Galaxies and dark matter go hand in hand – you typically don’t find one without the other. So when astronomers uncovered a galaxy that is almost completely devoid of dark matter, they were shocked.
Researchers classify the galaxy, dubbed NGC1052-DF2, as an ultra-diffuse galaxy. These barely visible galaxies are surprisingly common. However, no other galaxy of this type yet-discovered is so lacking in dark matter.
“Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy,” said lead author Pieter van Dokkum of Yale University.
“For decades, we thought that galaxies start their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC1052-DF2 challenges the standard ideas of how we think galaxies form.”
Van Dokkum and his team first spotted NGC1052-DF2 with the Dragonfly Telephoto Array, a custom-built telescope in New Mexico that they designed to find these ghostly galaxies. NGC1052-DF2 stood out in stark contrast when comparisons were made between images from the Dragonfly Telephoto Array and the Sloan Digital Sky Survey (SDSS). The Dragonfly images show a faint “blob-like” object, while SDSS data reveal a collection of relatively bright point-like sources.
To get a closer look at this inconsistency, the team dissected the light from several of the bright sources within NGC1052-DF2 using Keck Observatory’s Deep Imaging Multi-Object Spectrograph (DEIMOS) and Low-Resolution Imaging Spectrometer (LRIS), identifying 10 globular clusters. These clusters are large compact groups of stars that orbit the galactic core.
The spectral data obtained on the Keck telescopes revealed that the globular clusters were moving much slower than expected. The slower the objects in a system move, the less mass there is in that system. The team’s calculations show that all of the mass in the galaxy could be attributed to the mass of the stars, which means there is almost no dark matter in NGC1052-DF2.
“If there is any dark matter at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can account for all of the mass, and there doesn’t seem to be any room for dark matter.”
Galaxy formation is turbulent and violent, and van Dokkum suggests that the growth of the fledgling massive galaxy billions of years ago perhaps played a role in NGC1052-DF2’s dark-matter deficiency.
Another idea is that a cataclysmic event within the oddball galaxy, such as the birth of myriad massive stars, swept out all the gas and dark matter, halting star formation.
These possibilities are speculative, however, and don’t explain all of the characteristics of the observed galaxy, the researchers said.
Image credit: W. M. KECK OBSERVATORY/GEMINI OBSERVATORY/NSF/AURA/J. MILLER/J. POLLARD
Video credit: NASA/Goddard
Music credit: Music credit: “Reborn” by Maksim Tyutmanov [PRS] and Victoria Beits [PRS]; Atmosphere Music Ltd PRS; Score Addiction; Killer Tracks Production Music