arXiv subjects

Joseph Silk

Publications and source records attributed to Joseph Silk.

At least 361 records · Page 20Linked to original sources

arXiv2005

Observational Evidence for Extra Dimensions from Dark Matter

Recent astronomical observations of systems of dark matter, which have been cited as providing possible support for self-interacting cold dark matter, may provide evidence for the extra dimensions predicted by superstring scenarios. We find that the properties of the required dark matter self-interaction are precisely the consequences of a world with 3 large extra dimensions of size \~1nm, where gravity follows the r^{-5} law at scales below ~1nm. From the cross sections measured for various dark matter systems, we also constrain the mass of dark matter particles to be m_x ~ 3*10^{-16} proton mass, consistent with the mass of axions.

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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

LOFAR as a Probe of the Sources of Cosmological Reionisation

We propose use of the thickness of the ionisation front as a discriminant between alternative modes of reionisation in the early universe, by stars or by miniquasars. Assuming a photoionisation-recombination balance, we find that for miniquasar sources the transition from neutral to ionised intergalactic medium is extended and has two features. The first is a sudden steep increase in the neutral fraction with a typical width of 5-10 comoving megaparsecs, depending on the miniquasar power. The second feature is a long wing that represents a much slower transition from neutral fraction of $\approx 0.8$ to 1. The angular resolution of LOFAR is expected to resolve these scales and will, therefore, play an important role in discriminating the hard sources of ionising photons from the stellar ones.

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arXiv2005

Probing the Origins of Voids in the Distribution of Galaxies

If the voids that we see today in the distribution of galaxies existed at recombination, they will leave an imprint on the cosmic microwave background (CMB). On the other hand, if these voids formed much later, their effect on the CMB will be negligible and will not be observed with the current generation of experiments. In this paper presented at the 2004 Annual Scientific Meeting of the Astronomical Society of Australia, we discuss our ongoing investigations into voids of primordial origin. We show that if voids in the cold dark matter distribution existed at the epoch of decoupling, they could contribute significantly to the apparent rise in CMB power on small scales detected by the Cosmic Background Imager (CBI) Deep Field. Here we present our improved method for predicting the effects of primordial voids on the CMB in which we treat a void as an external source in the cold dark matter (CDM) distribution employing a Boltzmann solver. Our improved predictions include the effects of a cosmological constant (Lambda) and acoustic oscillations generated by voids at early times. We find that models with relatively large voids on the last scattering surface predict too much CMB power in an Einstein--de Sitter background cosmology but could be consistent with the current CMB observations in a Lambda-CDM universe.

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arXiv2005

The Polytropic Equation of State of Primordial Gas Clouds

The polytropic equation of state (EOS) of primordial gas clouds with modest enrichment is computed, motivated by the recent observations of very Fe-deficient stars, [Fe/H]~10^{-3.5}-10^{-5}, such as HE 0107-5240 and CS 29498-043. These stars are overabundant, relative to Fe, in C and O. We assume that the observed abundances of species like C, O, Si and Fe are representative of the gas from which the currently observed metal-deficient stars formed. Under this assumption, we find that this primordial metal abundance pattern has profound consequences for the thermal balance and chemical composition of the gas, and hence for the EOS of the parental cloud. The polytopic EOS is soft for low, [O/H]<10^{-3}, oxygen abundances, but stiffens to a polytropic index gamma large than unity for [O/H]>10^{-2} due to the large opacity in the CO and H2O cooling lines. It is further found that a regulating role is played by the presence and temperature of the dust, even when the overall carbon abundance is only [C/H]~10^{-2}. When the dust is warmer than the gas, a region with gamma~1.2 results around a density of ~10^4 cm^{-3}. When the dust is colder than the gas, a region with gamma~0.8 is found for a density of ~10^6 cm^{-3}. Implications for the primordial initial mass function (IMF) as well as the IMF in starburst galaxies, where the metallicity is super-solar, are explored and related to processes that influence the temperature of the ambient dust.

