arXiv subjects

Joseph Silk

Publications and source records attributed to Joseph Silk.

At least 343 records · Page 19Linked to original sources

arXiv2006

Suppressing cluster cooling flows by self-regulated heating from spatially distributed population of AGNs

(Abridged) Existing models invoking AGN activty to resolve the cooling flow conundrum in galaxy clusters focus exclusively on the role of the central galaxy. Such models require fine-tuning of highly uncertain microscopic transport properties to distribute the thermal thermal over the entire cluster cooling core. We propose that the ICM is instead heated by multiple, spatially distributed AGNs. There is mounting observational evidence for multiple AGNs in cluster environments. Active AGNs drive bubbles into the ICM. We identify three distinct interactions between the bubble and the ICM: (1) Upon injection, the bubbles expand rapidly in situ to reach pressure equilibrium with their surroundings, generating shocks and waves whose dissipation is the principal source of ICM heating. (2) Once inflated, the bubbles rise buoyantly at rate determined by a balance with the viscous drag force, which itself results in some additional heating. (3) Rising bubbles expand and compress their surroundings. This process is adiabatic and does not contribute to any additional heating; rather, the increased ICM density due to compression enhances cooling. Our model sidesteps the ``transport'' issue by relying on the spatially distributed galaxies to heat the cluster core. We include self regulation in our model by linking AGN activity in a galaxy to cooling characteristics of the surrounding ICM. We use a spherically symmetric one-dimensional hydrodynamical code to carry out a preliminary study illustrating the efficacy of the model. Our self-regulating scenario predicts that there should be enhanced AGN activity of galaxies inside the cooling regions compared to galaxies in the outer parts of the cluster. This prediction remains to be confirmed or refuted by observations.

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arXiv2006

Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies I

We explore the large angular scale temperature anisotropies in the cosmic microwave background due to expanding homogeneous local voids at redshift z~1. A compensated spherically symmetric homogeneous dust-filled void with radius \~3*10^2 h^{-1}Mpc, and density contrast ~-0.3 can be observed as a cold spot with a temperature anisotropy -1*10^{-5} surrounded by a slightly hotter ring. We find that a pair of these circular cold spots separated by ~50 degree can account both for the planarity of the octopole and for the alignment between the quadrupole and the octopole in the cosmic microwave background (CMB) anisotropy. The cold spot in the Galactic southern hemisphere which is anomalous at the ~3sigma level can be explained by such a large void at z~1. The observed north-south asymmetry in the large-angle CMB power can be attributed to the asymmetric distribution of these local voids between the two hemispheres. The statistical significance of the low quadrupole is further reduced in this interpretation of the large angular scale CMB anomalies.

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arXiv2006

Tidal Destruction of The First Dark Microhalos

We point out that the usual self-similarity in cold dark matter models is broken by encounters with individual normal galactic stars on sub-pc scale. Tidal heating and stripping must have redefined the density and velocity structures of the population of the Earth-mass dark matter halos, which are likely to have been the first bound structures to form in the Universe. The disruption rate depends strongly on {\it galaxy types} and the orbital distribution of the microhalos; in the Milky Way, stochastic radial orbits are destroyed first by stars in the triaxial bulge, microhalos on non-planar retrograde orbits with large pericenters and/or apocenters survive the longest. The final microhalo distribution in the {\it solar neighborhood} is better described as a superposition of filamentry microstreams rather than as a set of discrete spherical clumps in an otherwise homogeneous medium. We discuss its important consequences to our detections of microhalos by direct recoil signal and indirect annihilation signal.

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arXiv2006

Pregalactic Black Hole Formation with an Atomic Hydrogen Equation of State

The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M_h~10^7-10^9 Mo. For roughly isothermal collapse around 10^4 K, we find that line trapping of Lyman alpha (HI and HeII) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H2 abundance and very modest pollution by metals (<10^-4 Solar), fragmentation is likely to be inhibited for such an equation of state. We argue on purely thermodynamic grounds that a single black hole of ~0.02-0.003M_h can form at the center of a halo for z=10-20 when the free-fall time is less than the time needed for a resonantly scattered Lyman alpha photon to escape from the halo. The absence of H2 follows naturally from the high, 10^4 K, temperatures that are attained when Lyman alpha photons are trapped in the dense and massive halos that we consider. An H2 dissociating UV background is needed if positive feedback effects on H2 formation from X-rays occur. The black hole to baryon mass fraction is suggestively close to what is required for these intermediate mass black holes, of mass M_BH~10^4-10^6 Mo, to act as seeds for forming the supermassive black holes of mass ~0.001M_spheroid found in galaxies today.

