arXiv subjects

Ellis R. Owen

Publications and source records attributed to Ellis R. Owen.

At least 19 recordsLinked to original sources

arXiv2026

Cosmic ray heating of cold streams: Implications for the gas supply and growth of massive galaxies

Recent observations have demonstrated the presence of cosmic rays (CRs) in cosmic-web filaments. Cold streams supply gas inflows from these filaments into massive galaxies during the cosmic noon. As these streams are expected to be magnetised, external cosmic-web CRs may become entrained with this inflowing gas. We aim to determine whether this externally-supplied CR population can deposit energy to alter or disrupt the supply of cold gas to galaxies. We couple a spectrally-resolved CR transport calculation to a redshift-dependent analytical model of magnetised cold streams in galaxy haloes and investigate whether externally-supplied CRs can modify gas supply through this channel. We find CR energy deposition can alter the thermal state of cold streams. Dense stream cores remain largely resilient and only experience weak heating. Their temperature is raised by less than a factor of 10, which is insufficient to overcome radiative cooling at the stream-CGM interface. In more diffuse streams, and in partially mixed interface gas of the most massive haloes near the virial radius, CR heating becomes strong enough that radiative cooling can no longer balance it, and the gas is heated toward or above the mixing-layer temperature. This weakens the stability of the stream, making it more susceptible to disruption. Complete evaporation is possible only in extreme cases. Cold streams are therefore more vulnerable to CR heating at larger galactocentric radii, higher halo masses, and in more diffuse or partially-mixed stream material. By preferentially heating diffuse gas, externally supplied CRs may introduce additional selectivity into cold-gas accretion that modifies the gas supply and growth of massive galaxies. These CRs weaken fragile streams and erode their cold envelope, and may cause surviving cold flow components to appear thinner and more sharply confined far into galaxy haloes.

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arXiv2026

Revisiting Disk Winds in Active Galactic Nuclei as an Origin of Cosmic Gamma-ray and Neutrino Backgrounds

The origin of the cosmic neutrino background (CNB) and the cosmic gamma-ray background (CGB) remains uncertain. Accretion disk winds driven by active galactic nuclei (AGNs) have been proposed as possible contributors, but their predicted background levels depend sensitively on poorly constrained wind energetics and ambient densities. We revisit the AGN disk-wind scenario by constructing a lepto-hadronic wind model calibrated with radio and GeV gamma-ray data of nearby Fermi-LAT-detected Seyfert galaxies. In our framework, cosmic rays accelerated both at wind-driven forward and reverse shocks produce synchrotron, external-Compton, and hadronic emission. We also incorporate recent XRISM constraints on wind parameters. Applying our calibrated lepto-hadronic models to an AGN population synthesis model, we find that disk winds contribute at most $\lesssim 5\%$ of the CGB above 10 GeV and $\lesssim 10\%$ of the CNB around 100 TeV, suggesting that they are unlikely to dominate both backgrounds. Finally, we identify nearby Seyfert galaxies hosting ultrafast outflows as promising targets for future TeV gamma-ray and TeV$-$PeV neutrino observations, which would offer firm tests of the disk-wind scenario.

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arXiv2026

Multiwavelength Probes of Cosmic Ray Transport in Molecular Cloud Structures

We investigate how cosmic ray (CR) transport in molecular clouds and their substructures can be probed using multi-wavelength observations. The detailed microphysics regulating the penetration and coupling of CRs in dense molecular structures is unsettled. Self-generated turbulence can produce scattering and diffusive transport, while ion-neutral damping in cold, dense gas promotes ballistic CR propagation. We construct a self-consistent framework for CR transport and interactions in magnetized molecular clouds, considering three limiting propagation scenarios: ballistic transport, diffusion, and a hybrid configuration with a diffusive envelope and quasi-ballistic core. By forward-modeling pion-decay $γ$-ray emissivities, CR-driven ionization-rate profiles, and electron synchrotron emission in the hard X-ray band, we connect GeV attenuation and propagation signatures to independent diagnostics of secondary production and CR penetration. As an illustrative example, we apply our framework to the Taurus molecular cloud complex and selected embedded clumps. We show that CR scattering may be substantially enhanced on clump scales, with inferred CR diffusion coefficients suppressed relative to canonical interstellar medium (ISM) values at GeV energies. In this interpretation, CRs are more closely coupled with dense gas in the ISM, and a diffusive envelope boosts the effective gas column density encountered by the CRs. This increases the hadronic interaction rate in the cloud. In turn, the secondary CR electron injection is also increased, and CR ionization rates are elevated at higher densities. We show that a hard X-ray synchrotron emission component is also generated, which may be detectable with near-future facilities. Finally, we discuss how future $γ$-ray, X-ray, and ionization constraints will provide firm tests of CR propagation theories in molecular cloud environments.

