A team of researchers has further examined subtle distortions in ALMA's image of the gravitational lens SDP.81 and determined that they point to the presence of a dwarf dark galaxy in the halo of a larger galaxy approximately 4 billion light-years away.
The picture stems from ALMA's 2014 Long Baseline Campaign, in which astronomers studied numerous astronomical objects in order to test the telescope's high-resolution capabilities. In the current study, the image examined is an Einstein ring that was produced by gravitational lensing, a phenomenon that occurs when the gravity of a foreground galaxy bends the light emitted by another galaxy.
Gravitational lensing was predicted by Einstein's general theory of relativity and can be used to study galaxies that are too distant from our own to properly observe. In addition, the phenomenon also allows us to gather information on the nearby lensing galaxy through the examination of the way that its gravity distorts and focuses the light from distant objects.
"We can find these invisible objects in the same way that you can see rain droplets on a window," said Yashar Hezaveh, an astronomer from Stanford University in California and co-author of the study. "You know they are there because they distort the image of the background objects."
In the current image, distortions are created due to the gravitational influence of dark matter.
Although dark matter has yet to be directly observed and doesn't interact with visible light or other forms of electromagnetic radiation, it does have mass and thus can be identified through it gravitational influence.
Hezaveh and his team used thousands of computers working in parallel to detect anomalies that had consistent and measurable counterparts in each "band" of radio data. Using this data, the team was able to gain insight into the lensing of the galaxy's halo, the star-free region around the galaxy and a distinctive anomaly.
Examination of the anomaly's relationship to the larger galaxy, its estimated mass and lack of an optical counterparts led the team to believe that its root lies in a faint, dark-matter dominated satellite of the lensing galaxy that is likely a dark dwarf.
Current theories predict that most galaxies should possess similar dwarf galaxies, although detecting them is challenging, with astronomers only able to identify 40 or so of the thousands that should theoretically be present.
"This discrepancy between observed satellites and predicted abundances has been a major problem in cosmology for nearly two decades, even called a 'crisis' by some researchers," said Neal Dalal, a researcher from the University of Illinois and co-author of the study. "If these dwarf objects are dominated by dark matter, this could explain the discrepancy while offering new insights into the true nature of dark matter."
The current study suggests that most dwarf galaxies might be difficult to observe due to a composition that is mainly invisible dark matter, meaning they emit little or no light.
"Our current measurements agree with the predictions of cold dark matter," said Gilbert Holder, co-author of the study and researcher from McGill University in Montreal, Canada. "In order to increase our confidence we will need to look at many more lenses."
The findings were published Jan. 7 on the pre-print server arXiv.
