arXiv subjects

Chris Power

Publications and source records attributed to Chris Power.

At least 127 records · Page 7Linked to original sources

arXiv2014

Heating and Ionization of the Primordial Intergalactic Medium by High Mass X-ray Binaries

We investigate the influence of High Mass X-ray Binaries on their high redshift environments. Using a one-dimensional radiative transfer code, we predict the ionization and temperature profiles surrounding a coeval stellar population, composed of main sequence stars and HMXBs, at various times after its formation. We consider both uniform density surroundings, and a cluster embedded in a 10^8 solar mass NFW halo. HMXBs in a constant density environment produce negligible enhanced ionization because of their high-energy SEDs and short lifetimes. In this case, HMXBs only marginally contribute to the local heating rate. For NFW profiles, radiation from main sequence stars cannot prevent the initially ionized volume from recombining since it is unable to penetrate the high density galactic core. However, HMXB photons stall recombinations behind the front, keeping it partially ionized for longer. The increased electron density in these partially ionized regions promotes further cooling, resulting in lower IGM temperatures. In the context of this starburst model, we have shown that HMXBs do not make a major contribution to reionization or IGM heating. However, X-ray escape fractions are high in both density profile cases. Continuous star formation may result in the build up of X-rays over time, reducing the ionization timescale and potentially leading to low level ionization of the distant IGM.

astro-ph.GA↗

arXiv2014

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

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

astro-ph.GA↗

arXiv2014

Warm Dark Haloes Accretion Histories and their Gravitational Signatures

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

astro-ph.CO↗

arXiv2014

Gravitational lensing in WDM cosmologies: The cross section for giant arcs

The nature of the dark sector of the Universe remains one of the outstanding problems in modern cosmology, with the search for new observational probes guiding the development of the next generation of observational facilities. Clues come from tension between the predictions from ΛCDM and observations of gravitationally lensed galaxies. Previous studies showed that galaxy clusters in the ΛCDM are not strong enough to reproduce the observed number of lensed arcs. This work aims to constrain the warm dark matter cosmologies by means of the lensing efficiency of galaxy clusters drawn from these alternative models. The lensing characteristics of two samples of simulated clusters in the warm dark matter (ΛWDM) and cold dark matter (ΛCDM) cosmologies have been studied. The results show that even though the CDM clusters are more centrally concentrated and contain more substructures, the WDM clusters have slightly higher lensing efficiency than their CDM counterparts. The key difference is that WDM clusters have more extended and more massive subhaloes than CDM analogues. These massive substructures significantly stretch the critical lines and caustics and hence they boost the lensing efficiency of the host halo. Despite the increase in the lensing efficiency due to the contribution of massive substructures in the WDM clusters, this is not enough to resolve the arc statistics problem.

astro-ph.CO↗

arXiv2013

Quenching star formation in cluster galaxies

In order to understand the processes that quench star formation within rich clusters, we construct a library of subhalo orbits drawn from $Λ$CDM cosmological N-body simulations of four rich clusters. The orbits are combined with models of star formation followed by quenching in the cluster environment. These are compared with observed bulge and disc colours and stellar absorption linestrength indices of satellite galaxies. Models in which the bulge stellar populations depend only on the galaxy subhalo mass while the disc quenching depends on the cluster environment are acceptable fits to the data. An exponential disc quenching timescale of 3 - 3.5 Gyr is preferred. Models with short ($\lesssim 1$ Gyr) quenching timescales yield cluster-centric gradients in disc colours and Balmer line indices that are too steep compared to observations. We also examine models in which there is quenching in lower mass groups prior to cluster infall ("pre-processing"), finding that such models are a better fit to the data than models without pre-processing and require similar quenching times. The data slightly prefer models where quenching occurs only for galaxies falling within about 0.5 $r_{200}$. Finally, we have examined models with short quenching timescales of 1 Gyr, but a long delay time of 3 Gyr prior to quenching. All models with short quenching timescales, even such "delayed-then-rapid" quenching models, produce excessively red galaxies near the cluster core and are strongly disfavoured by the data. These results imply that the environments of rich clusters must impact star formation rates of infalling galaxies on relatively long timescales -- several times longer than a typical halo spends within the virial radius of a cluster. This scenario favours gentler quenching mechanisms such as slow "strangulation" over more rapid ram-pressure stripping.

