Galaxy [What is Galaxy?] formation cannot be understood today without the presence of an ubiquitous, but still mysterious component called dark matter. Astronomers have measured the amount of dark matter around galaxies and found that it varies between 10 and 300 times the amount of visible mass. However, a few years ago, the discovery of a very diffuse object, called Dragonfly 44, altered this perspective. This galaxy was found to have 10,000 times more dark matter than stars. Baffled by this discovery, astronomers have struggled to explore whether this object is truly anomalous or if something went wrong with the analysis of this galaxy. Now you have the answer.
An international team of astronomers led by the Kapteyn Astronomical Institute of the University of Groningen (Netherlands), with the participation of the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL) (Spain), has found that the The total number of globular clusters around Dragonfly 44, and therefore the dark matter content, is much lower than previous findings suggested, demonstrating that this galaxy is neither unique nor anomalous. The result was recently published in the Monthly Notices of the Royal Astronomical Society (MNRAS).
The Dragonfly 44 galaxy was discovered through a deep mapping of the Coma cluster, a region of the sky with a grouping of several thousand galaxies. From the beginning, the object caught the attention of researchers because the amount of inferred dark matter was almost as large as the measurement in our own Milky Way, the equivalent of a trillion suns.
However, instead of having around 100 billion stars, as is the case in our galaxy, DF44 only has 100 million stars, that is, a thousand times fewer. This makes the proposed dark matter content ten thousand times greater than its stellar content. Had it been true, it would have made it a unique object, with almost 100 times more dark matter than expected for the number of stars it has.
However, using a comprehensive analysis of the globular cluster system around Dragonfly 44, researchers have found that the total number of globular clusters is only 20 and that the total amount of dark matter in the system is about 300 times the luminous mass, which makes the object the expected one for this type of galaxy.
“The fact that in our work we only found 20 clusters, compared to the 80 that had been said to exist, drastically decreases the amount of dark matter that the galaxy is believed to have”, explains Ignacio Trujillo, IAC researcher and co-author of the article. “Moreover, with the number of globular clusters that we find, the amount of dark matter that Dragonfly 44 has is in agreement with what is expected for this type of galaxy. It goes from being a ratio of 1 to 10,000 to simply 1 to 300”.
“Dragonfly 44 has been an anomaly all these years that could not be explained with existing galaxy formation models. We now know that the previous results were incorrect and that DF44 is not extraordinary. It is time to move on,” says Teymoor Saifollahi, researcher at the Kapteyn Astronomical Institute and lead author of the study.
“Our work shows that this galaxy is not so unique or unusual. In this way, the galaxy formation model is able to explain it without the need to modify it,” says Michael A. Beasley, also a researcher at the IAC and co-author of the paper.
The total number of globular clusters is related to the total mass of the galaxies. That way, if researchers know the number of globular clusters, they will also know the amount of dark matter in a galaxy.
“However, we do not know why there is a relationship between the total number of globular clusters and the total mass of the galaxy. This is a purely observational fact. Although it may have to do with the initial amount of gas that makes up the stars and the globular clusters themselves. The more dark matter there is in a galaxy, the more gas it also has”, warns Johan H. Knapen, a researcher at the IAC.