Puzzling Galaxy Considered 99.9 Percent Dark Matter More Common Than Thought
An international team has narrowed down the number of globular clusters around a galaxy called Dragonfly 44 — and therefore the dark matter content — which is much less than previous findings showed, according to a recent study published in Monthly Notices of the Royal Astronomical Society (MNRAS).
Strange galaxy appeared to have 10,000 times more dark matter than stars
The formation of galaxies is not easy to grasp without the presence of a pervasive and mysterious component called dark matter. Astronomers have measured how much dark matter exists around galaxies, and discovered its abundance at between 10 to 300 times that of visible, ordinary matter, reports Phys.org.
However, a few years ago astronomers found a very diffuse object called Dragonfly 44, which changed this idea. The strange galaxy had 10,000 times more dark matter than stars — baffling astronomers worldwide.
Since then, astronomers have worked tirelessly to know whether the strange galaxy is an anomaly, or if an error was made during the analysis of the initial observations.
Now we know why.
Strange galaxy Dragonfly 44 isn't unique, far less dark matter
The international team behind the new study was under the leadership of the Kapteyn Institute of the University of Groningen (in the Netherlands), in addition to participation from researchers from the University of La Laguna (ULL), and the Instituto Astrofisica de Canarias (IAC).
Since scientists know the total number of globular clusters surrounding Dragonfly 44, they also know the dark matter content — and both are much less than previous findings suggested. This means the strange galaxy isn't anomalous or unique.
Dragonfly 44 has 300 times more dark matter than stars
Dragonfly 44 (DF44) was first discovered during a deep survey of the Coma cluster — which contains several thousand galaxies. From the beginning, the strange galaxy was considered as such because the quantity of dark matter inferred from initial observations was nearly equal to that of the Milky Way — roughly one billion solar masses.
However, unlike the Milky Way (which contains roughly one hundred thousand million stars), DF44 only has one hundred million stars — 1,000 times fewer. This means there is ten thousand times more dark matter than stars in DF44. If this was the way things really were, the strange galaxy would be very unique, with nearly 100 times more dark matter than was expected.
Regardless, the exhaustive analysis of the globular cluster system surrounding DF44 allowed researchers to narrow down the number of globular clusters to 20 — which means the total quantity of dark matter is roughly 300 times the amount of luminous (or normal) matter.
This means it's not outside of the typical ratio of normal versus dark matter for this kind of galaxy, after all.
Astronomers correct mistaken earlier thoughts on DF44
"The fact that in our work we found only 20 globular clusters, compared with the 80 previously claimed, reduces drastically the amount of dark matter which the galaxy is believed to contain," said Ignacio Trujillo, co-author of the study and IAC researcher.
"Moreover, with the number of globular clusters we found, the amount of dark matter in Dragonfly 44 is in agreement with what is expected for this type of galaxies. The ratio of visible to dark matter is no longer 1 in 10,000 but one in 300," Trujillo added.
"Dragonfly 44 has been an anomaly all these years that could not be explained with the existing galaxy formation models," said Teymoor Saifollahi, a researcher at the Kapteyn Institute and first author of the study. "Now we know that the previous results were wrong and that DF44 is not extraordinary[, i]t is time to move on."
Studying dark matter unveils other cosmic mysteries
The total number of globular clusters has a set relation to the total mass of an entire host galaxy. This means if the number of globular clusters is known, then the amount of dark matter may be calculated — especially if the visible matter comprises only a small fraction of the total mass.
As astronomers and astrophysicists work to comprehend the mysteries of dark matter, we should expect to unravel even more puzzles of the vast depths of outer space, thanks to the continuing advancement of physics — arguably the most interesting field in modern science.