ORCID Profile
0000-0001-6119-4871
Current Organisations
International Centre for Radio Astronomy Research
,
University of Western Australia
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In Research Link Australia (RLA), "Research Topics" refer to ANZSRC FOR and SEO codes. These topics are either sourced from ANZSRC FOR and SEO codes listed in researchers' related grants or generated by a large language model (LLM) based on their publications.
Astronomical and Space Sciences | Cosmology and Extragalactic Astronomy | Galactic Astronomy
Expanding Knowledge in the Physical Sciences | Expanding Knowledge in the Information and Computing Sciences |
Publisher: Oxford University Press (OUP)
Date: 21-08-2020
Abstract: The nature of the dark matter can affect the collapse time of dark matter haloes, and can therefore be imprinted in observables such as the stellar population ages and star formation histories of dwarf galaxies. In this paper, we use high-resolution hydrodynamical simulations of Local Group-analogue (LG) volumes in cold dark matter (CDM), sterile neutrino warm dark matter (WDM) and self-interacting dark matter (SIDM) models with the eagle galaxy formation code to study how galaxy formation times change with dark matter model. We are able to identify the same haloes in different simulations, since they share the same initial density field phases. We find that the stellar mass of galaxies depends systematically on resolution, and can differ by as much as a factor of 2 in haloes of a given dark matter mass. The evolution of the stellar populations in SIDM is largely identical to that of CDM, but in WDM early star formation is instead suppressed. The time at which LG haloes can begin to form stars through atomic cooling is delayed by ∼200 Myr in WDM models compared to CDM. It will be necessary to measure stellar ages of old populations to a precision of better than 100 Myr, and to address degeneracies with the redshift of reionization – and potentially other baryonic processes – in order to use these observables to distinguish between dark matter models.
Publisher: Oxford University Press (OUP)
Date: 26-09-2023
Publisher: Oxford University Press (OUP)
Date: 08-07-2017
Publisher: Oxford University Press (OUP)
Date: 07-09-2019
Abstract: We use two high-resolution N-body simulations, one assuming general relativity (GR) and the other the Hu–Sawicki form of f(R) gravity with $\\vert \\bar{f}_{\\mathrm{ R}} \\vert = 10^{-6}$, to investigate the concentration–formation time relation of dark matter haloes. We assign haloes to logarithmically spaced mass bins, and fit median density profiles and extract median formation times in each bin. At fixed mass, haloes in modified gravity are more concentrated than those in GR, especially at low masses and low redshift, and do not follow the concentration–formation time relation seen in GR. We assess the sensitivity of the relation to how concentration and formation time are defined, as well as to the segregation of the halo population by the amount of gravitational screening. We find a clear difference between halo concentrations and assembly histories displayed in modified gravity and those in GR. Existing models for the mass–concentration–redshift relation that have gained success in cold and warm dark matter models require revision in f(R) gravity.
Publisher: Oxford University Press (OUP)
Date: 07-07-2017
Publisher: Oxford University Press (OUP)
Date: 09-01-2019
Publisher: Oxford University Press (OUP)
Date: 08-03-2019
DOI: 10.1093/MNRAS/STZ691
Publisher: Oxford University Press (OUP)
Date: 28-12-2021
Abstract: We use a high-resolution cosmological dark matter-only simulation to study the orbital trajectories of haloes and subhaloes in the environs of isolated hosts. We carefully tally all apsis points and use them to distinguish haloes that are infalling for the first time from those that occupy more evolved orbits. We find that roughly 21 per cent of resolved subhaloes within a host’s virial radius are currently on first infall, and have not yet reached their first orbital pericentre roughly 44 per cent are still approaching their first apocentre after infall. For the range of host masses studied, roughly half of all accreted systems were pre-processed prior to infall, and about 20 per cent were accreted in groups. We confirm that the entire population of accreted subhaloes – often referred to as ‘associated’ subhaloes – extends far beyond the virial radii of their hosts, with roughly half currently residing at distances that exceed ≈1.2 × r200. Many of these backsplash haloes have gained orbital energy since infall, and occupy extreme orbits that carry them well past their initial turnaround radii. Such extreme orbits are created during the initial accretion and dissolution of loosely bound groups, but also through penetrating encounters between subhaloes on subsequent orbits. The same processes may also give rise to unexpectedly abrupt losses of orbital energy. These effects combine, giving rise to a large variation in the ratio of sequent apocentres for accreted systems. We find that, within two virial radii from host centres, the concentrations of first-infall haloes are remarkably similar to those of isolated field haloes, whereas backsplash haloes, as well as systems that were pre-processed, are considerably more concentrated.
