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I have lived in my neighborhood for a while now, but it is a place with a lot going on and so I am still constantly surprised to find that I live right next to railroads, department stores, cemeteries, or universities which I didn’t know existed for all of these years. This sort of thing doesn’t just happen in Brooklyn: it is true for the whole solar system. Astronomers just discovered the largest mega-structure in the Milky Way Galaxy, a swooping ribbon of hot gas and baby stars now known as “the Radcliffe Wave.” The wave begins 500 light-years below the Milky Way’s disk at a spot in the night sky around Orion, and runs through the constellations Taurus and Perseus to wind up near the constellation Cepheus (and 500 light years above the galactic plane).
The Radcliffe Wave is about 9,000 light-years in length–roughly a tenth the diameter of the galaxy–and is though to contain about 800 million stars (as a quick refresher, our own sun has a mass about 333,000 times that of Earth). Scientists have noticed pieces of the wave before, since it is a hot zone filled with tumultuous stellar nurseries where bright young stars emerge from vast clouds of gas, yet they did not realize it was a continuous ribbon.
The ribbon is relatively close to Earth, too. To quote João Alves, the co-author of the Nature article about the Radcliffe Wave, “The sun lies only 500 light-years from the wave at its closest point. It’s been right in front of our eyes all the time, but we couldn’t see it until now.” Five hundred light years is not exactly a drive to the strip mall (it is a distance of 4,730,000,000,000,000 kilometers!), but we have been through the Radcliffe Wave 13 million years ago and the solar system is projected to pass into it in again in another 13 million years.
Astronomers are interested in the wave, but they are even more interested in why is exists to begin with. Alves speculates that it was created in the same manner that ripples are made in the water of a pond when something exceedingly massive lands in it. What would be massive enough to make ripples in a galaxy? Another galaxy? Some sort of black hole biz? A giant hunk of dark matter? Who knows? (although this older post about giant voids in space might be somewhat instructive in talking about space’s busy neighborhoods too). We only just discovered the Radcliffe Wave and we will have to start working to figure out where it originated and what it means. After all, we will be there surfing it in a mere 13 million years. Kowabunga, space dudes!
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!
Hey, remember the super-massive black hole at the center of the galaxy? Well, scientists have been thinking about it too, and they concluded that other black holes should sink into the middle of the galaxy near to the central monster. To find out if this holds true, they utilized the Chandra X-Ray Observatory (an x-ray telescope located on a satellite in orbit around Earth) to observe stars near to the center of the galaxy. Black holes can’t be detected on their own, but if they interact with nearby stars they produce esoteric x-rays which can be detected (so long as the x-ray telescope is outside of a planetary atmosphere, which absorbs x-rays, thank goodness). Within the tiny (er, relatively tiny) three light year area which they scrutinized, the astronomers discovered dozens of black holes. Extrapolating this data leads them to conclude there are more than 10,000 black holes at the center of our galaxy. I wish I could contextualize this for you, but I just can’t… the concept of 10,000 super-dense gravity wells flattening and tearing all of the spacetime in the center of the galaxy into Swiss cheese is to disturbing for me to deal with (in any other way than blurting it out in a midnight blog). I’m not sure this universe is safe at all. I am going to go lie down.
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.
The constellation Sagittarius (from “Urania’s Mirror” a set of constellation cards published in England circa 1825)
My apologies for the blogging break last week. Usually I try to write a new post every weekday, but last week was a blogging holiday. To reinvigorate things after the lost week, let’s turn to a big subject—in fact a super-massive subject! Long ago, Ferrebeekeeper featured a post about Eta Carinae, a blue hypergiant with a hundred times the mass of the sun (which is itself a million times more massive than Earth). Stars like Eta Carinae are rarely formed and short lived—there are probably less than a dozen in our galaxy. However compared to the most massive object in the galaxy, Eta Carinae is puny and common. Twenty six thousand light years away from the solar system there exists a truly monstrous space object!
In 1974, Astronomers discovered an astronomical feature which was emitting exotic radio waves in the Sagittarius constellation. The scientists named the feature “Sagittarius A” and set out to determine what it was. Part of the feature seems to be the remnants of a star which had gone supernova. A second part of the feature is a cloud of ionized gas surrounded by an even larger torus of molecular gas. In the middle of Sagittarius A is something which is emitting most of the high energy electromagnetic radiation visible to radio telescopes. The cloud of ionized gas seems to be emptying into it and nearby stars orbit it with greater velocity than stars move anywhere else in the galaxy (in fact the object affects the proper motion of thousands of nearby stars). And yet the space object at the center of Sagittarius A has a diameter of only 44 million kilometers–a bit less than the distance between the middle of the sun and Mercury at its perihelion (when the rocky planet is closest to the sun). By calculating the proper motion of thousands of nearby stars, scientists determined that the mysterious object at the center of Sagittarius A (which they took to calling Sagitarrius A*) has mass of 4.31 million suns (i.e. solar masses). Whatever lies at the center of Sagittarius A–which I probably should have mentioned, is also the center of the Milky Way Galaxy–is smaller in volume than a large star, but has a mass which exceeds by many orders of magnitude even exotic hypergiants like Eta Carina.
Of course the only kinds of discrete objects which we know (or even hypothesize) to be capable of attaining such mass are black holes. It is believed that most (indeed probably all) galaxies have super-massive black hole at their centers. Smaller galaxies have small super massive black holes (forgive the oxymoron) but large galaxies have immense central black holes which can equal billions of solar masses. Radio astronomers have observed plumes of exotic electromagnetic radiation coming from the center of other galaxies, and they wondered where the Milky Way’s galactic center was located. It seems that a supernova near the galactic center blew away a great deal of the dust and gas on which the black hole would otherwise “feed” thereby making the galactic center of the Milky Way less energetic than the active center of farther (e.g. older) galaxies.
Artist’s Conception of Galactic Center
The super massive black holes which lie at the center of galaxies may be a result of the accretion of matter around stellar-sized black holes (which could grow quickly in matter-rich galactic cores) but most astrophysicists believe they are instead a primordial feature of the Big Bang around which galaxies themselves coalesced. The ultimate nature of super massive black holes remains unknown and seems to be tied to the nature and shape of our universe.
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