The Faintest Glow: Ultra Faint Dwarf Galaxies Quiz

UFDs are almost entirely "dry." They lost their gas billions of years ago, which is why they only contain ancient, low-mass stars and no new star-forming regions.

Studying UFDs provides the evidence required by the standard: the chemical composition of matter (low metallicity) and the motion of galaxies (velocity dispersion) both support our current scientific model of the universe.

The existence of tiny galaxies held together by dark matter proves that dark matter can "clump" into small structures, which is a key requirement of the Cold Dark Matter model.

Because UFDs are so small, when they orbit the Milky Way, our gravity can pull stars out of them, eventually dissolving the UFD into a "stellar stream."

Stars in UFDs are typically over 10 billion years old, whereas the Sun is only about 4.6 billion years old. These stars formed shortly after the Big Bang.

Discovered in 2005, Willman 1 was the first of a new class of galaxies that were much fainter than anyone previously thought possible, opening a new field of research in dark matter.

Tracking how these galaxies move across the sky over years helps us "weigh" the Milky Way and understand the distribution of dark matter throughout our entire local system.

Instead of looking at extremely distant, faint galaxies to see the early universe, we look at UFDs in our Local Group, which are "time capsules" from the same early era but much closer to home.

By plotting the color versus the brightness (magnitude) of stars, astronomers can see a distinct "Main Sequence" and "Red Giant Branch" that proves the stars are all at the same distance and have the same age.

During the era of reionization, the universe heated up. Tiny galaxies like UFDs didn't have enough gravity to keep their gas when it got hot, effectively "freezing" their evolution in time.

UFDs generally consist of very old stars and almost no gas. Their star formation was likely "quenched" by the heat of the early universe's reionization, preserving the chemical state of the cosmos as it was billions of years ago.

Big Bang simulations predict many small "clumps" of dark matter. UFDs are likely the visible versions of those clumps. Finding more of them helps align our observations with our theoretical models of the universe.

By using spectroscopy to find the radial velocity (motion toward/away from us), scientists can determine how much gravity—and therefore how much dark matter—is present in the UFD.

The Rubin Observatory (LSST) will conduct deep, wide-field surveys, allowing us to find thousands of "missing" ultra-faint satellites that are currently too dim for us to see.

Without the massive gravitational "well" provided by a dark matter halo, these tiny galaxies would have been torn apart long ago by the tidal forces of larger galaxies like the Milky Way.

UFD stars have very few elements heavier than helium (low metallicity). This provides a "direct link" to the primary nucleosynthesis of the Big Bang, confirming our models of how matter first formed.

UFDs are so dim and spread out that they blend into the background. We need high-resolution telescopes and complex data filtering to distinguish their stars from those belonging to our own galaxy.

Because UFDs have so few stars for their total gravitational weight, they are the most dark matter-dense objects discovered, making them the ultimate test subjects for dark matter theories.

Astronomers use algorithms to look for clusters of stars that share the same distance and age (color-magnitude diagrams). Finding a group of stars moving and "aging" together reveals the hidden galaxy.

UFDs have extremely low surface brightness. They are so diffuse that they often look like random "over-densities" of stars in the foreground of the Milky Way rather than distinct objects.