It doesn’t have as much dark matter. 10,000 times more dark matter than ordinary or visible matter? It was what scientists thought a few years ago about Dragonfly 44 ; something like an extreme cosmic anomaly. However, the study published in the Monthly Notices of the Royal Astronomical Society suggests that this galaxy is actually quite normal, and that the scientists who discovered it were wrong.
Today, galaxy formation is difficult to understand without the presence of an omnipresent but mysterious component called dark matter.
An international team led by the Kapteyn Institute of the University of Groningen (Holland), with the participation of the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL), discovered that the total number of globular clusters around Dragonfly 44 and therefore the dark matter content is much lower than previous findings had suggested, showing that this galaxy is neither unique nor anomalous.
A typical galaxy could have 10 to 300 times more dark matter than visible matter, which sets Dragonfly 44 apart from the norm. But it turns out that, according to the new study, the scientists who discovered Dragonfly 44 appear to have vastly overestimated the amount of dark matter it has. Its ratio would be 300 times the amount of visible matter (and not 10,000).
“The fact that in our work we find only 20 globular clusters, compared to the 80 previously claimed, drastically reduces the amount of dark matter that the galaxy is believed to contain,” comments Ignacio Trujillo, IAC researcher and co-author of the article .
Recall that the total number of globular clusters is related to the total mass of a galaxy. Therefore, if the number of globular clusters is measured, it is possible to identify the amount of dark matter, especially if the amount of visible matter is only a small fraction of the total.
“Furthermore, with the amount of globular clusters that we found, the amount of dark matter in Dragonfly 44 is in agreement with what was expected for this type of galaxy. The ratio of visible to dark matter is no longer 1 in 10,000 but 1 in 300 ”, adds Trujillo.
“ We now know that the previous results were incorrect and that DF44 is not extraordinary . It’s time to move on, ”said Teymoor Saifollahi, a researcher at the Kapteyn Astronomical Institute in the Netherlands and leader of the study.
Case closed
It’s disappointing that while it still has a high proportion of dark matter, Dragonfly 44 no longer stands out as much as a cosmic mystery. Fortunately, from a scientific point of view that is good news.
“Our work shows that this galaxy is not so singular or unexpected”, clarifies the researcher Michael Beasley, a researcher at the Instituto de Astrofísica de Canarias in Spain. “In this way, galaxy formation models can explain it without the need for modifications .”
Referencia: Teymoor Saifollahi et al, The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa3016