Publisher: Oxford University Press (OUP)
Date: 04-07-2019
Abstract: We study the impact of numerical parameters on the properties of cold dark matter haloes formed in collisionless cosmological simulations. We quantify convergence in the median spherically averaged circular velocity profiles for haloes of widely varying particle number, as well as in the statistics of their structural scaling relations and mass functions. In agreement with prior work focused on single haloes, our results suggest that cosmological simulations yield robust halo properties for a wide range of gravitational softening parameters, ϵ, provided: (1) ϵ is not larger than a ‘convergence radius’, rconv, which is dictated by two-body relaxation and determined by particle number, and (2) a sufficient number of time-steps are taken to accurately resolve particle orbits with short dynamical times. Provided these conditions are met, median circular velocity profiles converge to within ≈10 per cent for radii beyond which the local two-body relaxation time-scale exceeds the Hubble time by a factor $\\kappa \\equiv t_{\\rm relax}/t_{\\rm H}\\rm{\\,\\, \\buildrel\\gt \\over \\sim \\,\\,}0.177$, with better convergence attained for higher κ. We provide analytic estimates of rconv that build on previous attempts in two ways: first, by highlighting its explicit (but weak) softening-dependence and, second, by providing a simpler criterion in which rconv is determined entirely by the mean inter-particle spacing, l, for ex le better than 10 per cent convergence in circular velocity for $r\\rm{\\,\\, \\buildrel\\gt \\over \\sim \\,\\,}0.05\\, l$. We show how these analytic criteria can be used to assess convergence in structural scaling relations for dark matter haloes as a function of their mass or maximum circular speed.
Publisher: Oxford University Press (OUP)
Date: 14-08-2020
Abstract: We test a non-parametric higher order Jeans analysis method, GravSphere, on 32 simulated dwarf galaxies comparable to classical Local Group dwarfs like Fornax. The galaxies are selected from A Project Of Simulating The Local Environment (APOSTLE) suite of cosmological hydrodynamics simulations with cold dark matter (CDM) and self-interacting dark matter (SIDM) models, allowing us to investigate cusps and cores in density distributions. We find that, for CDM dwarfs, the recovered enclosed mass profiles have a bias of no more than 10 per cent, with a 50 per cent scatter in the inner regions and a 20 per cent scatter near the half-light radius, consistent with standard mass estimators. The density profiles are also recovered with a bias of no more than 10 per cent and a scatter of 30 per cent in the inner regions. For SIDM dwarfs, the mass and density profiles are recovered within our 95 per cent confidence intervals but are biased towards cuspy dark matter distributions. This is mainly due to a lack of sufficient constraints from the data. We explore the sources of scatter in the accuracy of the recovered profiles and suggest a χ2 statistic to separate successful models from biased ones. Finally, we show that the uncertainties on the mass profiles obtained with GravSphere are smaller than those for comparable Jeans methods and that they can be further improved if stronger priors, motivated by cosmological simulations, are placed on the velocity anisotropy. We conclude that GravSphere is a promising Jeans-based approach for modelling dark matter distributions in dwarf galaxies.
