Credit: Harvard-Smithsonian Center for Astrophysics
The center of the Milky Way galaxy is currently a quiet place where a supermassive black hole slumbers, only occasionally slurping small sips of hydrogen gas. But it wasn’t always this way. A new study shows that 6 million years ago, when the first human ancestors known as hominins walked the Earth, our galaxy’s core blazed forth furiously. The evidence for this active phase came from a search for the galaxy’s missing mass.
Measurements show that the Milky Way galaxy weighs about 1-2 trillion times as much as our Sun. About 5/6 of that is in the form of dark matter. The remaining 1/6 of our galaxy’s heft, or 150-300 billion solar masses, is normal matter...
The dark galaxy Dragonfly 44. The image on the left is from the Sloan Digital Sky Survey. Only a faint smudge is visible. The image on the right is a long exposure with the Gemini telescope, revealing a large, elongated object. Dragonfly 44 is very faint for its mass and consists almost entirely of dark matter. Credit: Images by Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey
Using the world’s most powerful telescopes, an international team of astronomers has found a massive galaxy that consists almost entirely of dark matter. The galaxy, Dragonfly 44, is located in the nearby Coma constellation and had been overlooked until last year because of its unusual composition: It is a diffuse “blob” about the size of the Milky Way, but with far fewer stars. “Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together,” said Yale University astronomer Pieter van Dokkum.
Van Dokkum’s team was able to get a good look at Dragonfly 44 thanks to the W.M. Keck Observatory and the Gemini North telescope, both in Hawaii. Astronomers used observations from Keck, taken over 6 nights, to measure the velocities of stars in the galaxy. They used the 8m Gemini North telescope to reveal a halo of spherical cluster of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way galaxy.
Star velocities are an indication of the galaxy’s mass. The faster the stars move, the more mass its galaxy will have. “Amazingly, the stars move at velocities that are far greater than expected for such a dim galaxy. It means that Dragonfly 44 has a huge amount of unseen mass,” said Roberto Abraham of the University of Toronto. Scientists initially spotted Dragonfly 44 with the Dragonfly Telephoto Array, a telescope invented and built by van Dokkum and Abraham.
Dragonfly 44’s mass is estimated to be 1 trillion times the mass of the Sun, or 2 tredecillion kg (a 2 followed by 42 zeros), which is similar to the mass of the Milky Way. However, only 1/100 of 1% of that is in the form of stars and “normal” matter. The other 99.99% is in the form of dark matter, a hypothesized material that remains unseen but may make up more than 90% of the universe.
Finding a galaxy composed mainly of dark matter is not new; ultra-faint dwarf galaxies have similar compositions. But those galaxies were roughly 10,000 times less massive than Dragonfly 44. “We have no idea how galaxies like Dragonfly 44 could have formed,” said Abraham. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”
Van Dokkum, Sol Goldman Family Professor of Astronomy and Physics at Yale, added: “Ultimately what we really want to learn is what dark matter is. The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle.” http://www.newswise.com/articles/scientists-discover-a-dark-milky-way
Collapsed datacubes showing HR 2562B in each of the four modes observed with GPI and reduced using KLIP. The K2 image is from February 2016 and demonstrates two possible solutions for the inner edge of the disk (38 and 75 AU with dashed and dotted-dashed lines respectively) assuming inclination of 78 degrees and position angle of 120 degrees. Credit: Konopacky et al., 2016.
Astronomers have detected a brown dwarf orbiting HR 2562 – a nearby star known to host a debris disk. HR 2562, 110 light years away, is an F5V star, about 30% more massive than the sun. It has a debris disk—a circumstellar belt of dust and planetesimals left over from planetary formation. The disk around HR 2562, spans from 38 to 75 AU away from the host star.
In early 2016, a team of researchers, led by Quinn Konop...
Massive galaxy cluster MACS J0416 seen in X-rays (blue), visible light (red, green, and blue), and radio light (pink). Credit: NASA/CXC/SAO/G.Ogrean/STScI/NRAO/AUI/NSF.
Researchers have discovered 4 of the most distant clusters of galaxies ever found, as they appeared when the universe was only 4 billion years old. This sample is now providing the best measurement yet of when and how fast galaxy clusters stop forming stars in the early universe. Clusters are rare regions of the universe consisting of hundreds of galaxies containing trillions of stars, as well as hot gas and mysterious dark matter. Spectroscopic observations from the ground using W. M. Keck Observatory in Hawaii and the Very Large Telescope in Chile confirmed the four candidates to be massive clusters...
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