Prof. Allison L. Strom

Chemistry is the next frontier in studying cosmic history

The Universe has a long history, spanning nearly 14 billion years. Over that time, a relatively uniform soup of hot sub-atomic particles has been transformed into a stunningly diverse landscape filled with complex and beautiful objects, like the Andromeda Galaxy you see above. The Universe today is populated by billions of galaxies with a wide range of colors, shapes, and sizes. No two galaxies are alike, just as no two people on Earth are exactly the same, but we still don't fully understand how present-day galaxies ended up the way they are and why some galaxies are so different from others.

Astronomers have only been studying the Universe outside our own galaxy, the Milky Way, since the 1920s. At that time, Edwin Hubble and others began measuring the distances to the curious "nebulae" they saw in images (spoiler alert: the Andromeda Galaxy plays an important role in that story too!). Now, less than a hundred years later, the study of galaxies outside the Milky Way—what we call "extragalactic astronomy"—is one of the largest sub-fields in modern astrophysics. Owing to dramatic advances in engineering and technology over the last few decades, we can use the combined power of giant telescopes and supercomputers to help us understand how galaxies grow and change over time.

One of the challenges inherent to observational astronomy is that we can only observe a given galaxy at one point in time, even though we want to know what it was like throughout its life. It's like trying to study human development when you only have snapshots of different people taken decades apart! To get around this problem, I use some of the largest telescopes in the world to learn about the internal properties of distant galaxies, including those that were forming more than 10 billion years ago, when the Universe was still very young. Using detailed spectroscopic observations of individual galaxies, we can look for evidence of what those galaxies were doing in the past. We can also use knowledge of their chemistry (how much oxygen, carbon, etc. is in a galaxy) to link them to galaxies that have the same "cosmic DNA", but that we observe at different points in the past. Combining what we know about galaxies throughout the Universe's history enables us to understand what the life of a typical galaxy might have looked like.

Now is an exciting time to be working on these problems, because we are building new telescopes that will let us peer back to the dawn of cosmic history and allow us to study galaxies everywhere in much greater detail than is currently possible. These facilities—like the James Webb Space Telescope, the Nancy Grace Roman Space Telescope, and the US Extremely Large Telescope Program—will help answer many of our outstanding questions about galaxies. They will also undoubtedly reveal new frontiers for current and future astronomers to explore over the next century of extragalactic astronomy.

The CECILIA Survey

The CECILIA (Chemical Evolution Constrained using Ionized Lines in Interstellar Aurorae) Survey will use NIRSpec on the James Webb Space Telescope (JWST) to observe very faint emission lines in high-redshift galaxies and use these observations to accurately determine the amount of oxygen in their interstellar medium for the first time. These observations are crucial for creating new tools for studying the chemistry of distant galaxies, where we typically have to rely on more easily-observed features that can more difficult to interpret. CECILIA will allow astronomers to learn about some of the youngest and most extreme galaxies in the known Universe.

CECILIA is named in honor of Cecilia Payne-Gaposchkin, the first woman to earn a PhD in astronomy at Harvard. In her 1925 thesis, she proposed that stars were composed primarily of hydrogen and helium, which contradicted the prevailing theory that the Sun and Earth had similar compositions. Senior astronomers at the time dismissed her results, and because she was a woman, Cecilia had far fewer career opportunities than her male peers. Despite these challenges, Cecilia continued to be active in research, and ultimately her work transformed the way astronomers thought about the chemistry of stars. She eventually went on to become a full professor at Harvard, the first woman to have received that honor, and today her doctoral work is recognized as "the most brilliant PhD thesis ever written in astronomy."

The CECILIA Survey is the intellectual descendent of Cecilia's pioneering work and will be one of the first projects conducted with JWST during its five year mission lifetime. Relative to prior space missions, more JWST science will be led by junior scientists, women, and underrepresented minorities than ever before. We are proud that CECILIA (led by two women, Gwen Rudie at the Carnegie Observatories and myself) is part of that step forward.

The Keck Baryonic Structure Survey

The Keck Baryonic Structure Survey (KBSS) is a large, targeted spectroscopic survey designed to jointly probe galaxies and their gaseous environments at the peak of galaxy assembly (z~2-3). The survey comprises 15 independent fields centered on a bright background quasar; the total survey area is 0.24 square degrees, comparable to many of the legacy fields. The KBSS galaxy sample is selected from deep optical and near-infrared imaging and subsequently followed up with spectroscopic observations in the rest-UV (with Keck/LRIS) and rest-optical (with Keck/MOSFIRE) bandpasses. My role since MOSFIRE's commissioning in 2012 has been to lead the near-infrared component of the KBSS survey, which is now one of the largest spectroscopic surveys of high-reshift galaxies.

