Tag Archives: dark galaxy

Newly found galaxy is almost entirely made out of dark matter

Both images show the Dragonfly 44 dark galaxy, only on the left it’s much fainter (Sloan Digital Sky Survey), while the on the right (Gemini telescope) was taken with a very long exposure. Credit Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey

A new Milky Way-sized galaxy has been found nearby in the Coma Berenices constellation – but this galaxy is more than meets the eye, astronomers say. Though the diffuse blob is big enough in size and mass, the galaxy holds very few stars. It should have been ripped apart millions of years ago in these conditions, but some mysterious force is holding it together. It didn’t take too long for scientists to discover what it was.

Using the W.M. Keck Observatory and the Gemini North telescope, both stationed in Hawaii, the international team of researchers measured the velocities of the stars harbored by the Dragonfly 44 galaxy. These velocities are used to calculate the mass of galaxies.

“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 co-author Roberto Abraham of the University of Toronto.

Collage of six cluster collisions with dark matter maps. Image by NASA.

Judging from the data, it seems Dragonfly 44’s mass is around 1 trillion times the mass of the sun or very similar to that of the Milky Way. Unlike our own galaxy, however, less than one percent of Dragonfly 44’s mass is made of stars or what astronomers call “normal” matter. The rest of the 99.99 percent has to be dark matter — an elusive form of matter that can’t be directly seen, not with our current means at least, but whose influence can be felt.

While finding a galaxy made almost entirely out of dark matter is very rare, it’s not new to science either. The scientists studying Dragonfly 44 say ultra-faint dwarf galaxies have a similar composition. Yet, galaxies belonging to this class are typically 10,000 times less massive than Dragonfly 44. Our own galaxy, it seems, might actually be unique, belonging to a yet unidentified class of dark galaxies.

“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.”

Yes, scientists have no idea how Dragonfly 44 got there or how it formed because … dark matter. Until we can unravel its nature, mysteries like Dragonfly 44 will remain partly unsolvable, which is why thousands of scientists around the world are dedicating their careers to untangling this invisible matter.

“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,” said Van Dokkum, the Sol Goldman Family Professor of Astronomy and Physics at Yale and co-author of the paper published in the Astrophysical Journal Letters.

Next, Van Dokkum and colleagues hope to find more such galaxies closer to home. They have a hunch that when dark matter interacts with itself, it generates signature ultraviolet light. In a ‘normal’ galaxy like the Miky Way, these signals are drowned by the noise of stars and dust, but a “dark galaxy” should be quite enough to maybe show us the first direct evidence of dark matter.

“We think that this galaxy is representative of an entirely new class of object,” van Dokkum said. “It’s not some weird singular galaxy that’s just a curiosity.”

This deep image shows the region of the sky around the quasar HE0109-3518. The quasar is labelled with a red circle near the centre of the image. The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation. The faint images of the glow from 12 dark galaxies are labelled with blue circles Click for ZOOM. (C) ESO

First evidence of dark galaxies from the early Universe spotted

An international team of astronomers may have come across the first sound evidence testifying the existence of dark galaxies – cosmic bodies from the early Universe long theorized by scientists in the past, but never before confirmed until now.

Dark galaxies are small, gas-rich galaxies that are very inefficient at forming stars themselves. Their name comes from the fact that they’re void of stars, thus no light is emitted, making them theoretically invisible. Scientists consider dark galaxies to have played a major role in star-rich galaxy formation during the early Universe expansion, feeding neighboring galaxies with precious gas required to birth stars.

Since dark galaxies don’t emit any light, confirming their existence has been always extremely difficult for scientists attempting such a feat. Previous studies of small absorption dips in the spectra of background light sources were thought to have hinted at dark galaxies, but this newly presented research is the first to provide rather tantalizing proof.

Using the European Southern Observatory’s Very Large Telescope (VLT) in northern Chile, the researchers saw the extremely faint fluorescent glow of the dark galaxies.

“Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it,” says Simon Lilly of ETH Zurich.

“We searched for the fluorescent glow of the gas in dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar. The light from the quasar makes the dark galaxies light up in a process similar to how white clothes are illuminated by ultraviolet lamps in a night club.”

This deep image shows the region of the sky around the quasar HE0109-3518. The quasar is labelled with a red circle near the centre of the image. The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation. The faint images of the glow from 12 dark galaxies are labelled with blue circles  Click for ZOOM. (C) ESO

This deep image shows the region of the sky around the quasar HE0109-3518. The quasar is labelled with a red circle near the centre of the image. The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation. The faint images of the glow from 12 dark galaxies are labelled with blue circles Click for ZOOM. (C) ESO

The telescope was directed towards a patch of the sky, around the bright quasar HE 0109-3518, where it mapped the region and looked for ultraviolet light released by hydrogen gas when subjected to radiation. Quasars are the brightest and most energetic objects in the Universe. The exposure time was enormous, but in the end it paid out for the astronomers.

“After several years of attempts to detect fluorescent emission from dark galaxies, our results demonstrate the potential of our method to discover and study these fascinating and previously invisible objects,” study lead author Sebastiano Cantalupo, from the University of California, Santa Cruz, said in a statement.

Their initial round of data returned 100 possible gaseous objects which lie within a few million light-years of the quasar. Eliminating objects where the emission might have been powered by internal star-formation in neighborliness galaxies, the team of researchers narrowed the list down to 12.

Also, the researchers were able to determine some of the dark galaxies’ properties. They speculate the mass of the gas in dark galaxies is about one billion times that of the sun, and that they’re 100 times less efficient at forming stars than most galaxies of the time. Their exact composition hasn’t been determined yet, since there’s no conclusive way of determining it. However, theoretically they’re composed of hydrogen, dust and dark matter.

“Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds,” Cantalupo said. “With this study, we’ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas.”

This research was presented in a paper entitled “Detection of dark galaxies and circum-galactic filaments fluorescently illuminated by a quasar at z=2.4”, by Cantalupo et al. to appear in Monthly Notices of the Royal Astronomical Society.

source: ESO