Sunday, April 15, 2018

The Dark Matter Lacking Galaxy, vol 2

The dark Matter Lacking galaxy is certainly good for some more controversy.

various astronomers have weighed in, and now also on astro-ph:

Thanks to Justin Read for this quick summary:

Three papers on the galaxy lacking DM:

Measured and found wanting: reconciling mass-estimates of ultra-diffuse galaxies  
With 10 tracers, there should be a factor ~10 error on the mass...

Current velocity data on dwarf galaxy NGC1052-DF2 do not constrain it to lack dark matter 

Where Nicholas Martin made good on his twitter thread (threat?) and generated a paper on the statistical treatment used in the Nature paper.

MOND and the dynamics of NGC1052-DF2

Of course the MOND people had to weigh in, mainly with the point that this isn't the stake to the heart of this theory. Apparently you have to cut off its head too. Or something similar.

and this has prompted Pieter van Dokkum to post more information and material on his website:

Especially, the actual table with redshifts and positions of the Globular clusters are posted here:

That does not sound like a big deal but to me it is. If you are being completely open in science (as we should be) then you make the reproduction of your approach and the test of other approaches as simple as possible. If I had one major gripe about the van Dokkum Nature paper, it was the lack of this table.

This allowed me to devise a rather simplistic homework assignment to calculate the mean redshift (systematic redshift) and the mean radius of the globular cluster distribution in kpc. One then takes the standard deviation of the distribution around the systematic velocity to obtain: 10.5 km/s and a radius r=3.6 kpc.

Yes that is just using Numpy's mean and std functions.

One can use this as an Jupyter notebook exercise for students to plot the data:

and the now infamous histogram:

Using our textbooks we get:

M = 7.5 {\sigma^2 r_h \over G}

and compare that to the stellar mass inferred from the Sersic fit and we find that this galaxy is lighter than the stars imply: something is definitely up.
So this is now a Dark Matter assignment in a new course I am building.

But that brings me to the current issue with this class of galaxies. We have way too few kinematic measurements of them, only done by one group and the implications are rather far-reaching. So, hopefully we'll see more data on these things soon. Because N=2 science in current circumstances is just embarrassing. 

Still not one of the most grumpy-old-man response I've seen:


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