KBSS contains more than 700 galaxies at z~2-3, with quality near-infrared spectroscopic observations of ~400 individual systems. The figure below shows the correlation beween two ratios of emission lines (the so-called "BPT" diagram) and comes from Strom et al. (2017). In that work, I detail the nebular properties of the z~2-3 KBSS galaxies and conclude that the primary difference with respect to local galaxies is an increase in the overall degree of excitation. At the same time, high-z KBSS galaxies appear to be more chemically evolved (with higher N/O and O/H) than local galaxies with similar excitation conditions, meaning that galaxies in the early Universe must have harder ionizing radiation fields than z~0 galaxies at fixed oxygen abundance. The most likely explanation for this trend is a systematic difference in the star-formation histories of galaxies at z~2-3 and z~0, even at fixed stellar mass.

What we learn from the "chemical DNA" of galaxies

The correlation between galaxies' stellar mass and their chemical enrichment (in elements like oxygen and iron) is a sensitive tracer of the baryonic physics that govern galaxy evolution. Specifically, the shape of this so-called "mass-metallicity relation" is evidence of outflows carrying enriched gas out of galaxies. The precise slope of this relation indicates the balance between these outflows and the accretion of unenriched gas, as well as how these processes may vary as a function of stellar mass. Many cosmological simulations make predictions of the relationship between stellar mass and O/H, and more recently both theorists and observers have been started comparing multiple elemental abundances for the same galaxies.

The figure below is from Strom et al. (2021) and shows the "mass-metallicity relation" for 195 high-redshift galaxies in three different elements: oxygen, nitrogen, and iron. This is the first time that abundance patterns have been reported for a large sample of individual galaxies. These elements are created by stars with difference initial masses and, as a result, have different formation timescales. Consequently, comparing these three relations can provide even more information about galaxies' star formation histories than studies of single elements. The similar slope of the relations for oxygen (O-MZR) and iron (Fe-MZR) indicate that most galaxies at these redshifts have young ages and chemistry dominated by the core-collapse supernovae that occur when massive stars die.

Mass-metallicity relations from Strom et al. (2021)

Measuring oxygen in distant galaxies

The emission line spectra of galaxies originates in the ionized gas surrounding young, massive stars and thus reflects the combined properties of both the gas and stars. Both local galaxies and high-redshift galaxies occupy relatively tight loci in multi-dimensional line-ratio space (like the BPT diagram), which implies strong correlations between the physical properties driving their spectra. However, it remains unclear if such correlations (for example, between ionization parameter and metallicity) are redshift-invariant, limiting the usefulness of empirical abundance calibrations based on z~0 samples.

There is also evidence to suggest that many of the emission line ratios observed for high-excitation nebulae respond more sensitively to changes in the shape and normalization of the ionizing radiation field than to changes in the gas-phase oxygen abundance. This effect is more pronounced at high-redshift because nearly all z~2-3 galaxies exhibit high levels of nebular excitation. Thus, efforts to measure their O/H must also consider their special ionization and excitation conditions.

With this in mind, I use a combination of BPASS stellar population models and photoionization modeling with Cloudy to obtain self-consistent estimates of O/H, N/H, Fe/H, and ionization parameter U for individual high-redshift galaxies. We proved the utility of this approach using deep composite spectra constructed from the rest-UV and rest-optical spectra of a representative subset of KBSS galaxies in Steidel, Strom, et al. (2016), and the results for individual galaxies are described in Strom et al. (2018).

The Subaru Prime Focus Spectrograph

The Prime Focus Spectrograph (PFS) is a massively-multiplexed, fiber-fed (2400 fibers over a ~1 degree diameter FoV) optical-NIR (0.38-1.26 micron) spectrograph now being commissioned at the Subaru Telescope. The PFS galaxy survey slated to begin in 2023 will deliver spectra for hundreds of thousands of galaxies during “cosmic afternoon” at z~0.5-2, the period 5-8 Gyr ago when many galaxies finished their assembly. PFS will open a discovery space at intermediate redshifts like that enabled at z~0 by the Sloan Digital Sky Survey (SDSS) in the 2000s, and I am excited to be part of the team leading this next-generation survey. My role will be to help lead analyses of the chemistry and physical conditions in this unprecedented sample of intermediate-redshift star-forming galaxies.

Allison L. Strom