Dark Matter in Dwarf Galaxies

Hi Particle Biters,

Last week there were some exciting new results from the Fermi-LAT collaboration in dark matter searches! Dark matter is an exciting topic – we believe that 85% of the known matter in the universe is stuff that we can’t see. “Dark matter” itself is a very broad idea. I’ll need to start by making some assumptions on the kind of dark matter we’re looking for. We assume a dark matter candidate exists as a particle that is both massive and interacts on the scale of the “weak” force (specifically a Weakly Interacting Massive Particle – WIMP). There are lots of reasons that motivate this type of a candidate (cosmic microwave background, baryon acoustic oscillations, large scale structure, gravitational lensing, and galactic rotation curves to name a few), and I can elaborate more in another post if readers are interested. For now, just trust me when I say WIMPs are a well motivated dark matter candidate.

Based on things like galactic rotation curves we can guess the distribution of dark matter in our galaxy – and the highest concentration is in the very center. The problem with looking fimg-thingor dark matter in the center of the galaxy is that there are lots of backgrounds. First, there are ~8 kilo-parsecs worth of spiral arms in between us and the center. That’s a lot of stuff to try to understand. Plus there is a super massive black hole – Sagittarius A* – and an unknown number of pulsars (for example) also in the region of the galactic center. Other good, less chaotic, places to search for WIMPs are small, old satellite galaxies orbiting the Milky Way. One class of these types of galaxies is called dwarf spheroidal galaxies (dsphs) (not to be confused with Disney-type dwarfs). They don’t have enough visible matter to be gravitationally bound (even though they are!), so we know that there must be a high concentration of dark matter in these systems. Below is a picture of the location of some known dsphs. In total about 25 are known that Fermi-LAT will analyze. (you can see Flip’s post for more info too!)


Many of these dsphs were discovered by the Sloan Digital Sky Survey (SDSS). SDSS is a 2.5 m optical telescope located at Apache Point Observatory in New Mexico. It has been surveying the northern sky since 2000. The Fermi-LAT collaboration then points at the location of these dsphs and looks for gamma-rays (high energy photons) which would indicate dark matter annihilation. Since these are old systems, there shouldn’t be any gamma-ray emission from these targets that isn’t from dark matter.

Fermi-DMdsphs-P8The Fermi-LAT collaboration has submitted the newest searches for these known dsphs in a paper on arXiv on March 9th. They used 6 years of data and the newest, best event analysis and reconstruction (called Pass 8). The results are on the left: the y-axis shows the cross section times the thermally averaged velocity that we are sensitive to and the x-axis shows different dark matter masses. The blue line shows the previous results obtained by Fermi-LAT. The dashed black line shows what we would expect to see given a specific model of dark matter. In this case we assume that the dark matter decays into b-quarks and anti-b quarks and then from then gamma rays are produced. The solid black line shows the limit of what we actually observed. (Unfortunately no discovery!! That would look like a sharp divergence from the expectation).

Although we haven’t found an indication of dark matter annihilation, we are just becoming sensitive to the “thermal relic” (or the amount of dark matter expected after the big bang shown in the dashed gray line ). So the next few years of these searches are going to be very exciting. I’ll also hint at a future post… there is currently another collaboration (the Dark Energy Survey, or DES), which similarly to SDSS will find more dsphs for us to use as targets – which will only improve our sensitivity. In the next post I’ll talk about these results.

I hope you’ve enjoyed this post! Please post any questions/comments.


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