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arXiv2026

The 'Forgotten' Neutrons: Implications for the Propagation of High-Energy Cosmic Rays in Magnetized Astrophysical and Cosmological Structures

Cosmological filaments, galaxy clusters, and galaxies are magnetized reservoirs of cosmic rays (CRs). The exchange of CRs across these structures is usually modeled assuming that they remain charged and magnetically confined. At high energies, hadronic interactions can convert CR protons to neutrons. This physics is routinely included in air-shower and ultra-high-energy (UHE) CR propagation Monte Carlo simulations used for composition studies but is rarely treated explicitly in propagation models of CR transport and exchange between magnetized reservoirs. CR neutrons are not affected by magnetic fields and can propagate ballistically over kpc-Mpc distances before decaying back into protons, with relativistic time dilation extending their effective decay length. We show how such charged-neutral switching modifies CR confinement and escape in four representative environments: a Milky Way-like galaxy, a starburst galaxy, a galaxy cluster, and a cosmological filament. By solving the transport of a confined CR proton population in each structure using a diffusion/streaming propagation approach with hadronic pp and p$γ$ interactions, and treating neutron production and decay as a stochastic Poisson ''jump'' process, we find that neutron-mediated steps can allow additional CR escape from large-scale cosmological structures at energies where charged-particle transport alone would predict strong CR confinement and attenuation in ambient radiation fields. These effects imply a qualitative shift in how ultra-high-energy CRs are transferred from embedded sources into filaments and voids once intermediate neutron propagation is considered, with consequences for the partitioning of CRs across the large-scale structure of the Universe.

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arXiv2026

Neutrinos as a new tool to characterise the Milky Way Centre

The Central Molecular Zone (CMZ), a star-forming region rich in molecular clouds located within hundreds of parsecs from the centre of our Galaxy, converts gas into stars less efficiently than anticipated. A key challenge in refining star-formation models is the lack of precise mapping of these dense molecular hydrogen clouds, where traditional tracers often yield inconsistent results. We demonstrate how, in the near future, neutrinos will emerge as a robust mass tracer due to worldwide advancements in neutrino telescopes. Neutrinos are produced alongside gamma-rays when cosmic-rays interact with molecular clouds. The neutrino production rate is proportional to the gas density without dependence on the complex properties of a cloud. Neutrinos also have the advantage of negligible absorption and unambiguous production channels, making it a method with the lowest systematic uncertainties. In an optimistic case where most gamma-ray emission from the Galactic Centre region originates from pion decays, we expect several tens of muon neutrinos to be detected in about two decades. Neutrinos from the CMZ will provide indications on the biases of traditional mass tracers and thus indirectly enhance the accuracy of gas measurements in far galaxies from which a neutrino signal is not detectable.

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arXiv2025

Second Discovery of GeV-TeV Connection from the Globular Cluster UKS 1

Using 16 years of data collected by Fermi Large Area Telescope and 1523 days of survey data from High Altitude Water Cherenkov (HAWC) Observatory, we discovered the long-sought second GeV-TeV connection towards the globular cluster (GC) UKS 1 (Shin et al. 2025). Gamma-ray spectroscopy suggests that the GeV emission can be attributed to both the pulsar magnetosphere and inverse Compton scattering (ICS) by the pulsar wind. In particular, the TeV peak is displaced from the cluster center by several tidal radii in the trailing direction of the proper motion of UKS 1. This alignment supports a scenario in which relativistic leptons, likely driven by a millisecond pulsar population, produce very-high-energy (VHE) gamma-rays via ICS within a bow shock tail. Our findings not only highlights GCs as potential sources of VHE gamma-rays, but also offers a rare opportunity to probe cosmic ray transport in the Milky Way by studying particle propagation and anisotropic gamma-ray production associated with the extended, offset TeV feature of UKS 1.

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arXiv2025

Cosmic Rays in Galaxy Halos: Impacts on Galactic Outflows and Baryon Cycling

Galaxies with high star-formation rate surface densities often host large-scale outflows that redistribute energy, momentum, and baryons between the interstellar medium and the halo, making them a key feedback channel regulating galaxy evolution. Despite their importance, the driving physics behind galactic outflows and their interaction with the surrounding halo is yet to be fully understood. In particular, the influence of a pre-existing reservoir of cosmic rays (CRs) in galaxy halos has not been clearly established. We determine the conditions required to launch outflows in the presence of halo CRs and investigate how CR pressure gradients modify outflow speeds. We find that CR halos suppress the development of large-scale, CR-driven winds and redirect CR feedback toward local recycling flows. Slow outflows are therefore more likely in young galaxies lacking extended CR halos, while fast winds in intense starbursts are dominated by momentum injection and largely unaffected by halo CRs.

