Researchers on the Dragonfly 44 study, a faint galaxy found in 2016 as comprising 99.99% of dark matter, closely re-revealed with less but more natural dark matter detection.
This means we don’t have to revise our models of galaxy formation, the researchers said, to try to figure out how they could produce such extreme aberrations – everything goes perfectly naturally.
Astronomer Timur Sivolahi of the Kapteyn Astronomical Institute in the Netherlands said: “Dragonfly 44 has been an anomaly all these years which cannot be explained by current galactic formation models. We now know that previous results were wrong and DF44 is not exceptional.”
Dark matter cannot be detected directly because it does not absorb, reflect or emit any electromagnetic radiation. However, based on the way that some things move due to gravity – the rotation of galaxies, and the way the path of light is curved – we can figure out how much mass is in galaxies. There is a mass greater than can be calculated in the population of ordinary matter that can be detected.
And we call the missing mass the dark matter. According to various measurements, approximately 85% of the matter in the universe is dark matter, although the proportions in galaxies vary according to species. It is a staple of our best models of galaxy formation. It is believed that the dark matter network could direct matter to galaxies in the early universe.
Dragonfly 44, located about 330 million light-years away in the coma group of galaxies, comes from a type known as the super-diffuse galaxy, which is much less bright than the Milky Way.
Astronomers, led by astronomer Peter van Ducum of Yale University, calculated the number of globular clusters around DF44, dense globular clusters of stars orbiting the center of the galaxy.
Because the number of globular clusters is related to the mass of the galaxy (although we’re not sure why), this allowed astronomers to calculate the mass of DF44 – and found that it was roughly the same as the mass of the Milky Way.
Given the low number of stars in DF44, this means that the galaxy is almost entirely composed of dark matter, making it very extreme. Thus, Civolahi and his team performed a recount of the globular masses, using the same Hubble Space Telescope observation data as the previous study. They applied strict parameters to define the DF44-associated globular groups, and ultimately they came out with a much smaller number.
“Moreover, with the number of globular clusters we have found, the amount of dark matter in Dragonfly 44 is consistent with what is expected for this type of galaxy,” said astrophysicist Ignacio Trujillo, from the Instituto de Astrofísica de Canarias in the Canary Islands. Visible to dark matter is 1 in 10,000 but it is one in 300. “
This is not Trujillo’s first encounter with super-diffuse galaxies with anomalous dark matter content. And last year, two such galaxies called DF2 and DF4, previously found (also by van Dokkum and colleagues) and containing little or no dark matter, were also likely normal.
“Our work shows that this galaxy is neither very singular nor unexpected,” said astrophysicist Michael Beasley, of the Astrophysics de Canaras Institute. “In this way, models of galaxy formation can be interpreted without the need for modification.”
However, such discussions are useful for science. Whatever the results, trying to discover them will help us improve our techniques for studying the universe.
Source: Royal Astronomical Society.