Our Milky Way galaxy is chock-full of dust. Stars are essentially dust-making factories that infuse the galaxy with a haze of dusty elements required for making planets and even life. But all that dust can make viewing the cosmos difficult. Telescopes that detect visible, or optical, light cannot see through the murkiness, and thus some of what goes on in the universe remains enshrouded.
Luckily for astronomers, infrared light, which has longer wavelengths than optical light, can sneak past dust. Several infrared-sensing telescopes, such as NASA's Spitzer Space Telescope, have taken advantage of this fact and revealed much of the so-called infrared sky, including hidden planets, stars, supermassive black holes, and more. The next frontier for infrared astronomy involves watching how the infrared sky changes over time, an effort that Mansi Kasliwal (MS '07, PhD '11), an assistant professor of astronomy at Caltech, refers to as opening up the dynamic infrared sky.
To that end, Kasliwal has planned a series of four small ground-based infrared telescopes that will reveal everything from never-before-seen star explosions, asteroids, and even infrared counterparts to stellar collisions that send ripples through space and time known as gravitational waves.
The ambitious plan begins with Palomar Gattini-IR, a robotic instrument now in operation at Palomar Observatory, and will eventually include two additional instruments, called WINTER (Wide-field Infrared Transient Explorer) and DREAMS (Dynamic Red All-Sky Monitoring Survey), both of which are under construction. The final step in the plan is to build an instrument destined for Antarctica, where the chilly temperatures lead to even crisper views of the infrared sky.
"We are changing the game," says Kasliwal. "We are building little telescopes that do big science."
Palomar Gattini-IR, or just Gattini for short, refers to the Italian word for kittens, gattini, and came from Kasliwal's collaborator Anna Moore, a professor of astronomy at Australian National University, who used the term to casually refer to her own fleet of small telescopes in the Antarctic. "The name just stuck," explains Kasliwal. Palomar Gattini-IR has been busy robotically scanning the skies from its perch in a small dome at Palomar Observatory since 2019 and has already produced some interesting results.
Stars That Go Bang
One recent paper accepted in The Astrophysical Journal reports the first real estimate of the number of nova explosions, or novae, that go off in our Milky Way galaxy per year (the answer is about 46). Novae are not as bright as supernovae, but powerful nonetheless and can briefly shine brighter than one million suns. They occur when a white dwarf, the burned-out core of a star, siphons enough material off a companion star to cause an explosion. These bursts are thought to seed our universe with many of the elements that make up our periodic table; in fact, novae are thought to be the main producers of lithium in our galaxy.
But novae can be hard to find because they often lie within the thick and dusty band of our Milky Way. Previous estimates of the rate of novae in our galaxy were wildly uncertain, with only about a dozen novae discovered each year.
"There was little consensus before now on the rate of novae in our galaxy," says Kishalay De (MS '18), a graduate student at Caltech and lead author of the Gattini study on novae. "The novae can be hidden behind huge columns of dust, so optical surveys could not find them."
The novae results demonstrate the power of an infrared survey like Gattini, which scans the whole Northern sky every two nights. The newfound novae were "insanely easy to pick out," according to Kasliwal, because they glow brightly when viewed in infrared light.
"This is truly a ground-breaking study," says Allen Shafter, a nova expert at San Diego State University. "Dust limits the reach of optical nova surveys to a relatively small volume of space near the sun. As a result, optical estimates of the Galactic nova rate require a large and uncertain extrapolation of the nova rate in the solar neighborhood to the full extent of our Milky Way galaxy. The new Gattini infrared nova study has greatly increased the volume of space that can be directly surveyed, thereby reducing the extent of the required extrapolation and resulting in a more accurate estimate of the Galactic nova rate than has been hitherto possible."
An Infrared Legacy
Caltech is a pioneer in the field of infrared astronomy. The late astronomy professors Gerry Neugebauer (PhD '60) and Robert Leighton (BS '41 and PhD '47) designed and built one of the world's first infrared telescopes. Later, Neugebauer and Tom Soifer (BS '68), the Harold Brown Professor of Physics, Emeritus, helped create the first space mission to perform an all-sky infrared survey mission, called IRAS (Infrared Astronomical Satellite), which launched in 1983 and led to the creation of Caltech's Infrared Processing and Analysis Center, now called simply IPAC.
Other IPAC infrared projects include the ground-based 2MASS (Two Micron All-Sky Survey), which scanned the entire sky from 1997 to 2001; NASA's Spitzer Space Telescope, a sister telescope to the Hubble Space Telescope that ceased operations in 2020; and NASA's WISE (Wide-field Infrared Survey Explorer), now called NEOWISE (Near-Earth Object WISE) and dedicated primarily to the search for asteroids.
