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Hey, did anybody notice there is a massive spooky ghost galaxy right next door to us? Well, actually the answer turns out to be “no: not really…not until November of last year.” It was only in November of 2018 that astronomers discovered Antlia 2, a galaxy which is a mere 130,000 light-years away from the Milky Way–which really is right next door in terms of cosmic distances (to contextualize this number, the diameter of the Milky Way itself is between 150,000 and 200,000 light years).
Ant2, as it is affectionately (?) called, is closer than the Large Magellanic Cloud, a dwarf galaxy with 30 billion stars which is 163,000 light years away and is generally regarded as our closest galactic neighbor. How did we miss this thing? And why are we calling it a ghost galaxy (aside from the fact that that sounds impossibly cool)?
Antlia 2 is a weird sort of celestial object. It has the lowest “surface brightness” of any known galaxy and is approximately 100 times more diffuse than any known ultra diffuse galaxy. Gee! that is really extremely diffuse. Antlia 2 is also a dark matter galaxy: the exiguous stars of which it consists are insufficient to hold it together without a great deal of unknown mystery mass. The ghost galaxy may be more than 99 percent dark matter. Additionally, Ant2 (insomuch as it exists) is hidden by the occlusion cloud above the spiral of the Milky Way. Only with the advanced astrometry readings of the European Space Agency’s satellite observatory Gaia were astronomers finally able to pinpoint this dark shadow in the sky above the southern constellation Antlia (which itself is named after an 18th century air pump).
What the jazz? This post is making less sense than usual, but I am not making any of it up…
I worry that this post is too abstruse for comfort. It concerns an all-but-invisible phantom galaxy made of unknown dark matter. The only reason we even found it to begin with is that astronomers were on the lookout for a hidden galaxy. Some unknown mass must have caused the stylish ruffles in the spiral arms of our own bright & lively Milky Way. Thus the fashion sensibility of space topologists helped us to find Antlia 2. Remind me about this thing in October. I want to draw some ghost galaxies to celebrate Halloween this year!
Previously discovered dwarf satellite galaxies (in blue) and the newly discovered candidates (in red) (Yao-Yuan Mao, Ralf Kaehler, Risa Wechsler (KIPAC/SLAC))
We have some new galactic neighbors! Well, actually maybe “new” is not the right term: they have been there for a long time but we only just now noticed. Astronomers are reporting the discovery of nine dwarf satellite galaxies orbiting the Milky Way like remoras stuck to a cosmic shark. These nine miniature galaxies are additional to the well-known Large and Small Magellanic Clouds—two dwarf galaxies which are located right next to the Milky Way (being respectively 160,000 and 200,000 light years away).
The new dwarf galaxies were discovered by a team of astronomers poring over data recovered from the Dark Energy Survey (a super-high resolution digital array which is part of the Victor M Blanco telescope in the Andes). The closest is a mere 97,000 light years from the Milky Way whereas the farthest lies 1.2 million light years away from us. The dwarf galaxies are a billion times fainter than the Milky Way. They are made up of millions (or hundreds of millions or even billions) of stars but are insignificant in size compared to the hundreds of billions of stars which constitute a true galaxy. Scientists believe that there are hundreds of similar miniature galaxies and pseudo-galaxies near the Milky Way, but they are dark and difficult to find (comparitively speaking).
The Large and Small Magellanic Clouds, near which the satellites were found. (image from European Southern Observatory)
I have been saying “dwarf galaxies” because I like the way it sounds (like the new galaxies live together in the woods in a little hut and work in the mines!), but actually only three of the new companions are definitely dwarf galaxies. The remaining six structures may be dwarf galaxies or they may merely be globular clusters—a far less euphonic phrase which indicates a group of stars which orbits a galactic core as a satellite. Unlike globular clusters, dwarf galaxies are held together by the gravitational mass of large quantities of dark matter (um, assuming it actually exists). Indeed dwarf galaxies seem to contain far greater quantities of dark matter than actual galaxies. This makes the newly discovered galactic neighbors a potentially useful focus for studying the properties of dark matter and refining our model of the universe.
News of the cosmos frequently involves inconceivably large numbers or gigantic objects beyond human imagination. This is particularly true of galaxies–gigantic systems of stars, gas clouds, black holes, and exotic unknown dark matter. Even the tiniest dwarf galaxies have tens of millions of stars and our lovely home galaxy, the Milky Way, has approximately 300 billion star systems! However the universe is a mysterious place and it frequently refutes conventional wisdom and prior expectations. This week astronomers from Hawaii’s Keck Observatory announced that they had discovered a ridiculously little galaxy with only a thousand stars. The adorable miniature galaxy, which has been dubbed Segue 2 is not a star cluster because it is surrounded by its own halo of dark matter, but it is many orders of magnitude smaller than any known galaxy. Astronomers are trying to determine whether it is a scrap of a larger galaxy which was ripped apart (!) or whether it is a baby galaxy which never fully coalesced. Astronomers hope that by learning more about Segue 2 and other hypothesized tiny galaxies they can find out more about the creation of the universe and the formation of elements.
“NO!” –image editors
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