re: Astronomers find the first and only known galaxy without dark matter

I got intrigued by this paper and tried to recreated their conclusion. Specifically I recalculated their velocity dispersion on which all their assumptions is based. Turns out they do some really funky stuff to get the very low intrinsic dispersion of 3km/s and 10km/s upper limit. The true value is closer to 12km/s with a 3 sigma limit at 20km/s!!! So then that new limit is twice as high and as velocity dispersion goes by the square into the mass you get at least a factor of 4 higher dynamical mass.

And then you add in that globular clusters are imperfect traces of the galaxy and their hyped conclusions become even more uncertain. they assume the globulars are in perfect equilibrium with the galaxy. But we know that is not true for many low-density galaxies, e.g. the clusters of the fornax dwarf galaxy give you a lower dispersion than it's stars.

They do not account for any of this. So this is very sketchy and what we see seems to be a low dark matter galaxy but the nowhere near the "DM free" galaxy that they hype it to be.

And now it's all over the media and made to be big thing yet it is at least doubtful. This type of unsubstantial hype just to create buzz on more than shaky grounds really makes me question being an astronomer sometimes. It's not about the quality of the work anymore just how well you sell it

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I got intrigued by this paper and tried to recreated their conclusion. Specifically I recalculated their velocity dispersion on which all their assumptions is based. Turns out they do some really funky stuff to get the very low intrinsic dispersion of 3km/s and 10km/s upper limit. The true value is closer to 12km/s with a 3 sigma limit at 20km/s!!! So then that new limit is twice as high and as velocity dispersion goes by the square into the mass you get at least a factor of 4 higher dynamical mass.

And then you add in that globular clusters are imperfect traces of the galaxy and their hyped conclusions become even more uncertain. they assume the globulars are in perfect equilibrium with the galaxy. But we know that is not true for many low-density galaxies, e.g. the clusters of the fornax dwarf galaxy give you a lower dispersion than it's stars.

They do not account for any of this. So this is very sketchy and what we see seems to be a low dark matter galaxy but the nowhere near the "DM free" galaxy that they hype it to be.

And now it's all over the media and made to be big thing yet it is at least doubtful. This type of unsubstantial hype just to create buzz on more than shaky grounds really makes me question being an astronomer sometimes. It's not about the quality of the work anymore just how well you sell it

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+1

That’s a comment from a Reddit thread on it he didn’t calculate that out just now

LINK

The author of the papers responded:

Hi, I am a co-author on this study and I think thats great that you ran the numbers yourself. I and some of the other team members would be curious to see your calculations, if you feel like sharing them. Though we actually agree with you that we cannot say for certain that the galaxy is dark matter free, only that its strongly deficient. I think perhaps the media has been overhyping the free bit and what we actually say in the paper is that
We conclude that NGC1052–DF2 is extremely deficient in dark matter, and a good candidate for a “baryonic galaxy” with no dark matter at all.
So we are not asserting that we have constrained the dark matter mass to be 0. However, the best fit to the kinematics is indeed obtained for M_halo = 0, so it could be dark matter free, but we don't have strong enough constraints to say that.
Regarding your calculations, we actually have done several alternative calculations of the mass in the Methods section of the paper under Robustness Tests and in one case got a similar value to you! It doesn't change the conclusions very much at all. In particular, we have tried calculating the velocity dispersion in several ways and each gives a slightly different result for the mass estimate. These small differences have to do with what assumptions are made about the intrinsic velocity distribution, the geometry, and the small relatively sample size. Again though, the largest discrepancy is about a factor of 4 (roughly what you got), which means that the dynamical mass is consistent with about 1 or 2 (maybe 3) times the stellar mass, which means that the galaxy still falls off the stellar-mass-halo-mass relation by a factor of >100, the lowest of any known galaxy by more than an order of magnitude! This is our main conclusion, which again is almost completely unchanged if you bump up the mass by a factor of a few; thats how extreme this object is.
I encourage those curious to read the paper.
I should also point out that we have several more observations planned to try to pin down the nature of this object. Stay tuned!

Seems like that galaxy is the perfect utopia. There is no dark matter there to be ruining everything.

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I got intrigued by this paper and tried to recreated their conclusion. Specifically I recalculated their velocity dispersion on which all their assumptions is based. Turns out they do some really funky stuff to get the very low intrinsic dispersion of 3km/s and 10km/s upper limit. The true value is closer to 12km/s with a 3 sigma limit at 20km/s!!! So then that new limit is twice as high and as velocity dispersion goes by the square into the mass you get at least a factor of 4 higher dynamical mass.

And then you add in that globular clusters are imperfect traces of the galaxy and their hyped conclusions become even more uncertain. they assume the globulars are in perfect equilibrium with the galaxy. But we know that is not true for many low-density galaxies, e.g. the clusters of the fornax dwarf galaxy give you a lower dispersion than it's stars.

They do not account for any of this. So this is very sketchy and what we see seems to be a low dark matter galaxy but the nowhere near the "DM free" galaxy that they hype it to be.

And now it's all over the media and made to be big thing yet it is at least doubtful. This type of unsubstantial hype just to create buzz on more than shaky grounds really makes me question being an astronomer sometimes. It's not about the quality of the work anymore just how well you sell it

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OT can make any topic racist

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So, there is more dark matter in the quad than in that galaxy?

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Only at Southern.

They're just making shite up at this point

We are slowly finding out how naive the human race is.

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Yes and no. There is substantial evidence that something with mass that we can’t see exists and makes up the majority of the universe. We don’t know what and it’s invisible aside from it’s effects to any methods we’ve discovered thus far to discover it, thus it’s "dark" matter. They can map where it is though by using gravitational lensing where it’s mass warps spacetime so that it bends light and we are able to see that.

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Sounds like someone follows the physics world that is about 10 years behind.