Dwarves and dark matter

Starting small can sometimes help tackle a big question, like the nature of dark matter. That’s the hope of Assistant Professor Alexie Leauthaud, whose research features dwarf galaxies, the most common—but also most elusive—galaxies in the Universe. Cosmologists refer to these relatively tiny collections of stars (e.g., one billion versus our Milky Way’s 200–400 billion) as “laboratories for dark matter,” Leauthaud said, because they contain copious amounts of the mysterious substance.

A sunset photograph shows the Cerro Tololo Inter-American Observatory (CTIO), a program of the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) in the mountains of northern Chile. More than 20 telescopes operate at the site; dominating the mountain peak in the foreground is the Víctor M. Blanco 4-m Telescope, now being used for the Merian Survey. Credit: CTIO/NOIRLab/NSF/AURA/B. Tafreshi (CC BY 4.0).

Taken with the Hubble Space Telescope, this image shows a dwarf galaxy, named UGC 685, located about 15 million light-years from Earth. The most common type of galaxy in the Universe, these relatively small collections of stars are a focus of research for assistant professor of astronomy and astrophysics Alexie Leauthaud. Credit: ESA/Hubble & NASA (CC BY 4.0).

With Princeton’s Jenny Greene, Leauthaud co-leads the Merian Survey, an international research collaboration that in March 2021 began using the Blanco Telescope in Chile to map and assess 100,000 dwarf galaxies. Captured through custom-made filters, the discovery of so many dwarf galaxies will enable the astronomers to use a technique called gravitational lensing to measure—for the first time—the amount of dark matter they contain. While light normally travels in a straight line, gravitational forces warp its path. Measuring this warp with gravitational lensing infers the mass of a galaxy: the bigger the deviation, the bigger the mass and how much dark matter is present, Leauthaud said.

This composite of six images, composed by graduate student **Yifei Luo**, shows three representative dwarf galaxies mapped and measured by the Merian Survey team in March 2021. The galaxy on the left is five times larger than the middle one, and ten times larger than the one on the right. Each top “color” image combines images obtained with different bands of light, while the corresponding bottom images contain a single color only. Credit: Courtesy of Alexie Leauthaud.

Given the massive amount of data the survey will collect, Leauthaud anticipates “exciting science,” including about the extent, distribution, and nature of dark matter and how it varies between galaxies. “We’re pushing into new territory,” she said.

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