arXiv subjects

Chris Power

Publications and source records attributed to Chris Power.

At least 145 records · Page 8Linked to original sources

arXiv2010

The Accretion Disc Particle Method for Simulations of Black Hole Feeding and Feedback

Black holes grow by accreting matter from their surroundings. However, angular momentum provides an efficient natural barrier to accretion and so only the lowest angular momentum material will be available to feed the black holes. The standard sub-grid model for black hole accretion in galaxy formation simulations - based on the Bondi-Hoyle method - does not account for the angular momentum of accreting material, and so it is unclear how representative the black hole accretion rate estimated in this way is likely to be. In this paper we introduce a new sub-grid model for black hole accretion that naturally accounts for the angular momentum of accreting material. Both the black hole and its accretion disc are modelled as a composite accretion disc particle. Gas particles are captured by the accretion disc particle if and only if their orbits bring them within its accretion radius R_acc, at which point their mass is added to the accretion disc and feeds the black hole on a viscous timescale t_visc. The resulting black hole accretion rate (dM/dt)_BH powers the accretion luminosity L_acc ~ (dM/dt)_BH, which drives black hole feedback. Using a series of controlled numerical experiments, we demonstrate that our new accretion disc particle method is more physically self-consistent than the Bondi-Hoyle method. We also discuss the physical implications of the accretion disc particle method for systems with a high degree of rotational support, and we argue that the M_BH-sigma relation in these systems should be offset from the relation for classical bulges and ellipticals, as appears to be observed.

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arXiv2010

The Accuracy of Subhalo Detection

With the ever increasing resolution of N-body simulations, accurate subhalo detection is becoming essential in the study of the formation of structure, the production of merger trees and the seeding of semi-analytic models. To investigate the state of halo finders, we compare two different approaches to detecting subhaloes; the first based on overdensities in a halo and the second being adaptive mesh refinement. A set of stable mock NFW dark matter haloes were produced and a subhalo was placed at different radii within a larger halo. SUBFIND (a Friends-of-Friends based finder) and AHF (an adaptive mesh based finder) were employed to recover the subhalo. As expected, we found that the mass of the subhalo recovered by SUBFIND has a strong dependence on the radial position and that neither halo finder can accurately recover the subhalo when it is very near the centre of the halo. This radial dependence is shown to be related to the subhalo being truncated by the background density of the halo and originates due to the subhalo being defined as an overdensity. If the subhalo size is instead determined using the peak of the circular velocity profile, a much more stable value is recovered. The downside to this is that the maximum circular velocity is a poor measure of stripping and is affected by resolution. For future halo finders to recover all the particles in a subhalo, a search of phase space will need to be introduced.

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arXiv2009

Simulations of momentum feedback by black hole winds

The observed super-massive black hole (SMBH) mass -- galaxy velocity dispersion ($M_{\rm bh} - σ$) correlation may be established when winds/outflows from the SMBH drive gas out of the potential wells of classical bulges. Here we present numerical simulations of this process in a static isothermal potential. Simple spherically symmetric models of SMBH feedback at the Eddington luminosity can successfully explain the $M_{\rm bh} - σ$ and nuclear cluster mass $M_{\rm NC}-σ$ correlations, as well as why larger bulges host SMBHs while smaller ones host nuclear star clusters. However these models do not specify how SMBHs feed on infalling gas whilst simultaneously producing feedback that drives gas out of the galaxy. More complex models with rotation and/or anisotropic feedback allow SMBHs to feed via a disc or regions not exposed to SMBH winds, but in these more realistic cases it is not clear why a robust $M_{\rm bh} - σ$ relation should be established. In fact, some of the model predictions contradict observations. For example, an isotropic SMBH wind impacting on a disc (rather than a shell) of aspect ratio $H/R \ll 1$ requires the SMBH mass to be larger by a factor $\sim R/H$, which is opposite to what is observed. We conclude that understanding how a SMBH feeds is as important a piece of the puzzle as understanding how its feedback affects its host galaxy. Finally, we note that in aspherical cases the SMBH outflows induce differential motions in the bulge. This may pump turbulence that is known to hinder star formation in star forming regions. SMBH feedback thus may not only drive gas out of the bulge but also reduce the fraction of gas turned into stars.

