Probing the Tiny: A New Look at the Boötes II Dwarf Galaxy
In this installment of the Pristine Dwarf-Galaxy survey, Nicolas Longeard and collaborators dive into one of the Milky Way’s faintest and most mysterious companions: the ultra-faint dwarf galaxy Boötes II (Boo II). Using high-quality spectroscopic data from the VLT’s FLAMES instrument, the team sets out to paint a clearer picture of Boo II’s stars—their speeds, their chemical makeup, and even their membership in this elusive galaxy.
A Hidden Galaxy Comes into Focus
Boo II is one of the faintest dwarf galaxies orbiting the Milky Way. Astronomers are especially interested in galaxies like Boo II because their properties can help test theories about how galaxies form and how the Universe evolved. However, finding and studying the stars that belong to Boo II is no easy task. It lies about 66,000 light-years away and is easily confused with stars from our own galaxy. This study builds on previous work, including that of Bruce et al. (2023), by using advanced techniques to better select and analyze Boo II’s true members.
Finding the Right Stars
To choose which stars to observe, the team used several layers of data. First, they looked at the stars' brightness and color—key clues about their age and metal content. They also used precise motion measurements from the Gaia space telescope to estimate which stars move like they belong to Boo II. Most importantly, they used a tool called the Pristine survey, which can detect stars that are especially poor in metals—just like the stars expected in an ancient dwarf galaxy. Stars with higher metal content were likely to be from the Milky Way and were excluded.
Peering into the Spectrum
Observations were made using the FLAMES instrument on the Very Large Telescope in Chile. This tool spreads out a star’s light to reveal dark absorption lines caused by elements in the star’s atmosphere. By studying these features—especially the Calcium Triplet lines—the team could calculate how fast each star is moving (its radial velocity) and how metal-rich it is. After carefully processing and analyzing the spectra of 39 stars, they found nine new members of Boo II, bringing the total number of confirmed members to a new high. Two of these stars are classified as "Extremely Metal-Poor" ([Fe/H] < –3.0), making them some of the most pristine stars ever found in Boo II.
The Galaxy's Motion and Makeup
Using their larger and more accurate sample, the researchers measured Boo II’s average motion and how much variation exists among the stars’ speeds—known as the velocity dispersion. They found an average velocity of about –127 km/s and a velocity dispersion of about 5.6 km/s. This suggests that Boo II is held together by a significant amount of dark matter. Importantly, the team found no strong evidence of a "velocity gradient," which would have hinted that the galaxy is being pulled apart or disrupted by the Milky Way.
Building a Better Galactic Model
Why do all these measurements matter? Astronomers use them to compare real galaxies like Boo II to computer simulations of galaxy formation. When the data match the models, scientists gain confidence in their understanding of how galaxies—especially the tiniest ones—form and survive in a Universe dominated by dark matter. The precise measurements of Boo II’s metal content and motions help test these models, while the discovery of extremely metal-poor stars provides targets for future studies on early chemical enrichment in galaxies.
A Path Forward
This study not only strengthens our grasp of Boo II’s structure but also helps refine techniques for finding and analyzing stars in other ultra-faint dwarf galaxies. The approach combining Pristine photometry, Gaia motions, and high-resolution spectroscopy could be applied to similar systems, deepening our understanding of the Milky Way’s smallest companions. With each new member star identified, astronomers get closer to understanding the building blocks of galaxies—and the cosmic history written in their stars.
Source: Longeard
