arXiv subjects

Joseph Silk

Publications and source records attributed to Joseph Silk.

At least 145 records · Page 8Linked to original sources

arXiv2018

Pevatron at the Galactic Center: Multi-Wavelength Signatures from Millisecond Pulsars

Diffuse TeV emission has been observed by H.E.S.S. in the Galactic Center region, in addition to the GeV gamma rays observed by Fermi. We propose that a population of unresolved millisecond pulsars located around the Galactic Center, suggested as possible candidates for the diffuse Galactic Center excess observed by Fermi, accelerate cosmic rays up to very high energies, and are thus also responsible for the TeV excess. We model analytically the diffusion of these accelerated protons and their interaction with the molecular clouds, producing gamma rays. The spatial and spectral dependences of the gamma rays produced can reproduce the H.E.S.S. observations, for a population of $\sim 10^4-10^5$ millisecond pulsars above the cosmic-ray luminosity $10^{34}\,{\rm erg\,s}^{-1}$, with moderate acceleration efficiency. More precise measurements at the highest energies would allow us to constrain the properties of the pulsar population, such as the magnetic field or initial spin distributions.

astro-ph.HE↗

arXiv2018

The Most Massive Galaxies and Black Holes Allowed by $Λ$CDM

Given a galaxy's stellar mass, its host halo mass has a lower limit from the cosmic baryon fraction and known baryonic physics. At z>4, galaxy stellar mass functions place lower limits on halo number densities that approach expected $Λ$CDM halo mass functions. High-redshift galaxy stellar mass functions can thus place interesting limits on number densities of massive haloes, which are otherwise very difficult to measure. Although halo mass functions at z<8 are consistent with observed galaxy stellar masses if galaxy baryonic conversion efficiencies increase with redshift, JWST and WFIRST will more than double the redshift range over which useful constraints are available. We calculate maximum galaxy stellar masses as a function of redshift given expected halo number densities from $Λ$CDM. We apply similar arguments to black holes. If their virial mass estimates are accurate, number density constraints alone suggest that the quasars SDSS J1044-0125 and SDSS J010013.02+280225.8 likely have black hole mass -- stellar mass ratios higher than the median z=0 relation, confirming the expectation from Lauer bias. Finally, we present a public code to evaluate the probability of an apparently $Λ$CDM-inconsistent high-mass halo being detected given the combined effects of multiple surveys and observational errors.

astro-ph.GA↗

arXiv2018

Signatures of primordial black holes as seeds of supermassive black holes

It is broadly accepted that Supermassive Black Holes (SMBHs) are located in the centers of most massive galaxies, although there is still no convincing scenario for the origin of their massive seeds. It has been suggested that primordial black holes (PBHs) of masses $\gtrsim 10^{2} M_\odot$ may provide such seeds, which would grow to become SMBHs. We suggest an observational test to constrain this hypothesis: gas accretion around PBHs during the cosmic dark ages powers the emission of high energy photons which would modify the spin temperature as measured by 21cm Intensity Mapping (IM) observations. We model and compute their contribution to the standard sky-averaged signal and power spectrum of 21cm IM, accounting for its substructure and angular dependence for the first time. If PBHs exist, the sky-averaged 21cm IM signal in absorption would be higher, while we expect an increase in the power spectrum for $\ell~\gtrsim 10^2-10^3$. We also forecast PBH detectability and measurement errors in the abundance and Eddington ratios for different fiducial parameter configurations for various future experiments, ranging from SKA to a futuristic radio array on the dark side of the Moon. While the SKA could provide a detection, only a more ambitious experiment would provide accurate measurements.

astro-ph.CO↗

arXiv2018

Cluster-Void Degeneracy Breaking: Modified Gravity in the Balance

Combining galaxy cluster and void abundances is a novel, powerful way to constrain deviations from General Relativity and the $Λ$CDM model. For a flat $w$CDM model with growth of large-scale structure parameterized by the redshift-dependent growth index $γ(z) = γ_0 + γ_1 z/(1+z)$ of linear matter perturbations, combining void and cluster abundances in future surveys with Euclid and the 4-metre Multi-Object Spectroscopic Telescope (4MOST) could improve the Figure of Merit for ($w, γ_0, γ_1$) by a factor of 20 compared to individual abundances. In an ideal case, improvement on current cosmological data is a Figure of Merit factor $600$ or more.

