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Publication list

See my ORCiD profile and ADS for full publication list. Some recent highlighted papers are below.


The extragalactic γ-ray background: imprints from the physical properties and evolution of star-forming galaxy populations
Ellis R. Owen, Albert K. H. Kong, Khee-Gan Lee

Published in Monthly Notices of the Royal Astronomical Society; Volume 513, Issue 2, pp.2335-2348

Star-forming galaxies (SFGs) harbour an abundant reservoir of cosmic rays (CRs). At GeV energies, these CRs undergo interactions with their environment to produce gamma-rays, and the unresolved gamma-ray emission from populations of SFGs forms a component of the isotropic extragalactic gamma-ray background (EGB). In this work, we investigate the contribution to the 0.01 - 50 GeV EGB from SFG populations located up to redshift z=3. We find this is dominated by starbursts, while the contribution from main sequence SFGs is marginal at all energies. We also demonstrate that most of the gamma-ray contribution from SFGs emanates from low mass galaxies, with over 80 per cent of the emission originating from galaxies with stellar masses below 10e8 solar masses. Many of these galaxies are located at relatively high redshift, with their peak EGB contribution arising ∼700 Myr before the noon of cosmic star-formation. We find that the precise redshift distributions of EGB sources at different energies imprint intensity signatures at different angular scales, which may allow their contribution to be distinguished using analyses of small-scale EGB intensity anisotropies, particularly if the diffuse EGB is dominated by hadronic CR-driven gamma-ray emission from SFGs. We show that the EGB is sensitive to the evolution of low mass populations of galaxies, particularly around z~2.5, and that it provides a new means to probe the engagement of CRs in these galaxies before the high noon of cosmic star-formation.

Published in Monthly Notices of the Royal Astronomical Society; Volume 510, Issue 4, pp.5834-5853

The evolutionary behaviour and multiwavelength emission properties of bubbles around galaxies, such as the Fermi bubbles of the Milky Way, is unsettled. We perform 3D magneto-hydrodynamical simulations to investigate the evolution of leptonic galaxy bubbles driven by a 0.3-Myr intense explosive outburst from the nucleus of Milky-Way-like galaxies. Adopting an ageing model for their leptonic cosmic rays, we post-process our simulations to compute the multiwavelength emission properties of these bubbles. We calculate the resulting spectra emitted from the bubbles from radio frequencies to gamma-rays, and construct emission maps in four energy bands to show the the development of the spatial emission structure of the bubbles. The simulated bubbles show a progression in their spectral properties as they age. In particular, the TeV gamma-ray emission is initially strong and dominated by inverse Compton scattering, but falls rapidly after ∼1 Myr. In contrast, the radio synchrotron emission remains relatively stable and fades slowly over the lifetime of the bubble. Based on the emission properties of our post-processed simulations, we demonstrate that gamma-ray observations will be limited in their ability to detect galaxy bubbles, with only young bubbles around nearby galaxies being within reach. However, radio observations with, e.g. the upcoming Square Kilometer Array, would be able to detect substantially older bubbles at much greater distances, and would be better placed to capture the evolutionary progression and diversity of galaxy bubble populations.


Outflows from starburst galaxies with various driving mechanisms and their X-ray properties
B. P. Brian Yu, Ellis R. Owen, Kuo-Chuan Pan, Kinwah Wu, Ignacio Ferreras

Published in Monthly Notices of the Royal Astronomical Society; Volume 508, Issue 4, pp.5092-5113

Outflows in starburst galaxies driven by thermal-mechanical energy, cosmic rays, and their mix are investigated with 1D and 2D hydrodynamic simulations. We show that these outflows could reach a stationary state, after which their hydrodynamic profiles asymptotically approach previous results obtained semi-analytically for stationary outflow configurations. The X-rays from the simulated outflows are computed, and high-resolution synthetic spectra and broad-band light curves are constructed. The simulated outflows driven by thermal mechanical pressure and CRs have distinguishable spectral signatures, in particular, in the sequence of the keV K α lines of various ions and in the L-shell Fe emission complex. We demonstrate that broad-band colour analysis in X-rays is a possible alternative means to probe outflow driving mechanisms for distant galaxies, where observations may not be able to provide sufficient photons for high-resolution spectroscopic analyses.

Ultra high-energy cosmic rays from beyond the Greisen-Zatsepin-Kuz'min horizon
Ellis R. Owen, Qin Han, Kinwah Wu, Y. X. Jane Yap, Pooja Surajbali

Published in the Astrophysical Journal; Volume 922, Issue 1, id.32, pp. 15

A study investigating where the sources of the highest energy particles in our Universe originate from. We find more than half of the detected cosmic rays on Earth, at ultra high-energies originate from a large number of distant source populations. Much of the flux has been attenuated by the CMB and extra-galactic background light, but the residual still can produce a strong background component.

