arXiv subjects

James E. Taylor

Publications and source records attributed to James E. Taylor.

At least 73 records · Page 4Linked to original sources

arXiv2005

Massive black hole remnants of the first stars II: optical and X-ray signatures in present-day galactic haloes

The first stars forming in minihaloes at redshifts greater than 20 may have been very massive and could have left behind massive black hole (MBH) remnants. In a previous paper we investigated the hierarchical merging of these MBHs and their associated haloes, using a semi-analytical approach consisting of a hierarchical merger tree algorithm and explicit prescriptions for the dynamics of merged substructure inside a larger host halo following a merger. One of the results was the prediction of a number of MBHs orbiting throughout present-day galactic haloes. In addition we determined the mass accretion rate of these MBHs assuming that they could in principle remain embedded in a gaseous core that is a remnant of the haloes with which the MBHs were associated before merging with the host system. On this basis we determine in this paper the bolometric, optical and X-ray luminosity functions for the accreting MBHs using thin disk and advection dominated accretion flow models. Our predicted MBH X-ray emissions are then compared with observations of ultra-luminous X-ray sources in galaxies. We find that the slope and normalisation of the predicted X-ray luminosity function are consistent with the observations.

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arXiv2005

Massive black hole remnants of the first stars III: observational signatures from the past

The first stars forming in minihaloes at redshifts greater than 20 may have been very massive and could have left behind massive black hole (MBH) remnants. In previous papers we investigated the hierarchical merging of these `seed' MBHs and their associated haloes, using a semi-analytical approach consisting of a hierarchical merger tree algorithm and explicit prescriptions for the dynamics of merged substructure inside a larger host halo following a merger. We also estimated accretion luminosities for these MBHs and found them to be consistent with observations of ultra-luminous X-ray point sources. Here we compute the strength of gravitational wave events as MBHs merge to grow from seed mass MBH at high redshifts to those that we predict to reside in galaxy haloes today. If MBHs merge efficiently we predict that some 10^4 - 10^5 events per year could be observed, that fall just within the sensitivity limits of the proposed LISA gravitational wave observatory. The collapse of the first massive stars to form MBHs may be accompanied by gamma ray bursts (GRBs). If this is the case and GRBs are in principle observable out to the redshifts of first star formation, we find that about 10^5 - 10^6 GRBs per year could be detected. As merging MBH binaries reach their last stable orbits before final coalescence a fraction of the gravitational wave energy may be released as a pulse of gamma rays (for instance, through interaction with material enveloping a merging MBH binary). This fraction has to be in the range between 10^{-14} - 10^{-10} for MBH mergers to be less luminous than GBRs, yet detectable out to cosmological distances with upcoming GRB detector missions.

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arXiv2005

Earth-mass dark halos are torn into dark mini-streams by stars

The promising neutralino dark matter particles generically condense into numerous earth-mass dark halos with smaller substructures suppressed by free streaming\cite{berezinsky}. The recent {\it Letter to Nature}\cite{moore} claims that these 0.01pc-sized dense halos emerged at redshifts 60--26 are rarely destructed inside galaxies, hence the nearest halos at about 0.1pc from Earth are bright in annihilation-powered gamma-rays, but are inaccessible to direct detections. However, most mini-halos reaching solar neighbourhood should experience strong impulses by individual stars in the Galactic disk and bulge, and have been torn into pc-long tidal streams over a Hubble time with only modest overdensity, reducing indirect annihilation signals. Sweeping across the solar system per century, mini-streams leave directional and temporal signatures for direct searches of neutralinos.

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arXiv2004

Substructure and Tidal Debris in Local Galaxies: Models and Observations

One of the generic predictions of modern cosmological models is that large galaxies should have experienced many mergers with smaller galaxies at some point in their past. Debris from such encounters will leave spatially distinct substructure in the stellar haloes of nearby galaxies, detectable for a few orbital periods after the final merger. In the case of the Milky Way, kinematic data from surveys such as RAVE and satellite missions such as GAIA will allow us to probe much more of the merger history, and to connect the properties of the stellar halo with those of local dwarf galaxies. To estimate what these programmes may discover, we review current observations of minor mergers in nearby galaxies, and compare these with predictions from a semi-analytic model of galaxy formation.

