Markarian 1216 (Mrk 1216 for short) is a compact elliptical galaxy about 316 million light-years away. New data from NASA’s Chandra X-ray Observatory have helped astronomers reveal that Mrk 1216 has more dark matter packed into its core than expected after being isolated for billions of years.
Hubble’s visible-light image of the compact elliptical galaxy Markarian 1216. Image credit: NASA / STScI.
Mrk 1216 belongs to a family of elliptically shaped galaxies that are more densely packed with stars in their centers than most other galaxies.
Astronomers think they have descended from reddish, compact galaxies called red nuggets that formed about a billion years after the Big Bang, but then stalled in their growth about 10 billion years ago.
If this explanation is correct, then the dark matter in Mark 1216 and its galactic cousins should also be tightly packed.
To test this idea, University of California at Irvine astronomers David Buote and Aaron Barth studied the X-ray brightness and temperature of hot gas at different distances from Mrk 1216’s center, so they could weigh how much dark matter exists in the middle of the galaxy.
“When we compared the Chandra data to our computer models, we found a much stronger concentration of dark matter was required than we find in other galaxies of similar total mass,” Dr. Buote said.
“This tells us the history of Mrk 1216 is very different from the typical galaxy. Essentially all of its stars and dark matter was assembled long ago with little added in the past 10 billion years.”
Chandra’s X-ray image of Markarian 1216. Image credit: NASA / CXC / University of California at Irvine / D. Buote.
According to the new study, a halo of dark matter formed around the center of Mrk 1216 about 3 or 4 billion years after the Big Bang. The halo is expected to have extended over a larger region than the stars in the galaxy.
The formation of such a red nugget galaxy was typical for a wide range of elliptical galaxies seen today.
However, unlike Mrk 1216, most giant elliptical galaxies continued to gradually grow in size when smaller galaxies merged with them over cosmic time.
“The old ages and dense concentration of the stars in compact elliptical galaxies like Mrk 1216 seen relatively nearby provided the first key evidence that they are the descendants of the red nuggets seen at great distances,” Dr. Barth said.
“We think the compact size of the dark matter halo seen here clinches the case.”
The Mrk 1216 data also provide useful information about dark matter.
The astronomers interpreted the Chandra data using both standard, Newtonian models of gravity and an alternative theory known as modified Newtonian dynamics (MOND).
The results showed that both theories of gravity required about the same extraordinary amount of dark matter in the center of Mrk 1216, effectively removing the need for the MOND explanation.
“In the future we hope to go a step further and study the nature of dark matter,” Dr. Buote said.
“The dense accumulation of dark matter in the middle of Mrk 1216 may provide an interesting test for non-standard theories that predict less centrally concentrated dark matter, such as for dark matter particles that interact with each other by an additional means other than gravity.”
The findings were published in the Astrophysical Journal.
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David A. Buote & Aaron J. Barth. 2019. The Extremely High Dark Matter Halo Concentration of the Relic Compact Elliptical Galaxy Mrk 1216. ApJ 877, 91; doi: 10.3847/1538-4357/ab1008