astro-ph.CO↗

arXiv2013

Seeking Observable Imprints of Small-Scale Structure on the Properties of Dark Matter Haloes

The characteristic prediction of the Cold Dark Matter (CDM) model of cosmological structure formation is that the Universe should contain a wealth of small-scale structure -- low-mass dark matter haloes and subhaloes. However, galaxy formation is inefficient in their shallow potential wells and so we expect these low-mass haloes and subhaloes to be dark. Can we tell the difference between a Universe in which low-mass haloes are present but dark and one in which they never formed, thereby providing a robust test of the CDM model? We address this question using cosmological simulations to examine how properties of low-mass haloes that are potentially accessible to observation, such as their spatial clustering, rate of accretions and mergers onto massive galaxies and the angular momentum content of massive galaxies, differ between a LCDM model and dark matter models in which low-mass halo formation is suppressed. Adopting an effective cut-off mass scale M_cut below which small-scale power is suppressed in the initial conditions, we study dark matter models in which M_cut varies between 5e9 Msol/h and 1e11 Msol/h, equivalent to the host haloes of dwarf and low mass galaxies. Our results show that both the clustering strength of low-mass haloes around galaxy-mass primaries and the rate at which they merge with these primaries is sensitive to the assumed value of M_cut; in contrast, suppressing low-mass halo formation has little influence on the angular momentum content of galaxy-mass haloes -- it is the quiescence or violence of a halo's assembly history that has a more marked effect. However, we expect that measuring the effect on spatial clustering or the merger rate is likely to be observationally difficult for realistic values of M_cut, and so isolating the effect of this small-scale structure would appear to be remarkably difficult to detect, at least in the present day Universe.

astro-ph.CO↗

arXiv2013

Confronting Cold Dark Matter Predictions with Observed Galaxy Rotations

The rich statistics of galaxy rotations as captured by the velocity function (VF) provides invaluable constraints on galactic baryon physics and the nature of dark matter (DM). However, the comparison of observed galaxy rotations against cosmological models is prone to subtle caveats that can easily lead to misinterpretations. Our analysis reveals full statistical consistency between ~5000 galaxy rotations, observed in line-of-sight projection, and predictions based on the standard cosmological model (LCDM) at the mass-resolution of the Millennium simulation (HI line-based circular velocities above ~50 km/s). Explicitly, the HI linewidths in the HI Parkes All Sky Survey (HIPASS) are found consistent with those in S3-SAX, a post-processed semi-analytic model for the Millennium simulation. Previously found anomalies in the VF can be plausibly attributed to (1) the mass-limit of the Millennium simulation, (2) confused sources in HIPASS, (3) inaccurate inclination measurements for optically faint sources, and (4) the non-detectability of gas-poor early-type galaxies. These issues can be bypassed by comparing observations and models using linewidth source counts rather than VFs. We investigate if and how well such source counts can constrain the temperature of DM.

astro-ph.CO↗

arXiv2013

HMFcalc: An Online Tool for Calculating Dark Matter Halo Mass Functions

The dark matter halo mass function (HMF) is a characteristic property of cosmological structure formation models, quantifying the number density of dark matter haloes per unit mass in the Universe. A key goal of current and planned large galaxy surveys is to measure the HMF and to use it to test theories of dark matter and dark energy. We present a new web application for calculating the HMF -- the frontend HMFcalc and the engine hmf. HMFcalc has been designed to be flexible, efficient and easy to use, providing observational and theoretical astronomers alike with the means to explore standard functional forms of the HMF or to tailor their own. We outline the theoretical background needed to compute the HMF, we show how it has been implemented in hmf, and finally we provide worked examples that illustrate HMFcalc's versatility as an analysis tool.

astro-ph.CO↗

arXiv2013

How well do we know the Halo Mass Function?