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arXiv2005

Population III and the Near-Infrared Background Excess

We make a critical assessment of models that attribute the recently detected near-infrared background ``excess'' (NIRBE) to the redshifted light from Population III objects. To supply the required 25 keV/baryon at redshift 9, Pop III massive stars must form with an efficiency exceeding 30% in all ``minihalos'' with virial temperatures above a few hundred kelvins: to avoid excessive metal pollution, most of the baryons once in Pop III stars must end up in intermediate-mass black holes (IMBHs). Gas accretion onto such IMBHs must either be inhibited or lead to early miniquasars with steep UV/X-ray spectra, in order not to overproduce the present-day unresolved soft X-ray background. In the latter case (NIRBE dominated by "X-ray quiet miniquasars"), the total mass density of IMBHs at redshift 9 must be >50 times higher than the mass density of supermassive black holes observed today in the nuclei of galaxies. A stellar-dominated NIRBE is less economical energetically: more than 5% of all baryons in the universe must be processed into Population III stars. We survey various aspects of the Population III hypothesis for the NIRBE, and show that the ionizing photon budget required to account for the NIRBE is much larger than that required to explain the high electron scattering optical depth measured by the WMAP satellite.

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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

A new search for features in the primordial power spectrum

We develop a new approach toward a high resolution non-parametric reconstruction of the primordial power spectrum using WMAP cosmic microwave background temperature anisotropies that we confront with SDSS large-scale structure data in the range k~0.01-0.1 h/Mpc. We utilise the standard LambdaCDM cosmological model but we allow the baryon fraction to vary. In particular, for the concordance baryon fraction, we compare indications of a possible feature at k~0.05 h/Mpc in WMAP data with suggestions of similar features in large scale structure surveys.

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arXiv2004

Dark Matter and Galaxy Formation: Challenges for the Next Decade

The origin of the galaxies represents an important focus of current cosmological research, both observational and theoretical. Its resolution involves a comprehensive understanding of star formation, galaxy dynamics, the cosmology of the very early universe, and the nature of the dark matter. In this review, I will focus on those aspects of dark matter that are relevant for understanding galaxy formation, and describe the outlook for detecting the most elusive component, non-baryonic dark matter.

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arXiv2004

Possible Evidence for MeV Dark Matter In Dwarf Spheroidals

It has been recently proposed that the observed 511 keV emission from the Galactic bulge could be the product of very light (1-100 MeV) annihilating dark matter particles. Other possible explanations for this signal are associated with stellar objects, such as hypernovae. In order to distinguish between annihilating light dark matter scenario and more conventional astrophysical sources for the bulge emission, we here propose the study of dwarf spheroidals such as Sagittarius. These galaxies have typical luminosities of $10^5 L_\odot$ but mass-to-light ratios of $\sim 100$. As there are comparatively few stars, the prospects for 511 keV emission from standard astrophysical scenarios are minimal. The dwarf spheroidals do, however, contain copious amounts of dark matter. INTEGRAL/SPI has observed the Sagittarius region. Analysis of this data for 511 keV emission will provide a test of MeV dark matter which can distinguish between annihilating dark matter and more standard astrophysics. The observation of such a signal from Sagittarius should be a ``smoking gun'' for MeV dark matter.

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arXiv2004

Towards simulating star formation in the interstellar medium

As a first step to a more complete understanding of the local physical processes which determine star formation rates (SFRs) in the interstellar medium (ISM), we have performed controlled numerical experiments consisting of hydrodynamical simulations of a kilo-parsec scale, periodic, highly supersonic and "turbulent" three-dimensional flow. Using simple but physically motivated recipes for identifying star forming regions, we convert gas into stars which we follow self-consistently as they impact their surroundings through supernovae and stellar winds. We investigate how various processes (turbulence, radiative cooling, self-gravity, and supernovae feedback) structure the ISM, determine its energetics, and consequently affect its SFR. We find that the one-point statistical measurement captured by the probability density function (PDF) is sensitive to the simulated physics. Aside from its effect on the density structure of the medium, including self-gravity and/or supernovae feedback changes the dynamics of the medium by halting the decay of the kinetic energy. Since we find that the SFR depends most strongly on the underlying velocity field, the SFR declines in the runs lacking a means to sustain the kinetic energy, and the subsequent high density constrasts. This strong dependence on the gas velocity dispersion is in agreement with Silk's formula for the SFR (Silk 2001) which also takes the hot gas porosity, and the average gas density as important parameters. Measuring the porosity of the hot gas for the runs with supernovae feedback, we compare Silk's model for the SFR to our measured SFR and find agreement to better than a factor two.