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arXiv2004

The Evolution of Substructure in Galaxy, Group and Cluster Haloes III: Comparison with Simulations

In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of halo formation based on merger trees. In this paper, we compare the predictions of our model with results from self-consistent numerical simulations. We find that in general the two methods agree extremely well, particularly once numerical effects and selection effects in the choice of haloes are taken into account. As expected from the original analyses of the simulations, we see some evidence for artificial overmerging in the innermost regions of the simulated haloes, either because substructure is being disrupted artificially or because the group-finding algorithms used to identify substructure are not detecting all the bound clumps in the highest-density regions. Our analytic results suggest that greater mass and force resolution may be required before numerical overmerging becomes negligible in all current applications. We discuss the implications of this result for observational and experimental tests of halo substructure, such as the analysis of discrepant magnification ratios in strongly lensed systems, terrestrial experiments to detect dark matter particles directly, or indirect detection experiments searching for positrons, gamma-rays, neutrinos or other dark matter decay products.

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arXiv2004

The Evolution of Substructure in Galaxy, Group and Cluster Haloes II: Global Properties

In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of dark matter halo formation based on merger trees. In this paper, we present the basic predictions of our full model of halo formation. We first describe the overall properties of substructure in galaxy, group or cluster haloes at the present day. We then discuss the evolution of substructure, and the effect of the mass accretion history of an individual halo on the mass function and orbital grouping of its subhalo population. We show, in particular, that the shape of the subhalo mass function is strongly correlated with the formation epoch of the halo. In a third paper in this series we will compare the results of our semi-analytic method with the results of self-consistent numerical simulations of halo formation.

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arXiv2004

Predicting Substructure in CDM Haloes

Observations of multiple-image gravitational lens systems suggest that the projected mass distributions of galaxy haloes may contain substantial inhomogeneities. The fraction of the halo mass in dense substructure is still highly uncertain, but could be as large as a few percent. While halo substructure is seen in numerical simulations of CDM haloes, little of this substructure survives in the innermost regions of haloes, and thus the observational claims for substructure at small projected radii are slightly surprising. There is evidence, however, that even the highest-resolution simulations published to-date are still limited by numerical effects that heat and disrupt substructure artificially in high-density regions. By comparing numerical and semi-analytic models of halo substructure, we show that current simulations probably underestimate the mass fraction in substructure at small projected radii, by a factor of at least 2-3. We discuss the prospects for using lensing observations as a fundamental test of the nature of dark matter.

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arXiv2004

Can Supersymmetry Naturally Explain the Positron Excess?

It has often been suggested that the cosmic positron excess observed by the HEAT experiment could be the consequence of supersymmetric dark matter annihilating in the galactic halo. Although it is well known that evenly distributed dark matter cannot account for the observed excess, if substantial amounts of local dark matter substructure are present, the positron flux would be enhanced, perhaps to the observed magnitude. In this paper, we attempt to identify the nature of the substructure required to match the HEAT data, including the location, size and density of any local dark matter clump(s). Additionally, we attempt to assess the probability of such substructure being present. We find that if the current density of neutralino dark matter is the result of thermal production, very unlikely ($\sim 10^{-4}$ or less) conditions must be present in local substructure to account for the observed excess.

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arXiv2003

Small-scale Substructure in Dark Matter Haloes: Where Does Galaxy Formation Come to an End?

Models of structure formation based on cold dark matter predict that most of the small dark matter haloes that first formed at high redshift would have merged into larger systems by the present epoch. Substructure in present-day haloes preserves the remains of these ancient systems, providing the only direct information we may ever have about the low-mass end of the power spectrum. I describe some recent attempts to model halo substructure down to very small masses, using a semi-analytic model of halo formation. I make a preliminary comparison between the model predictions, observations of substructure in lensed systems, and the properties of local satellite galaxies.

