This is part 2 in my “series” on dark matter in dwarf galaxies. In my previous post, I explained a bit about WIMP-like dark matter and why we look for its signature in these particular type of small galaxies (dsphs) that are orbiting the Milky Way. About 2 months ago, the Dark Energy Survey (DES) collaboration released its first year of data to the public. Similar to SDSS, DES is also a surveying the optical-near infrared sky using the 4 m Victor M. Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile. The important distinction between SDSS and DES is that while SDSS observes the northern galactic latitudes, DES observes southern galactic latitudes. Since this is a whole new region of observation, we expect a lot of new exciting things to come out of the data… And sure enough exciting things came. Eight new dsphs candidates were discovered and published in the first data release orbiting. Dwarf galaxies are very old (> 13 billion years old) and have little gas, dust and star formation. I say candidates, because to confirm that these dsph candidates not something else, follow-up observations with other telescopes have to be done.
However, that doesn’t mean that we (we being everyone since the Fermi data is public) can’t have a look at these potential dark matter targets. On the same day that the new DES candidate dsphs were released, the Fermi-LAT team had a look. Of the eight candidates, most were far away (~100 kpc or ~300k light years). This distance makes looking for dark matter difficult because a signal will be very weak. However, there was one candidate that was only 32 kpc away (DES J0335.6-5403 or Reticulum II), making it the most interesting search target. You can see the counts map of Reticulum II on the right.
The results (on the left) showed that there was no clear WIMP-like dark matter signature coming from any of the candidates (shucks!!). However, the closest target wasn’t totally boring. Another team found a small excess (~2 sigma) in Reticulum II. When the Fermi-LAT team compared analysis methods, we found that there results were optimistic, yet not inconsistent with ours. This got the New York Times’s writer Dennis Overbye excited :).
The good news is that DES is going to continue for at least 4 more years, which means we’ll have many more opportunities to search for dark matter in dsphs. What we need to find is nearby dsphs. And even more exciting, the Large Synoptic Survey Telescope (LSST) will start taking in the 2020s. This telescope will have access to ~half of the sky (more on the LSST in a future post ;)). This will give us many more targets in the years to come, so stay tuned!