astro-ph.CO↗

arXiv2018

Limits on Runaway Growth of Intermediate Mass Black Holes from Advanced LIGO

There is growing evidence that intermediate-mass black holes (IMBHs), defined here as having a mass in the range M=500-10^5 Msun, are present in the dense centers of certain globular clusters (GCs). Gravitational waves (GWs) from their mergers with other IMBHs or with stellar BHs in the cluster are mostly emitted in frequencies <10 Hz, which unfortunately is out of reach for current ground-based observatories such as advanced LIGO (aLIGO). Nevertheless, we show that aLIGO measurements can be used to efficiently probe one of the possible formation mechanisms of IMBHs in GCs, namely a runaway merger process of stellar seed BHs. In this case, aLIGO will be sensitive to the lower-mass rungs of the merger ladder, ranging from the seed BH mass to masses >~50-300 Msun, where the background from standard mergers is expected to be very low. Assuming this generic IMBH formation scenario, we calculate the mass functions that correspond to the limiting cases of possible merger trees. Based on estimates for the number density of GCs and taking into account the instrumental sensitivity, we show that current observations do not effectively limit the occupation fraction f_occ of IMBHs formed by runaway mergers of stellar BHs in GCs. However, we find that a six-year run of aLIGO at design sensitivity will be able to probe down to f_occ<3% at a 99.9% confidence level, either finding evidence for this formation mechanism, or necessitating others if the fraction of GCs that harbor IMBHs is higher.

astro-ph.HE↗

arXiv2018

Limits on primordial black holes from $μ$ distortions in cosmic microwave background

If primordial black holes (PBHs) form directly from inhomogeneities in the early Universe, then the number in the mass range $10^5 -10^{12}M_{\odot}$ is severely constrained by upper limits to the $μ$ distortion in the cosmic microwave background (CMB). This is because inhomogeneities on these scales will be dissipated by Silk damping in the redshift interval $5\times 10^4\lesssim z\lesssim2\times 10^6$. If the primordial fluctuations on a given mass scale have a Gaussian distribution and PBHs form on the high-$σ$ tail, as in the simplest scenarios, then the $μ$ constraints exclude PBHs in this mass range from playing any interesting cosmological role. Only if the fluctuations are highly non-Gaussian, or form through some mechanism unrelated to the primordial fluctuations, can this conclusion be obviated.

astro-ph.CO↗

arXiv2018

Unique probe of dark matter in the core of M87 with the Event Horizon Telescope

We demonstrate the unprecedented capabilities of the Event Horizon Telescope (EHT) to image the innermost dark matter profile in the vicinity of the supermassive black hole at the center of the M87 radio galaxy. We present the first model of the synchrotron emission induced by dark matter annihilations from a spiky profile in the close vicinity of a supermassive black hole, accounting for strong gravitational lensing effects. Our results show that the EHT should readily resolve dark matter spikes if present. Moreover, the photon ring surrounding the silhouette of the black hole is clearly visible in the spike emission, which introduces observable small-scale structure into the signal. We find that the dark matter-induced emission provides an adequate fit to the existing EHT data, implying that in addition to the jet, a dark matter spike may account for a sizable portion of the millimeter emission from the innermost (subparsec) region of M87. Regardless, our results show that the EHT can probe very weakly annihilating dark matter. Current EHT observations already constrain very small cross sections, typically down to a few 10^{-31} cm^{3} s^{-1} for a 10 GeV candidate, close to characteristic values for p-wave-suppressed annihilation. Future EHT observations will further improve constraints on the DM scenario.

astro-ph.GA↗

arXiv2018

Intermediate mass black holes and dark matter at the Galactic center

Could there be a large population of intermediate-mass black holes (IMBHs) formed in the early universe? Whether primordial or formed in Population III, these are likely to be very subdominant compared to the dark matter density, but could seed early dwarf galaxy/globular cluster and supermassive black hole formation. Via survival of dark matter density spikes, we show here that a centrally concentrated relic population of IMBHs, along with ambient dark matter, could account for the Fermi gamma-ray "excess" in the Galactic center because of dark matter particle annihilations.

astro-ph.GA↗

arXiv2018

FSD: Frequency Space Differential measurement of CMB spectral distortions

Although the Cosmic Microwave Background agrees with a perfect blackbody spectrum within the current experimental limits, it is expected to exhibit certain spectral distortions with known spectral properties. We propose a new method, Frequency Space Differential (FSD) to measure the spectral distortions in the CMB spectrum by using the inter-frequency differences of the brightness temperature. The difference between the observed CMB temperature at different frequencies must agree with the frequency derivative of the blackbody spectrum, in the absence of any distortion. However, in the presence of spectral distortions, the measured inter-frequency differences would also exhibit deviations from blackbody which can be modeled for known sources of spectral distortions like $y\,\&\,μ$. Our technique uses FSD information for the CMB blackbody, $y$, $μ$ or any other sources of spectral distortions to model the observed signal. Successful application of this method in future CMB missions can provide an alternative method to extract spectral distortion signals and can potentially make it feasible to measure spectral distortions without an internal blackbody calibrator.

