arXiv subjects

Gianfranco Bertone

Publications and source records attributed to Gianfranco Bertone.

At least 109 records · Page 6Linked to original sources

arXiv2008

Consequences of dark matter self-annihilation for galaxy formation

Galaxy formation requires a process that continually heats gas and quenches star formation in order to reproduce the observed shape of the luminosity function of bright galaxies. To accomplish this, current models invoke heating from supernovae, and energy injection from active galactic nuclei. However, observations of radio-loud active galactic nuclei suggest that their feedback is likely to not be as efficient as required, signaling the need for additional heating processes. We propose the self-annihilation of weakly interacting massive particles that constitute dark matter as a steady source of heating. In this paper, we explore the circumstances under which this process may provide the required energy input. To do so, dark matter annihilations are incorporated into a galaxy formation model within the Millennium cosmological simulation. Energy input from self-annihilation can compensate for all the required gas cooling and reproduce the observed galaxy luminosity function only for what appear to be extreme values of the relevant key parameters. The key parameters are: the slope of the inner density profile of dark matter haloes and the outer spike radius. The inner density profile needs to be steepened to slopes of -1.5 or more and the outer spike radius needs to extend to a few tens of parsecs on galaxy scales and a kpc or so on cluster scales. If neutralinos or any thermal relic WIMP with s-wave annihilation constitute dark matter, their self-annihilation is inevitable and could provide enough power to modulate galaxy formation. Energy from self-annihilating WIMPs could be yet another piece of the feedback puzzle along with supernovae and active galactic nuclei.

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arXiv2008

Dark Matter Candidates: A Ten-Point Test

An extraordinarily rich zoo of non-baryonic Dark Matter candidates has been proposed over the last three decades. Here we present a 10-point test that a new particle has to pass, in order to be considered a viable DM candidate: I.) Does it match the appropriate relic density? II.) Is it {\it cold}? III.) Is it neutral? IV.) Is it consistent with BBN? V.) Does it leave stellar evolution unchanged? VI.) Is it compatible with constraints on self-interactions? VII.) Is it consistent with {\it direct} DM searches? VIII.) Is it compatible with gamma-ray constraints? IX.) Is it compatible with other astrophysical bounds? X.) Can it be probed experimentally?

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arXiv2007

Black Holes as Dark Matter Annihilation Boosters

We review the consequences of the growth and evolution of Black Holes on the distribution of stars and Dark Matter (DM) around them. We focus in particular on Supermassive and Intermediate Mass Black Holes, and discuss under what circumstances they can lead to significant overdensities in the surrounding distribution of DM, thus effectively acting as DM annihilation boosters.

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arXiv2007

Compact Stars as Dark Matter Probes

We discuss the consequences of the accretion of dark matter (DM) particles on compact stars such as white dwarfs and neutron stars. We show that in large regions of the DM parameter space, these objects are sensitive probes of the presence of DM and can be used to set constraints both on the DM density and on the physical properties of DM particles.

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arXiv2007

Dark Matter: A Multidisciplinary Approach

We review the current status of accelerator, direct and indirect Dark Matter (DM) searches, focusing on the complementarity of different techniques and on the prospects for discovery. After taking a census of present and upcoming DM-related experiments, we review the motivations to go beyond an "accelerator-only" approach, and highlight the benefits of multidisciplinarity in the quest for DM.

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arXiv2007

Towards Closing the Window on Strongly Interacting Dark Matter: Far-Reaching Constraints from Earth's Heat Flow

We point out a new and largely model-independent constraint on the dark matter scattering cross section with nucleons, applying when this quantity is larger than for typical weakly interacting dark matter candidates. When the dark matter capture rate in Earth is efficient, the rate of energy deposition by dark matter self-annihilation products would grossly exceed the measured heat flow of Earth. This improves the spin-independent cross section constraints by many orders of magnitude, and closes the window between astrophysical constraints (at very large cross sections) and underground detector constraints (at small cross sections). In the applicable mass range, from about 1 to about 10^{10} GeV, the scattering cross section of dark matter with nucleons is then bounded from above by the latter constraints, and hence must be truly weak, as usually assumed.

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arXiv2007

Gamma-Rays from Decaying Dark Matter

We study the prospects for detecting gamma-rays from decaying Dark Matter (DM), focusing in particular on gravitino DM in R-parity breaking vacua. Given the substantially different angular distribution of the predicted gamma-ray signal with respect to the case of annihilating DM, and the relatively poor (of order 0.1$^\circ$) angular resolution of gamma-ray detectors, the best strategy for detection is in this case to look for an exotic contribution to the gamma-ray flux at high galactic latitudes, where the decaying DM contribution would resemble an astrophysical extra-galactic component, similar to the one inferred by EGRET observations. Upcoming experiments such as GLAST and AMS-02 may identify this exotic contribution and discriminate it from astrophysical sources, or place significant constraints on the mass and lifetime of DM particles.

