Dark Matter in Triangulum II, Dead Dwarf Satellite of Milky Way Galaxy

Summary: Dead dwarf satellite Triangulum II has the universe's highest dark matter concentration, says a paper in The Astrophysical Journal Letters Nov. 20, 2015.

3D map of web-like large-scale distribution of dark matter; map reveals loose network of dark matter filaments, gradually collapsing under relentless pull of gravity and growing clumpier over time; box's 3 axes correspond to sky position (in right ascension and declination), and distance from Earth increasing from left to right (as measured by cosmological redshift). Clumping of dark matter becomes more pronounced, moving right to left across volume map, from early Universe to more recent Universe: NASA/ESA/Richard Massey (California Institute of Technology), Public Domain, via Hubble Space Telescope

A dead dwarf satellite appears to hold the universe’s record for highest concentrations of dark matter, according to a paper published Nov. 20, 2015, by The Astrophysical Journal Letters in its open-access edition.

The honor belongs to Triangulum II, a recently discovered compact stellar system at the edge of the Milky Way Galaxy. The faint, small dwarf galaxy 117,000 light years away from Earth contains about 1,000 stars. The DEIMOS (Deep Extragalactic Imaging Multi-Object Spectrograph) on W. M. Keck Observatory’s telescope II near the summit of Mauna Kea in Hawaii detects in Triangulum II’s galactic center six stars that are sufficiently luminous for examination of stellar velocity, inference of gravitational force and measurement of galactic mass.

Co-authors Judith G. Cohen and Evan N. Kirby of the California Institute of Technology in Pasadena, Puragra Guhathakurta of the University of California Lick Observatory at Mount Hamilton and Joshua D. Simon of the Observatories of the Carnegie Institution of Washington in Pasadena estimate a total mass far greater than may be expected from considering only how observable matter makes Triangulum II appear.

The five co-authors find Triangulum II’s high mass and low luminosity similar to that of Segue 1, a Milky Way satellite and the universe’s faintest galaxy. Dead galaxies such as Segue 1 and Triangulum II generate no new stars. Both have a handful of central stars moving inexplicably faster than the other 1,000 stars.

One theory is dark matter, whose presence may be suspected when stars move unexpectedly fast. Supersymmetric WIMPs (weakly interacting massive particles) of dark matter jostle one another and, in the mutual annihilation, may produce gamma rays.

Astrophysicists know that dead, faint galaxies have theoretical likelihoods of emissions detectable by space-based gamma-ray observatories.

Results and theory lead Evan Kirby, Assistant Professor of Astronomy and study leader, to summarize: “The total mass I measured was much, much greater than the mass of the total number of stars -- implying that there’s a ton of densely packed dark matter contributing to the total mass. The ratio of dark matter to luminous matter is the highest of any galaxy we know.”

Triangulum II models dynamic equilibrium or tidal disruption. Ten co-researchers at France’s University of Strasbourg note in “Triangulum II: A Very Metal-Poor and Dynamically Hot Stellar System” of Oct. 16, 2015, for The Astrophysical Journal Letters: “We favor the scenario in which Tri II is a dwarf galaxy that is either disrupting or embedded in a stellar stream but cannot completely rule out that it could be a disrupting globular cluster.”

Dr. Kirby observes: “If it turns out that those [13] outer stars aren’t actually moving faster than the [6] inner ones, then the galaxy could be in what’s called dynamic equilibrium. That would make it the most excellent candidate for detecting dark matter with gamma rays.”

Caltech FIRE (Feedback in Realistic Environments) simulation showing predicted distribution of stars (left) and dark matter (right) around a galaxy like the Milky Way. The red circle shows a dwarf galaxy like Triangulum II with a lot of dark matter but very few stars: A. Wetzel and P. Hopkins, Caltech, usage restrictions: with image credit, via EurekAlert!

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

dark matter 3D: NASA/ESA/Richard Massey (California Institute of Technology), Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:COSMOS_3D_dark_matter_map.png; via Hubble Space Telescope @ http://spacetelescope.org/images/heic0701b/

Caltech FIRE (Feedback in Realistic Environments) simulation showing predicted distribution of stars (left) and dark matter (right) around a galaxy like the Milky Way. The red circle shows a dwarf galaxy like Triangulum II with a lot of dark matter but very few stars: A. Wetzel and P. Hopkins, Caltech, usage restrictions: with image credit, via EurekAlert! @ https://www.eurekalert.org/multimedia/pub/103633.php?from=312204

For further information:

Caltech @Caltech. 18 November 2015. "Caltech's Evan Kirby finds #DarkMatter dominates in nearby dwarf galaxy." Twitter.
Available @ https://twitter.com/Caltech/status/667055959806599168

Dajose, Lori. 18 November 2015. "Dark Matter Dominates in Nearby Dwarf Galaxy." Caltech > News.
Available @ http://www.caltech.edu/news/dark-matter-dominates-nearby-dwarf-galaxy-48790

GeoBeats News. 21 November 2015. "Dead Galaxy Could Contain A Wealth of Dark Matter." YouTube.
Available @ http://www.youtube.com/watch?v=jYOug1V6-Mo

Kirby, Evan N., et al. 16 November 2015. "Triangulum II: Possibly A Very Dense Ultra-Faint Dwarf Galaxy." The Astrophysical Journal Letters, vol. 814, no. 1. DOI: 10.1088/2041-8205/814/I/L7
Available @ http://arxiv.org/pdf/1510.04433v1.pdf

Navarro, Alyssa. 20 November 2015. "Dead Galaxy At Edge Of Milky Way Hosts Highest Concentration Of Dark Matter." Tech Times.
Available @ http://www.techtimes.com/articles/108824/20151120/dead-galaxy-at-edge-of-milky-way-hosts-highest-concentration-of-dark-matter.htm