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arXiv2006

A simple model for the size-evolution of elliptical galaxies

We use semi-analytical modelling of galaxy formation to predict the redshift-size-evolution of elliptical galaxies. Using a simple model in which relative sizes of elliptical galaxies of a given mass correlate with the fraction of stars formed in a star burst during a major merger event, we are able to reproduce the observed size redshif t evolution. The size evolution is a result of the amount of cold gas available during the major merger. Mergers at high redshifts are gas-rich and produce ellipticals with smaller sizes. In particular we find a power-law relation between the sizes at different redshifts, with the power-law index giving a measure of the relative amount of dissipation during the mergers that lead to the formation of an elliptical. The size evolution is found to be stronger for more massive galaxies as they involve more gas at high redshifts when they form compared to less massive ellipticals. Local ellipticals more massive than $5 \times 10^{11}$ M$_{\odot}$ will be approximately 4 times larger than their counterparts at $z=2$. Our results indicate that the scatter in the size of similar massive present day elliptical galaxies is a result of their formation epoch, with smaller ellipticals being formed earlier.

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arXiv2006

Local dark matter clumps and the cosmic ray positron excess

It has been proposed that the excess in cosmic ray positrons at approximately 8 GeV, observed on both flights of the HEAT balloon experiment, may be associated with the annihilation of dark matter within the Milky Way halo. In this paper we demonstrate how the self-annihilation of neutralino dark matter within local substructure can account for this excess, and estimate the annihilation cross-section for several benchmark minimal supersymmetric (MSSM) models. We also demonstrate the effect on the permitted parameter space as tidal stripping effects and destruction of substructure by mergers becomes increasingly severe.

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arXiv2006

Luminosity Function of Faint Globular Clusters in M87

We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit \textit{Hubble Space Telescope (HST)} WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.

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arXiv2006

On the First Generation of Stars

We argue that the first stars may have spanned the conventional mass range rather than be identified with the Very Massive Objects (100-1000 solar masses) favoured by numerical simulations. Specifically, we find that magnetic field generation processes acting in the first protostellar systems suffice to produce fields that exceed the threshold for MRI instability to operate and thereby allow the MRI dynamo to generate equipartition-amplitude magnetic fields on protostellar mass scales below 50 solar masses. Such fields allow primordial star formation to occur at essentially any metallicity by regulating angular momentum transfer, fragmentation, accretion and feedback in much the same way as occurs in conventional molecular clouds.

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arXiv2006

Galaxy Formation and Dark Matter

The challenge of dark matter may be addressed in two ways; by studying the confrontation of structure formation with observation and by direct and indirect searches. 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. Galaxy formation theory is driven by phenomenology and by numerical simulations of dark matter clustering under gravity. Once the complications of star formation are incorporated, the theory becomes so complex that the brute force approach of numerical simulations needs to be supplemented by incorporation of such astrophysical processes as feedback by supernovae and by active galactic nuclei. I present a few semi-analytical perspectives that may shed some insight into the nature of galaxy formation.

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arXiv2006

Non-parametric reconstruction of the primordial power spectrum at horizon scales from WMAP data

We extend to large scales a method proposed in previous work that reconstructs non-parametrically the primordial power spectrum from cosmic microwave background data at high resolution. The improvement is necessary to account for the non-gaussianity of the Wilkinson Microwave Anisotropy Probe (WMAP) likelihood due primarily to cosmic variance. We assume the concordance LambdaCDM cosmology, utilise a smoothing prior and perform Monte Carlo simulations around an initial power spectrum that is scale-free and with spectral index n_s=0.97, very close to the concordance spectrum. The horizon scale for the model we are considering corresponds to the wavenumber k_h=4.52X10^{-4} Mpc^{-1}. We find some evidence for the presence of features and we quantify the probabilities of exceeding the observed deviations in WMAP data with respect to the fiducial models. We detect the following marginal departures from a scale-free (spectral index n_s=0.97) initial spectrum: a cut-off at 0.0001<k<0.001 Mpc^{-1} at 79.5% (92%), a dip at 0.001<k<0.003 Mpc^{-1} at 87.2% (98%) and a bump at 0.003<k<0.004 Mpc^{-1} at 90.3% (55.5%) confidence level. These frequentist confidence levels are calculated by integrating over the distribution of the Monte Carlo reconstructions built around the fiducial models. The frequentist analysis finds the low k cutoff of the estimated power spectrum to be about 2.5 sigma away from the n_s=0.97 model, while in the Bayesian analysis the model is about 1.5 sigma away from the estimated spectrum. (The sigma's are different for the two different methods.)