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arXiv2008

A correlation between spin parameter and dark matter halo mass

Using a set of high-resolution dark matter only cosmological simulations we found a correlation between the dark matter halo mass M and its spin parameter lambda for objects forming at redshifts z > 10: the spin parameter decreases with increasing mass. However, halos forming at later times do not exhibit such a strong correlation, in agreement with the findings of previous studies. While we presented such a correlation in a previous study using the Bullock et al. (2001) spin parameter defintion we now defer to the classical definition showing that the results are independent of the definition.

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arXiv2008

Dynamics of substructures in warm dark-matter cosmologies

We performed cosmological simulations based upon both a cold dark matter (CDM) and a warm dark matter (WDM) model. The focus of our investigations lies with selected spatial and kinematic properties of substructure halos (subhalos) orbiting within host halos, that form in both dark-matter cosmologies. We aim at using the dynamics of the subhalos as a probe of the respective cosmology.

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arXiv2008

The Anisotropic Distribution of Satellite Galaxies

We identify satellites of isolated galaxies in SDSS and examine their angular distribution. Using mock catalogues generated from cosmological N-body simulations, we demonstrate that the selection criteria used to select isolated galaxies and their satellites must be very strict in order to correctly identify systems in which the primary galaxy dominates its environment. The criteria used in many previous studies instead select predominantly group members. We refine a set of selection criteria for which the group contamination is estimated to be less than 7% and present a catalogue of the resulting sample. The angular distribution of satellites about their host is biased towards the major axes for spheroidal galaxies and probably also for red disc galaxies, but is isotropic for blue disc galaxies, i.e. it is the colour of the host that determines the distribution of its satellites rather than its morphology. The similar anisotropy measured in this study as in studies that were dominated by groups implies that group-specific processes are not responsible for the angular distribution. Satellites that are most likely to have been recently accreted show a tendancy to lie along the same axis as the surrounding large scale structure. The orientations of isolated early and intermediate-type galaxies also align with the surrounding large scale structures. We discuss the origin of the anisotropic satellite distribution and consider the implications of our results, critically assessing the respective roles played by the orientation of the visible galaxy within its dark matter halo; anisotropic accretion of satellites from the larger scale environment; and the biased nature of satellites as tracers of the underlying dark matter subhalo population. (Abridged)

astro-ph↗

arXiv2008

On the relation between radial alignment of dark matter subhalos and host mass in cosmological simulations

We explore the dependence of the radial alignment of subhalos on the mass of the host halo they orbit in. As the effect is seen on a broad range of scales including massive clusters as well as galactic systems it only appears natural to explore this phenomenon by means of cosmological simulations covering the same range in masses. We have 25 well resolved host dark matter halos at our disposal ranging from 10^15 Msun/h down to 10^12 Msun/h each consisting of order of a couple of million particles within the virial radius. We observe that subhalos tend to be more spherical than isolated objects. Both the distributions of sphericity and triaxiality of subhalos are Gaussian distributed with peak values of s approx. 0.80 and T approx. 0.56, irrespective of host mass. Interestingly we note that the radial alignment is independent of host halo mass and the distribution of \cosθ(i.e. the angle between the major axis E_a of each subhalo and the radius vector of the subhalo in the reference frame of the host) is well fitted by a simple power law P(\cosθ) proportional to \cos^4θwith the same fitting parameters for all host halos.

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arXiv2008

The Dynamics of Subhalos in Warm Dark Matter Models

We present a comparison of the properties of substructure halos (subhalos) orbiting within host halos that form in Cold Dark Matter (CDM) and Warm Dark Matter (WDM) cosmologies. Our study focuses on selected properties of these subhalos, namely their anisotropic spatial distribution within the hosts; the existence of a "backsplash'' population; the age-distance relation; the degree to which they suffer mass loss; and the distribution of relative (infall) velocities with respect to the hosts. We find that the number density of subhalos in our WDM model is suppressed relative to that in the CDM model, as we would expect. Interestingly, our analysis reveals that backsplash subhalos exist in both the WDM and CDM models. Indeed, there are no statistically significant differences between the spatial distributions of subhalos in the CDM and WDM models. There is evidence that subhalos in the WDM model suffer enhanced mass loss relative to their counterparts in the CDM model, reflecting their lower central densities. We note also a tendency for the (infall) velocities of subhalos in the WDM model to be higher than in the CDM model. Nevertheless, we conclude that observational tests based on either the spatial distribution or the kinematics of the subhalo population are unlikely to help us to differentiate between the CDM model and our adopted WDM model.

