Analysis of the very high resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the gravitational lens SDP.81 reveals a dwarf dark-matter dominated galaxy in the halo of a foreground lensing galaxy roughly 4 billion light-years away.
Composite image of the gravitational lens SDP.81 showing the distorted ALMA image of the more distant galaxy (red arcs) and the Hubble optical image of the nearby lensing galaxy (blue center object). By analyzing the distortions in the ring, astronomers have determined that a dark dwarf galaxy (data indicated by white dot near left lower arc segment) is lurking nearly 4 billion light-years away. Image credit: Y. Hezaveh, Stanford University / ALMA / NRAO / ESO / NAOJ / NASA / ESA / Hubble Space Telescope.
In 2014, astronomers studied a variety of astronomical objects to test ALMA’s new, high-resolution capabilities.
One of these experimental images was that of an Einstein ring, which was produced by the gravity of a massive foreground galaxy bending the light emitted by the galaxy SDP.81 (also known as HATLAS J090311.6+003906) nearly 12 billion light-years away.
In a new paper accepted for publication in the Astrophysical Journal (arXiv.org preprint), Stanford University astronomer Dr. Yashar Hezaveh and co-authors explain how detailed analysis of this widely publicized image uncovered signs of a hidden dwarf dark galaxy in the halo of the foreground galaxy.
Current theories suggest that dark matter, which makes up about 80% of the mass of the Universe, is made of as-yet-unidentified particles that don’t interact with visible light or other forms of electromagnetic radiation.
Dark matter does, however, have appreciable mass, so it can be identified by its gravitational influence.
For their analysis, Dr. Hezaveh and co-authors harnessed thousands of computers working in parallel for many weeks, including NSF’s Blue Waters, to search for subtle anomalies that had a consistent and measurable counterpart in each ‘band’ of radio data.
From these combined computations, they were able to piece together an unprecedented understanding of the lensing galaxy’s halo and discovered a distinctive clump less than one-thousandth the mass of the Milky Way.
Because of its relationship to the larger galaxy, estimated mass, and lack of an optical counterpart, the astronomers believe this gravitational anomaly may be caused by an extremely faint, dark-matter dominated satellite of the lensing galaxy.
Computer models of the evolution of the Universe indicate that by measuring the ‘clumpiness’ of dark matter, it’s possible to measure its temperature. So by counting the number of small dark matter clumps around distant galaxies, astronomers can infer the temperature of dark matter, which has an important bearing on the smoothness of our Universe.
“If these halo objects are simply not there, then our current dark matter model cannot be correct and we will have to modify what we think we understand about dark matter particles,” said co-author Dr. Daniel Marrone, from the University of Arizona.
“This study suggests, however, that the majority of dwarf galaxies may simply not be seen because they’re mainly composed of invisible dark matter and emit little if any light.”
“Our current measurements agree with the predictions of cold dark matter. In order to increase our confidence we will need to look at many more lenses,” said co-authors Dr. Gilbert Holder, from McGill University.
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Yashar D. Hezaveh et al. 2016. Detection of lensing substructure using ALMA observations of the dusty galaxy SDP.81. ApJ, accepted for publication; arXiv: 1601.01388
