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Heart of Lonesome Galaxy is Brimming with Dark Matter

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Posted June 4, 2019

Data from NASA’s Chandra X-ray Observatory (left) have helped astronomers reveal that a galaxy has more dark matter packed into its core than expected after being isolated for billions of years. The image on the right shows the galaxy called Markarian 1216 (abbreviated as Mrk 1216) in visible light from NASA’s Hubble Space Telescope over the same field of view.

Markarian 1216 in optical and X-Ray spectres.

Markarian 1216. Credit: X-ray: NASA/CXC/Univ. of CA Irvine/D. Buote; Optical: 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 red, 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 evolution is correct, then the dark matter in Mrk 1216 and its galactic cousins should also be tightly packed. To test this idea for the first time, a pair of astronomers 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. The brighter colors at the center of the Chandra image represent the increased density of hot gas in the galaxy’s core.

According to the new study, a halo, or fuzzy sphere, of dark matter formed around the stars in the center of Mrk 1216 about 3 or 4 billion years after the Big Bang. The formation of such a red nugget 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.

Previously, astronomers estimated that the supermassive black hole in Mrk 1216 is more massive than expected for a galaxy of its mass. This most recent study, however, concluded that the black hole mass is likely to be less than about four billion times the mass of the Sun, which means it may not be unusually massive for a galaxy as large as Mrk 1216.

Researchers also searched for signs of outbursts from the supermassive black hole in the center of the galaxy. They saw hints of cavities in the hot gas similar to those observed in other massive galaxies and galaxy clusters like Perseus, but more data are needed to confirm their presence.

A paper by David Buote and Aaron Barth (both of the University of California at Irvine) describing these results appeared in the June 1st issue of The Astrophysical Journal and is available online. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.

Source: Chandra X-Ray Observatory

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