M. Riley Owens

Astronomer

GitHub LinkedIn ORCiD ResearchGate

Research

Unveiling ionizing photon escape with high-redshift lensed galaxies

My primary research interest is how individual star-forming regions regulate the production and escape of hydrogen-ionizing photons (commonly just ionizing photons or the Lyman continuum (LyC)). This is important to understand because it has significant implications for cosmic reionization (z ~ 6–10), wherein the first generations of stars and active galactic nuclei (AGN) ionized the intergalactic medium (IGM). The relative contribution of stars versus AGN, the role of massive, luminous galaxies versus light, faint ones, and the time evolution of both dichotomies remain unclear.

Directly studying galaxies during reionization is difficult. The galaxies are extremely distant and generally faint, and the increasingly opaque IGM as near as z > 2 typically makes it impossible to directly observe their LyC photons. This means estimates of LyC escape for reionization galaxies must generally rely on proxies for LyC escape like rest-ultraviolet (UV) colors, dust content, ionization state, etc., which do not always have a clear relation to LyC escape. And although the angular diameter evolution is favorable to reionization galaxies, they are typically unresolved, preventing spatially resolved analyses of their interiors, which is critical to understand LyC production and escape methods.

Studying nearby galaxies instead is promising because many are spatially resolved, but they become unresolved by z=0.1, and they are not necessarily fair comparisons to reionization galaxies, typically requiring criteria to select for galaxies analogous to reionization galaxies. These criteria can suffer from biases. Their LyC is also totally inaccessible to ground-based instruments, and the only state-of-the-art instrument that can access their LyC is the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph, which has an on-sky aperture far too large to constrain LyC escape to individual regions of a galaxy, and often a limited wavelength coverage below the Lyman limit.

My work uses high-redshift, gravitationally lensed galaxies at cosmic noon (z ~ 1–3) to offer an optimal middle ground between directly studying reionization and studying local galaxies. These galaxies offer many advantages. They are much closer in time to reionization, so we expect them to generally be more comparable to reionization galaxies. Their foreground lenses magnify their light by as much as tens or hundreds of times, and increase the angular size of the galaxy, routinely permitting the resolution of individual star-forming regions, and in extreme cases, individual stars. At these redshifts, the LyC from these galaxies is directly accessible with the sharp imaging of optical instruments on HST, and the rest-UV through rest-optical is well-captured by optical and near-infrared instruments on ground-based telescopes, offering a complementary wealth of spectral diagnostics constraining outflows, gas conditions, stellar populations, and much more. This combination is a powerful tool to characterize the environments that leak LyC photons. I combine sharp space-based lens modeling from my collaborators with the rest-UV signatures of outflows, nebular emission, and neutral gas morphology to characterize the ISM as it relates to the escape of LyC photons.

Relevant papers: Mainali et al. (2022), Kim et al. (2023), Rivera-Thorsen et al. (2024)

My primary research interest is how the smallest physical mechanisms in galaxies regulate the production and escape of hydrogen-ionizing photons (commonly just ionizing photons or the Lyman continuum (LyC)). This is important to understand because it has significant implications for cosmic reionization, an ancient cosmological epoch wherein the first generations of ionizing sources ionized most of the universe. The relative contribution of different types of ionizing sources and their contribution's time dependence is still unclear.

Directly studying galaxies during reionization (remember: light has a speed limit, so that we observe galaxies how they appeared when they first emitted their light arriving to us) is difficult. Not only are the galaxies extremely distant and often faint, but it is typically impossible to directly observe their ionizing photons. They were mostly spent ionizing the universe. This makes it difficult to predict from their observable characteristics how many ionizing photons they produce and emit. Additionally, these galaxies are typically unresolved on the sky, meaning we can't see their internal structure to understand how internal mechanics of a galaxy affects its ionizing photon production and escape. Galaxies are busy places made with many building blocks interacting in complex manners, meaning it is critical to peer into galactic interiors to robustly understand galaxies.

That wasn't very encouraging to hear. So what else can we do?

Owing to their proximity, nearby galaxies are excellent choices to scrutinize physical processes in galaxies in detail. They are large on the sky, and modern telescopes can easily make out their tiny features and shape. But for these galaxies, it is impossible to detect their ionizing photons with ground-based telescopes since the Earth's atmosphere absorbs their ionizing photons. Space-based telescopes can circumvent this, but apart from space-based telescopes being more difficult to build, deploy, and operate, ultraviolet detectors are technically challenging to construct, which can severely limit the wavelength range of ionizing photons they can detect, and their ability to distinguish small details on the sky. Furthermore, nearby galaxies are often much more evolved than galaxies from reionization, making a direct comparison an 'apples to oranges' one. Just studying nearby galaxies thought to be analogous to those from reionization can help resolve this, but this is still subject to possible biases of what we think makes nearby galaxies analogous to galaxies during reionization.

My work uses distant, gravitationally lensed galaxies to offer an optimal middle ground between directly studing reionization and studying local galaxies. The largest assemblies of mass in the universe, galaxy clusters, can magnify the light from galaxies behind them by tens or hundreds of times, and also 'zoom in' on background galaxies, so that they appear much larger on the sky than they otherwise would. In this manner, it is often possible to study the interiors of lensed galaxies. The great distance to these lensed galaxies also causes a redshift of their light due to the expansion history of the universe. This means that their light stretches to longer wavelengths as it travels to us. This redshift is strong enough for these galaxies that it is feasible to observe their ionizing photons with optical detectors that offer excellent image quality and wavelength coverage. Because these galaxies are so distant, they can make fairer comparisons to galaxies during reionization, since we see them separated by much less time. I combine the superior image quality of space-based telescopes with ground-based spectroscopy, and use the two to pin down the tiny mechanics that make galaxies leak ionizing photons.

See my list below of authored and co-authored journal articles and white papers. Click the chevrons for a brief review of the publication's results. For a complete breakdown of my citation statistics and publication record, visit my ORCiD filter on NASA/ADS.

Co-author

First author

Co-author

Principal investigator

Co-investigator

Impact

Unveiling ionizing photon escape with high-redshift lensed galaxies

My work uses high-redshift, gravitationally lensed galaxies at cosmic noon (z ~ 1–3) to offer an optimal middle ground between directly studing reionization and studying local galaxies. These galaxies offer many advantages. They are much closer in time to reionization, so we expect them to generally be more comparable to reionization galaxies. Their foreground lenses magnify their light by as much as tens or hundreds of times, and increase the angular size of the galaxy, routinely permitting the resolution of individual star-forming regions, and in extreme cases, individual stars. At these redshifts, the LyC from these galaxies is directly accessible with the sharp imaging of optical instruments on HST, and the rest-UV through rest-optical is well-captured by optical and near-infrared instruments on ground-based telescopes, offering a complementary wealth of spectral diagnostics constraining outflows, gas conditions, stellar populations, and much more. This combination is a powerful tool to characterize the environments that leak LyC photons. I combine sharp space-based lens modeling from my collaborators with the rest-UV signatures of outflows, nebular emission, and neutral gas morphology to characterize the ISM as it relates to the escape of LyC photons.

Gallery

This is where I keep photo albums from my travels. Hover over each destination's thumbnail for more information, and click to see the complete album. The albums appear in reverse chronological order from left to right and top to bottom. All pictures are mine unless credited otherwise.