‘Echo Mapping’ in Faraway Galaxies Could Measure Vast Cosmic Distances

Matter swirling around supermassive black holes creates bursts of light that “echo” in nearby dust clouds. These traveling signals could serve as a new cosmic yardstick.

When you look up at the night sky, how do you know whether
the specks of light that you see are bright and far away, or relatively faint
and close by? One way to find out is to compare how much light the object actually
emits with how bright it appears. The difference between its true luminosity
and its apparent brightness reveals an object’s distance from the

Measuring the luminosity of a celestial object is challenging,
especially with black holes, which don’t emit light. But the supermassive black
holes that lie at the center of most galaxies provide a loophole: They often pull
lots of matter around them, forming hot disks that can radiate brightly.
Measuring the luminosity of a bright disk would allow astronomers to gauge the
distance to the black hole and the galaxy it lives in. Distance measurements
not only help scientists create a better, three-dimensional map of the
universe, they can also provide information about how and when objects formed.

In a new study, astronomers used a technique that
some have nicknamed “echo mapping” to measure the luminosity of black
hole disks in over 500 galaxies. Published last month in the Astrophysical Journal, the study adds support to the idea that this approach
could be used to measure the distances between Earth and these faraway

The process of echo mapping, also known as
reverberation mapping, starts when the disk of hot plasma (atoms that have lost
their electrons) close to the black hole gets brighter, sometimes even
releasing short flares of visible light (meaning wavelengths that can be seen
by the human eye). That light travels away from the disk and eventually runs
into a common feature of most supermassive black hole systems: an enormous cloud
of dust in the shape of a doughnut (also known as a torus). Together, the disk
and the torus form a sort of bullseye, with the accretion disk wrapped tightly
around the black hole, followed by consecutive rings of slightly cooler plasma
and gas, and finally the dust torus, which makes up the widest, outermost ring
in the bullseye. When the flash of light from the accretion disk reaches the inner
wall of the dusty torus, the light gets absorbed, causing the dust to heat up
and release infrared light. This brightening of the torus is a direct response
to or, one might say an “echo” of the changes happening in the disk.

The distance from the accretion disk to the inside
of the dust torus can be vast – billions or trillions of miles. Even light,
traveling at 186,000 miles (300,000 kilometers) per second, can take months or
years to cross it. If astronomers can observe both the initial flare of visible
light in the accretion disk and the subsequent infrared brightening in the torus,
they can also measure the time it took the light to travel between those two
structures. Because light travels at a standard speed, this information also
gives astronomers the distance between the disk and the torus.

Scientists can then use the distance measurement to
calculate the disk’s luminosity, and, in theory, its distance from Earth. Here’s
how: The temperature in the part of the disk closest to the black hole can
reach tens of thousands of degrees – so high that even atoms are torn apart and
dust particles can’t form. The heat from the disk also warms the area around
it, like a bonfire on a cold night. Traveling away from the black hole, the
temperature decreases gradually.

Astronomers know that dust forms when the
temperature dips to about 2,200 degrees Fahrenheit (1,200 Celsius); the bigger the
bonfire (or the more energy the disk radiates), the farther away from it the
dust forms. So measuring the distance between the accretion disk and the torus
reveals the energy output of the disk, which is directly proportional to its

Because the light can take months or years to
traverse the space between the disk and the torus, astronomers need data that
spans decades. The new study relies on nearly two decades of visible-light
observations of black hole accretion disks, captured by several ground-based
telescopes. The infrared light emitted by the dust was detected by NASA’s Near Earth Object Wide Field Infrared Survey
(NEOWISE), previously
named WISE. The spacecraft surveys the entire sky about once every six months,
providing astronomers with repeated opportunities to observe galaxies and look
for signs of those light “echoes.” The study used 14 surveys of the
sky by WISE/NEOWISE, collected between 2010 and 2019. In some galaxies, the
light took more than 10 years to traverse the distance between the accretion
disk and the dust, making them the longest echoes ever measured outside the
Milky Way galaxy.

Galaxies Far, Far Away

The idea to use echo mapping to measure the
distance from Earth to far away galaxies is not new, but the study makes substantial
strides in demonstrating its feasibility. The largest single survey of its
kind, the study confirms that echo mapping plays out in the same way in all
galaxies, regardless of such variables as a black hole’s size, which can vary
significantly across the universe. But the technique isn’t ready for prime

Due to multiple factors, the authors’ distance measurements
lack precision. Most notably, the authors said, they need to understand more
about the structure of the inner regions of the dust doughnut encircling the
black hole. That structure could affect such things as which specific
wavelengths of infrared light the dust emits when the light first reaches it.

The WISE data doesn’t span the entire infrared
wavelength range, and a broader dataset could improve the distance
measurements. NASA’s Nancy Grace Roman Space Telescope, set to launch in the mid-2020s, will provide targeted
observations in different infrared wavelength ranges. The agency’s upcoming SPHEREx
(which stands for
Spectro-Photometer for the History of the Universe, Epoch of Reionization and
Ices Explorer) will survey the entire sky in multiple infrared wavelengths and
could also help improve the technique.

“The beauty of the echo mapping technique is
that these supermassive black holes aren’t going away anytime soon,” said Qian
Yang, a researcher at the University of Illinois at Urbana-Champaign and lead
author of the study, referring to the fact that black hole disks may experience
active flaring for thousands or even millions of years. “So we can measure
the dust echoes over and over again for the same system to improve the distance

Luminosity-based distance measurements can already
be done with objects known as “standard candles,” which have a known
luminosity. One example is a type of exploding star called a Type 1a supernova,
which played a critical role in the discovery of dark energy (the name given to
the mysterious driving force behind the universe’s accelerating expansion). Type
1a supernova all have about the same luminosity, so astronomers only need to
measure their apparent brightness to calculate their distance from Earth.

With other standard candles, astronomers can
measure a property of the object to deduce its specific luminosity. Such is the
case with echo mapping, where each accretion disk is unique but the technique
for measuring the luminosity is the same. There are benefits for astronomers to
being able to use multiple standard candles, such as being able to compare
distance measurements to confirm their accuracy, and each standard candle has
strengths and weaknesses.

“Measuring cosmic distances is a fundamental
challenge in astronomy, so the possibility of having an extra trick up one’s
sleeve is very exciting,” said Yue Shen, also a researcher at the University of
Illinois at Urbana-Champaign and co-author of the paper.

Launched in 2009, the WISE spacecraft was placed
into hibernation in 2011 after completing its primary mission. In Sept. 2013,
NASA reactivated the spacecraft with the primary goal of scanning for
near-Earth objects, or NEOs, and the mission and spacecraft were renamed
NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and
operated WISE for NASA’s Science Mission Directorate. The mission was selected
competitively under NASA’s Explorers Program managed by the agency’s Goddard
Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, a
division of Caltech, and the University of Arizona, supported by NASA’s
Planetary Defense Coordination Office.

News Media Contact

Calla Cofield

Jet Propulsion Laboratory, Pasadena, Calif.




Source: Jet Propulsion Laboratory

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