Galactic Demographics: Studying Galaxies at the Population Level

Our understanding of the dynamics of the universe is about to be revolutionized, thanks to the Vera C. Rubin Observatory. Come 2024, this observatory is slated to begin its Legacy Survey of Space and Time (LSST), a campaign that will observe the full sky every three nights over the course of a decade. The survey will capture an unprecedented amount of astrophysical data, opening our eyes to the cosmos as never before.

Groups of researchers worldwide have banded together in forming science collaborations to provide scientific expertise for the Rubin Observatory. Many Kavli Institute-affiliated researchers belong to these science collaborations. This article is part of a series profiling these researchers and the advances they expect the groundbreaking observatory will deliver.

As people are to Earth, galaxies are to the universe.

While not an analogy you will ever see on a standardized test, the comparison holds up surprisingly well. Just as people are what populate the planet (albeit from a narrow anthropomorphic view), galaxies are what populate the cosmos. Humans concentrate in big cities, similar to how galaxies group together in structures called clusters. People change as they age, and so do galaxies. Interactions between people and between galaxies can also leave each other altered (usually only mentally with people, and exclusively physically, of course, with galaxies).

Even from a broad numerical perspective, just a couple orders of magnitude differentiate the populations of these biological and astrophysical entities: There are about eight billion alive on our planet right now, compared to some 100 billion galaxies exist in the universe. Were it not for the fact that people die, unlike galaxies (though galaxies can "disappear" when subsumed by larger galaxies), the numbers would be stunningly similar, with estimates placing the total number of our species to have ever lived right at the 100-billion mark.

All of which is to say that in order to understand the totality of the universe, we humans must understand our galactic quasi-counterparts. LSST is expected to be very helpful in this regard. Accordingly, researchers have created a Galaxies Science Collaboration to leverage how the survey can reveal galactic formation and transformation over galaxies' eons-spanning existences.

Given her interests in these sorts of galactic matters, Yuanyuan Zhang is a member of the Galaxies Science Collaboration. Zhang has studied galaxy clusters, galaxy evolution, and cosmological implications through optical surveys like the Sloan Digital Sky Survey and the Dark Energy Survey—predecessors in many ways to the LSST. Now Zhang looks forward to the big-picture perspective LSST will unprecedently bring by frequently observing the whole southern sky, collecting valuable information on millions upon millions of galaxies in the process.

"I am particularly interested in a couple of questions: how do galaxies interact with their neighbors and their general environment, and how do those interactions change the galaxies over time?" says Zhang, who joins the National Science Foundation's NOIRLab, having wrapped up her work as a Schramm Postdoctoral Fellow at Fermilab where she was also an associate fellow at the Kavli Institute for Cosmological Physics at the University of Chicago.

The people-as-galaxies analogy informs the LSST work. Unlike observatories such as the Hubble Space Telescope or the new James Webb Space Telescope, which deliver excellent information for individual galaxies or groups, LSST instead works at the galactic population level. Space telescopes "can provide very detailed data for some galaxies or galaxy clusters," Zhang explains, but "LSST wins in statistics." She goes on: "If the Hubble Space Telescope or James Webb Space Telescope are good at writing detailed 'biographies' of galaxies or galaxy clusters, LSST is good at writing a demographics survey report."

Hubble and Webb are indeed designed to perform deep observational dives into galactic research subjects. Narrow fields of view and dedicated observing time—think of long camera exposures—can suss out galaxies' distinctive, personal structure and dynamics, as well as characterizations about the kinds of stars within them. The stellar population information in turn speaks to the chemical and astrophysical conditions in galaxies, while also providing evidence of their interaction histories with other galaxies.

LSST will instead capture broader trends, sorting galaxies out within different environments and epochs of the cosmos. In particular, Zhang expects LSST to change the game significantly for studying how galaxies evolve inside galaxy clusters. In these dense congregations, galaxies experience far more interactions than those isolated galaxies that have mostly kept to themselves in the cosmic hinterlands.

Advantageously, galaxy clusters are places where large quantities of the mysterious substance called dark matter co-occur. Scientists only know of this matter's existence through the gravitational effects it exerts on the regular matter we can directly observe. Were it not for this extra gravitation from postulated dark matter, single galaxies would fling their stars out into space, and giant groups of galaxies could not stay conglomerated over billions of years. Examining these clusters could offer critical insight into the nature of dark matter, constraining its possible properties and getting us closer to gleaning what it must be and thus how it could be verified.

Finding galaxy clusters where individual galaxies are undergoing clear interactions with the ample stores of dark matter on hand, however, has been challenging—a needle-in-the-haystack kind of problem that LSST's gathering power should alleviate.

"Those galaxy clusters are very rare," says Zhang, "but given the data-collection ability of LSST, [the survey] will find hundreds of thousands of them, making it possible to study them statistically."

Like how outlying populations can reveal important sociological information not readily gleanable in more average groups, LSST will advance our knowledge of astrophysics and cosmology broadly through identifying special galactic groupings. (It's no accident that Zhang is also a member of the LSST Dark Energy Science Collaboration, which is exploring how the universe is expanding over time.)

Learning more about the universe writ large through its galactic "people" will ultimately circle back to us, Earth's people, and our species' place in the grandness of it all.