Exploding stars generate dramatic light shows. Infrared telescopes like
Spitzer can see through the haze and to give a better idea of how often
these explosions occur.
You’d think that supernovae – the death throes of massive stars and among
the brightest, most powerful explosions in the universe – would be hard to
miss. Yet the number of these blasts observed in the distant parts of the
universe falls way short of astrophysicists’ predictions.
A new study using data from NASA’s recently retired Spitzer Space Telescope
reports the detection of five supernovae that, going undetected in optical
light, had never been seen before. Spitzer saw the universe in infrared
light, which pierces through dust clouds that block optical light – the kind
of light our eyes see and that unobscured supernovae radiate most brightly.
To search for hidden supernovae, the researchers looked at Spitzer
observations of 40 dusty galaxies. (In space, dust refers to grain-like
particles with a consistency similar to smoke.) Based on the number they
found in these galaxies, the study confirms that supernovae do indeed occur
as frequently as scientists expect them to. This expectation is based on
scientists’ current understanding of how stars evolve. Studies like this are
necessary to improve that understanding, by either reinforcing or
challenging certain aspects of it.
“These results with Spitzer show that the optical surveys we’ve long relied
on for detecting supernovae miss up to half of the stellar explosions
happening out there in the universe,” said Ori Fox, a scientist at the Space
Telescope Science Institute in Baltimore, Maryland, and lead author of the
new study, published in the Monthly Notices of the Royal Astronomical
Society. “It’s very good news that the number of supernovae we’re seeing
with Spitzer is statistically consistent with theoretical predictions.”
The “supernova discrepancy” – that is, the inconsistency between the number
of predicted supernovae and the number observed by optical telescopes – is
not an issue in the nearby universe. There, galaxies have slowed their pace
of star formation and are generally less dusty. In the more distant reaches
of the universe, though, galaxies appear younger, produce stars at higher
rates, and tend to have higher amounts of dust. This dust absorbs and
scatters optical and ultraviolet light, preventing it from reaching
telescopes. So researchers have long reasoned that the missing supernovae
must exist and are just unseen.
“Because the local universe has calmed down a bit since its early years of
star-making, we see the expected numbers of supernovae with typical optical
searches,” said Fox. “The observed supernova-detection percentage goes down,
however, as you get farther away and back to cosmic epochs where dustier
galaxies dominated.”
Detecting supernovae at these far distances can be challenging. To perform a
search for supernovae shrouded within murkier galactic realms but at less
extreme distances, Fox’s team selected a local set of 40 dust-choked
galaxies, known as luminous and ultra-luminous infrared galaxies (LIRGs and
ULIRGs, respectively). The dust in LIRGs and ULIRGs absorbs optical light
from objects like supernovae but allows infrared light from these same
objects to pass through unobstructed for telescopes like Spitzer to detect.
The researchers’ hunch proved correct when the five never-before-seen
supernovae came to (infrared) light. “It’s a testament to Spitzer’s
discovery potential that the telescope was able to pick up the signal of
hidden supernovae from these dusty galaxies,” said Fox.
“It was especially fun for several of our undergraduate students to
meaningfully contribute to this exciting research,” added study co-author
Alex Filippenko, a professor of astronomy at the University of California,
Berkeley. “They helped answer the question, ‘Where have all the supernovae
gone?’”
The types of supernovae detected by Spitzer are known as “core-collapse
supernovae,” involving giant stars with at least eight times the mass of the
Sun. As they grow old and their cores fill with iron, the big stars can no
longer produce enough energy to withstand their own gravity, and their cores
collapse, suddenly and catastrophically.
The intense pressures and temperatures produced during the rapid cave-in
forms new chemical elements via nuclear fusion. The collapsing stars
ultimately rebound off their ultra-dense cores, blowing themselves to
smithereens and scattering those elements throughout space. Supernovae
produce “heavy” elements, such as most metals. Those elements are necessary
for building up rocky planets, like Earth, as well as biological beings.
Overall, supernova rates serve as an important check on models of star
formation and the creation of heavy elements in the universe.
“If you have a handle on how many stars are forming, then you can predict
how many stars will explode,” said Fox. “Or, vice versa, if you have a
handle on how many stars are exploding, you can predict how many stars are
forming. Understanding that relationship is critical for many areas of study
in astrophysics.”
Next-generation telescopes, including NASA’s Nancy Grace Roman Space
Telescope and the James Webb Space Telescope, will detect infrared light,
like Spitzer.
“Our study has shown that star formation models are more consistent with
supernova rates than previously thought,” said Fox. “And by revealing these
hidden supernovae, Spitzer has set the stage for new kinds of discoveries
with the Webb and Roman space telescopes.”
Reference:
“A Spitzer survey for dust-obscured supernovae” by Ori D Fox, Harish
Khandrika, David Rubin, Chadwick Casper, Gary Z Li, Tamás Szalai, Lee Armus,
Alexei V Filippenko, Michael F Skrutskie, Lou Strolger and Schuyler D Van
Dyk, 21 June 2021, Monthly Notices of the Royal Astronomical Society.
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