6 ms·

Researchers solve mystery of the galaxy with no dark matter

64 points by curtis 7y ago

8bitsrule 7y ago
Using five independent methods to estimate the distance of the object, they found that all of them coincided in one conclusion... I wish they had listed the five methods, I can't think of that many ... (?!)
- T-A 7y ago
  https://arxiv.org/abs/1806.10141 https://arxiv.org/abs/1806.10141
p1necone 7y ago
I want to know more about how it was measured incorrectly the first time.
- pdonis 7y ago
  This paper is not based on new measurements. It's based on a new analysis of existing data.
  - thedudeabides5 7y ago
    looks like they overshot by around 50% Anyone able to explain what the new analysis was that led that existing data to be interpreted so differently?
sriku 7y ago
A naive question (not read the actual paper or know much about the field) - if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics?
- idlewords 7y ago
  There are several different features of galaxies that dark matter explains. One of them is the speed with which stars orbit the center of a galaxy—this "rotation curve" is independent of how far the stars are from the galactic center, where if there were no dark matter, you would expect the outermost stars to go slower. That is an effect that is not affected by errors in measuring the distance to remote galaxies. We can see it in our own galaxy! https://www.e-education.psu.edu/astro801/content/l8_p8.html https://www.e-education.psu.edu/astro801/content/l8_p8.html In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason. Nobody likes dark matter as a hypothesis, everyone remembers analogous complications in the history of science that proved to be signposts to a better theory, and everyone knows that a Nobel prize waits for anyone who can find the correct explanation.
  - trhway 7y ago
    >One of them is the speed with which stars orbit the center of a galaxy do we really orbit it? I mean is it a stable orbit (that is the option which requires DM ) or a just a spiral trajectory (no DM required)? The spiral galaxies look like a fireworks wheel https://depositphotos.com/69366109/stock-video-fireworks-pinwheel-catherine-wheel.html https://depositphotos.com/69366109/stock-video-fireworks-pin... , and without DM our "orbital" speed being higher than otherwise expected Keplerian orbital speed means that we're just moving away on a spiral trajectory from the center the same way like the sparks fly away on a spiral trajectory from the center of the fireworks wheel.
    - zaarn 7y ago
      There are plenty of non-spiral galaxies where we observe the same. Our sun moving away from the galactic center is also unlikely as if it had been much further in during it's creation phase, it would have been unable to create planets that support life. A galactic year is only a few million earth years, so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. That points to a stable orbit around the galaxy. If it was a firework wheel our current position would mean that sun would have been traveling to the outside of the galaxy far below escape velocity or would have been unable to produce life-bearing planets.
whatshisface 7y ago
If MOND was dealt a near-lethal blow with the discovery of a galaxy that had no dark matter (implying that dark matter was a thing you could put in and take away, not a universal law like MOND), now that the anomalous galaxy is gone, does that mean MOND is back?
- cwmma 7y ago
  MOND has other issues, not least of which it only explains galaxy rotational curves, it doesn't explain the larger structures of the universe (which dark matter does explain)
  - ncmncm 7y ago
    Large-scale structure has been attributed to plasma-dynamic effects. It's not a popular hypothesis, apparently in large part because plasma dynamics is hard. So plasma is generally agreed to be there (to the exclusion of everything else, really -- except, y'know, dark matter), but is not allowed to (be said to) do anything dynamical and hard to model. It will take a future generation of plasma-dynamics-loving astrophysicists to change the ... er, dynamic. Presuming we live that long.
privong 7y ago
It's worth nothing that the authors of the original NGC1052-DF4 article claiming little to no dark matter have disputed the conclusions of the Trujillo+2019 (i.e., the paper the phys.org article is based on): https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstract https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstra... I think it's safe to say that the jury is still out on this one. Edit: One reason this is difficult is that you can think of galaxies as having two major contributions to the line-of sight velocity we observe from earth: expansion of the Universe ("Hubble flow") and "peculiar" velocity (motion due to the local gravitational field in which the galaxy lives). The latter velocities are typically a few hundred km/s (or later if the galaxy is in a cluster). NGC 1052 has an observed recession velocity of 1510 km/s (https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hconst=67.8&omegam=0.308&omegav=0.692&wmap=4&corr_z=1 https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hcons...). Assuming a peculiar velocity of 300 km/s, that means the peculiar velocity is on the order of 20% of the total velocity. But we don't know what component of the peculiar velocity is along the line of sight or whether it's positive or negative. So with these numbers it's "hubble flow" velocity could be from 1200 km/s to 1800 km/s. If you naive plug those two recession velocities into a cosmology model (e.g., the latest model derived from Planck observations) the distance range you get is 17.3-26.1 Mpc, or a ~40% uncertainty. So we cannot rely just on the Universe's measured expansion to get a sufficiently reliable distance estimate. So one must use other techniques to measure distances. They each have their own advantages and disadvantages. In general most distance estimates are build on the "Cosmic distance ladder" and accumulate uncertainty as you move up the ladder: https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder
- _Codemonkeyism 7y ago
  What I don't understand here is: You have a galaxy where mass doesn't add up. There are rivaling explanations, one being that you estimated the distance wrong, the other that there is no Dark Matter. Isn't the first one the more likely one you should investigate?
  - privong 7y ago
    > There are rivaling explanations, one being that you estimated the distance wrong, the other that there is no Dark Matter. Isn't the first one the more likely one you should investigate? If you look back at the original paper claiming the discovery of the dark matter free galaxy, they do explore several different ways of estimating the distance and argue that they're consistent and imply the more distant value. So that's something they had considered and attempted to cross-check between different distance indicators. Later came the Trujillo paper saying essentially, hey you didn't consider "X" which argues that the galaxy is closer. The authors of the original paper then published a Research Note (that I'd liked in my original post) saying that "X" doesn't agree with the other data. So people did consider distance errors before claiming the absence of dark matter. But getting accurate galaxy distances Is Hard and there are multiple ways to do it. The original paper considered the distance estimates they could think of and they were essentially consistent. So they did investigate that first.
cobbzilla 7y ago
Weren’t these all sparse elliptic galaxies? Didn't they all lack a SMB core? Is it possible they are whisps of parts of galaxies thrown off in a galactic collision? Like two spiral arms synchronizing and flying off, then coalescing far from the dark matter, which consolidates in the merged galaxy. These degenerate galaxies might then understandably have little/no dark matter of their own.
- ncmncm 7y ago
  That's called a "just so" story. Scientists are not gentle about those. It's all, "show me the math or GTFO!". Understandable, really.
  - cobbzilla 7y ago
    christ it was an honest/curious question. i luv HN, really
    - ncmncm 7y ago
      It was a valid question. But the details matter too much for it to have an answer.