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arXiv2005

Ultraluminous Starbursts from SMBH-induced outflows

I argue that there are two modes of global star formation. Disks and smaller spheroids form stars relatively inefficiently as a consequence of supernova-triggered negative feedback via a sequence of ministarbursts (S mode), whereas massive spheroids formed rapidly with high efficiency via the impact of AGN jet-triggered positive feedback (J mode) that generates and enhances ultraluminous starbursts. Supermassive black hole growth by accretion is favoured in the gas-rich protospheroid environment as mergers build up the mass of the host galaxy and provide a centrally concentrated gas supply. Quasi-spherical outflows arise and provide the source of porosity as the energetic jets from the accreting central SMBH are isotropised by the inhomogeneous interstellar medium in the protospheroid core. Super-Eddington outflows occur and help generate both the SMBH at high redshift and the strong positive feedback on protospheroid star formation that occurs as dense interstellar clouds are overpressured and collapse. SMBH form before the bulk of spheroid stars, and the correlation between spheroid velocity dispersion and supermassive black hole mass arises as AGN-triggered outflows limit the gas reservoir for spheroid star formation. The super-Eddington phase plausibly triggers a top-heavy IMF in the region of influence of the SMBH. The Compton-cooled Eddington-limited outflow phase results in a spheroid core whose phase space density scales as the inverse 5/2 power of the core mass, and whose mass scales as the 3/2 power of SMBH mass. This latter scaling suggests that SMBH growth (and hence spheroid formation) is anti-hierarchical.

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arXiv2005

Did Boomerang hit MOND?

Purely baryonic dark matter dominated models like MOND based on modification of Newtonian gravity have been successfully in reproducing some dynamical properties of galaxies. More recently, a relativistic formulation of MOND proposed by Bekenstein seems to agree with cosmological large scale structure formation. In this work, we revise the agreement of MOND with observations in light of the new results on the Cosmic Microwave Anisotropies provided by the 2003 flight of Boomerang. The measurements of the height of the third acoustic peak, provided by several small scale CMB experiments have reached enough sensitivity to severely constrain models without cold dark matter. Assuming that acoustic peak structure in the CMB is unchanged and that local measurements of the Hubble constant can be applied, we find that the cold dark matter is strongly favoured with Bayesian probability ratio of about one in two hundred.

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arXiv2005

Cosmic rays, lithium abundance and excess entropy in galaxy clusters

We consider the production of $^6$Li in spallation reactions by cosmic rays in order to explain the observed abundance in halo metal-poor stars. We show that heating of ambient gas by cosmic rays is an inevitable consequence of this process, and estimate the energy input required to reproduce the observed abundance of $^6$Li/H$\sim 10^{-11}$ to be of order a few hundred eV per particle. We draw attention to the possibility that this could explain the excess entropy in gas in galaxy groups and clusters. The evolution of $^6$Li and the accompanying heating of gas is calculated for structures collapsing at the present epoch with injection of cosmic rays at high redshift. We determine the energy required to explain the abundance of $^6$Li at $z \sim 2$ corresponding to the formation epoch of halo metal-poor stars, and also an increased entropy level of $\sim 300$ keV cm$^2$ necessary to explain X-ray observations of clusters. The energy budget for this process is consistent with the expected energy output of radio-loud AGNs, and the diffusion length scale of cosmic-ray protons responsible for heating is comparable to the size of regions with excess entropy. We also discuss the constraints imposed by the extragalactic gamma-ray background.

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arXiv2005

AGN Feedback Causes Downsizing

We study the impact of outflows driven by active galactic nuclei (AGN) on galaxy formation. Outflows move into the surrounding intergalactic medium (IGM) and heat it sufficiently to prevent it from condensing onto galaxies. In the dense, high-redshift IGM, such feedback requires highly energetic outflows, driven by a large AGN. However, in the more tenuous low-redshift IGM, equivalently strong feedback can be achieved by less energetic winds (and thus smaller galaxies). Using a simple analytic model, we show that this leads to the anti-hierarchical quenching of star-formation in large galaxies, consistent with current observations. At redshifts prior to the formation of large AGN, galaxy formation is hierarchical and follows the growth of dark-matter halos. The transition between the two regimes lies at the z ~ 2 peak of AGN activity.

