Object 91: Antlia 2
Podcast release date: 06 February 2023
Right ascension: 09:35:32.8
Corresponding Earth location: Farmland on the southern outskirts of Newbridge, Victoria, Australia
The coordinates for this episode point to a location within the constellation Antlia, one of the stupid constellations invented by the eighteenth century astronomer Nicolas Louis de Lacaille . I've mentioned Lacaille several times before as a prolific creator of stupid constellations. All of his constellations are named after eighteenth century objects, like Horologium the clock or Telescopium the telescope, and they basically contain stars so faint and so disorganized that they don't look like the constellations they are supposed to represent. Antlia is supposed to represent an eighteenth century air pump, but it might as well represent an eighteenth century butter churn, or a twenty first century smartphone, or actress Michelle Yeoh.
The specific object featured in this episode is a very nearby, very low brightess dwarf spheroidal galaxy orbiting the Milky Way. It basically looks like a small elliptical galaxy except that it's extremely faint. Since this object was the second such dwarf galaxy discovered in the constellation Antlia, it was given the designation Antlia 2. Sometimes this is writtten with a Roman numeral II, but most of the time, it's written with an Arabic numeral 2, which is slightly weird because a lot of other dwarf galaxies get designations that use Roman numerals.
Anyway, if people have listened to the first episode in this podcast series, where I discuss the Leo IV Dwarf Galaxy (which gets a roman numeral), the story of the discovery of Antlia 2 is somewhat similar. To recap, the Milky Way, like a lot of other spiral galaxies, has a lot of dwarf galaxies orbiting it. The Large and Small Magellanic Clouds are actually two objects that are visible to the naked eye, so those galaxies were known to prehistoric people, although they are named after Ferdinand Magellan because he was one of the first Europeans to document their existence. After this, a few other galaxies were bright enough to be identified by eighteenth or nineteenth century astronomers looking through telescopes from this time period, although those astronomers would not have understood that these objects were outside our own galaxy. With the advent of photographic surveys in the twentieth century, astronomers were able to identify quite a few more nearby but faint dwarf galaxies, and at this point, they understood that these were objects in orbit around the Milky Way . However, the identification of the faintest dwarf galaxies could only be done in the twenty first century using not only digital imagery and other modern measurement techniques but also statistical methods that were needed to show that some stars seen in some areas of the sky are actually part of another galaxy and not just a slightly overdense part of our own .
Antlia 2 is one of the types of nearby dwarf galaxies that is so faint that it could only be identified using twenty first century statistical methods. Its discovery was published in 2019 by a group of people led by Gabriel Torrealba working with data from the Gaia spacecraft . Gaia was constructed by the European Space Agency and launched in 2013, and it was designed to make extremely accurate, extremely precise measurements of the distances and velocities of stars in and near our galaxy as well as make measurements of the brightnesses of those stars , and it was so sensitive that it detected many of the stars within Antlia 2. The way the stars in Antlia 2 stood out was they all were located at the same distance of about 410 light years (125 kpc) , they all seemed to be moving in a different direction from everything else in the constellation Antlia, and many of them had very similar colors.
One of the things that was immediately apparent about Antlia 2 was that it was the most diffuse galaxy ever discovered (or, in other words, this galaxy appears to have the lowest density of any galaxy ever found up to this point in time). The galaxy has a diameter of 1.26 degrees (or about 2.5 times the width of the Moon) as seen from Earth in the night sky , and its physical diameter is about 9000 light years. This is actually similar in size to the Large Magellanic Cloud, but the Large Magellanic Cloud is much denser and much, much brighter. Just to re-iterate, the Large Magellanic Cloud can be seen without a telescope, while Antlia 2 was only identified using a statistical analysis. Antlia 2's total mass is estimated to be about 150 million times the mass of the Sun . For context, this would make Antlia 2 only 100 times more massive than a relatively large globular cluster in the Milky Way, but given that such a globular cluster would have a volume 27000 times smaller, it would be 270 denser . You can kind of understand how Antlia 2 is really diffuse in comparison. If I were to compare Antlia 2 to the Large Magellanic Cloud again, I would say that the Large Magellanic Cloud is 1000 times more massive and therefore 1000 times more dense . However, this clearly makes Antlia 2 look so embarassingly wimpy, so I'm not going to make this comparison. Related to all of this, Antlia 2 also appears to have very little dark matter, which is rather unusual compared to most other dwarf galaxies in the Local Group or most other galaxies in general .
Antlia 2 is so diffuse-looking that astronomers actually have a hard time explaining how the galaxy got this way. Most other dwarf galaxies that get too close to the Milky Way tend to get disrupted by our galaxy's gravitational forces until they no longer look like anything. Their outer layers get stripped away, making them smaller, and they get stretched out like strands of spaghetti until they cease to look ellipsoidal. Antlia 2 looks like it is being tidally disrupted by the Milky Way, but the way it is undergoing this tidal disruption simply looks different [3, 8]. It still looks elliptical, and it also doesn't seem to have shrunk in size; it only seems to have lost mass. This is going to keep the people who do computer simulations of galactic interactions awake at night just trying to figure out how something like Antlia 2 could actually exist.
What is really interesting is that Antlia 2 might be the first of a new class of really diffuse dwarf galaxies that astronomers previously never seen before. Other as-of-yet undiscovered diffuse dwarf galaxies may be in orbit around the Milky Way, the Andromeda Galaxy, and other spiral and elliptical galaxies. This is going to be an exciting topic in extragalactic astrophysics for years to come.
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