The parameters governing the standard ΛCold Dark Matter cosmological model have been constrained with unprecedented accuracy by precise measurements of the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites. Each new data release has refined further our knowledge of quantities -- such as the matter density parameter Ω_M -- that are imprinted on the dark matter halo mass function (HMF), a powerful probe of dark matter and dark energy models. In this letter we trace how changes in the cosmological parameters over the last decade have influenced uncertainty in our knowledge of the HMF. We show that this uncertainty has reduced significantly since the 3rd WMAP data release, but the rate of this reduction is slowing. This is limited by uncertainty in the normalisation σ_8, whose influence is most pronounced at the high mass end of the mass function. Interestingly, we find that the accuracy with which we can constrain the HMF in terms of the cosmological parameters has now reached the point at which it is comparable to the scatter in HMF fitting functions. This suggests that the power of the HMF as a precision probe of dark matter and dark energy hinges on more accurate determination of the theoretical HMF. Finally, we assess prospects of using the HMF to differentiate between Cold and Warm Dark Matter models based on ongoing improvements in measurements of Ω_M, and we comment briefly on optimal survey strategies for constraining dark matter and dark energy models using the HMF.

astro-ph.CO↗

arXiv2013

Modelling the Growth of Supermassive Black Holes in Cosmological Simulations

There is strong evidence that supermassive black holes reside in all galaxies that contain a stellar spheroid and their mass is tightly correlated with properties such as stellar bulge mass and velocity dispersion. There are also strong theoretical arguments that feedback from supermassive black holes plays an important role in shaping the high mass end of the galaxy mass function, hence to accurately model galaxies we also need to model the black holes. We present a comparison of two black hole growth models implemented within a large-scale, cosmological SPH simulation including star formation and feedback. One model is a modified Bondi-Hoyle prescription that grows black holes based on the smooth density of local gas, while the other is the recently proposed Accretion Disc Particle (ADP) method. This model swallows baryonic particles that pass within an accretion radius of the black hole and adds them to a subgrid accretion disc. Black holes are then grown by material from this disc. We find that both models can reproduce local scaling relations, although the ADP model is offset from the observed relations at high black hole masses. The total black hole mass density agrees between models to within a factor of three, but both struggle to reproduce the black hole mass function. The simulated mass functions are too steep and underestimate the number of intermediate and high mass black holes. In addition, the ADP model swallows excessive amounts of material at the resolution of large-scale, cosmological simulations producing unrealistically large accretion discs. Future work needs to be performed to improve the black hole mass function within simulations. This should be done through the mass growth and feedback as they are strongly coupled and should not be treated as separate entities.

astro-ph.CO↗

arXiv2012

A Robust Measure of Cosmic Structure beyond the Power-Spectrum: Cosmic Filaments and the Temperature of Dark Matter

We discover that the mass of dark matter particles mDM is imprinted in phase-correlations of the cosmic density field more significantly than in the 2-point correlation. In particular, phase-correlations trace mDM out to scales about five times larger than the 2-point correlation. This result relies on a new estimator l(r) of pure phase-information in Fourier space, which can be interpreted as a parameter-free and scale-invariant tracer of filament-like structure. Based on simulated density fields we show how mDM can, in principle, be measured using l(r), given a suitably reconstructed density field.

astro-ph.CO↗

arXiv2012

Feedback from High-Mass X-Ray Binaries on the High Redshift Intergalactic Medium : Model Spectra

Massive stars at redshifts z > 6 are predicted to have played a pivotal role in cosmological reionization as luminous sources of ultra-violet (UV) photons. However, the remnants of these massive stars could be equally important as X-ray luminous (L_X 1e38 erg/s) high-mass X-ray binaries (HMXBs). Because the absorption cross section of neutral hydrogen decreases sharply with photon energy (proportional to the inverse cube), X-rays can escape more freely than UV photons from the star-forming regions in which they are produced, allowing HMXBs to make a potentially significant contribution to the ionizing X-ray background during reionization. In this paper, we explore the ionizing power of HMXBs at redshifts z > 6 using a Monte Carlo model for a coeval stellar population of main sequence stars and HMXBs. Using the archetypal Galactic HMXB Cygnus X-1 as our template, we propose a composite HMXB spectral energy distribution consisting of black-body and power-law components, whose contributions depend on the accretion state of the system. We determine the time-dependent ionizing power of a combined population of UV-luminous stars and X-ray luminous HMXBs, and deduce fitting formulae for the boost in the population's ionizing power arising from HMXBs; these fits allow for simple implementation of HMXB feedback in numerical simulations. Based on this analysis, we estimate the contribution of high redshift HMXBs to the present-day soft X-ray background, and we show that it is a factor of ~100-1000 smaller than the observed limit. Finally, we discuss the implications of our results for the role of HMXBs in reionization and in high redshift galaxy formation.