astro-ph.CO↗

arXiv2017

Searching for Secluded Dark Matter with H.E.S.S., Fermi-LAT, and Planck

Short-lived mediators are often used to describe dark matter interactions with Standard Model particles. When the dark matter mass is heavier than the mass of the mediator, it may self-annihilate into short-lived mediators, and in some cases this might be the dominant annihilation channel. This scenario is known as secluded dark matter. We use Fermi-LAT observations of dwarf spheroidal galaxies, H.E.S.S. data from the Galactic center, and Planck measurements of the Cosmic Microwave Background to constrain secluded dark matter. We explore the interplay between these experiments and we assess the impact of the mediator mass on our bounds, an often overlooked yet very important point. In particular, we exclude pair -annihilation cross-sections greater or on the order of $σv \sim 4 \times 10^{-27} {\rm cm^3/s}$ for dark matter masses around $10$~GeV and greater or on the order of $σv \sim \times 10^{-25} {\rm cm^3/s}$ for dark matter masses around a TeV. Our findings supersede previous constraints which use Fermi-LAT data, and constitute the first limits on secluded dark sectors using the H.E.S.S. telescope. We also show that one can fit TeV gamma-ray observations from H.E.S.S. with secluded dark matter annihilations, with the mediator mass impacting the best-fit dark matter particle mass. Our findings indicate that any assessment of secluded dark sectors in the context of indirect detection significantly depends on the choice of the mediator mass.

hep-ph↗

arXiv2017

Measuring Polarized Emission in Clusters in the CMB S4 Era

The next generation of CMB experiments (CMB Stage-4) will produce a Sunyaev-Zel'dovich (SZ) cluster catalog containing $\sim10^{5}$ objects, two orders of magnitudes more than currently available. In this paper, we discuss the detectability of the polarized signal generated by scattering of the CMB quadrupole on the cluster electron gas using this catalog. We discuss the possibility of using this signal to measure the relationship between cluster optical depth and mass. We find that the area of observation of S4 maximizes the signal-to-noise (S/N) on the polarized signal but that this S/N is extremely small for an individual cluster, of order 0.5% for a typical cluster in our catalog, the main source of noise being the residual primordial E-mode signal. However, we find that the signal could be detected using the full cluster catalog and that the significance of the result will increase linearly with the size of the CMB S4 telescope mirror.

astro-ph.CO↗

arXiv2017

Ultrahigh-energy cosmic rays from tidally-ignited white dwarfs

Ultra-high-energy cosmic rays (UHECRs) can be accelerated by tidal disruption events of stars by black holes. We suggest a novel mechanism for UHECR acceleration wherein white dwarfs (WDs) are tidally compressed by intermediate-mass black holes (IMBHs), leading to their ignition and subsequent explosion as a supernova. Cosmic rays accelerated by the supernova may receive an energy boost when crossing the accretion-powered jet. The rate of encounters between WDs and IMBHs can be relatively high, as the number of IMBHs may be substantially augmented once account is taken of their likely presence in dwarf galaxies. Here we show that this kind of tidal disruption event naturally provides an intermediate composition for the observed UHECRs, and suggest that dwarf galaxies and globular clusters are suitable sites for particle acceleration to ultra-high energies.

astro-ph.HE↗

arXiv2017

Fermionic Light Dark Matter particles and the New Physics of Neutron Stars

Dark Matter constitutes most of the matter in the presently accepted cosmological model for our Universe. The extreme conditions of ordinary baryonic matter, namely high density and compactness, in Neutron Stars make these objects suitable to gravitationally accrete such a massive component provided interaction strength between both, luminous and dark sectors, at current experimental level of sensitivity. We consider several different DM phenomenological models from the myriad of those presently allowed. In this contribution we review astrophysical aspects of interest in the interplay of ordinary matter and a fermionic light Dark Matter component. We focus in the interior nuclear medium in the core and external layers, i.e. the crust, discussing the impact of a novel dark sector in relevant stellar quantities for (heat) energy transport such as thermal conductivity or emissivities.

astro-ph.HE↗

arXiv2017

AGN feedback in dwarf galaxies?

