Cosmic ray interactions and their associated deposition of energy and momentum are a crucial agent in controlling the physical properties and evolution of galaxies and their environment. My research considers the engagement of cosmic rays with galactic and circum-galactic media and phenomena, with a focus on cosmic ray interactions, their micro-physics and their observable signatures.
Cosmic rays in galaxies
Vibrantly star-forming galaxies are rich in energetic cosmic rays. These are accelerated by diffusive shock acceleration processes in supernova remnants. The cosmic rays interact to release energy into their environment, with their thermalization regulated by the production of secondary leptons, momentum transfer by scattering in magnetic fields and direct collision and ionization processes. In order to investigate the role energetic cosmic rays have in the thermal evolution of young galaxies, I developed a model to compute the distribution of cosmic rays in an evolving galactic magnetic field, by solving their transport equation for an idealized galaxy configuration. I demonstrated that cosmic rays become entangled within the interstellar environment of their host galaxy
just a few million years after the onset of star-formation. Thus they are refocused to deliver a powerful heating effect inside a galaxy. I found the heating power of the cosmic rays in interstellar gas of a highly star-forming galaxy could be comparable to heating effect of stars in a hot, ionized interstellar medium. However, the dense molecular cloud complexes that harbor the sites of star-formation are typically shielded from starlight, leaving cosmic rays as the sole agent able to deliver feedback to these regions. [read more in Owen et al. 2018].
Image Credit: ESA/Herschel/SPIRE/PACS/Gould Belt Survey/D. Arzoumanian (CEA Saclay)
I introduced a detailed model to assess the patterns of cosmic ray propagation, heating and ionization in these dense, magnetized molecular cloud complexes. This is a necessary step to properly link cosmic ray micro-physical feedback processes to galactic-scale consequences. Detailed observations of molecular clouds in external galaxies are not available. Thus, I turned instead to the Milky Way to apply and test my models. I developed a novel method to empirically-derive cosmic ray diffusion parameters through molecular clouds, based on the dispersion statistics of magnetic field fluctuations. Using
targets in the Milky Way as a test-bed, I computed an empirical cosmic
ray diffusion parameter and used this to model the distribution of cosmic rays and the heating and ionization patterns they would deliver inside cloud complexes. Although this indicated only negligible cosmic ray heating would arise in Milky Way environments, much more powerful heating would be expected in star-forming galaxies, where cosmic ray energy densities are higher. [read more in Owen et al. 2021a, Owen et al. 2021b].
High energy processes and phenomena around galaxies
Cosmic rays modify the physical properties of circum-galactic and intergalactic media (CGM and IGM, respectively). A recent study investigated the development of these flows using 1D and 2D hydrodynamical simulations. I used these simulations to construct synthetic X-ray emission spectra from their hot gas. Their X-ray spectra were sensitive to the local thermal conditions of the outflow and, hence, its stage of evolution and the fraction of driving power provided by CRs. A novel broad-band X-ray `color' method to differentiate between flow driving mechanisms observationally could then be introduced. This will open-up the possibility to study the internal physics of outflows from distant galaxies, without a need to spatially or spectrally resolve them. This will be particularly important
Image Credit: M82: X-ray: NASA/CXC/JHU/D.Strickland; Optical: NASA/ESA/STScI/AURA/The Hubble Heritage Team; IR: NASA/JPL-Caltech/Univ. of AZ/C. Engelbracht
Cosmic rays themselves are also affected by the dynamics and properties of their environment. Within the fast-flowing magnetized plasma of a galactic outflow, cosmic rays can be transported by advection away from the interstellar medium (ISM) of their host galaxy and into the CGM or beyond. By solving the transport equation within an outflow, I showed that, conservatively, 10% of the cosmic ray luminosity of a star-forming galaxy can typically be transferred to its CGM by advection, where it would thermalize and also provide additional outward pressure. This would impact on the recycling of gases between the CGM and ISM and could even modify entrained multi-phase structures. Any cosmic rays that are not advected can still indirectly impact the CGM. For example, I found that CR secondary electrons arising from hadronic interactions would up-scatter background photons in high-redshift (z~7) galaxies. This could sustain a strong X-ray glow, affecting the local galactic ecosystem. I found that this could even account for the quenching timescale inferred for some high-redshift galaxies. [read more in Owen et al. 2019a, Owen et al. 2019b, Wu, Li, Owen et al. 2020].