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arXiv2003

The Satellite-substructure Connection

We describe our recent attempts to model substructure in dark matter halos down to very small masses, using a semi-analytic model of halo formation. The results suggest that numerical simulations of halo formation may still be missing substructure in the central regions of halos due to purely numerical effects. If confirmed, this central 'overmerging' problem will have important consequences for the interpretation of lensing measurements of substructure. We also show that the spatial distribution of subhalos relative to the satellite companions of the Milky Way rules out at least one simple model of how dwarf galaxies form in low-mass halos.

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arXiv2003

Massive black hole remnants of the first stars I: abundance in present-day galactic haloes

We investigate the possibility that present-day galaxies and their dark matter haloes contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars in the Universe. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. The relation between SMBH and galactic bulge mass in our model displays the same slope as that found in observations. Many MBHs will not reach the centre of the host halo, however, but continue to orbit within it. In doing so MBHs may remain associated with remnants of the satellite halo systems of which they were previously a part. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with galactic disks, we follow the hierarchical merging of MBH systems and their subsequent dynamical evolution inside the respective host haloes. We predict the mass and abundance of MBHs in present-day galactic haloes, and also estimate the MBH mass accretion rates as well as bolometric luminosities for two different accretion scenarios. MBHs that have not undergone recent merging will retain associated dark matter cusps that were enhanced by black hole accretion growth, and may be possible sources of gamma rays via neutralino annihilations.

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arXiv2003

Observing the Sunyaev-Zel'dovich Effect Closer to Home

Hot gas trapped in a dark matter halo will produce a decrement in the surface brightness of the microwave background, the Sunyaev-Zel'dovich (SZ) effect. While massive clusters produce the strongest central SZ decrements, we point out that a local galaxy halo, specifically the halo of M31, may be one of the brightest integrated SZ sources in the sky. For various realistic gas distributions consistent with current X-ray limits, we show that the integrated SZ decrement from M31 will be comparable to decrements already detected in more distant sources, provided its halo contains an appreciable quantity of hot gas. A measurement of this decrement would provide direct information on the mass, spatial distribution and thermodynamic state of hot gas in a low-mass halo, and could place important constraints on current models of galaxy formation. Detecting such an extended (~ 10 degree), low-amplitude signal will be challenging, but should be possible with all-sky SZ maps from satellite missions such as the Wilkinson Microwave Anisotropy Probe or the Planck Surveyor.

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arXiv2003

The Evolution of Substructure in Galaxy, Group and Cluster Haloes I: Basic Dynamics

The hierarchical mergers that form the haloes of dark matter surrounding galaxies, groups and clusters are not entirely efficient, leaving substantial amounts of dense substructure, in the form of stripped halo cores or `subhaloes', orbiting within these systems. Using a semi-analytic model of satellite dynamics, we study the evolution of haloes as they merge hierarchically, to determine how much substructure survives merging and how the properties of individual subhaloes change over time. We find that subhaloes evolve, due to mass loss, orbital decay, and tidal disruption, on a characteristic time-scale equal to the period of radial oscillations at the virial radius of the system. Subhaloes with realistic densities and density profiles lose 25-45 per cent of their mass per pericentric passage, depending on their concentration and on the circularity of their orbit. As the halo grows, the subhalo orbits also grow in size and become less bound. Based on these general patterns, we suggest a method for including realistic amounts of substructure in semi-analytic models based on merger trees. We show that the parameters in the resulting model can be fixed by requiring self-consistency between different levels of the merger hierarchy. In a companion paper, we will compare the results of our model with numerical simulations of halo formation.

