## Tuesday, March 10, 2015 ... /////

### Dwarf galaxy 3.7-sigma evidence for dark matter claimed

A new astro-ph paper cross-linked to hep-ph,

Evidence for Gamma-ray Emission from the Newly Discovered Dwarf Galaxy Reticulum 2
by Geringer-Sameth and 6 co-authors (Carnegie-Mellon/Brown/Cambridge), boasts that they have looked into the Fermi-LAT telescope data more carefully than the Fermi folks and found something that looks like a 3.7-sigma (or, with model-independent background estimates, 2.3-sigma) deviation from the expected gamma-ray backgrounds at frequencies $2$-$10\GeV$.

The approximate shape of Reticulum 2, a dwarf galaxy, and the light it emits.

If that deviation is a sign of new physics, and it is a huge "If" indeed, the interpretation is that the signal is due to a pairwise self-annihilating particle whose mass is a few times $100\GeV$.

The signal-resembling observations have been made in Reticulum 2, a dwarf galaxy. One must realize that Reticulum 2 was "cherry-picked" as the "most signal-like dwarf galaxy" among a few dozens of dwarf galaxies which should reduce your faith (or certainty) that the signal is real.

An excess: in the middle of the picture, the red crosses seem to be above the solid black line.

Reticulum means a "small reticle" in Latin, and a "reticle" is a web, typically a system of thin lines (with a cross) in the circular visual field of a telescope. More importantly, Reticulum 2 (Ret2) is a dwarf galaxy which is really close – it is about 30 kpc away from Pilsen. ;-) It's further than Prague but it is very close – almost exactly equal to the diameter of the Milky Way (according to Wolfram Alpha). Ret2 is the third closest dwarf galaxy after Segue 1 (Seg1, 23 kpc) and Sagittarius (24 kpc).

Ret2 is one of the 9 new low-luminosity objects. It (or they) may be viewed as "satellites of the Milky Way". Among the 9 guys, it's probably the most obvious one. Along with Eridanus 2 and no one else, it shows a statistically significant elongation (others are round) which may perhaps increase the density at some points and improve the odds that self-annihilation of dark matter takes place there.

The signal-like optimistic interpretation is that a new particle, such as a superpartner, pairwise annihilates first into a pair of Standard Model $X\bar X$ particles before they create some gamma-rays. We don't know what the $X\bar X$ pair is. Depending on whether $X\bar X$ is $b\bar b$ or $\tau^+\tau^-$ or $hh$ or $\mu^+\mu^-$, the mass $m$ and $J\langle\sigma v \rangle$ of the dark matter particle may have different values – the mass is distributed in rather wide intervals roughly between $50$ and $1,000\GeV$.

It's obviously incorrect to say that the dark matter has been discovered. After all, the significance level wouldn't be sufficient for a hard discovery even if one forgot about all the other possible reasons to question the bold claim suggested in this paper (Ret2 was cherry-picked; Fermi folks haven't noticed that; even according to these 7 authors, it may be due to astrophysical sources, etc.). Nevertheless, the evidence has some beef and it's plausible that the signal is real and in the future, with the hindsight, these folks will be quoted as those who already saw the actual proof of the dark matter.

Thanks to Matt Buckley

#### snail feedback (12) :

I'm not a physicist, so please forgive my ignorant comment: I suspect dark matter is just as likely to exist as Gravity is likely to be in inisotropic. Maybe the fundamental solution needs to be revisited.

"between 50 and 1000GeV" Between a chromium atom and 5.77 top quarks. The particle is stable for the current life of the universe and has no interaction except gravitation. It felicitously self-annihilates without providing universal background radiation or a beacon near galactic central black holes.

Does it also fart rainbows?

I've never quite understood what thermal equilibrium of the universe means physically if dark matter essentially only interacts gravitationally, or the consequences on cosmic expansion. Or how dark matter gets cold. So in trying to picture dark matter decay, it seems like dark matter should be fairly hot, implying many heavy unstable particles still around to decay. From that it seems like the difficulty in spotting the radiation is already fairly strong evidence against the predominance of primordial dark matter. Is there some reason why decay of unstable dark matter would be faster?

Refresher course for the simple minded. If dark matter does not interact except gravitationally how can they produce photons? Also, is it possible that the dark matter world is as complex -- with atoms, molecules, etc., or something comparable -- as the world we know?

no, skittles.

Why do you suspect that these possibilities are equal? I'd say that's because you don't understand them enough to feel the difference.
You can't just state that the fundamental principles should be changed. You should actually construct a viable model. And the fundamental principles are fundamental because they are very hard to dismiss. That's the theoretical problem with MOND (putting aside the observations) it's not so trivial to make consistent fundamental theory for it as you may think. And be sure, you will have to introduce new fields and particles.
On the other hand some dark matter candidates like SUSY particles would be the truly revolutionary discovery

Interesting article. But I do not understand the reason for cherry picking low luminosity objects. I would think that emission of lot of visible light would not obscure detection of gamma rays!

I like your depiction of the dwarf galaxy, Reticulum 2 ;). I think this is what we call a Leprechaun in Ireland. There was another preprint yesterday from the Fermi-LAT and DES collaborations where they reported no excess of gamma ray radiation from 8 dwarf galaxies found by the Dark Energy Survey (http://arxiv.org/pdf/1503.02632v1.pdf). In this preprint Reticulum 2 is DES J0335.6􀀀5403.

Low luminosity objects will have fewer astrophysical backgrounds like neutron stars, et cetera. The concentration of baryons to dark matter seems to drop in these objects, so the signal to noise of any dark matter signal could go up, all other things being equal.

Thanks for the reply. I understand their hypothesis now. Then, if a convincing signal of dark matter is not found in these objects, their hypothesis of a smaller baryon to dark matter in these objects would be proven wrong.