These previous infrared surveys catalogued millions of never-before-seen asteroids, stars, galaxies, and other objects, and had better resolutions than Gattini, but they did not scan the whole sky as quickly.
"We're doing a large chunk of what 2MASS did every night," says De. "Gattini is the first-ever survey of the dynamic, or changing, infrared sky. We have traded in resolution for a wide field of view to enable us to regularly capture the whole night sky." Gattini's telescope is only 30 centimeters in size but its field of view is a whopping 25 square degrees, 40 times larger than any past or current infrared telescopes.
"Caltech is a pioneer for both infrared astronomy and time-domain astronomy, so it only makes sense that we would combine the two in the first dynamic infrared sky survey," says Kasliwal. Time-domain astronomy refers to nightly surveys of the changing sky; Caltech's Zwicky Transient Facility (ZTF) is a key instrument in this growing field, but unlike Gattini, it detects optical light.
From the Ground Up
The Gattini instrument was built at Caltech by Kasliwal and her team, including both graduate and undergraduate students. It was first installed at Palomar in 2018 and took some time to calibrate and set up to work automatically. "We left the telescope on its own to operate robotically," says De. "Then the data came pouring down from the sky to our computers thanks to our data pipeline."
One of the challenges in designing a survey instrument like Gattini is the development of software. Gattini's software automatically sifts through enormous amounts of data to detect changes in the sky every night. De spent six months developing the software and data pipeline for the project as part of his PhD thesis.
"These software techniques are of prime importance to future space-based telescopes as well," says De, "because they remove the blurring caused by the earth's atmosphere and hence can in principle get extremely sharp images."
Now that Gattini is up and running, astronomers have been mining its data for use in various projects. For instance, Caltech professor of astronomy Lynne Hillenbrand and her team used the instrument's data to help discover a rare bursting young star hidden by clouds of dust. Hillenbrand's group had previously discovered a similar star with the help of NEOWISE.
"Gattini can uniquely detect objects that are so buried in dust to not be seen in visible light, and which brighten so rapidly that only Gattini scans the sky fast enough to pick them out," says Hillenbrand.
Next up in Kasliwal's plan to open the dynamic infrared skies are WINTER and DREAMS. WINTER, which is currently being built at MIT under the leadership of Kasliwal's collaborator Rob Simcoe (PhD '04), a professor of physics, is scheduled to begin operations at Palomar in the fall of 2021. DREAMS is being built by a team led by Moore in Australia and is scheduled to begin operations at Siding Springs Observatory in 2022. Both telescopes will use next-generation infrared detectors that are more efficient than those on Gattini.
The final step is to build an infrared survey telescope in Antarctica that will take advantage of the frigid air. "The night sky is blindingly bright in infrared light, but it's 40 times darker in Antarctica at infrared wavelengths, which is partly due to the cold temperatures," explains Kasliwal.
Another reason for building a survey telescope at the South Pole is because, together with those in the North, they will cover the entire sky. "It's always nighttime somewhere," she says.
A Goldmine of a Find
One of Kasliwal's dreams is to be able to identify cataclysmic mergers of neutron stars, dramatic events that produce what astronomers call kilonovas. These explosions are even more powerful than novae, and are thought to generate a significant amount of the universe's heaviest elements, including gold and platinum. Kasliwal's team identified one such explosion along with other groups back in 2017, when LIGO (Laser Interferometer Gravitational-wave Observatory) first identified the gravitational waves produced by the collision. The occasion marked the first time that both gravitational waves and light were detected from the same event, and helped usher in the field of multi-messenger astronomy (where gravitational waves, light, and neutrinos are the messengers).
Since that time, LIGO has detected dozens of additional gravitational-wave events, but none have been seen simultaneously in light. Kasliwal suspects this may be due to the fact that kilonovas inherently produce much more infrared than optical light and are thus being missed by optical telescopes. Each step in Kasliwal's plan—Gattini, WINTER, DREAMS, and a future instrument in Antarctica—has the ability to sleuth out the hidden kilonovas with increasing sensitivities. It is also possible that one of the telescopes may even catch a long-sought neutron star and black hole merger, which could be even more luminous in infrared light than neutron star collisions.
"There is a lot you can do with small ground-based telescopes," she says. "Our small teams are very agile, and enable us to have some fun, take risks, and try something new. We have the freedom to dream big."
Palomar Gattini-IR is funded by Caltech, Australian National University, the Mt. Cuba Foundation, the Heising-Simons Foundation, and the US-Israel Binational Science Foundation. The instrument is a collaborative project among Caltech, Australian National University, University of New South Wales, Columbia University, University of Chinese Academy of Sciences, and the Weizmann Institute of Science.