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arXiv2008

On the Correlation between Spin Parameter and Halo Mass

We report on a correlation between virial mass M and spin parameter lambda for dark matter halos forming at redshifts z > 10. We find that the spin parameter decreases with increasing halo mass. Interestingly, our analysis indicates that halos forming at later times do not exhibit such a strong correlation, in agreement with the findings of previous studies. We briefly discuss the implications of this correlation for galaxy formation at high redshifts and the galaxy population we observe today.

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arXiv2008

The Tidal Streams of Disrupting Subhaloes in Cosmological Dark Matter Haloes

(abridged) We present a detailed analysis of the properties of tidally stripped material from disrupting substructure haloes or subhaloes in a sample of high resolution cosmological N-body host haloes ranging from galaxy- to cluster-mass scales. We focus on devising methods to recover the infall mass and infall eccentricity of subhaloes from the properties of their tidally stripped material (i.e. tidal streams). Our analysis reveals that there is a relation between the scatter of stream particles about the best-fit debris plane and the infall mass of the progenitor subhalo. This allows us to reconstruct the infall mass from the spread of its tidal debris in space. We also find that the spread in radial velocities of the debris material (as measured by an observer located at the centre of the host) correlates with the infall eccentricity of the subhalo, which allows us to reconstruct its orbital parameters. We devise an automated method to identify leading and trailing arms that can, in principle at least, be applied to observations of stellar streams from satellite galaxies. This method is based on the energy distribution of material in the tidal stream. Using this method, we show that the mass associated with leading and trailing arms differ. While our analysis indicates that tidal streams can be used to recover certain properties of their progenitor subhaloes (and consequently satellites), we do not find strong correlations between host halo properties and stream properties. This likely reflects the complicated relationship between the stream and the host, which in a cosmological context is characterised by a complex mass accretion history, an asymmetric mass distribution and the abundance of substructure. [...]

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arXiv2008

Dark Matter Halo Profiles in Scale-Free Cosmologies

We explore the dependence of the central logarithmic slope of dark matter halo density profiles $α$ on the spectral index $n$ of the linear matter power spectrum $P(k)$ using cosmological $N$-body simulations of scale-free models (i.e. $P(k) \propto k^n$). For each of our simulations we identify samples of well resolved haloes in dynamical equilibrium and we analyse their mass profiles. By parameterising the mass profile using a ``generalised'' Navarro, Frenk & White profile in which the central logarithmic slope $α$ is allowed to vary while preserving the $r^{-3}$ asymptotic form at large radii, we obtain preferred central slopes for haloes in each of our models. There is a strong correlation between $α$ and $n$, such that $α$ becomes shallower as $n$ becomes steeper. However, if we normalise our mass profiles by $r_{-2}$, the radius at which the logarithmic slope of the density profile is -2, we find that these differences are no longer present. We conclude that there is no evidence for convergence to a unique central asymptotic slope, at least on the scales that we can resolve.

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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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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

The Impact of Box Size on the Properties of Dark Matter Haloes in Cosmological Simulations

We investigate the impact finite simulation box size has on the structural and kinematic properties of Cold Dark Matter haloes forming in cosmological simulations. Our approach involves generating a single realisation of the initial power spectrum of density perturbations and studying how truncation of this power spectrum on scales larger than L_cut affects the structure of dark matter haloes at z=0. In particular, we have examined the cases of L_cut = f_cut L_box with f_cut=1 (i.e. no truncation), 1/2, 1/3 and 1/4. In common with previous studies, we find that the suppression of long wavelength perturbations reduces the strength of clustering, as measured by a suppression of the 2-point correlation function xi(r), and reduces the numbers of the most massive haloes, as reflected in the depletion of the high mass end of the mass function n(M). Interestingly, we find that truncation has little impact on the internal properties of haloes. The masses of high mass haloes decrease in a systematic manner as L_cut is reduced, but the distribution of concentrations is unaffected. On the other hand, the median spin parameter is ~50% lower in runs with f_cut<1. We argue that this is an imprint of the linear growth phase of the halo's angular momentum by tidal torquing, and that the absence of any measurable trend in concentration and the weak trend observed in halo shape reflect the importance of virialisation and complex mass accretion histories for these quantities. These results are of interest for studies that require high mass resolution and statistical samples of simulated haloes, such as simulations of the population of first stars. Our analysis shows that large-scale tidal fields have relatively little effect on the internal properties of Cold Dark Matter haloes and hence may be ignored in such studies.

