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Class of Large But Very Dim Galaxies Discovered (nature.com)
schwit1 writes from a report via Nature: Astronomers have now detected and measured a new class of large but very dim galaxy that previously was not expected to exist. Nature reports: "'[Ultradiffuse]' galaxies came to attention only last year, after Pieter van Dokkum of Yale University in New Haven, Connecticut, and Roberto Abraham of the University of Toronto in Canada built an array of sensitive telephoto lenses named Dragonfly. The astronomers and their colleagues observed the Coma galaxy cluster 101 megaparsecs (330 million light years) away and detected 47 faint smudges. 'They can't be real,' van Dokkum recalls thinking when he first saw the galaxies on his laptop computer. But their distribution in space matched that of the cluster's other galaxies, indicating that they were true members. Since then, hundreds more of these galaxies have turned up in the Coma cluster and elsewhere. Ultradiffuse galaxies are large like the Milky Way -- which is much bigger than most -- but they glow as dimly as mere dwarf galaxies. It's as though a city as big as London emitted as little light as Kalamazoo, Michigan." More significantly, they have now found that these dim galaxies can be as big and as massive as the biggest bright galaxies, suggesting that there are a lot more stars and mass hidden out there and unseen than anyone had previously predicted.
Amazing! Dark matter is just regular matter that was dimmer than other galaxies according to you! Where's your nobel prize?
Isn't that exactly what you just said before?
Anyway, one of the things dark matter is hoped to explain is the rotation curves of galaxies we can already see. I don't see how these newly discovered ultra diffuse galaxies can do that.
systemd is Roko's Basilisk.
The missing mass is at a galactic level, not at the universe level. Stars in the galaxies, including our own, are moving too fast in their orbits around the galactic core to not shoot off into the space between galaxies if only visible matter is assumed to account for the gravity of each galaxy holding the stars in their orbits.
Other than the gravitation of each galaxy required to be 1000% of what the visible matter contributes, nothing we do to the existing formulas for gravity can account for it without changing the entire dynamics of the universe to something that doesn't match the observations.
The source for the missing 90% gravity isn't visible as stars, dust, planets, gas etc and we've coined the term dark matter as a category to describe this source.
These new dim galaxies cannot contribute to the dark matter as it is required in the structure of all galaxies. It might assist with understanding inflation as that is a universe-wide effect.
Trying to become famous by taking photos. Visit my homepage please.
It's not. There are hree primary motivations for the belief that dark matter must exist:
1) Galactic dynamics. Stars within galaxies are orbiting far too fast given the luminous matter in those galaxies -- far too fast. On the assumption of Newtonian mechanics, we need an overabundance of unobserved matter to matter of around four times, and this needs to be distributed spherically (regardless of the distribution of the galaxy itself). It is simply not possible for that matter to be normal matter; we would see, for instance, frequent microlensing events as the matter passes in front of distant stars. Note that this "dark matter" does not have to literally be dark matter in the form of a particulate matter; it could be anything that replicates the motion of stars within galaxies. MOND, for instance, does an admirable job of fitting practically all (if not all) galaxy rotation curves with a single parameter, which is a bit better than particulate theories can.
2) Cluster dynamics. Galaxies within clusters are *also* orbiting far too fast given the luminous galaxies in the clusters. Here the "microlensing" issue is no longer micro -- galaxies lens rather more visibly. The dark matter distribution is no longer spherical. Note that we have observations, the Bullet Cluster being the most famous, where we can compare the mass distribution as recovered from lensing with the luminous matter and they're in very different places. In this instance, MOND doesn't work -- it breaks down entirely. That does not mean that the "dark matter" must be particulate, but it certainly can't actually be MONDian.
3) Cosmology. Cosmology provides the strongest evidence that something behaving as dark matter has to exist. Cosmological observations tell us that the universe is flat, to within a percent or so. At the same time, we observe the abundances of primordial hydrogen, helium, lithium and the like. These abundances are exquisitely sensitive to the amount of normal matter in the universe. That is, the amount of standard model matter, not just "luminous matter". To fit the observations we're restricted to approximately 5% of the critical density being made up of normal, standard model matter. But to fit observations of the cosmic microwave background and of the large-scale distribution of matter we not only need to be at the critical density but we need around 25%-30% of the universe to be made up of matter that gravitates like normal matter does, and about 70%-75% in something that is beginning to act "anti-gravitationally". That is, entirely independently from galaxy observations, cosmology states we need between four and five times of the matter in the universe in the form of "dark matter". Again, this does not need to be particulate. The conclusions are predicated on a relatively naive interpretation of general relativity. Changing either the theory of gravity, or the way in which it is applied, can change the conclusions, but nothing is especially convincing and in particular little is more convincing than the possible existence of a lightest supersymmetric particle, which would fit observations across all scales.
In each case it's useful to note that the galactic "dark matter" does not in fact have to be from the same source as the cosmological "dark matter". It's also important to note that in all cases its existence is deduced from comparing observations with a model explicitly based on a particular theory. As such, I would advise against taking either "dark" or "matter" as literal descriptions of what's going on. What we can say, with certainty, is that if you relatively naively apply Newtonian mechanics to galaxies or galaxy clusters, and if you relatively naively apply general relativistic dynamics to a universe which is smooth on extremely large scales, you independently come to the same conclusion: something in both cases acts as if there were an invisible distribution of gravitating matter, with an abundance about four or five times that of our normal, well-understood standard model m
Hey, looka that!
Dark matter turns out to be a lot of dim matter. Thanks for shining a light on this subject.
Dark Galaxies Matter, you insensitive clod!
#BlackMatterLives
And how does dark matter explain this? If you cant prove it even exists?
Dark matter is not an explanation, it's a placeholder. What we've observed is a gravitational pull we can't explain, to which there are three possible explanations:
1) Our formula for gravity is wrong
2) There is matter of known types we haven't detected
3) There is one or more unknown types of matter at play
We've looked hard at modifying the theory of gravity but all the proposed modifications cause it to fail other results that are correct today. So assuming the formula is correct, we can estimate how much mass is "missing" and we label this dark matter. And then search the particles we know to see what other effects they'd have, to get some upper bound on how much of the dark matter it is. And then we find it doesn't add up to 100%, not even close. Most explanations rush this part and go right to all the potential candidates for what the rest is.
So we've found more of the traditional matter, that's neat. We know it's out there, we don't know exactly where and how much. But we know it's not enough to explain everything, not in the places it needs to be. I'm not sure how to explain it in a good way, it's like proving that you can't go to the moon with a horse and carriage. It's not about how many horses there are, there could be an infinite number to give you infinite horsepower and it still wouldn't work. You need a different kind of propulsion. Same way with dark matter, no matter how much traditional matter you add it doesn't work. You need something else.
Live today, because you never know what tomorrow brings
You don't understand how we use parallax. We measure how much the object shifts 6 months apart, when we have moved 2 AU. Further distances will shift less, and the amount that an object moves that far away is too small for us to measure with current technology. Plus, it requires a backdrop of relatively much-more-distant objects to actually measure the shift (they too move, but so much less so that it's close enough for a good estimate). The other galaxies out there that would be the backdrop are not orders of magnitude further away necessarily, making the measurement even more difficult.
So yes, in this context a milli arc-second does indeed correspond to a fixed distance. The Earth is what moves to make the parallax, and that is a fixed distance. The distance to the object is not required to know what the angular measurement means; rather, the angular measurement given our displacement on Earth is precisely what we use to determine the distance to the object. You're thinking of it backwards.