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arXiv2004

Feedback and the Initial Mass function

I describe a turbulence-inspired model for the stellar initial mass function which includes feedback and self-regulation via protostellar outflows. A new aspect of the model provides predictions of the star formation rate in molecular clouds and gas complexes. A similar approach is discussed for self-regulation on kiloparsec scales via supernova input, and an expression is presented for the global star formation rate that depends on the turbulent pressure of the interstellar medium.

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arXiv2004

Searching for Dark Matter with Future Cosmic Positron Experiments

Dark matter particles annihilating in the Galactic halo can provide a flux of positrons potentially observable in upcoming experiments, such as PAMELA and AMS-02. We discuss the spectral features which may be associated with dark matter annihilation in the positron spectrum and assess the prospects for observing such features in future experiments. Although we focus on some specific dark matter candidates, neutralinos and Kaluza-Klein states, we carry out our study in a model independent fashion. We also revisit the positron spectrum observed by HEAT.

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arXiv2004

Cosmic Star Formation, Reionization, and Constraints on Global Chemical Evolution

Motivated by the WMAP results indicating an early epoch of reionization, we consider alternative cosmic star formation models which are capable of reionizing the early intergalactic medium. We develop models which include an early burst of massive stars (with several possible mass ranges) combined with standard star formation. We compute the stellar ionizing flux of photons and we track the nucleosynthetic yields for several elements: D, He4, C, N, O, Si, S, Fe, Zn. We compute the subsequent chemical evolution as a function of redshift, both in the intergalactic medium and in the interstellar medium of forming galaxies, starting with the primordial objects which are responsible for the reionization. We apply constraints from the observed abundances in the Lyman alpha forest and in Damped Lyman alpha clouds in conjunction with the ability of the models to produce the required degree of reionization. We also consider possible constraints associated with the observations of the two extremely metal-poor stars HE 0107-5240 and CS22949-037. We confirm that an early top-heavy stellar component is required, as a standard star formation model is unable to reionize the early Universe and reproduce the abundances of the very metal-poor halo stars. A bimodal (or top-heavy) IMF (40 - 100 M_\odot) is our preferred scenario compared to the extreme mass range (\ga 100 M_\odot) often assumed to be responsible for the early stages of reionization. A mode of even more extreme stellar masses in the range (\ge 270 M_\odot) has also been considered. All massive stars in this mode collapse entirely into black holes, and as a consequence, chemical evolution and reionization are de-correlated. [Abstract abbreviated.]

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arXiv2004

Particle Dark Matter: Evidence, Candidates and Constraints

In this review article, we discuss the current status of particle dark matter, including experimental evidence and theoretical motivations. We discuss a wide array of candidates for particle dark matter, but focus on neutralinos in models of supersymmetry and Kaluza-Klein dark matter in models of universal extra dimensions. We devote much of our attention to direct and indirect detection techniques, the constraints placed by these experiments and the reach of future experimental efforts.

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arXiv2004

Have Atmospheric Cerenkov Telescopes Observed Dark Matter?

Two ground-based experiments have recently independently detected TeV $γ$-rays from the direction of the Galactic center. The observations made by the VERITAS and CANGAROO collaborations are unexpected, although not impossible to interpret in terms of astrophysical sources. Here we examine in detail whether the observed $γ$-rays may arise from the more exotic alternative of annihilations of dark matter particles clustered in the center of the Galaxy.

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arXiv2004

Neutrinos from Dark Matter annihilations at the Galactic Centre

We discuss the prospects for detection of high energy neutrinos from dark matter annihilation at the Galactic centre. Despite the large uncertainties associated with our poor knowledge of the distribution of dark matter in the innermost regions of the Galaxy, we determine an upper limit on the neutrino flux by requiring that the associated gamma-ray emission does not exceed the observed flux. We conclude that if dark matter is made of neutralinos, a neutrino flux from dark matter annihilations at the GC will not be observable by Antares. Conversely, the positive detection of such a flux would either require an alternative explanation, in terms of astrophysical processes, or the adoption of other \DM candidates, disfavouring the case for neutralinos.

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