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arXiv2005

Reconstruction of primordial density fields

The Monge-Ampere-Kantorovich (MAK) reconstruction is tested against cosmological N-body simulations. Using only the present mass distribution sampled with particles, and the assumption of homogeneity of the primordial distribution, MAK recovers for each particle the non-linear displacement field between its present position and its Lagrangian position on a primordial uniform grid. To test the method, we examine a standard LCDM N-body simulation with Gaussian initial conditions and 6 models with non-Gaussian initial conditions: a chi-squared model, a model with primordial voids and four weakly non-Gaussian models. Our extensive analyses of the Gaussian simulation show that the level of accuracy of the reconstruction of the nonlinear displacement field achieved by MAK is unprecedented, at scales as small as about 3 Mpc. In particular, it captures in a nontrivial way the nonlinear contribution from gravitational instability, well beyond the Zel'dovich approximation. This is also confirmed by our analyses of the non-Gaussian samples. Applying the spherical collapse model to the probability distribution function of the divergence of the displacement field, we also show that from a well-reconstructed displacement field, such as that given by MAK, it is possible to accurately disentangle dynamical contributions induced by gravitational clustering from possible initial non-Gaussianities, allowing one to efficiently test the non-Gaussian nature of the primordial fluctuations. In addition, a simple application of MAK using the Zel'dovich approximation allows one to also recover accurately the present-day peculiar velocity field on scales of about 8 Mpc.

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arXiv2005

Mini-dark halos with intermediate mass black holes

We argue that the Milky Way (MW) contains thousands intermediate mass black holes (IMBHs) and minihalos with a fraction of IMBHs still being enshrouded in extremely dense mini-spikes of dark matter (DM) particles. Each containing $10^6\msun$ of dark particles and no baryons in a sphere of 50 pc radius, the minihalos are dense enough to survive the Milky Way tide with the nearest minihalo within 2 kpc from the Sun. The IMBH is formed off-centre in a minihalo by gas accretion, and its growth adiabatically compresses a finite density of surrounding dark matter into a $r^{-1.5}$ mini-spike. Some IMBHs recentre on their minihalos after dynamical friction, and some IMBHs are ejected by birth kicks. Detectable by GLAST, the mini-spikes and minihalos should stand out the background and dominate the neutralino annihilation in the smooth MW and satellite galaxies. If they are the unidentified EGRET sources, upper limits can be set on the branching ratio of neutralino annihilation. The supermassive BH of the MW, if orginates from an IMBH, is also likely enshrouded with a mini-spike.

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arXiv2005

A New Signature of Dark Matter Annihilations: Gamma-Rays from Intermediate-Mass Black Holes

We study the prospects for detecting gamma-rays from Dark Matter (DM) annihilations in enhancements of the DM density (mini-spikes) around intermediate-mass black holes with masses in the range $10^2 \lsim M / \msun \lsim 10^6$. Focusing on two different IMBH formation scenarios, we show that, for typical values of mass and cross section of common DM candidates, mini-spikes, produced by the adiabatic growth of DM around pregalactic IMBHs, would be bright sources of gamma-rays, which could be easily detected with large field-of-view gamma-ray experiments such as GLAST, and further studied with smaller field-of-view, larger-area experiments like Air Cherenkov Telescopes CANGAROO, HESS, MAGIC and VERITAS. The detection of many gamma-ray sources not associated with a luminous component of the Local Group, and with identical cut-offs in their energy spectra at the mass of the DM particle, would provide a potential smoking-gun signature of DM annihilations and shed new light on the nature of intermediate and supermassive Black Holes.

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arXiv2005

New constraints on MOND from galaxy clusters

We revisit the application of Modified Newtonian Dynamics (MOND) to galaxy clusters. We confront the high quality X-ray data for eight clusters of galaxies observed by the \xmm satellite with the predictions of MOND. We obtain a ratio of the MOND dynamical mass to the baryonic mass of $M_m/M_b=4.94\pm 0.50$ in the outer parts (i.e $r\sim 0.5$ ~ R$_{vir}$), in the concordance cosmological model where the predicted asymptotic ratio, if any baryons are present, is $7.7^{+1.4}_{-1.1}$ (at $r\geq 0.3$ ~ R$_{vir}$). We confirm that the MOND paradigm lowers the discrepancy between the binding mass and the baryonic mass in clusters by a factor of $\sim 1.6$ at about half the virial radius. However, at this radius about 80% of the mass is still missing, and as pointed out by \citet{sanders03}, this necessitates a component of dark baryons or neutrinos in the cluster core. Concerning the neutrino hypothesis, application of the new data requires a minimum neutrino mass of $m_ν>1.74\pm 0.34$ eV to fill this gap. The corresponding 2$σ$ lower limit of $m_ν>1.06$ eV is marginally inconsistent with the current constraints from the cluster number counts, and from the CMB and large scale structure data. MOND must invoke neutrinos to represent the main component that account for the missing mass problem in clusters.

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