Characterizing the signatures of star-forming galaxies in the extragalactic γ-ray background
Ellis R. Owen, Khee-Gan Lee, Albert K. H. Kong

Published in Monthly Notices of the Royal Astronomical Society; Volume 506, Issue 1, pp.52-72

In this work, we modelled the contribution distant populations of star-forming galaxies could make to the isotropic gamma-ray background. We show that the dust and the extra-galactic background light are important factors in determining the spectrum observed, but can be well described by simple parameterised models. We further demonstrate how anisotropies can be used to reveal new insights into characteristics of star-forming galaxies as an unresolved source population in the gamma-ray background.

Observational signatures of cosmic ray interactions in molecular clouds
Ellis R. Owen, Alvina Y. L. On, Shih-Ping Lai, Kinwah Wu

Published in the Astrophysical Journal; Volume 913, Issue 1, id.52, pp. 25

We introduce a new method to assess the propagation of cosmic rays in molecular cloud complexes, using information derived from polarised dust emission/absorption signatures from these regions (which presumably trace local magnetic field structures). We apply our method to the IC 5146 molecular cloud complex in Cygnus, to derive effective cosmic ray propagation parameters, and discuss the associated cosmic ray driving heating and ionisation patterns they may cause in this region, as well as their observational signatures.


A hydrodynamical study of outflows in starburst galaxies with different driving mechanisms
B. P. Brian Yu, Ellis R. Owen, Kinwah Wu, Ignacio Ferreras

Published in Monthly Notices of the Royal Astronomical Society; Volume 492, Issue 3, pp.3179-3193

Outflows from starburst galaxies can be driven by thermal pressure, radiation, and cosmic rays. We present an analytic phenomenological model that accounts for these contributions simultaneously to investigate their effects on the hydrodynamical properties of outflows. We assess the impact of energy injection, wind opacity, magnetic field strength, and the mass of the host galaxy on flow velocity, temperature, density, and pressure profiles. For an M82-like wind, a thermally dominated driving mechanism is found to deliver the fastest and hottest wind. Radiation-driven winds in typical starburst-galaxy configurations are unable to attain the higher flow velocities and temperatures associated with thermal and cosmic ray-driven systems, leading to higher wind densities which would be more susceptible to cooling and fragmentation at lower altitudes. High opacity winds are more sensitive to radiative driving, but terminal flow velocities are still lower than those achieved by other driving mechanisms at realistic opacities. We demonstrate that variations in the outflow magnetic field can influence its coupling with cosmic rays, where stronger fields enable greater streaming but less driving near the base of the flow, instead with cosmic rays redirecting their driving impact to higher altitudes. The gravitational potential is less important in M82-like wind configurations, and substantial variations in the flow profiles only emerge at high altitude in massive haloes. This model offers a more generalized approach to examine the large-scale hydrodynamical properties for a wide variety of starburst galaxies.

Charge-exchange emission and cold clumps in multiphase galactic outflows
Kinwah Wu, Kaye Jiale Li, Ellis R. Owen, Ji Li, Shuinai Zhang, Graziella Branduardi-Raymont

Published in Monthly Notices of the Royal Astronomical Society; Volume 491, Issue 4, pp.5621-5635

Large-scale outflows from starburst galaxies are multiphase, multicomponent fluids. Charge-exchange lines that originate from the interfacing surface between the neutral and ionized components are a useful diagnostic of the cold dense structures in the galactic outflow. From the charge-exchange lines observed in the nearby starburst galaxy M82, we conduct surface-to-volume analyses and deduce that the cold dense clumps in its galactic outflow have flattened shapes, resembling a hamburger or a pancake morphology rather than elongated shapes. The observed filamentary H α features are therefore not prime charge-exchange line emitters. They are stripped material torn from the slow-moving dense clumps by the faster moving ionized fluid, which are subsequently warmed and stretched into elongated shapes. Our findings are consistent with numerical simulations that have shown that cold dense clumps in galactic outflows can be compressed by ram pressure, and also progressively ablated and stripped before complete disintegration. We have shown that some clumps could survive their passage along a galactic outflow. These are advected into the circumgalactic environment, where their remnants would seed condensation of the circumgalactic medium to form new clumps. The infall of these new clumps back into the galaxy and their subsequent re-entrainment into the galactic outflow form a loop process of galactic material recycling.


