Newly found "dwarf" galaxies having an outsized impact

Mar 16, 2015

(NB: Based on a SLAC NAL press release )



Primary researchers and authors of the DES and FERMI analyses of the new dwarf galaxies --- top row: KIPAC alums (L to R) Keith Bechtol, Alex Drlica-Wagner, and KIPAC faculty Risa Wechsler, and bottom row: current KIPAC postdocs (L to R) Andrea Albert, Matthew Wood (underneath a model of the FERMI satellite), and Research Staff Member Eli Rykoff.



After Ten Years of "Discovery Drought", Eight New Milky Way Companions Found 

Researchers have discovered a set of eight celestial objects orbiting our home galaxy, the Milky Way, that appear to be rare dwarf satellite galaxies.  Dwarf galaxies are the smallest known galaxy structures and if the conditions are right, may hold the key to understanding dark matter.

New dwarfs are shown in red in the lower right in the figure of the Milky Way above -- shown as if we would be able to see our own Galaxy from the outside.

The discovery was made independently by scientists of the Dark Energy Survey (DES) – a collaboration that includes researchers from the Department of Energy’s SLAC National Accelerator Laboratory – and a research group from the University of Cambridge in the UK. The teams reported their findings on March 9 in two scientific papers.

“This result is particularly exciting because dwarf galaxies are one of the best places to look for dark matter,” says DES researcher Risa Wechsler from Stanford University’s Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) and SLAC, one of the study’s co-authors. “Finding more of these galaxies will also help us test and refine our models of how dark matter impacts the evolution of the universe and the formation of galaxies.”  

KIPAC researchers also played a key role in two additional studies, one by the Large Area Telescope (LAT) collaboration, and one performed jointly by the LAT and DES collaborations, which did not so far find any signs of dark matter particles either in previously known dwarf galaxies or in the newly found objects. These results [ 1  ,  2  ] published on March 9, 2015 will help constrain future searches for mysterious dark matter. 

Discovery of Dwarf Galaxy Candidates

On an astronomical scale, dwarf galaxies are very small and therefore difficult to spot. The smallest ones contain only a few hundred stars, compared to the tens of billions of stars in the Milky Way. The newly discovered objects are a billion times dimmer than the Milky Way and a million times less massive. The closest of them is about 95,000 light-years away. 

New dwarfs shown in the DES "footprint"  (i.e. where DES has scanned the sky thus far) shown in gray.

Scientists have seen more than two dozen of these satellite galaxies around the Milky Way. About half were discovered in 2005 and 2006 by the Sloan Digital Sky Survey, the predecessor of DES. However, almost none have been discovered in the past five years. 

DES found the new objects in a different portion of the sky, never examined so closely before, by closely analyzing photographs taken by the world’s most powerful digital camera mounted on a telescope in Chile. 

While more analysis is required to confirm any of the observed objects as dwarf satellite galaxies, the researchers note their size, low surface brightness and significant distance from the center of the Milky Way as evidence that they are excellent candidates.

The DES team identified the potential dwarf galaxies by adapting a method developed in large part by KIPAC’s Eli Rykoff, one of the study’s co-authors. “I wrote a program that finds galaxies in galaxy clusters by associating galaxies of the same color that are located in the same region of the sky,” Rykoff says. “A few years ago, Risa pointed out that we could use the same technique to look for the association of stars in dwarf galaxies.” 

The study’s two lead authors, Alex Drlica-Wagner from Fermi National Accelerator Laboratory and Keith Bechtol from the University of Chicago, both former KIPAC graduate students, followed up on the idea and applied it to the DES analysis.   


Dwarf Galaxies and Dark Matter Research

Newly discovered dwarf galaxies would also present scientists with more opportunities to look for signatures of dark matter, which accounts for about 85 percent of all matter in the universe but whose nature is unknown.

“We know that dwarf galaxies are dominated by unseen dark matter because they are much more massive than all their visible stars taken together,” says KIPAC’s Matthew Wood, who led the dark matter analysis of the LAT studies. 

The LAT is the principal scientific instrument on NASA’s Fermi Gamma-ray Space Telescope spacecraft. SLAC assembled the LAT and runs its data processing center. Scientists use the LAT to search for gamma rays as potential indicators of dark matter particles annihilating each other in space. Since dwarf galaxies are rich in dark matter and do not host other gamma-ray sources, they are ideal objects for this search. 

In the first of the two LAT papers, the researchers reported that they did not see any gamma rays coming from 15 previously known dwarf satellite galaxies – a result that limits the properties hypothetical dark matter particles can have.

In the second study, the researchers analyzed the region of the sky in which DES found the dwarf galaxy candidates. Again, no significant gamma-ray emission was detected. “If these objects are confirmed as dwarf galaxies, they will increase the sensitivity of our analysis and further constrain the properties of dark matter particles,” says KIPAC’s Andrea Albert, who helped develop a new technique to estimate the object’s dark matter content.  

The discovery of eight potential dwarf galaxies resulted from an analysis of the first year of DES data alone. Scientists expect that the experiment will find between 10 and 100 of these galaxies over its planned lifetime of five years. The more targets DES finds, the more clues it will give to researchers about mysterious dark matter.

Small Sized, but Shining Brightly

Whether or not dark matter signatures are seen from these new dwarfs, one thing that is clear is that they are already having an outsized impact on several subfields of astronomy and astrophysics -- and KIPAC researchers were integral to all phases of their discovery and subsequent analysis.