Publisher: Oxford University Press (OUP)
Date: 22-09-2018
Abstract: We study the vertical structure of polytropic centrifugally supported gaseous discs embedded in cold dark matter (CDM) haloes. At fixed radius, R, the shape of the vertical density profile depends weakly on whether the disc is self-gravitating (SG) or non-self-gravitating (NSG). The disc ‘characteristic’ thickness, zH, set by the midplane sound speed and circular velocity, zNSG = (cs/Vc)R, in the NSG case, and by the sound speed and surface density, $z_{\\rm SG} = c_{\\rm s}^2/G\\Sigma$, in SG discs, is smaller than zSG and zNSG. SG discs are typically Toomre unstable, NSG discs are stable. Exponential discs in CDM haloes with roughly flat circular velocity curves ‘flare’ outwards. Flares in mono abundance or coeval populations in galaxies like the Milky Way are thus not necessarily due to radial migration. For the polytropic equation of state of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations, discs that match observational constraints are NSG for Md & 3 × 109 M⊙ and SG at higher masses, if fully gaseous. We test these analytic results using a set of idealized smoothed particle hydrodynamic simulations and find excellent agreement. Our results clarify the role of the gravitational softening on the thickness of simulated discs, and on the onset of radial instabilities. EAGLE low-mass discs are NSG so the softening plays no role in their vertical structure. High-mass discs are expected to be SG and unstable, and may be artificially thickened and stabilized unless gravity is well resolved. Simulations with spatial resolution high enough to not compromise the vertical structure of a disc also resolve the onset of their instabilities, but the converse is not true.
Publisher: Oxford University Press (OUP)
Date: 15-07-2019
Abstract: The impact of 2-body scattering on the innermost density profiles of dark matter haloes is well established. We use a suite of cosmological simulations and idealized numerical experiments to show that 2-body scattering is exacerbated in situations where there are two species of unequal mass. This is a consequence of mass segregation and reflects a flow of kinetic energy from the more to less massive particles. This has important implications for the interpretation of galaxy sizes in cosmological hydrodynamic simulations, which nearly always model stars with less massive particles than are used for the dark matter. We compare idealized models as well as simulations from the eagle project that differ only in the mass resolution of the dark matter component, but keep subgrid physics, baryonic mass resolution, and gravitational force softening fixed. If the dark matter particle mass exceeds the mass of stellar particles, then galaxy sizes – quantified by their projected half-mass radii, R50 – increase systematically with time until R50 exceeds a small fraction of the redshift-dependent mean interparticle separation, l (${\\rm R_{50}} \\gtrsim 0.05\\times l$). Our conclusions should also apply to simulations that adopt different hydrodynamic solvers, subgrid physics, or adaptive softening, but in that case may need quantitative revision. Any simulation employing a stellar-to-dark matter particle mass ratio greater than unity will escalate spurious energy transfer from dark matter to baryons on small scales.
Publisher: Oxford University Press (OUP)
Date: 10-01-2023
Abstract: We use a suite of idealized N-body simulations to study the impact of spurious heating of star particles by dark matter (DM) particles on the kinematics and morphology of simulated galactic discs. We find that spurious collisional heating leads to a systematic increase of the azimuthal velocity dispersion (σϕ) of stellar particles and a corresponding decrease in their mean azimuthal velocities ($\\overline{v}_\\phi$). The rate of heating is dictated primarily by the number of DM halo particles (or equivalently, by the DM particle mass at fixed halo mass) and by radial gradients in the local DM density along the disc it is largely insensitive to the stellar particle mass. Galaxies within haloes resolved with fewer than ≈106 DM particles are particularly susceptible to spurious morphological evolution, irrespective of the total halo mass (with even more particles required to prevent heating of the galactic centre). Collisional heating transforms galactic discs from flattened structures into rounder spheroidal systems, causing them to lose rotational support in the process. It also affects the locations of galaxies in standard scaling relations that link their various properties: at fixed stellar mass, it increases the sizes of galaxies, and reduces their mean stellar rotation velocities and specific angular momenta. Our results urge caution when extrapolating simulated galaxy scaling relations to low masses where spurious collisional effects can bias their normalization, slope, and scatter.