astro-ph.CO↗

arXiv2012

Thermal instabilities in cooling galactic coronae: fuelling star formation in galactic discs

We investigate the means by which cold gas can accrete onto Milky Way mass galaxies from a hot corona of gas, using a new smoothed particle hydrodynamics code, 'SPHS'. We find that the 'cold clumps' seen in many classic SPH simulations in the literature are not present in our SPHS simulations. Instead, cold gas condenses from the halo along filaments that form at the intersection of supernovae-driven bubbles from previous phases of star formation. This positive feedback feeds cold gas to the galactic disc directly, fuelling further star formation. The resulting galaxies in the SPH and SPHS simulations differ greatly in their morphology, gas phase diagrams, and stellar content. We show that the classic SPH cold clumps owe to a numerical thermal instability caused by an inability for cold gas to mix in the hot halo. The improved treatment of mixing in SPHS suppresses this instability leading to a dramatically different physical outcome. In our highest resolution SPHS simulation, we find that the cold filaments break up into bound clumps that form stars. The filaments are overdense by a factor of 10-100 compared to the surrounding gas, suggesting that the fragmentation results from a physical non-linear instability driven by the overdensity. This 'fragmenting filament' mode of disc growth has important implications for galaxy formation, in particular the role of star formation in bringing cold gas into disc galaxies.

astro-ph.GA↗

arXiv2012

The observed $M - σ$ relations imply that SMBHs grow by cold chaotic accretion

We argue that current observations of $M - σ$ relations for galaxies can be used to constrain theories of super-massive black holes (SMBH) feeding. In particular, assuming that SMBH mass is limited only by the feedback on the gas that feeds it, we show that SMBHs fed via a planar galaxy scale gas flow, such as a disc or a bar, should be much more massive than their counterparts fed by quasi-spherical inflows. This follows from the relative inefficiency of AGN feedback on a flattened inflow. We find that even under the most optimistic conditions for SMBH feedback on flattened inflows, the mass at which the SMBH expels the gas disc and terminates its own growth is a factor of several higher than the one established for quasi-spherical inflows. Any beaming of feedback away from the disc and any disc self-shadowing strengthens this result further. Contrary to this theoretical expectation, recent observations have shown that SMBH in pseudobulge galaxies (which are associated with barred galaxies) are typically under- rather than over-massive when compared with their classical bulge counterparts at a fixed value of $σ$. We conclude from this that SMBHs are not fed by large (100 pc to many kpc) scale gas discs or bars, most likely because such planar flows are turned into stars too efficiently to allow any SMBH growth. Based on this and other related observational evidence, we argue that most SMBHs grow by chaotic accretion of gas clouds with a small and nearly randomly distributed direction of angular momentum.

astro-ph.CO↗

arXiv2012

Modelling supermassive black hole growth: towards an improved sub-grid prescription

Accretion onto supermassive black holes (SMBHs) in galaxy formation simulations is frequently modelled by the Bondi-Hoyle formalism. Here we examine the validity of this approach analytically and numerically. We argue that the character of the flow where one evaluates the gas properties is unlikely to satisfy the simple Bondi-Hoyle model. Only in the specific case of hot virialised gas with zero angular momentum and negligible radiative cooling is the Bondi-Hoyle solution relevant. In the opposite extreme, where the gas is in a state of free-fall at the evaluation radius due to efficient cooling and the dominant gravity of the surrounding halo, the Bondi-Hoyle formalism can be erroneous by orders of magnitude in either direction. This may impose artificial trends with halo mass in cosmological simulations by being wrong by different factors for different halo masses. We propose an expression for the sub-grid accretion rate which interpolates between the free-fall regime and the Bondi-Hoyle regime, therefore taking account of the contribution of the halo to the gas dynamics.

astro-ph.IM↗

arXiv2011

The Dynamical State of Dark Matter Haloes in Cosmological Simulations I: Correlations with Mass Assembly History