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arXiv2025

Starburst-Driven Galactic Outflows -- Unveiling the Suppressive Role of Cosmic Ray Halos

Aims. We investigate the role of cosmic ray (CR) halos in shaping the properties of starburst-driven galactic outflows. Methods. We develop a microphysical model for galactic outflows driven by a continuous central feedback source, incorporating a simplified treatment of CRs. The model parameters are linked to the effective properties of a starburst. By analyzing its asymptotic behavior, we derive a criterion for launching starburst-driven galactic outflows and determine the corresponding outflow velocities. Results. We find that in the absence of CRs, galactic outflows can only be launched if the star-formation rate (SFR) surface density exceeds a critical threshold proportional to the dynamical equilibrium pressure. In contrast, CRs can always drive slow outflows. CRs dominate in systems with SFR surface densities below the critical threshold but become negligible in highly star-forming systems. However, in older systems with established CR halos, the CR contribution to outflows diminishes once the outflow reaches the galactic scale height, rendering CRs ineffective in sustaining outflows in such systems. Conclusions. Over cosmic time, galaxies accumulate relic CRs in their halos, providing additional non-thermal pressure support that suppresses low-velocity CR-driven outflows. We predict that such low-velocity outflows are expected only in young systems that have not yet built up significant CR halos. In contrast, fast outflows in starburst galaxies, where the SFR surface density exceeds the critical threshold, are primarily driven by momentum injection and remain largely unaffected by CR halos.

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arXiv2025

GeV-TeV Connections in Galaxies: Evolutionary Signatures from Pulsars in Globular Clusters

The dominant mechanisms underlying high-energy $γ$-ray emission from galaxies vary by galaxy type. In starbursts, a major contribution comes from neutral pion decay. This is driven by interactions between interstellar gas and hadronic cosmic rays (CRs), which are accelerated in strong shocks associated star formation activity and stellar remnants. Leptonic $γ$-ray emission can also arise from electrons directly energized in interstellar shocks, produced via charged pion decays, or emitted by pulsars and their surrounding halos. In quiescent galaxies, pulsars and their halos can represent a major $γ$-ray source class, with millisecond pulsars predominantly located in globular clusters (GCs) being particularly important. Recent detections of very high-energy (VHE) emission from Galactic GCs suggests they may also contribute to the TeV $γ$-ray flux from evolved galaxies. We consider a scenario where this VHE emission from GCs is powered by electrons accelerated in communal stellar/pulsar wind cluster termination shocks. These electrons undergo inverse Compton scattering as they propagate into GC magnetotails. Our results show that the high-energy emission from GCs can be an important contributor to the GeV and TeV flux from massive, quiescent galaxies. The relative strength of each component depends on the global galactic properties and its evolutionary history.

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arXiv2025

The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Using Geant4 Monte Carlo particle simulations, we model the lunar gamma-ray spectrum. Our results demonstrate its consistency with Fermi-LAT observations, and predict that next-generation MeV gamma-ray instruments will detect both the lunar MeV continuum and several key spectral line features, notably the $1.779~\mathrm{MeV}$ line from $\mathrm{^{28}Si}$ de-excitation enhanced by the lunar surface composition, the $e^+e^-$ annihilation line, and radioactive decay lines from $\mathrm{^{22}Na}$ ($τ\approx3.75\,\mathrm{yr}$) and long-lived $\mathrm{^{26}Al}$ ($τ\approx1\,\mathrm{Myr}$). These gamma-ray lines are sensitive to CRs with energies $\lesssim1\,\mathrm{GeV\,nuc^{-1}}$, offering unique temporal probes of CR activity over different timescales. Observations of the lunar MeV gamma-ray spectrum will therefore open a new window to study the current irradiation of the solar-terrestrial environment by low-energy CRs and its long-term temporal evolution.

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arXiv2025

High energy extragalactic multimessenger backgrounds from starburst and dead galaxies

Starburst galaxies are $γ$-ray sources. Canonically, their emission is driven by hadronic cosmic rays (CRs) interacting with interstellar gas, forming $γ$-rays via the decay of neutral pions. Charged pions are also formed in this process. They decay into secondary leptons, including electrons and neutrinos. Starburst galaxies are therefore also expected to be neutrino sources, and their high-energy $γ$-ray emission may include a secondary leptonic component. Leptonic $γ$-rays may also originate from electrons directly energized by shocks within the interstellar medium of galaxies, or from pulsars and their surrounding halos. In the Milky Way, pulsars/pulsar halos are the dominant $γ$-ray source class. They are associated with stellar remnants or old stellar populations, and are presumably abundant in old galaxies. In this work, we show that the collective high-energy emission from galaxies can account for only a fraction of extragalactic neutrinos, but can form a major component of the extragalactic $γ$-ray background. Contrary to the traditional view, a substantial fraction of this radiation may originate from leptonic processes, including from old, quiescent galaxies.