Starburst and post-starburst high-redshift protogalaxies: the feedback impact of high energy cosmic rays
Ellis R. Owen, Kinwah Wu, Xiangyu Jin, Pooja Surajbali, Noriko Kataoka

Published in Astronomy & Astrophysics; Volume 626, Article A85

Quenching of star-formation has been identified in many starburst and post-starburst galaxies, indicating burst-like star-formation histories (SFH) in the primordial Universe. Galaxies undergoing violent episodes of star-formation are expected to be rich in high energy cosmic rays (CRs). We have investigated the role of these CRs in such environments, particularly how they could contribute to this burst-like SFH via quenching and feedback. These high energy particles interact with the baryon and radiation fields of their host via hadronic processes to produce secondary leptons. The secondary particles then also interact with ambient radiation fields to generate X-rays through inverse-Compton scattering. In addition, they can thermalise directly with the semi-ionised medium via Coulomb processes. Heating at a rate of ∼10e-25 erg cm-3 s-1 can be attained by Coulomb processes in a star-forming galaxy with one core-collapse SN event per decade, and this is sufficient to cause quenching of star-formation. At high-redshift, a substantial amount of CR secondary electron energy can be diverted into inverse-Compton X-ray emission. This yields an X-ray luminosity of above 10e41 erg s-1 by redshift z = 7 which drives a further heating effect, operating over larger scales. This would be able to halt inflowing cold gas filaments, strangulating subsequent star-formation. We selected a sample of 16 starburst and post-starburst galaxies at 7 ≲ z ≲ 9 and determine the star-formation rates they could have sustained. We applied a model with CR injection, propagation and heating to calculate energy deposition rates in these 16 sources. Our calculations show that CR feedback cannot be neglected as it has the strength to suppress star-formation in these systems. We also show that their currently observed quiescence is consistent with the suffocation of cold inflows, probably by a combination of X-ray and CR heating.

Hadronic interactions of energetic charged particles in protogalactic outflow environments and implications for the early evolution of galaxies
Ellis R. Owen, Xiangyu Jin, Kinwah Wu, Suetyi Chan

Published in Monthly Notices of the Royal Astronomical Society; Volume 484, Issue 2, pp.1645-1671

We investigate the interactions of energetic hadronic particles with the media in outflows from star-forming protogalaxies. These particles undergo pion-producing interactions which can drive a heating effect in the outflow, while those advected by the outflow also transport energy beyond the galaxy, heating the circumgalactic medium. We investigate how this process evolves over the length of the outflow and calculate the corresponding heating rates in advection-dominated and diffusion-dominated cosmic ray transport regimes. In a purely diffusive transport scenario, we find the peak heating rate reaches 10e-26 erg cm-3 s-1 at the base of the outflow where the wind is driven by core-collapse supernovae at an event rate of 0.1 yr-1, but does not extend beyond 2 kpc. In the advection limit, the peak heating rate is reduced to 10e-28 erg cm-3 s-1, but its extent can reach to tens of kpc. Around 10 per cent of the cosmic rays injected into the system can escape by advection with the outflow wind, while the remaining cosmic rays deliver an important interstellar heating effect. We apply our cosmic ray heating model to the recent observation of the high-redshift galaxy MACS1149-JD1 and show that it could account for the quenching of a previous starburst inferred from spectroscopic observations. Re-ignition of later star-formation may be caused by the presence of filamentary circumgalactic inflows which are reinstated after cosmic ray heating has subsided.


Interactions between ultra-high-energy particles and protogalactic environments
Ellis R. Owen, Idunn B. Jacobsen, Kinwah Wu, Pooja Surajbali

Published in Monthly Notices of the Royal Astronomical Society; Volume 481, Issue 1, pp.666-687

We investigate the interactions of energetic hadronic particles (cosmic ray protons) with photons and baryons in protogalactic environments, where the target photons are supplied by the first generations of stars to form in the galaxy and the cosmological microwave background, while the target baryons are the interstellar and circumgalactic medium. We show that pair-production and photo-pion processes are the dominant interactions at particle energies above 10e19 eV, while proton-proton (pp) interaction pion-production dominates at the lower energies in line with expectations from, for example gamma-ray observations of star-forming galaxies and dense regions of our own galaxy's interstellar medium. We calculate the path lengths for the interaction channels and determine the corresponding rates of energy deposition. We have found that protogalactic magnetic fields and their evolution can significantly affect the energy transport and energy deposition processes of cosmic rays. Within a Myr after the onset of star-formation the magnetic field in a protogalaxy could attain a strength sufficient to confine all but the highest energy particles within the galaxy. This enhances the cosmic ray-driven self-heating of the protogalaxy to a rate of around 10e-24 erg cm-3 s-1 for a galaxy with strong star-forming activity that yields one core collapse supernova event per year. This heating power exceeds even that due to radiative emission from the protogalaxy's stellar populations. However, in a short window before the protogalaxy is fully magnetized, energetic particles could stream across the galaxy freely, delivering energy into the circumgalactic and intergalactic medium.