64 points by curtis 7y ago

Using five independent methods to estimate the distance of the object, they found that all of them coincided in one conclusion... I wish they had listed the five methods, I can't think of that many ... (?!)

T-A 7y ago
https://arxiv.org/abs/1806.10141 https://arxiv.org/abs/1806.10141

p1necone 7y ago

I want to know more about how it was measured incorrectly the first time.

pdonis 7y ago
This paper is not based on new measurements. It's based on a new analysis of existing data.
- thedudeabides5 7y ago
  looks like they overshot by around 50% Anyone able to explain what the new analysis was that led that existing data to be interpreted so differently?

sriku 7y ago

A naive question (not read the actual paper or know much about the field) - if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics?

idlewords 7y ago
There are several different features of galaxies that dark matter explains. One of them is the speed with which stars orbit the center of a galaxy—this "rotation curve" is independent of how far the stars are from the galactic center, where if there were no dark matter, you would expect the outermost stars to go slower. That is an effect that is not affected by errors in measuring the distance to remote galaxies. We can see it in our own galaxy! https://www.e-education.psu.edu/astro801/content/l8_p8.html https://www.e-education.psu.edu/astro801/content/l8_p8.html In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason. Nobody likes dark matter as a hypothesis, everyone remembers analogous complications in the history of science that proved to be signposts to a better theory, and everyone knows that a Nobel prize waits for anyone who can find the correct explanation.
- trhway 7y ago
  >One of them is the speed with which stars orbit the center of a galaxy do we really orbit it? I mean is it a stable orbit (that is the option which requires DM ) or a just a spiral trajectory (no DM required)? The spiral galaxies look like a fireworks wheel https://depositphotos.com/69366109/stock-video-fireworks-pinwheel-catherine-wheel.html https://depositphotos.com/69366109/stock-video-fireworks-pin... , and without DM our "orbital" speed being higher than otherwise expected Keplerian orbital speed means that we're just moving away on a spiral trajectory from the center the same way like the sparks fly away on a spiral trajectory from the center of the fireworks wheel.
  - zaarn 7y ago
    There are plenty of non-spiral galaxies where we observe the same. Our sun moving away from the galactic center is also unlikely as if it had been much further in during it's creation phase, it would have been unable to create planets that support life. A galactic year is only a few million earth years, so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. That points to a stable orbit around the galaxy. If it was a firework wheel our current position would mean that sun would have been traveling to the outside of the galaxy far below escape velocity or would have been unable to produce life-bearing planets.