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arXiv2002

The Structural Evolution of Substructure

We investigate the evolution of substructure in cold dark matter halos using N-body simulations of tidal stripping of substructure halos (subhalos) within a static host potential. We find that halos modeled following the Navarro, Frenk & White (NFW) mass profile lose mass continuously due to tides from the massive host, leading to the total disruption of satellite halos with small tidal radii. The structure of stripped NFW halos depends mainly on the fraction of mass lost, and can be expressed in terms of a simple correction to the original NFW profile. We apply these results to substructure in the Milky Way, and conclude that the dark matter halos surrounding its dwarf spheroidal (dSph) satellites have circular velocity curves that peak well beyond the luminous radius at velocities significantly higher than expected from the stellar velocity dispersion. Our modeling suggests that the true tidal radii of dSphs lie well beyond the putative tidal cutoff observed in the surface brightness profile, suggesting that the latter are not really tidal in origin but rather features in the light profile of limited dynamical relevance. For Draco, in particular, our modeling implies that its tidal radius is much larger than derived by Irwin & Hatzidimitriou (1995), lending support to the interpretation of recent Sloan survey data by Odenkirchen et al. (2001). Similarly, our model suggests that Carina's halo has a peak circular velocity of ~55 km/s, which may help explain how this small galaxy has managed to retain enough gas to undergo several bursts of star formation. Our results imply a close correspondence between the most massive subhalos expected in a CDM universe and the known satellites of the Milky Way, and suggest that only subhalos with peak circular velocities below 35 km/s lack readily detectable luminous counterparts.

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arXiv2002

Massive black hole remnants of the first stars in galactic haloes

We investigate the possibility that present-day galactic haloes contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. Many MBHs will not reach the centre of the main halo, however, but continue to orbit within satellite subhaloes. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with the galactic disk, we follow the dynamics of the satellites and their MBHs and determine the abundance and distribution of MBHs in present-day haloes of various masses. Considering two different accretion scenarios we also compute the bolometric luminosity function for the MBHs.

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arXiv2002

Modelling the Formation of Individual Galaxies: A Morphology Problem for CDM?

We use a semi-analytic model of halo formation to study the dynamical history of giant field galaxies like the Milky Way. We find that in a concordance LCDM cosmology, most isolated disk galaxies have remained undisturbed for 8-10 Gyr, such that the age of the Milky Way's thin disk is unremarkable. Many systems also have older disk components which have been thickened by minor mergers, consistent with recent observations of nearby field galaxies. We do have a considerable problem, however, reproducing the morphological mix of nearby galaxies. In our fiducial model, most systems have disk-to-bulge mass ratios of order 1, and look like S0s rather than spirals. This result depends mainly on merger statistics, and is unchanged for most reasonable choices of our model parameters. We discuss two possible solutions to this morphology problem in LCDM.

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arXiv2002

The Clumpiness of Cold Dark Matter: Implications for the Annihilation Signal

We examine the expected signal from annihilation events in realistic cold dark matter halos. If the WIMP is a neutralino, with an annihilation cross-section predicted in minimal SUSY models for the lightest stable relic particle, the central cusps and dense substructure seen in simulated halos may produce a substantial flux of energetic gamma rays. We derive expressions for the relative flux from such events in simple halos with various density profiles, and use these to calculate the relative flux produced within a large volume as a function of redshift. This flux peaks when the first halos collapse, but then declines as small halos merge into larger systems of lower density. Simulations show that halos contain a substantial amount of dense substructure, left over from the incomplete disruption of smaller halos as they merge together. We calculate the contribution to the flux due to this substructure, and show that it can increase the annihilation signal substantially. Overall, the present-day flux from annihilation events may be an order of magnitude larger than predicted by previous calculations. We discuss the implications of these results for current and future gamma-ray experiments.

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arXiv2002

A Semi-analytic Model of Halo Dynamics

We have developed a new semi-analytic model for the formation and evolution of structure on galaxy, group and cluster scales. The model combines merger trees with a detailed, spatially resolved description of the dynamical evolution of halo substructure. It reproduces the results of numerical simulations of mergers and halo formation at a fraction of the computational expense. We use this model to study the formation of the dwarf galaxies in the Local Group and the structural components of the Milky Way. We find that reionization can explain the scarcity of Local Group dwarfs, although the reionization epoch is constrained to be high -- z_ri >= 12. We also find that disk disruption at recent times is rare, such that many galactic disks have an old, thin component.

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