Dwarf galaxy anomalies, such as their abundance and cusp-core problems, remain a prime challenge in our understanding of galaxy formation. The inclusion of baryonic physics could potentially solve these issues, but the efficiency of stellar feedback is still controversial. We analytically explore the possibility of feedback from Active Galactic Nuclei (AGN) in dwarf galaxies and compare AGN and supernova (SN) feedback. We assume the presence of an intermediate mass black hole within low mass galaxies and standard scaling relations between the relevant physical quantities. We model the propagation and properties of the outflow and explore the critical condition for global gas ejection. Performing the same calculation for SNe, we compare the ability of AGN and SNe to drive gas out of galaxies. We find that a critical halo mass exists below which AGN feedback can remove gas from the host halo and that the critical halo mass for AGN is greater than the equivalent for SNe in a significant part of the parameter space, suggesting that AGN could provide an alternative and more successful source of negative feedback than SNe, even in the most massive dwarf galaxies.

astro-ph.GA↗

arXiv2017

The natural emergence of the correlation between H2 and star formation rate surface densities in galaxy simulations

In this study, we present a suite of high-resolution numerical simulations of an isolated galaxy to test a sub-grid framework to consistently follow the formation and dissociation of H$_2$ with non-equilibrium chemistry. The latter is solved via the package KROME, coupled to the mesh-less hydrodynamic code GIZMO. We include the effect of star formation (SF), modelled with a physically motivated prescription independent of H$_2$, supernova feedback and mass losses from low-mass stars, extragalactic and local stellar radiation, and dust and H$_2$ shielding, to investigate the emergence of the observed correlation between H$_2$ and SF rate surface densities. We present two different sub-grid models and compare them with on-the-fly radiative transfer (RT) calculations, to assess the main differences and limits of the different approaches. We also discuss a sub-grid clumping factor model to enhance the H$_2$ formation, consistent with our SF prescription, which is crucial, at the achieved resolution, to reproduce the correlation with H$_2$. We find that both sub-grid models perform very well relative to the RT simulation, giving comparable results, with moderate differences, but at much lower computational cost. We also find that, while the Kennicutt-Schmidt relation for the total gas is not strongly affected by the different ingredients included in the simulations, the H$_2$-based counterpart is much more sensitive, because of the crucial role played by the dissociating radiative flux and the gas shielding.

astro-ph.GA↗

arXiv2017

Massive Black Holes from Dissipative Dark Matter

We show that a subdominant component of dissipative dark matter resembling the Standard Model can form many intermediate-mass black hole seeds during the first structure formation epoch. We also observe that, in the presence of this matter sector, the black holes will grow at a much faster rate with respect to the ordinary case. These facts can explain the observed abundance of supermassive black holes feeding high-redshift quasars. The scenario will have interesting observational consequences for dark substructures and gravitational wave production.

astro-ph.CO↗

arXiv2017

Dark matter line searches towards dwarf galaxies with H.E.S.S

High energy $γ$-rays are powerful probes in the search for annihilations of dark matter (DM) par- ticles in dense environments. In several DM particle models their annihilation produces characteristic features such as lines, bumps or cut-offs in their energy spectrum. The High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes is perfectly suited to search for such features from multi-TeV mass DM particles. The Dwarf Spheroidal Galaxies (dSphs) of the Local Group are the most common satellites of the Milky Way and assumed to be gravitationally bound dominantly by DM, with up to O(10 3 ) times more mass in DM than in visible matter. Over the past decade, several observational campaigns on dwarf satellite galaxies were launched by H.E.S.S. amounting to more than 140 hours of exposure in total. The observations are reviewed here. In the absence of clear signals, the expected spectral and spatial morphologies of signal and background are used to derive constraints on the DM particle annihilation cross- section for particle models producing line-like signals. The combination of the data of all the dwarf galaxies allows a significant improvement in the HESS sensitivity.

astro-ph.HE↗

arXiv2017

Gravitational Waves from Stellar Black Hole Binaries and the Impact on Nearby Sun-like Stars

We investigate the impact of resonant gravitational waves on quadrupole acoustic modes of Sun-like stars located nearby stellar black hole binary systems (such as GW150914 and GW151226). We find that the stimulation of the low-overtone modes by gravitational radiation can lead to sizeable photometric amplitude variations, much larger than the predictions for amplitudes driven by turbulent convection, which in turn are consistent with the photometric amplitudes observed in most Sun-like stars. For accurate stellar evolution models, using up-to-date stellar physics, we predict photometric amplitude variations of $1$ -- $10^3$ ppm for a solar mass star located at a distance between 1 au and 10 au from the black hole binary, and belonging to the same multi-star system. The observation of such a phenomenon will be within the reach of the Plato mission because telescope will observe several portions of the Milky Way, many of which are regions of high stellar density with a substantial mixed population of Sun-like stars and black hole binaries.

astro-ph.GA↗