Sgr A* better DM halo/core of 56KeV Fermions than BH

[.Scott]

TL;DR Summary

An Italian team looks at the track of stars near Sgr A* and finds their motion is better explained by a core element of a galactic Dark Matter halo of 56KeV fermions than by a black hole.

A team in Italy has been studying the paths of stars affected by Sgr A* and finds that Sgr A* is better modeled by a galactic halo and core of 56KeV Dark Matter fermions by a Black Hole.

Their paper is available in arxiv which also reports that it has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. It's only 6 pages long.
... but apparently it's not there yet.
It has also but written up in a "Science Alert" article.

The selected fermion mass of 56KeV is constrained more by overall Milky Way galactic gravity than by the Sgr A* observations. But the 56KeV is within the range of values that betters the BH model.

So ...
1) Haven't there been Dark Matter searches that would have found 56KeV fermions?
2) I'm a bit lost on what happens when normal (non-dark) matter gets pulled in by a fermionic DM core. I can see how dark matter would have trouble forming a BH - since it has no good method for the kind of frictional braking that can lower the orbit of non-dark matter. But what about all the non-dark matter that's attracted? Does it just get ejected back out? Does it eventually go through some intense process - like spaghetification - that converts it to DM?

 

[ohwilleke]

The paper and its abstract are as follows (the emphasis in the abstract is mine):

[Submitted on 13 May 2021 (v1), last revised 14 May 2021 (this version, v2)]

Hinting a dark matter nature of Sgr A* via the S-stars​

E. A. Becerra-Vergara, C. R. Argüelles, A. Krut, J. A. Rueda, R. Ruffini

The motion data of the S-stars around the Galactic center gathered in the last 28 yr imply that Sgr A* hosts a supermassive compact object of about 4×106 M⊙, a result awarded with the Nobel Prize in Physics 2020. A non-rotating black hole (BH) nature of Sgr A* has been uncritically adopted since the S-star orbits agree with Schwarzschild geometry geodesics. The orbit of S2 has served as a test of General Relativity predictions such as the gravitational redshift and the relativistic precession. The central BH model is, however, challenged by the G2 post-peripassage motion and by the lack of observations on event-horizon-scale distances robustly pointing to its univocal presence. We have recently shown that the S2 and G2 astrometry data are better fitted by geodesics in the spacetime of a self-gravitating dark matter (DM) core - halo distribution of 56 keV-fermions, "darkinos", which also explains the outer halo Galactic rotation curves. This Letter confirms and extends this conclusion using the astrometry data of the 17 best-resolved S-stars, thereby strengthening the alternative nature of Sgr A* as a dense core of darkinos.

Comments:	Accepted for publication in MNRAS Letters
Subjects:	Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)
Cite as:	arXiv:2105.06301 [astro-ph.GA]

1. Don't take the paper too seriously. This is not a widely accepted conclusion and the paper itself acknowledges in its abstract that it is a minority interpretation.

2. The reduced chi-square values of the fits to both models (traditional black holes and the non-black hole model) are both consistent with the data at a two sigma level after considering look elsewhere effects, even if the dark matter fit is slightly better. See Table 1 at page 9 of the pre-print showing just one star (S2) out of 17 with a reduced chi-square fit of worse than 2 (which is what would be expected by random chance) and a difference of only 0.28 in the reduced chi-square of that fit.

The only real big discrepancy between the two hypotheses is the fit of another single star (G2) which has a reduced chi-square fit of 20 in the DM model proposed and 41 in the standard Black Hole analysis. But this ignores the fact that the G2 object is a horrible fit compared to every single other object in the vicinity of Sgr A* than any other known object, by a wide margin, under both models. It is far more likely that the G2 discrepancy in both cases is due to systemic error and a misplaced theoretical assumption shared by the models, than it is that the DM model proposed is a "correct" theory. Rather than rolling this into a global fit, that should have been treated as an outlier and analyzed separately.

Even if there is a technically "statistically significant" preference for the post-hoc DM model created to explain S2 and G2 than to the black hole model established a century ago by Einstein, looking at the data as a whole and recognizing that lots of astronomy data is not only inaccurate but has understated error bars, the argument for a DM explanation for these two specific star's rotation curves isn't very convincing.

3. Truly "sterile" dark matter that has no non-gravitational interactions with ordinary Standard Model matter, wouldn't be possible to detect in direct dark matter detection experiments or in particle colliders.

Also, any estimate of the mass of truly "sterile" dark matter is based on an estimate of the mean velocity of the dark matter that is inferred from gravitational dynamics and quantum effects, which in turn is mostly derived from a viral theorem relating mean velocity to mass in a thermal freeze out scenario. To the extent the the dark matter isn't generated by thermal freeze out, inferring mass is more difficult.

Dark matter particles of 56 keV is what would usually be called "warm dark matter" and there have been serious efforts to constrain the parameter space of warm dark matter from galaxy dynamics that would disfavor warm dark matter with a rest mass this large. But, the paper in question doesn't engage at all with any of this literature to either determine that it rules out their model, or that the analysis applied to other dark matter models isn't applicable to their dark matter model for a particular reason which is articulated in the paper.

4. Most dark matter particle models do not contemplate the possibility that ordinary matter is converted to dark matter, or visa versa. In particular, the LambdaCDM model also called the "Standard Model of Cosmology" contemplates that for all practical cosmology purposes, the quantity of dark matter in the universe, and the quantity of ordinary baryonic matter in the universe, is fixed just moments after the Big Bang, and that the quantity of it globally, or in any decent sized local area, does not change materially thereafter. If "Cold Dark Matter" does annihilate, the creation and annihilation of this non-baryonic matter must be in something very close to equilibrium over almost the entire history of the universe in this model.

 

[mfb]

The event horizon telescope has recorded data from Sgr A*, so this could be a short-living hypothesis. It would be odd if the Milky Way has such a cluster while M87 has a black hole. Maybe the collapse mechanism they discuss can lead to such a difference, but this isn't discussed quantitatively.

It's easy to get a better fit if you add another degree of freedom. Allow the central black hole to rotate and it's likely your fit quality improves a bit, too.

I would like to see why this dense cluster wouldn't capture things - stars at the very least, but probably also black holes that would then grow from more infalling baryonic and dark matter.