Reticulum II: Welcome to the neighborhood

This plot shows the positions of stars surrounding the newly discovered dwarf galaxy Reticulum II. Points outlined in black represent stars for which high-resolution optical spectra provided velocity measurements. Red points represent stars that were confirmed to be members of the new dwarf galaxy, while gray points are non-members. (Points that are not outlined do not have velocity measurements.)

The number of dark matter-dominated Milky Way satellite dwarf galaxies was increased by one this week. Scientists discovered the newest dwarf galaxy, Reticulum II, in data from the Dark Energy Survey. However, the DES data alone were not enough to confirm that Reticulum II was indeed a dark matter-dominated dwarf galaxy. Determining the dark matter content of Reticulum II required an extensive campaign combining observations from some of the largest telescopes in the world.

Researchers determine the dark matter content of dwarf galaxies by measuring the velocities of the stars in these objects. The higher the velocity of the stars, the more mass is required to keep the stars gravitationally bound. Stellar velocities are determined from the Doppler shift of elemental lines, which produce sharp features in the spectrum of visible light coming from the stars. Reticulum II was targeted with high- and medium-resolution spectroscopy by the Magellan 6.5-meter telescope, the Gemini 8.1-meter telescope and the VLT 8.2-meter telescope, all located in Chile. The result: Reticulum II has 470 times more mass than can be accounted for by its stars alone. This makes Reticulum II the first spectroscopically confirmed dwarf galaxy discovered outside of the Sloan Digital Sky Survey.

If dark matter is composed of weakly interacting massive particles, it may annihilate to produce Standard Model particles, including gamma rays. Regions of high dark matter density, such as dwarf galaxies, would then shine in gamma rays produced from dark matter annihilation. The strength of the gamma ray signal from each dwarf galaxy would be related to the distance and dark matter content of that galaxy. While nearby and highly dark matter-dominated, Reticulum II actually has a smaller dark matter content than several other previously known dwarf galaxies. This makes it unlikely to detect a gamma ray signal from dark matter annihilation in Reticulum II without seeing a similar signal in other nearby dwarf galaxies with greater dark matter content.

In addition to Reticulum II, researchers have found seven more dwarf galaxy candidates in the DES data. Since March 10, three additional dwarf galaxy candidates were announced using data from other surveys. Interestingly, two of these three additional candidates used the Dark Energy Camera for photometric confirmation. While spectroscopy is necessary to confirm that these candidates are indeed dwarf galaxies, it is already clear that DECam is a powerful instrument for understanding dark matter.

Alex Drlica-Wagner

These scientists worked on this analysis. Top row, from left: Josh Simon (Carnegie Observatories), Alex Drlica-Wagner (Fermilab), Ting Li (Texas A&M U). Bottom row, from left: Brian Nord (Fermilab), Keith Bechtol (U Chicago).