Publisher: Oxford University Press (OUP)
Date: 08-08-2020
Abstract: Modelling the molecular gas that is routinely detected through CO observations of high-redshift galaxies constitutes a major challenge for ab initio simulations of galaxy formation. We carry out a suite of cosmological hydrodynamic simulations to compare three approximate methods that have been used in the literature to track the formation and evolution of the simplest and most abundant molecule, H2. Namely, we consider (i) a semi-empirical procedure that associates H2 to dark-matter haloes based on a series of scaling relations inferred from observations, (ii) a model that assumes chemical equilibrium between the H2 formation and destruction rates, and (iii) a model that fully solves the out-of-equilibrium rate equations and accounts for the unresolved structure of molecular clouds. We study the impact of finite spatial resolution and show that robust H2 masses at redshift $z$ ≈ 4 can only be obtained for galaxies that are sufficiently metal enriched in which H2 formation is fast. This corresponds to H2 reservoirs with masses $M_{\\mathrm{H_2}}\\gtrsim 6\\times 10^9$ M⊙. In this range, equilibrium and non-equilibrium models predict similar molecular masses (but different galaxy morphologies) while the semi-empirical method produces less H2. The star formation rates as well as the stellar and H2 masses of the simulated galaxies are in line with those observed in actual galaxies at similar redshifts that are not massive starbursts. The H2 mass functions extracted from the simulations at $z$ ≈ 4 agree well with recent observations that only s le the high-mass end. However, our results indicate that most molecular material at high $z$ lies yet undetected in reservoirs with $10^9\\lt M_{\\mathrm{H}_2}\\lt 10^{10}$ M⊙.
Publisher: Oxford University Press (OUP)
Date: 29-09-2022
Abstract: The concentrations of dark matter haloes provide crucial information about their internal structure and how it depends on mass and redshift – the so-called concentration–mass–redshift relation, denoted c(M, z). We present here an extensive study of the cosmology-dependence of c(M, z) that is based on a suite of 72 gravity-only, full N-body simulations in which the following cosmological parameters were varied: σ8, ΩM, Ωb, ns, h, Mν, w0, and wa. We characterize the impact of these parameters on concentrations for different halo masses and redshifts. In agreement with previous works, and for all cosmologies studied, we find that there exists a tight correlation between the characteristic densities of dark matter haloes within their scale radii, r−2, and the critical density of the universe at a suitably defined formation time. This finding, when combined with excursion set modelling of halo formation histories, allows us to accurately predict the concentrations of dark matter haloes as a function of mass, redshift, and cosmology. We use our simulations to test the reliability of a number of published models for predicting halo concentration and highlight when they succeed or fail to reproduce the cosmological c(M, z) relation.
Publisher: Oxford University Press (OUP)
Date: 10-07-2019
Abstract: We examine the formation of dark matter (DM) cores in dwarf galaxies simulated with the eagle model of galaxy formation. As in earlier work, we find that the star formation (SF) gas density threshold (ρth) plays a critical role. At low thresholds (LT), gas is unable to reach densities high enough to dominate the gravitational potential before being dispersed by feedback from supernovae. LT runs show little effect on the inner DM profile, even in systems with extended and bursty SF, two ingredients often cited as critical for core formation. For higher thresholds, gas is able to dominate the gravitational potential before being ejected by feedback. This can lead to a substantial reduction in the inner DM content, but only if the gas is gravitationally important over an extended period of time, allowing the halo to contract before gas removal. Rapid assembly and removal of gas in short SF bursts is less effective at altering the inner DM content. Subsequent gas accretion may draw DM back in and reform a cusp, unless SF is bursty enough to prevent it, preserving the core. Thus, for the eagle SF + feedback model, there is no simple relation between core formation and SF history, contrary to recent claims. The dependence of the inner DM content of dwarfs on ρth hinders robust predictions and the interpretation of observations. A simulation of a $(12 \\rm \\ Mpc)^3$ volume with high ρth results in dwarfs with sizeable cores over a limited halo mass range, but with insufficient variety in mass profiles to explain the observed ersity of dwarf galaxy rotation curves.
Publisher: Oxford University Press (OUP)
Date: 14-02-2020
Abstract: Linking the properties of galaxies to the assembly history of their dark matter haloes is a central aim of galaxy evolution theory. This paper introduces a dimensionless parameter s ∈ [0, 1], the ‘tree entropy’, to parametrize the geometry of a halo’s entire mass assembly hierarchy, building on a generalization of Shannon’s information entropy. By construction, the minimum entropy (s = 0) corresponds to smoothly assembled haloes without any mergers. In contrast, the highest entropy (s = 1) represents haloes grown purely by equal-mass binary mergers. Using simulated merger trees extracted from the cosmological N-body simulation SURFS, we compute the natural distribution of s, a skewed bell curve peaking near s = 0.4. This distribution exhibits weak dependences on halo mass M and redshift z, which can be reduced to a single dependence on the relative peak height δc/σ(M, z) in the matter perturbation field. By exploring the correlations between s and global galaxy properties generated by the SHARK semi-analytic model, we find that s contains a significant amount of information on the morphology of galaxies – in fact more information than the spin, concentration, and assembly time of the halo. Therefore, the tree entropy provides an information-rich link between galaxies and their dark matter haloes.