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arXiv2007

Impact of astrophysical processes on the gamma-ray background from dark matter annihilations

We study the impact of astrophysical processes on the gamma-ray background produced by the annihilation of dark matter particles in cosmological halos, with particular attention to the consequences of the formation of supermassive black holes. In scenarios where these objects form adiabatically from the accretion of matter on small seeds, dark matter is first compressed into very dense ``spikes'', then its density progressively decreases due to annihilations and scattering off of stellar cusps. With respect to previous analyses, based on non-evolving halos, the predicted annihilation signal is higher and significantly distorted at low energies, reflecting the large contribution to the total flux from unevolved spikes at high redshifts. The peculiar spectral feature arising from the specific redshift distribution of the signal, would discriminate the proposed scenario from more conventional astrophysical explanations. We discuss how this affects the prospects for detection and demonstrate that the gamma-ray background from DM annihilations might be detectable even in absence of a signal from the Galactic center.

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arXiv2007

Gamma-Rays from Dark Matter Mini-Spikes in M31

The existence of a population of wandering Intermediate Mass Black Holes (IMBHs) is a generic prediction of scenarios that seek to explain the formation of Supermassive Black Holes in terms of growth from massive seeds. The growth of IMBHs may lead to the formation of DM overdensities called "mini-spikes", recently proposed as ideal targets for indirect DM searches. Current ground-based gamma-ray experiments, however, cannot search for these objects due to their limited field of view, and it might be challenging to discriminate mini-spikes in the Milky Way from the many astrophysical sources that GLAST is expected to observe. We show here that gamma-ray experiments can effectively search for IMBHs in the nearby Andromeda galaxy (also known as M31), where mini-spikes would appear as a distribution of point-sources, isotropically distributed in a \thickapprox 3^{\circ} circle around the galactic center. For a neutralino-like DM candidate with a mass m_χ=150 GeV, up to 20 sources would be detected with GLAST (at 5σ, in 2 months). With Air Cherenkov Telescopes such as MAGIC and VERITAS, up to 10 sources might be detected, provided that the mass of neutralino is in the TeV range or above.

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arXiv2006

GLAST sensitivity to Point Sources of Dark Matter Annihilation

We study the prospects for detecting gamma-rays from point sources of Dark Matter annihilation with the space satellite GLAST. We simulate the instrument response to the gamma-ray spectrum arising from the annihilation of common Dark Matter candidates, and derive full-sky sensitivity maps for the {\it detection} of point sources and for the {\it identification} of the Dark Matter (as opposed to astrophysical) origin of the gamma-ray emission. These maps represent a powerful tool to assess the detectability of point sources, i.e. sources with angular size smaller than the angular resolution of GLAST, ~ 0.1 degrees, in {\it any} DM scenario. As an example, we apply the obtained results to the so-called 'mini-spikes' scenario, where the annihilation signal originates from large Dark Matter overdensities around Intermediate Mass Black Holes. We find that if these objects exist in the Galaxy, not only GLAST should be able to detect them over a timescale as short as 2 months, but in many cases it should be possible to determine with good accuracy the mass of the annihilating Dark Matter particles, while null searches would place stringent constraints on this scenario.

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arXiv2006

Collisionally Regenerated Dark Matter Structures in Galactic Nuclei

We show that the presence of a rho~1/r^{3/2} dark matter overdensity can be robustly predicted at the center of any galaxy old enough to have grown a power-law density cusp in the stars via the Bahcall-Wolf mechanism. Using both Fokker-Planck and direct N-body integrations, we demonstrate collisional generation of these dark matter "crests" (Collisionally REgenerated STtructures) even in the extreme case that the density of both stars and dark matter were previously lowered by slingshot ejection from a binary supermassive black hole. The time scale for collisional growth of the crest is approximately the two-body relaxation time as defined by the stars, which is < 10 Gyr at the centers of stellar spheroids with luminosities comparable to that of the Milky Way bulge or less. The presence of crests can robustly be predicted in such galaxies, unlike the steeper enhancements, called "spikes," produced by the adiabatic growth of black holes. We discuss special cases where the prospects for detecting dark matter annihilations from the centers of galaxy haloes are significantly affected by the formation of crests.