Using a statistical sample of dark matter haloes drawn from a suite of cosmological N-body simulations of the Cold Dark Matter (CDM) model, we quantify the impact of a simulated halo's mass accretion and merging history on two commonly used measures of its dynamical state, the virial ratio eta and the centre of mass offset Delta r. Quantifying this relationship is important because the degree to which a halo is dynamically equilibrated will influence the reliability with which we can measure characteristic equilibrium properties of the structure and kinematics of a population of haloes. We begin by verifying that a halo's formation redshift zform correlates with its virial mass Mvir and we show that the fraction of its recently accreted mass and the likelihood of it having experienced a recent major merger increases with increasing Mvir and decreasing zform. We then show that both eta and Delta r increase with increasing Mvir and decreasing zform, which implies that massive recently formed haloes are more likely to be dynamically unrelaxed than their less massive and older counterparts. Our analysis shows that both eta and Delta r are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Delta r provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at z~0. We find that Delta r < 0.04 is sufficient to pick out dynamically relaxed haloes at z=0. Finally, we assess our results in the context of previous studies, and consider their observational implications.

astro-ph.CO↗

arXiv2011

Cosmic evolution of the atomic and molecular gas content of galaxies

We study the evolution of the cold gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model GALFORM in the LCDM framework. We calculate the molecular-to-atomic hydrogen mass ratio, H2/HI, in each galaxy using two different approaches; the pressure-based empirical relation of Blitz & Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply them to consistently calculate the star formation rates of galaxies. We find that the model based on the Blitz & Rosolowsky law predicts an HI mass function, CO(1-0) luminosity function, correlations between the H2/HI ratio and stellar and cold gas mass, and infrared-CO luminosity relation in good agreement with local and high redshift observations. The HI mass function evolves weakly with redshift, with the number density of high mass galaxies decreasing with increasing redshift. In the case of the H2 mass function, the number density of massive galaxies increases strongly from z=0 to z=2, followed by weak evolution up to z=4. We also find that the H2/HI ratio of galaxies is strongly dependent on stellar and cold gas mass, and also on redshift. The slopes of the correlations between H2/HI and stellar and cold gas mass hardly evolve, but the normalisation increases by up to two orders of magnitude from z=0-8. The strong evolution in the H2 mass function and the H2/HI ratio is primarily due to the evolution in the sizes of galaxies and secondarily, in the gas fractions. The predicted cosmic density evolution of HI agrees with the observed evolution inferred from DLAs, and is dominated by low/intermediate mass halos. We find that previous theoretical studies have largely overestimated the redshift evolution of the global H2/HI ratio due to limited resolution. We predict a maximum of rho_H2/rho_HI~1.2 at z~3.5.

astro-ph.CO↗

arXiv2011

Feeding SMBHs through supersonic turbulence and ballistic accretion

It has long been recognised that the main obstacle to accretion of gas onto supermassive black holes (SMBHs) is large specific angular momentum. It is feared that the gas settles in a large scale disc, and that accretion would then proceed too inefficiently to explain the masses of the observed SMBHs. Here we point out that, while the mean angular momentum in the bulge is very likely to be large, the deviations from the mean can also be significant. Indeed, cosmological simulations show that velocity and angular momentum fields of gas flows onto galaxies are very complex. Furthermore, inside bulges the gas velocity distribution can be further randomised by the velocity kicks due to feedback from star formation. We perform hydrodynamical simulations of gaseous rotating shells infalling onto an SMBH, attempting to quantify the importance of velocity dispersion in the gas at relatively large distances from the black hole. We implement this dispersion by means of a supersonic turbulent velocity spectrum. We find that, while in the purely rotating case the circularisation process leads to efficient mixing of gas with different angular momentum, resulting in a low accretion rate, the inclusion of turbulence increases this accretion rate by up to several orders of magnitude. We show that this can be understood based on the notion of "ballistic" accretion, whereby dense filaments, created by convergent turbulent flows, travel through the ambient gas largely unaffected by hydrodynamical drag. We derive a simple analytical formula that captures the numerical results to within a factor of a few. Rescaling our results to astrophysical bulges, we argue that this "ballistic" mode of accretion could provide the SMBHs with a sufficient supply of fuel without the need to channel the gas via large-scale discs or bars, and therefore that star formation in bulges can be a strong catalyst for SMBH accretion.

astro-ph.HE↗