Publisher: Oxford University Press (OUP)
Date: 06-02-2020
Abstract: We address the issue of numerical convergence in cosmological smoothed particle hydrodynamics simulations using a suite of runs drawn from the eagle project. Our simulations adopt subgrid models that produce realistic galaxy populations at a fiducial mass and force resolution, but systematically vary the latter in order to study their impact on galaxy properties. We provide several analytic criteria that help guide the selection of gravitational softening for hydrodynamical simulations, and present results from runs that both adhere to and deviate from them. Unlike dark matter-only simulations, hydrodynamical simulations exhibit a strong sensitivity to gravitational softening, and care must be taken when selecting numerical parameters. Our results – which focus mainly on star formation histories, galaxy stellar mass functions and sizes – illuminate three main considerations. First, softening imposes a minimum resolved escape speed, vϵ, due to the binding energy between gas particles. Runs that adopt such small softening lengths that $v_\\epsilon \\gtrsim 10\\, {\\rm km\\, s^{-1}}$ (the sound speed in ionized ${\\sim }10^4\\, {\\rm K}$ gas) suffer from reduced effects of photoheating. Secondly, feedback from stars or active galactic nuclei may suffer from numerical overcooling if the gravitational softening length is chosen below a critical value, ϵeFB. Thirdly, we note that small softening lengths exacerbate the segregation of stars and dark matter particles in halo centres, often leading to the counterintuitive result that galaxy sizes increase as softening is reduced. The structure of dark matter haloes in hydrodynamical runs respond to softening in a way that reflects the sensitivity of their galaxy populations to numerical parameters.
Publisher: Oxford University Press (OUP)
Date: 31-10-2021
Abstract: We use the second Gaia data release to investigate the kinematics of 17 ultra-faint dwarf galaxies (UFDs) and 154 globular clusters (GCs) in the Milky Way, focusing on the differences between static and evolving models of the Galactic potential. An evolving potential modifies a satellite’s orbit relative to its static equivalent, though the difference is small compared to existing uncertainties on orbital parameters. We find that the UFD Boötes II is likely on its first passage around the Milky Way. Depending on the assumed mass of the Milky Way, the UFDs Triangulum II, Hydrus I, Coma Berenices, Draco II, and Ursa Major II, as well as the GC Pyxis, may also be on first infall so may be useful for constraining the mass of the Galaxy. We identify a clear kinematic distinction between metal-rich ([Fe/H] & −1.1) and metal-poor GCs ([Fe/H] ≤ −1.1). Although most metal-rich clusters occupy predominately prograde orbits, with low eccentricities (e ≈ 0.35) and similar specific angular momenta and orbital planes as the Galactic disc, seven show potentially retrograde orbits, the origin of which is unclear. Metal-poor clusters have more erse orbits, higher eccentricities (e ≈ 0.65), and half of them have orbital planes offset from the disc by 60° to 120°—twice as many as the metal-poor GCs. The UFDs have similar θ and ϕ to the metal-poor GCs, suggesting a similar origin. We provide a catalogue of orbital parameters for UFDs and GCs for two different Galaxy masses and their observational uncertainties.
