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Starts With A Bang

The Smallest Galaxy Ever Discovered Demonstrates The Unequivocal Need For Dark Matter

Small, faint galaxies, like the Draco dwarf spheroidal galaxy shown here, have much more dark matter in them relative to the normal matter found inside than larger galaxies. For the smallest galaxies of all, this is the most extreme example of dark matter. (BERNHARD HUBL / ASTROPHOTON.COM)

It’s not the only one of its kind, but it’s definitely makes no sense without dark matter.


Throughout the Universe, galaxies and star clusters come in all different sizes and masses.

This NASA/ESA Hubble Space Telescope image shows a massive galaxy cluster, PLCK_G308.3–20.2, glowing brightly in the darkness. Although the largest, brightest galaxies in this cluster are the easiest to see, they are vastly outnumbered by smaller, fainter structures with much lower luminosities, and cannot be seen in this image. All told, even with the limits of the Hubble Space Telescope, we’re seeing fewer than 10% of the total galaxies expected to be out there. (ESA/HUBBLE & NASA, RELICS; ACKNOWLEDGEMENT: D. COE ET AL.)

While the brightest ones are always the easiest to see, the faint ones are far greater in number.

The dwarf galaxy UGC 5340 is forming stars irregularly, likely due to a gravitational interaction with a companion galaxy that is not pictured here. Many other galaxies, much more massive but also more distant, can be seen off in the background. (NASA, ESA, AND THE LEGUS TEAM)

Even within our Local Group, with just 2 or 3 large galaxies, some 60 dwarf galaxies and hundreds of globular clusters abound.

Our Local Group of galaxies is dominated by Andromeda and the Milky Way, but there’s no denying that Andromeda is the biggest, the Milky Way is #2, Triangulum is #3, and the LMC is #4. Perhaps 60 other smaller dwarf galaxies abound, and hundreds if not thousands of globular clusters and other even smaller structures make up our corner of the Universe. (ANDREW Z. COLVIN)

Only the stars emit visible light, but all forms of matter exert their gravity.

A galaxy cluster can have its mass reconstructed from the gravitational lensing data available. Most of the mass is found not inside the individual galaxies, shown as peaks here, but from the intergalactic medium within the cluster, where dark matter appears to reside. The time-delay observations of the Refsdal supernova cannot be explained without dark matter in this galaxy cluster, nor can the individual rapid motions of galaxies within the cluster, an observation dating all the way back to Fritz Zwicky in 1933. (A. E. EVRARD. NATURE 394, 122–123 (09 JULY 1998))

The smallest independent structures known today are Segue 1 and Segue 3.

Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy’s history, has heated it. The dark matter-to-normal matter ratio of ~3400-to-1 is the greatest ratio ever seen in the dark matter-favoring direction. (MARLA GEHA AND KECK OBSERVATORIES)

Both of these are small, dwarf galaxies, occupying space in the Milky Way’s galactic halo.

A map of the nearest globular clusters surrounding the Milky Way’s center. The globular clusters closest to the galactic center have a higher metal content than the ones on the outskirts, but measuring the 3D motions of these clusters enables us to infer how much mass is present, total, throughout the Milky Way. It is still debated whether Segue 1 and Segue 3 are best classified as faint globular clusters or very small galaxies. (WILLIAM E. HARRIS / MCMASTER U., AND LARRY MCNISH / RASC CALGARY)

They’re both incredibly intrinsically faint: only a few hundred times as luminous as the Sun.

The globular cluster Messier 75, showing a huge central concentration, is over 13 billion years old. Many globular clusters have stellar populations that are in excess of 12 or even 13 billion years, but the closest ones (i.e., the ones in or around the Milky Way) can have their individual stellar motions measured. (HST / FABIAN RRRR, WITH DATA FROM THE HUBBLE LEGACY ARCHIVE)

With no more than 1000 stars apiece, spread out over just a few light-years in space, they’re the most extreme galaxies known today.

Dwarf galaxy NGC 5477 is one of many irregular dwarf galaxies. The blue regions are indicative of new star formation, but many such galaxies have formed no new stars in many billions of years. A galaxy’s entire history and properties over that history must be understood to determine how they arrived in its present configuration. (ESA/HUBBLE AND NASA)

But we can track the motions of the individual stars inside, inferring each galaxy’s total mass.

Many nearby galaxies, including all the galaxies of the local group (mostly clustered at the extreme left), display a relationship between their mass and velocity dispersion that indicates the presence of dark matter. NGC 1052-DF2 is the first known galaxy that appears to be made of normal matter alone, and was later joined by DF4 earlier in 2019. Galaxies like Segue 1 and Segue 3, however, are very high up and clustered towards the left of this chart; these are the most dark matter-rich galaxies known: the smallest and lowest-mass ones. (DANIELI ET AL. (2019), ARXIV:1901.03711)

Segue 1 holds the extreme record, requiring 600,000 solar masses total for just ~175 solar masses in stars.

Dwarf galaxies, like the one imaged here, have a much greater than 5-to-1 dark matter to normal matter ratio, as bursts of star formation have expelled much of the normal matter. (ESO / DIGITIZED SKY SURVEY 2)

Star formation can eject the excess normal matter, explaining why there’s no gas seen inside.

When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2–10, located 30 million light years away. If there isn’t enough overall gravitation in these objects to hang onto the matter that doesn’t form stars, it will be ejected due to these bursts of star formation. (X-RAY (NASA/CXC/VIRGINIA/A.REINES ET AL); RADIO (NRAO/AUI/NSF); OPTICAL (NASA/STSCI))

All that gravitation must come from somewhere. Only the dark matter explanation fits perfectly.

This large, fuzzy-looking galaxy is so diffuse that astronomers call it a “see-through” galaxy because they can clearly see distant galaxies behind it. The ghostly object, catalogued as NGC 1052-DF2, thought to be dark matter-free, can only exist alongside galaxies like Segue 1 and Segue 3 in a Universe where dark matter exists, but a galaxy’s formation history can occur in different ways. (NASA, ESA, AND P. VAN DOKKUM (YALE UNIVERSITY))

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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