whatshisface 7y ago

If MOND was dealt a near-lethal blow with the discovery of a galaxy that had no dark matter (implying that dark matter was a thing you could put in and take away, not a universal law like MOND), now that the anomalous galaxy is gone, does that mean MOND is back?

cwmma 7y ago
MOND has other issues, not least of which it only explains galaxy rotational curves, it doesn't explain the larger structures of the universe (which dark matter does explain)
- ncmncm 7y ago
  Large-scale structure has been attributed to plasma-dynamic effects. It's not a popular hypothesis, apparently in large part because plasma dynamics is hard. So plasma is generally agreed to be there (to the exclusion of everything else, really -- except, y'know, dark matter), but is not allowed to (be said to) do anything dynamical and hard to model. It will take a future generation of plasma-dynamics-loving astrophysicists to change the ... er, dynamic. Presuming we live that long.

privong 7y ago

It's worth nothing that the authors of the original NGC1052-DF4 article claiming little to no dark matter have disputed the conclusions of the Trujillo+2019 (i.e., the paper the phys.org article is based on): https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstract https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstra... I think it's safe to say that the jury is still out on this one. Edit: One reason this is difficult is that you can think of galaxies as having two major contributions to the line-of sight velocity we observe from earth: expansion of the Universe ("Hubble flow") and "peculiar" velocity (motion due to the local gravitational field in which the galaxy lives). The latter velocities are typically a few hundred km/s (or later if the galaxy is in a cluster). NGC 1052 has an observed recession velocity of 1510 km/s (https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hconst=67.8&omegam=0.308&omegav=0.692&wmap=4&corr_z=1 https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hcons...). Assuming a peculiar velocity of 300 km/s, that means the peculiar velocity is on the order of 20% of the total velocity. But we don't know what component of the peculiar velocity is along the line of sight or whether it's positive or negative. So with these numbers it's "hubble flow" velocity could be from 1200 km/s to 1800 km/s. If you naive plug those two recession velocities into a cosmology model (e.g., the latest model derived from Planck observations) the distance range you get is 17.3-26.1 Mpc, or a ~40% uncertainty. So we cannot rely just on the Universe's measured expansion to get a sufficiently reliable distance estimate. So one must use other techniques to measure distances. They each have their own advantages and disadvantages. In general most distance estimates are build on the "Cosmic distance ladder" and accumulate uncertainty as you move up the ladder: https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder

_Codemonkeyism 7y ago
What I don't understand here is: You have a galaxy where mass doesn't add up. There are rivaling explanations, one being that you estimated the distance wrong, the other that there is no Dark Matter. Isn't the first one the more likely one you should investigate?
- privong 7y ago
  > There are rivaling explanations, one being that you estimated the distance wrong, the other that there is no Dark Matter. Isn't the first one the more likely one you should investigate? If you look back at the original paper claiming the discovery of the dark matter free galaxy, they do explore several different ways of estimating the distance and argue that they're consistent and imply the more distant value. So that's something they had considered and attempted to cross-check between different distance indicators. Later came the Trujillo paper saying essentially, hey you didn't consider "X" which argues that the galaxy is closer. The authors of the original paper then published a Research Note (that I'd liked in my original post) saying that "X" doesn't agree with the other data. So people did consider distance errors before claiming the absence of dark matter. But getting accurate galaxy distances Is Hard and there are multiple ways to do it. The original paper considered the distance estimates they could think of and they were essentially consistent. So they did investigate that first.

cobbzilla 7y ago

Weren’t these all sparse elliptic galaxies? Didn't they all lack a SMB core? Is it possible they are whisps of parts of galaxies thrown off in a galactic collision? Like two spiral arms synchronizing and flying off, then coalescing far from the dark matter, which consolidates in the merged galaxy. These degenerate galaxies might then understandably have little/no dark matter of their own.

ncmncm 7y ago
That's called a "just so" story. Scientists are not gentle about those. It's all, "show me the math or GTFO!". Understandable, really.
- cobbzilla 7y ago
  christ it was an honest/curious question. i luv HN, really
  - ncmncm 7y ago
    It was a valid question. But the details matter too much for it to have an answer.