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),
Owens 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.
