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 there are still many unanswered questions about why that is and how present-day galaxies ended up the way they are.

Astronomers have only been intentionally 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 challenge, 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 chemical abundance patterns (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 Giant Magellan Telescope---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 Keck Baryonic Structure Survey (KBSS)

Quasar field

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 now encompasses observations of more than 1100 individual galaxies.

Near-infrared spectoscropy of individual distant galaxies

KBSS contains more than 700 galaxies at z~2-3, with quality near-infrared spectroscopic observations of ~400 individual systems. The figure below comes from Strom et al. (2017), where 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.

N2-BPT

Measuring oxygen in distant galaxies

The nebular 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, 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.

Part of my ongoing work with the KBSS sample involves using a combination of BPASSv2 stellar population models and photoionization modeling with Cloudy to obtain self-consistent estimates of O/H, N/O, and ionization parameter U for individual 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, and the results for individual galaxies are described in Strom et al. (2018).

MOSPEC, an IDL-based analysis tool for MOSFIRE spectra

MOSPEC

MOSPEC is an interactive analysis tool developed in IDL specifically for MOSFIRE spectroscopy and is designed to reproduce many of the central functionalities of the splot task in IRAF. MOSPEC allows the user to extract 1D spectra from the 2D spectrograms produced by the MOSFIRE data reduction pipeline, using either the default aperture based on the CSU mask design file or an aperture defined in real-time by the user. In the case of emission-line galaxies (such as those in KBSS), MOSPEC can also be used to model and measure line fluxes for a specified list of emission lines.

If you are interested in using MOSPEC for your observations, please contact me for access to the beta version designed for public use.

About me




Allison Strom

Astronomer

My research focuses on the chemical enrichment of distant galaxies, primarily by analyzing their rest-UV and rest-optical spectra with ground-based telescopes like Keck, Magellan, and Subaru. I am also interested in the overlap between extragalactic observational science and theoretical predictions, not only of galaxy formation and evolution, but also concerning stellar evolution. Galaxies in the early Universe are powerful laboratories for studying stellar populations with unique properties, particularly with respect to their chemical composition and energetic feedback on their surroundings.

Advocate

In addition to research, I also engage in public outreach and seek to promote diversity, equity, and inclusion (DEI) in the scientific community. I have given public talks at venues across the LA area, partnered with the local YWCA, and even talked about the history of astronomy on public-access television. As co-chair of the DEI series at Carnegie Observatories, I have led and organzied workshops around topics like inclusive mentoring and allyship. As a local alumna of Caltech, I am also an active member of the Women Mentoring Women program and a mentor through the LA chapter of Step Up Women's Network.

Contact



Department of Astrophysical Sciences
4 Ivy Lane
Princeton University, Princeton, NJ 08544

Office 213
Telephone: (907) 230-7994
Email: allison.strom [at] astro.princeton.edu