Publisher: Oxford University Press (OUP)
Date: 24-04-2020
Abstract: We use a compilation of disc galaxy rotation curves to assess the role of the luminous component (‘baryons’) in the rotation curve ersity problem. As in earlier work, we find that rotation curve shape correlates with baryonic surface density: high surface density galaxies have rapidly rising rotation curves consistent with cuspy cold dark matter haloes slowly rising rotation curves (characteristic of galaxies with inner mass deficits or ‘cores’) occur only in low surface density galaxies. The correlation, however, seems too weak to be the main driver of the ersity. In addition, dwarf galaxies exhibit a clear trend, from ‘cuspy’ systems where baryons are unimportant in the inner mass budget to ‘cored’ galaxies where baryons actually dominate. This trend constrains the various scenarios proposed to explain the ersity, such as (i) baryonic inflows and outflows during galaxy formation (ii) dark matter self-interactions (iii) variations in the baryonic mass structure coupled to rotation velocities through the ‘mass discrepancy–acceleration relation’ (MDAR) or (iv) non-circular motions in gaseous discs. Together with analytical modelling and cosmological hydrodynamical simulations, our analysis shows that each of these scenarios has promising features, but none seems to fully account for the observed ersity. The MDAR, in particular, is inconsistent with the observed trend between rotation curve shape and baryonic importance either the trend is caused by systematic errors in the data or the MDAR does not apply. The origin of the dwarf galaxy rotation curve ersity and its relation to the structure of cold dark matter haloes remains an open issue.
Publisher: Oxford University Press (OUP)
Date: 17-07-2023
Abstract: We use the eagle cosmological simulations to study the evolution of the vertical velocity dispersion of cold gas, σz, in central disc galaxies and its connection to stellar feedback, gravitational instabilities, cosmological gas accretion, and galaxy mergers. To isolate the impact of feedback, we analyse runs that turn off stellar and (or) active galactic nuclei feedback in addition to a run that includes both. The evolution of σz and its dependence on stellar mass and star formation rate in eagle are in good agreement with observations. Galaxies hosted by haloes of similar virial mass, $\\rm M_{200}$, have similar σz values even in runs where feedback is absent. The prevalence of local instabilities in discs is uncorrelated with σz at low redshift and becomes only weakly correlated at high redshifts and in galaxies hosted by massive haloes. σz correlates most strongly with the specific gas accretion rate onto the disc as well as with the degree of misalignment between the inflowing gas and the disc’s rotation axis. These correlations are significant across all redshifts and halo masses, with misaligned accretion being the primary driver of high gas turbulence at redshifts z ≲ 1 and for halo masses $\\rm M_{200} \\lesssim 10^{11.5} {\\rm M}_{\\odot }$. Galaxy mergers increase σz, but because they are rare in our s le, they play only a minor role in its evolution. Our results suggest that the turbulence of cold gas in eagle discs results from a complex interplay of different physical processes whose relative importance depends on halo mass and redshift.
Publisher: Oxford University Press (OUP)
Date: 31-08-2023
Publisher: Oxford University Press (OUP)
Date: 02-10-2021
Abstract: We use idealized N-body simulations of equilibrium stellar discs embedded within course-grained dark matter (DM) haloes to study the effects of spurious collisional heating on disc structure and kinematics. Collisional heating artificially increases the vertical and radial velocity dispersions of disc stars, as well as the thickness and size of discs the effects are felt at all galacto-centric radii. The integrated effects of collisional heating are determined by the mass of DM halo particles (or equivalently, by the number of particles at fixed halo mass), their local density and characteristic velocity dispersion, but are largely insensitive to the stellar particle mass. The effects can therefore be reduced by increasing the mass resolution of DM in cosmological simulations, with limited benefits from increasing the baryonic (or stellar) mass resolution. We provide a simple empirical model that accurately captures the effects of spurious collisional heating on the structure and kinematics of simulated discs, and use it to assess the importance of disc heating for simulations of galaxy formation. We find that the majority of state-of-the-art zoom simulations, and a few of the highest-resolution, smallest-volume cosmological runs, are in principle able to resolve thin stellar discs in Milky Way-mass haloes, but most large-volume cosmological simulations cannot. For ex le, DM haloes resolved with fewer than ≈106 particles will collisionally heat stars near the stellar half-mass radius such that their vertical velocity dispersion increases by ≳ 10 per cent of the halo’s virial velocity in approximately one Hubble time.
Location: Australia
Start Date: 10-2021
End Date: 09-2024
Amount: $645,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 06-2017
End Date: 06-2023
Amount: $652,000.00
Funder: Australian Research Council
View Funded Activity