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arXiv2025

Cosmic ray neutrons in magnetized astrophysical structures

Cosmic rays are often modeled as charged particles. This allows their non-ballistic propagation in magnetized structures to be captured. In certain situations, a neutral cosmic ray component can arise. For example, cosmic ray neutrons are produced in considerable numbers through hadronic pp and p$γ$ interactions. At ultrahigh energies, the decay timescales of these neutrons is dilated, allowing them to traverse distances on the scale of galactic and cosmological structures. Unlike charged cosmic rays, neutrons are not deflected by magnetic fields. They propagate ballistically at the speed of light in straight lines. The presence of a neutral baryonic cosmic ray component formed in galaxies, clusters and cosmological filaments can facilitate the escape and leakage of cosmic rays from magnetic structures that would otherwise confine them. We show that, by allowing confinement breaking, the formation of cosmic-ray neutrons by high-energy hadronic interactions in large scale astrophysical structures can modify the exchange of ultra high-energy particles across magnetic interfaces between galaxies, clusters, cosmological filaments and voids.

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arXiv2025

A possible GeV-TeV connection in the direction of the Globular Cluster UKS 1

Using public data collected by the Fermi Large Area Telescope (LAT) over 16 years, and the 1523 days of survey data (3HWC) from the High Altitude Water Cherenkov (HAWC) observatory, we searched for possible GeV-TeV connections in globular clusters (GCs). In addition to the confirmed $γ-$ray GCs in the 4FGL catalog, we report a GeV detection at the position of UKS 1 with a post-trial probability of $\sim8\times10^{-5}$ of it being a fluctuation. Its spectrum within this energy range is well described by a power-law model with $Γ\simeq2.3\pm0.5$. Furthermore, this GeV feature appears to extend southeast in a direction towards the Galactic plane. From the 3HWC survey data, we have also identified a TeV feature in the direction of UKS 1. It is well-resolved from any known Very High Energy (VHE) source. The post-trial probability that this feature is a fluctuation is $\sim3\times10^{-4}$. If confirmed, this would be the second detection of a TeV feature in the proximity of a GC. While the GeV emission mostly coincides with the center of UKS 1, the TeV peak is displaced from the cluster center by several tidal radii in the trailing direction of the GC's proper motion. Given the supersonic speed of UKS 1 at $\sim270$ km s$^{-1}$, our findings are consistent with a scenario where the VHE $γ-$rays are produced by inverse Compton scattering between relativistic particles and ambient soft photon fields during the course of their propagation away from the head of the bow shock.

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arXiv2025

The Disk Wind Contribution to the Gamma-Ray emission from the nearby Seyfert Galaxy GRS 1734-292

Radio-quiet Seyfert galaxies have been detected in GeV gamma-rays by the $Fermi$ Large Area Telescope (LAT), but the origin of much of this emission is unclear. We consider the nearby example, the Seyfert galaxy GRS 1734-292, which exhibits weak starburst and jet activities that are insufficient to explain the observed gamma-ray flux. With the first detailed multi-wavelength study of this source, we demonstrate that an active galactic nucleus (AGN) disk wind can account for its gamma-ray emission. Using a lepto-hadronic emission model based on a shocked ambient medium and a shocked wind region created by an AGN accretion disk wind, we identify two viable scenarios that are consistent with the $Fermi$-LAT data and multi-wavelength observations: a hadronic $pp$-dominated scenario and a leptonic external Compton-dominated scenario. Both of these show that future observations with the Cherenkov Telescope Array (CTA) and the Southern Wide-field Gamma-ray Observatory (SWGO) could detect TeV emission from a disk wind in GRS 1734-292. Such a detection would substantially improve our understanding of cosmic ray acceleration efficiency in AGN disk wind systems, and would establish radio-quiet Seyfert galaxies as cosmic ray accelerators capable of reaching ultra-high energies.

