔZ=2.1 \pm 0.4$ at the 68% confidence level. Our derived value agrees with determinations from HII regions and with theoretical predictions from stellar yields with standard assumptions for the initial mass function. The amount of helium in stars determines how long they live and therefore how fast they will enrich the insterstellar medium with fresh material.

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