Star-formation history in MACS 1149-JD1, a gravitationally lensed galaxy at z = 9.11. CR containment and heating power co-evolve with the magnetic field strength. Three star-formation history models are shown. Even the most moderate (green) is sufficient to yield CR feedback powerful enough to quench the galaxy for 100 million years. The inferred star-forming activity of the galaxy at z = 9.11 is shown by the solid orange line, after a quenched epoch. See Owen et al. 2019a for details.
Cosmic rays can also drive energy emission from bubbles around galaxies. An example this phenomena are the Fermi bubbles of our Galaxy. I investigated the spatial and multi-wavelength spectral emission from similar bubbles around external galaxies by performing MHD simulations, and then post-processed their results to show that bubbles located around nearby galaxies would be detectable in gamma-rays and at radio frequencies, with their observable radio emission persisting for 7 million years, out to distances of 10s of Mpc. This opens-up the possibility of using future radio observations (e.g. with SKA) to study the properties of galaxy bubbles, and capture their evolutionary progression. I further modeled hadronic cosmic ray emission processes in these bubbles, and compared their spectral emission features with their leptonic counterparts and considered possible ways hadronic and leptonic bubbles could be distinguished [read more in Owen & Yang 2021a, Owen & Yang 2021b].
2D projection plot of the radio emission from our simulated galaxy bubble, integrated over 8.3-15.3 GHz, computed with a post-processing approach. The left panel shows the emission at 1 Myr, while the right panel shows the emission at 7 Myr. The emission is leptonic in origin in the model, and is dominated by synchrotron at these radio frequencies.
The origins and characteristics of energetic backgrounds
A significant product of cosmic ray interactions is their emission of gamma-rays. As these are produced quickly and directly by hadronic interaction processes, it can be considered that gamma-ray emission can be used as a tracer for cosmic ray engagement and feedback within galaxies. I showed that gamma-ray emitting galaxy populations embed a signal in the extra-galactic gamma-ray background (EGB), at scales set by the galaxy power spectrum. Being a function of redshift, this would allow EGB contributions from galaxy populations at different redshifts to be distinguished, according to the spatial scale of their EGB impressions. I modeled the gamma-ray emission properties of populations of star-forming galaxies, accounting for internal gamma-ray absorption effects within interstellar media. I showed this emission can be well-parametrised by just a few key quantities (star-formation rate, concentration, interstellar gas density and the cosmic ray spectral index), which allows for rapid physical modeling of EGB intensity statistics. I used this model to compute the EGB spectra and intensity anisotropies that would arise from parametrized star-forming galaxy source populations. [read more in Owen, Lee & Kong 2021a, Owen, Lee & Kong 2021b].
Total star-forming galaxy contribution to the EGB between 0.1−50 GeV (line 1), of which the contribution from cascade emission due to inverse-Compton scattered EBL radiation fields is indicated (line 2). Comparison with 50 months of Fermi-LAT data for the the total observed diffuse EGB (taken from Ajello et al. 2015, with original data from Ackermann et al. 2015) is shown, together with model predictions for all blazar contributions (band 3, with the lower, middle and upper grey lines showing the three models presented in Ajello et al. 2015), the all AGN and starburst galaxy contributions computed by Makiya et al. 2011 (lines 4 and 5, respectively), the total SFG contribution calculated by Chakraborty & Fields 2013 (line 6), and the total, direct and cascade emission from SFG nuclei computed by Peretti et al. 2020 (lines 7, 8 and 9, respectively).
Alongside star-forming galaxies, other sources (e.g. Blazars) contribute high energy radiation and particles to cosmic backgrounds. I investigated the contribution of distant source populations of ultra high-energy (UHE) cosmic rays to the flux arriving on Earth. When accounting for the interactions of UHE cosmic ray nuclei with cosmological radiation fields (e.g. the CMB), their propagation length would practically be limited to a few 10s of Mpc. However, I showed that this effective particle
`horizon' could be extended when accounting for the photo-degradation cascade of nuclei along UHE cosmic ray propagation paths. Moreover, despite their strong attenuation, I showed the residual flux of UHE nuclei arriving on Earth is significant, and naturally forms an isotropic UHE cosmic ray `background', upon which weak anisotropies associated with nearby cosmic accelerators would be superposed. The impact of different redshift distributions of UHE CR source populations would leave a signature in their mean nuclear mass <lnA> as a function of energy, with photo-degradation processes (e.g. photo-spallation) building up more mass of smaller nuclei for more distant populations of sources. [read more in Owen et al. 2021c].
Average UHE CR mass composition, <ln A >, for four source classes, compared to data obtained with the Pierra Auger Observatory