Dwarf Dark Galaxy Hidden in ALMA Gravitational Lens Image
April 16, 2016 - 11:45 AM
(Credit: Y. Hezaveh, Stanford Univ.; ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble Space Telescope)
Subtle distortions hidden in ALMA’s stunning image of the gravitational lens SDP.81
are telltale signs that a dwarf dark galaxy is lurking in the halo of a
much larger galaxy nearly 4 billion light-years away. This discovery
paves the way for ALMA to find many more such objects and could help
astronomers address important questions on the nature of dark matter.
2014, as part of ALMA’s Long Baseline Campaign, astronomers studied a
variety of astronomical objects to test the telescope'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 another galaxy nearly 12
billion light-years away.
This phenomenon, called gravitational
lensing, was predicted by Einstein’s general theory of relativity and it
offers a powerful tool for studying galaxies that are otherwise too
distant to observe. It also sheds light on the properties of the nearby
lensing galaxy because of the way its gravity distorts and focuses light
from more distant objects.
In a new paper accepted for publication in the Astrophysical Journal,
astronomer Yashar Hezaveh at Stanford University in California and his
team explain how detailed analysis of this widely publicized image
uncovered signs of a hidden dwarf dark galaxy in the halo of the more
"We can find these invisible objects in the same
way that you can see rain droplets on a window. You know they are there
because they distort the image of the background objects,” explained
Hezaveh. In the case of a rain drop, the image distortions are caused by
refraction. In this image, similar distortions are generated by the
gravitational influence of dark matter.
Current theories suggest
that dark matter, which makes up about 80 percent 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, the researchers
harnessed thousands of computers working in parallel for many weeks,
including the National Science Foundation's most powerful supercomputer,
Blue Waters, to search for subtle anomalies that had a consistent and
measurable counterpart in each "band" of radio data. From these combined
computations, the researchers were able to piece together an
unprecedented understanding of the lensing galaxy’s halo, the diffuse
and predominantly star-free region around the galaxy, and discovered a
distinctive clump less than one-thousandth the mass of the Milky Way.
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. According to theoretical predictions,
most galaxies should be brimming with similar dwarf galaxies and other
companion objects. Detecting them, however, has proven challenging. Even
around our own Milky Way, astronomers can identify only 40 or so of the
thousands of satellite objects that are predicted to be present.
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 of the University of
Illinois, a member of the team. "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," he added.
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.
these halo objects are simply not there," notes co-author Daniel Marrone
of the University of Arizona, "then our current dark matter model
cannot be correct and we will have to modify what we think we understand
about dark matter particles."
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," said team member Gilbert Holder of McGill University in
Montreal, Canada. "In order to increase our confidence we will need to
look at many more lenses."
"This is an amazing demonstration of
the power of ALMA," said Hezaveh. "We are now confident that ALMA can
efficiently discover these dwarf galaxies. Our next step is to look for
more of them and to have a census of their abundance to figure out if
there is any possibility of a warm temperature for dark matter
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
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