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arXiv2006

Dark Matter: the Connection with Gamma-Ray Astrophysics

We review the status of indirect Dark Matter searches, focusing in particular on the connection with gamma-ray Astrophysics, and on the prospects for detection with the upcoming space telescope GLAST and Air Cherenkov Telescopes such as CANGAROO, HESS, MAGIC and VERITAS. After a brief introduction where we review the fundamental motivations for indirect searches, we tackle the question of whether it is possible to obtain strong enough evidence from astrophysical observations, to claim discovery of Dark Matter particles. To this purpose, we discuss some recent conflicting claims that have generated some confusion in the field, and present new strategies that may provide the long-awaited smoking-gun for Dark Matter.

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arXiv2006

Prospects for detecting Dark Matter with neutrino telescopes in Intermediate Mass Black Holes scenarios

Current strategies of indirect Dark Matter detection with neutrino telescopes are based on the search for high-energy neutrinos from the Solar core or from the center of the Earth. Here, we propose a new strategy based on the detection of neutrinos from Dark Matter annihilations in 'mini-spikes' around Intermediate Mass Black Holes. Neutrino fluxes, in this case, depend on the annihilation cross-section of Dark Matter particles, whereas solar and terrestrial fluxes are sensitive to the scattering cross-section off nucleons, a circumstance that makes the proposed search complementary to the existing ones. We discuss the prospects for detection with upcoming under-water and under-ice experiments such as ANTARES and IceCube, and show that several, up to many, sources could be detected with both experiments. A kilometer scale telescope in the Mediterranean appears to be ideally suited for the proposed search.

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arXiv2006

Gamma ray bursts and the origin of galactic positrons

A recent observation of the 511 keV electron-positron annihilation line from the Galactic bulge has prompted a debate on the origin of the galactic positrons responsible for this emission. Assuming equilibrium between annihilation and injection of positrons in the Galaxy, we investigate the possibility that positrons were produced by past gamma ray bursts (GRBs). We compare the positron annihilation rate inferred from the observed 511 keV line with the predicted injection rate due to electron-positron pairs leaking out of individual GRBs and those pairs produced by GRB photons in the dense molecular clouds at the Galactic center. We conclude that the proposed scenario is marginally consistent with observations, and can reproduce the observed normalization of the annihilation line only for higher-than-average values of the GRB rate in the Galaxy and/or the molecular clouds optical depth for pair production.

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arXiv2005

Time-Dependent Models for Dark Matter at the Galactic Center

The prospects of indirect detection of dark matter at the galactic center depend sensitively on the mass profile within the inner parsec. We calculate the distribution of dark matter on sub-parsec scales by integrating the time-dependent Fokker-Planck equation, including the effects of self-annihilations, scattering of dark matter particles by stars, and capture in the supermassive black hole. We consider a variety of initial dark matter distributions, including models with very high densities ("spikes") near the black hole, and models with "adiabatic compression" of the baryons. The annihilation signal after 10 Gyr is found to be substantially reduced from its initial value, but in dark matter models with an initial spike, order-of-magnitude enhancements can persist compared with the rate in spike-free models, with important implications for indirect dark matter searches with GLAST and Air Cherenkov Telescopes like HESS and CANGAROO.

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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

Dark Matter Dynamics and Indirect Detection

Non-baryonic, or "dark," matter is believed to be a major component of the total mass budget of the universe. We review the candidates for particle dark matter and discuss the prospects for direct detection (via interaction of dark matter particles with laboratory detectors) and indirect detection (via observations of the products of dark matter self-annihilations), focusing in particular on the Galactic center, which is among the most promising targets for indirect detection studies. The gravitational potential at the Galactic center is dominated by stars and by the supermassive black hole, and the dark matter distribution is expected to evolve on sub-parsec scales due to interaction with these components. We discuss the dominant interaction mechanisms and show how they can be used to rule out certain extreme models for the dark matter distribution, thus increasing the information that can be gleaned from indirect detection searches.

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arXiv2004

Gamma-Ray Constraint on Galactic Positron Production by MeV Dark Matter

The Galactic positrons, as observed by their annihilation gamma-ray line at 0.511 MeV, are difficult to account for with astrophysical sources. It has been proposed that they are produced instead by dark matter annihilation or decay in the inner Galactic halo. To avoid other constraints, these processes are required to occur "invisibly," such that the eventual positron annihilation is the only detectable signal. However, electromagnetic radiative corrections to these processes inevitably produce real gamma rays (``internal bremsstrahlung"); this emission violates COMPTEL and EGRET constraints unless the dark matter mass is less than about 20 MeV.

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