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arXiv2005

Galaxy Groups: Proceedings from a Swinburne University Workshop

We present the proceedings from a 2-day workshop held at Swinburne University on the 24th-25th of May 2005. The workshop participants highlighted current Australian research on both theoretical and observational aspects of galaxy groups. These proceedings include short 1-page summaries of a number of the talks presented at the workshop. The talks presented ranged from reconciling N-body simulations with observations, to the HI content of galaxies in groups and the existence of ``dark galaxies''. The formation and existence of ultra-compact dwarfs in groups, and a new supergroup in Eridanus were also discussed.

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arXiv2005

The Distribution of Satellite Galaxies: The Great Pancake

The 11 known satellite galaxies within 250 kpc of the Milky Way lie close to a great circle on the sky. We use high resolution N-body simulations of galactic dark matter halos to test if this remarkable property can be understood within the context of the cold dark matter cosmology. We construct halo merger trees from the simulations and use a semianalytic model to follow the formation of satellite galaxies. We find that in all 6 of our simulations, the 11 brightest satellites are indeed distributed along thin, disk-like structures analogous to that traced by the Milky Way's satellites. This is in sharp contrast to the overall distributions of dark matter in the halo and of subhalos within it which, although triaxial, are not highly aspherical. We find that the spatial distribution of satellites is significantly different from that of the most massive subhalos but is similar to that of the subset of subhalos that had the most massive progenitors at earlier times. The elongated disk-like structure delineated by the satellites has its long axis aligned with the major axis of the dark matter halo. We interpret our results as reflecting the preferential infall of satellites along the spines of a few filaments of the cosmic web.

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arXiv2005

How Concentrated Are The Haloes Of Low Surface Brightness Galaxies In The Cold Dark Matter Model?

We investigate whether a correlation exists between the concentration and spin parameters of dark matter haloes forming in LCDM N-body simulations. In particular, we focus on haloes with virial masses characteristic of the hosts of Low Surface Brightness (LSB) and High Surface Brightness (HSB) galaxies. Our analysis demonstrates that concentration and spin are anti-correlated. If we assume that a galaxy disk forms in one of these haloes from baryons that approximately conserved angular momentum during their dissipation into the rotationally supported disk, then it is possible to estimate the disk's central surface density. For an appropriate choice of mass-to-light ratio, we can place constraints on the central surface brightness of the disk and hence identify the analogues of the host haloes of LSB and HSB galaxies. We find that our LSB galaxy analogues occupy haloes that have lower concentrations than might be expected based on consideration of the mass-concentration relation for the LCDM cosmology. The distribution of concentrations peaks at c~6, in good agreement with observational data, although there are important differences between the shapes of the simulated and observationally inferred distributions. This suggests that LSB galaxies inhabit a biased subsample of the halo population, and that this bias may be an important ingredient for resolving the current debate regarding the structure of LSB galaxy dark matter haloes.

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arXiv2004

Disk Galaxy Rotation Curves in Triaxial CDM Halos

We use N-body hydrodynamical simulations to study the structure of disks in triaxial potentials resembling CDM halos. Our analysis focuses on the accuracy of the dark mass distribution inferred from rotation curves derived from simulated long-slit spectra. We consider a massless disk embedded in a halo with axis ratios of 0.5:0.6:1.0 and with its rotation axis aligned with the minor axis of the halo. Closed orbits for the gaseous particles deviate from coplanar circular symmetry, resulting in a variety of long-slit rotation curve shapes, depending on the orientation of the disk relative to the line of sight. Rotation curves may thus differ significantly from the spherically-averaged circular velocity profile of the dark matter halo. "Solid-body" rotation curves--typically interpreted as a signature of a constant density core in the dark matter distribution--are obtained about 25% of the time for random orientations although the dark matter follows the cuspy density profile proposed by Navarro, Frenk & White (NFW). We conclude that the discrepancies reported between the shape of the rotation curve of low surface brightness galaxies and the structure of CDM halos may be resolved once the complex effects of halo triaxiality on the dynamics of the gas component is properly taken into account.

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