Look up at the night sky. In favorable viewing conditions you’ll be able to detect a large band across the sky. This is our home, the Milky Way. Our galaxy has a diameter of over 100,000 light years and based on current estimates it contains about 250 billion stars. The Milky Way is classified as a spiral galaxy because of its large spiral arms. The Milky Way also has a bar-shaped core, thus making it a barred spiral galaxy. Using data from the New Horizons spacecraft, astronomers now estimate the total galactic population to be roughly 200 billion (far fewer than the initial estimate of 2 trillion) and have discovered many other types of galaxies, including elliptical, lenticular, irregular, etc. In this installment I’ll be discussing the most common galaxy type, and one you may not have heard of: dwarf galaxies.
Excluding the Milky Way, what is the closest Galaxy to Earth? Most will say the Andromeda Galaxy, and that’s a reasonable guess. It’s estimated to be 220,000 light years in diameter, which makes it appear very large in the night sky. However, Andromeda is not actually the closest galaxy. The Andromeda Galaxy is 2.537 million light years away, which makes it only the 19th nearest galaxy to the Earth. So what galaxies are closer? Dwarf galaxies!
A dwarf galaxy is a small, dim galaxy on a scale between a globular cluster and an average large galaxy such as ours. And although they are much smaller, dwarves have the same morphological types as larger galaxies. The Milky Way is orbited by over 50 satellite galaxies, all of which are considered dwarf galaxies. These vary in size, from a few hundred to several thousand light years in diameter.
The largest of the Milky Way’s satellite galaxies and the most well-known dwarf galaxy is the Large Magellanic Cloud. This galaxy has a diameter of 14,000 light years, which makes it the fourth largest galaxy in the Local Group, behind the Triangulum, Milky Way, and Andromeda Galaxies. The Large Magellanic Cloud can be spotted in a dark sky location in the southern hemisphere, and should appear as a smudge near the Milky Way. The galaxy is actually very large in the night sky, with an apparent size at about 10 by 10 degrees. This is many times larger than the Andromeda Galaxy at 3 by 1 degrees.
The Large Magellanic Cloud is classified as a barred irregular galaxy. This means it is irregular but it has a bar at the center like barred spiral galaxies, such as our own. This bar probably exists because the Large Magellanic Cloud interacted with the Milky Way and the Small Magellanic Cloud in the past. The Small Magellanic Cloud is another irregular dwarf galaxy about half the size of the larger cloud in light years, but with only a few hundred million stars compared to the Large Magellanic Cloud’s 30 billion.
Only 8 of the 50 closest dwarf galaxies to the Milky Way are over 1 kiloparsec, or about 3260 light years. Some dwarf galaxies can be extremely small, such as the Segue 1 galaxy at only 0.06 kilo-parsecs or about 200 light years in diameter. This is about the same size as a large globular cluster, such as Messier 3, which is also about 200 light years in diameter. There is no clear boundary between a cluster and a galaxy, but dwarf galaxies are many times dimmer than globular clusters of equivalent size. The Segue 1 dwarf galaxy is one of the darkest known galaxies orbiting the Milky Way. Its integrated luminosity is 300 times the Sun’s, and its mass is currently estimated to be 600,000 solar masses. This gives it a mass to light ratio of 3400, which means it is 3400 times more massive than can be accounted for from observations. The Keck Observatory pointed their 10-meter telescope to Segue 1 in 2011 and could only find 1,000 stars. Astronomers knew there had to be something missing because of the speed of the stars were traveling. Instead of moving at 209 kilometers per second relative to the Milky Way, the stars were going as slow as 194 kilometers per second and as fast as 224 kilometers per second. This indicated the presence of a large amount of unseen dark matter accelerating the stars. Similar results were found in Segue 2 in 2013, another extremely small and faint dwarf galaxy. These galaxies also provide the first evidence that dark matter arranges itself into clumps. Those clumps in turn form larger “halos” comprised of thousands of clumps. These dark matter halos surround the Milky Way and all large galaxies.
Dwarf galaxies have created a lot of confusion in the scientific community. Cosmological simulations with dark matter done in the early 2000s indicated that there should be at least 500 dwarf galaxies around the Milky Way, and thousands more in our Local Group. Yet researchers have only been able to identify about 50 of these dwarves. For decades, astronomers were puzzled by this problem, which they called the “missing satellites” problem. They proposed various complicated theories to explain it, including the existence of “invisible galaxies” that are too faint to observe because they have so little luminous matter.
But recent discoveries have flipped everything upside down. In 2016, physicist Andrew Wetzel and his team created new simulations that included both normal matter and dark matter. They concluded that after 14 billion years, only 13 dwarf satellite galaxies should remain around the Milky Way. So instead of there being too few dwarf galaxies, there were now too many! In the following year, Wetzel’s team compared simulations done with and without normal matter. They found that without normal matter in dwarf galaxies, they can’t have a strong magnetic field. Without a strong magnetic field, dwarf galaxies can pass through the Milky Way unharmed, but those with a strong magnetic field cannot. With normal matter included in the model, the center of the Milky Way has much stronger gravitational pull and most of the dwarf galaxies get torn apart.
Since the Milky Way only consumes dwarf galaxies close to it, it is assumed that there are many more galaxies farther out. This quote from Annika Peter explains the situation pretty well: “The Milky Way is a lot like the Cookie Monster. It’s going to eat all the cookies in the house, but probably not walk to the bakery to eat the ones out there.” Using this information, Peter estimated there are 800-1700 Milky Way satellites that we cannot currently detect. However, when simulating the formation of satellites in the early universe, her team estimated there were only 100-300. A later study in November of 2018 led by James Bullock suggested that there should even fewer satellite galaxies still orbiting ours. Only about 30 should exist. The rest should have been destroyed. These discrepancies between studies mean that the problem isn’t fully solved.
But there has been progress recently. Astronomers previously thought that the threshold for a dark matter halo to have enough gravity to create stars and form a dwarf galaxy was about 500 million times the mass of the sun. Bullock believes it’s significantly lower, at just 30 million times. If small amounts of dark matter can attract enough ordinary matter to create galaxies, then his simulations match up to the observations. Both the shape of the galaxies’ orbits and the number of simulated dark matter halos are the same in the observations and in Bullock’s simulations. These “mini-halos” in the dwarf galaxies could help us better understand dark matter. Beth Willman, who discovered the first ultrafaint dwarf galaxy, said “I feel like we’re really at the cusp of being able to extend what we’re learning from these ultrafaint dwarfs to learning something about dark matter.”
I hope you enjoyed this installment on dwarf galaxies. I will be back with more installments on other topics soon.