Jan 8, 2020 01:49 AM
(This post was last modified: Jan 8, 2020 01:54 AM by C C.)
https://www.vice.com/en_us/article/pkedv...es-history
EXCERPT: In November, astronomers at the Chinese Academy of Sciences in Beijng published a paper identifying 19 galaxies which might violate the most fundamental theory of how the universe first formed. They had been searching the sky for yet-undiscovered galaxies which seem to be lacking the usual dark matter component, aiming to add more evidence to a baffling phenomenon scientists had begun observing last year. And they claimed to have found a whole group of them.
Until recently, it was almost unanimously assumed that huge amounts of invisible dark matter was the key to galaxy formation; the gravitational effects experienced and induced by clumps of dark matter in the universe produced swirling disks of gas clouds, stars and dark matter. Dark matter had to make up the majority of matter in these galaxies, according to standard models of the universe, otherwise they would never have formed.
But since 2016, researchers keep stumbling upon galaxies that don’t seem to be dominated by dark matter. Some calculations imply that these galaxies lack dark matter entirely. Either way, galaxies which have far less dark matter than expected would give physicists a lot of explaining to do: why are they lacking dark matter, and how did they form in the first place?
Unlike the researchers in Beijing, Yale professor Pieter van Dokkum wasn’t trying to find anomalies among the "ultra diffuse" galaxies he studies—those with very few stars. After all, before van Dokkum made a chance discovery in 2018, there was very little reason to expect to find any galaxies which had formed and continued to exist without dark matter.
In fact, van Dokkum and his team expected to find dark matter everywhere they looked. Unlike our own star-packed Milky Way, where dark matter is only on the periphery, ultra-diffuse galaxies should be densely filled with dark matter. Their analysis instead identified, for the first time, a galaxy without any dark matter at all. “The way the stars move should be entirely dominated by the amount of dark matter,” he says. “The fact we found this deficit was not a subtle result.”
Though van Dokkum's findings were and remain controversial, new analysis of galaxies observed through giant telescopes keeps poking holes in accepted theories. If nothing else, the latest study by researchers in Beijing highlights still-open questions about the shape and structure of galaxies.
[...] it is indeed possible for very tiny galaxies to form without the presence of dark matter, but not the way galaxies were formed just after the Big Bang. Rather than gas and stars clumping together due to gravitational effects, these galaxies form when an existing galaxy collides with another and residual matter stripped from the colliding galaxies forms a new galaxy—all without any dark matter. In contrast to galaxies formed by the tidal-stripping effects ... these galaxies would live for a relatively short time by cosmic standards: a few hundred years. Examples have already been identified for decades, he says, but these aren’t the sort of dark-matter-deficient galaxies that challenge the current understanding of our universe.
“People have already found things they have called galaxies without dark matter, but they are transient features which then disappear,” he says. “The novelty would be to find a galaxy which was formed originally in the early universe without dark matter, and I don’t think we have any strong candidates yet.”
[...] The “fun thing” about the ultra-diffuse dwarf galaxies van Dokkum and others are studying is that they are “nearby and bright," van Dokkum said, which makes it relatively easy to investigate their behaviour. With the powerful telescopes available today, van Dokkum hopes not only to make more accurate measurements of galaxy velocities and distance, but also identify the epoch in which the galaxies formed, using new data from the Hubble telescope. This empirical evidence would rule out a whole class of explanations for the apparent dark matter deficiency, one way or another. (MORE - details)
EXCERPT: In November, astronomers at the Chinese Academy of Sciences in Beijng published a paper identifying 19 galaxies which might violate the most fundamental theory of how the universe first formed. They had been searching the sky for yet-undiscovered galaxies which seem to be lacking the usual dark matter component, aiming to add more evidence to a baffling phenomenon scientists had begun observing last year. And they claimed to have found a whole group of them.
Until recently, it was almost unanimously assumed that huge amounts of invisible dark matter was the key to galaxy formation; the gravitational effects experienced and induced by clumps of dark matter in the universe produced swirling disks of gas clouds, stars and dark matter. Dark matter had to make up the majority of matter in these galaxies, according to standard models of the universe, otherwise they would never have formed.
But since 2016, researchers keep stumbling upon galaxies that don’t seem to be dominated by dark matter. Some calculations imply that these galaxies lack dark matter entirely. Either way, galaxies which have far less dark matter than expected would give physicists a lot of explaining to do: why are they lacking dark matter, and how did they form in the first place?
Unlike the researchers in Beijing, Yale professor Pieter van Dokkum wasn’t trying to find anomalies among the "ultra diffuse" galaxies he studies—those with very few stars. After all, before van Dokkum made a chance discovery in 2018, there was very little reason to expect to find any galaxies which had formed and continued to exist without dark matter.
In fact, van Dokkum and his team expected to find dark matter everywhere they looked. Unlike our own star-packed Milky Way, where dark matter is only on the periphery, ultra-diffuse galaxies should be densely filled with dark matter. Their analysis instead identified, for the first time, a galaxy without any dark matter at all. “The way the stars move should be entirely dominated by the amount of dark matter,” he says. “The fact we found this deficit was not a subtle result.”
Though van Dokkum's findings were and remain controversial, new analysis of galaxies observed through giant telescopes keeps poking holes in accepted theories. If nothing else, the latest study by researchers in Beijing highlights still-open questions about the shape and structure of galaxies.
[...] it is indeed possible for very tiny galaxies to form without the presence of dark matter, but not the way galaxies were formed just after the Big Bang. Rather than gas and stars clumping together due to gravitational effects, these galaxies form when an existing galaxy collides with another and residual matter stripped from the colliding galaxies forms a new galaxy—all without any dark matter. In contrast to galaxies formed by the tidal-stripping effects ... these galaxies would live for a relatively short time by cosmic standards: a few hundred years. Examples have already been identified for decades, he says, but these aren’t the sort of dark-matter-deficient galaxies that challenge the current understanding of our universe.
“People have already found things they have called galaxies without dark matter, but they are transient features which then disappear,” he says. “The novelty would be to find a galaxy which was formed originally in the early universe without dark matter, and I don’t think we have any strong candidates yet.”
[...] The “fun thing” about the ultra-diffuse dwarf galaxies van Dokkum and others are studying is that they are “nearby and bright," van Dokkum said, which makes it relatively easy to investigate their behaviour. With the powerful telescopes available today, van Dokkum hopes not only to make more accurate measurements of galaxy velocities and distance, but also identify the epoch in which the galaxies formed, using new data from the Hubble telescope. This empirical evidence would rule out a whole class of explanations for the apparent dark matter deficiency, one way or another. (MORE - details)