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arXiv2024

Energetic particles and high-energy processes in cosmological filaments and their astronomical implications

Large-scale cosmic filaments connect galaxies, clusters and voids. They are permeated by magnetic fields with a variety of topologies. Cosmic rays with energies up to $10^{20}\;\!{\rm eV}$ can be produced in astrophysical environments associated with star-formation and AGN activities. The fate of these cosmic rays in filaments, which cannot be directly observed on Earth, are rarely studied. We investigate the high-energy processes associated with energetic particles (cosmic rays) in filaments, adopting an ecological approach that includes galaxies, clusters/superclusters and voids as key cosmological structures in the filament ecosystem. We derive the phenomenology for modelling interfaces between filaments and these structures, and investigate how the transfer and fate of energetic cosmic ray protons are affected by the magnetism of the interfaces. We consider different magnetic field configurations in filaments and assess the implications for cosmic ray confinement and survival against hadronic pion-producing and photo-pair interactions. Our analysis shows that the fate of the particles depends on the location of their origin within a filament ecosystem, and that filaments act as `highways', channelling cosmic rays between galaxies, galaxy clusters and superclusters. Filaments can also operate as cosmic `fly paper', capturing cosmic ray protons with energies up to $10^{18}\;\!{\rm eV}$ from cosmic voids. Our analysis predicts the presence of a population of $\sim 10^{12}-10^{16}\;\!{\rm eV}$ cosmic ray protons in filaments and voids accumulated continually over cosmic time. These protons do not suffer significant energy losses through photo-pair or pion-production, nor can they be cooled efficiently. Instead, they form a cosmic ray fossil record of the power generation history of the Universe.

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arXiv2023

Neutrino Imaging of the Galactic Centre and Millisecond Pulsar Population

In this work, we consider the possible presence of a large population of millisecond pulsars in the Galactic Centre. Their direct detection would be challenging due to severe pulse broadening caused by scattering of radiation. We propose a new method to constrain their population with neutrino imaging of the Galactic Centre. Millisecond pulsars are proposed cosmic-ray accelerators. The high-energy protons they produce will collide with the baryonic matter in the central molecular zone to create charged and neutral pions that decay into neutrinos and $γ$-rays, respectively. The specific neutrino and $γ$-ray fluxes must be below their corresponding observed values, allowing us to put a conservative upper limit on the millisecond pulsar population of N_MSP < 10,000 within a galacto-centric radius of 20 pc. This upper limit is sensitive to the proton acceleration efficiency of the pulsars, but is less dependent on the particle injection spectral index and the choice of mass tracers. The population will be better constrained when high resolution neutrino observations of the Galactic Centre become available. The presence of these millisecond pulsars can account for the $γ$-ray excess in the Galactic Centre.

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arXiv2023

Cosmic ray calorimetry in star-forming galaxy populations and implications for their contribution to the extra-galactic $γ$-ray background

Star-forming galaxies (SFGs) have been established as an important source population in the extra-galactic $γ$-ray background (EGB). Their intensive star-formation creates an abundance of environments able to accelerate particles, and these build-up a rich sea of cosmic rays (CRs). Above GeV energies, CR protons can undergo hadronic interactions with their environment to produce $γ$-rays. SFGs can operate as CR proton "calorimeters", where a large fraction of the CR energy is converted to $γ$-rays. However, CRs also deposit energy and momentum to modify the thermal and hydrodynamic conditions of the gas in SFGs, and can become a powerful driver of outflows. Such outflows are ubiquitous among some types of SFGs, and have the potential to severely degrade their CR proton calorimetry. This diminishes their contribution to the EGB. In this work, we adopt a self-consistent treatment of particle transport in outflows from SFGs to assess their calorimetry. We use 1D numerical treatments of galactic outflows driven by CRs and thermal gas pressure, accounting for the dynamical effects and interactions of CRs. We show the impact CR-driven flows have on the relative contribution of SFG populations to the EGB, and investigate the properties of SFGs that contribute most strongly.

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arXiv2023

Cosmic ray processes in galactic ecosystems

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal-mechanical and radiative feedback have been widely considered, however cosmic rays (CRs) are also powerful energy carriers in galactic ecosystems. Resolving the capability of CRs to operate as a feedback agent is therefore essential to advance our understanding of the processes regulating galaxies. The effects of CRs are yet to be fully understood, and their complex multi-channel feedback mechanisms operating across the hierarchy of galaxy structures pose a significant technical challenge. This review examines the role of CRs in galaxies, from the scale of molecular clouds to the circum-galactic medium. An overview of their interaction processes, their implications for galaxy evolution, and their observable signatures is provided and their capability to modify the thermal and hydrodynamic configuration of galactic ecosystems is discussed. We present recent advancements in our understanding of CR processes and interpretation of their signatures, and highlight where technical challenges and unresolved questions persist. We discuss how these may be addressed with upcoming opportunities.

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