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| A gravitationally lensed view of a ring-shaped slice of the galaxy SPT0418-47, as seen by the ALMA array in Chile. Recent observations by NASA's James Webb Space Telescope reveal that the galaxy has a satellite that's rich in star formation. (Image credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.) |
Scanning the first images of a well-known early galaxy taken by NASA's James
Webb Space Telescope (JWST), Cornell astronomers were intrigued to see a
blob of light near its outer edge.
Their initial focus, and the infrared observatory's target, was SPT0418-47,
one of the brightest dusty, star-forming galaxies in the early universe, its
distant light bent and magnified by a foreground galaxy's gravity into a
circle, called an Einstein ring.
But a deeper dive into the early JWST data released last fall produced a
serendipitous discovery: a companion galaxy previously hidden behind the
light of the foreground galaxy, one that surprisingly seems to have already
hosted multiple generations of stars despite its young age, estimated at 1.4
billion years old.
"We found this galaxy to be super-chemically abundant, something none of us
expected," said Bo Peng, a doctoral student in astronomy, who led the data
analysis. "JWST changes the way we view this system and opens up new venues
to study how stars and galaxies formed in the early universe."
Peng is the lead author of "Discovery of a Dusty, Chemically Mature
Companion to z~4 Starburst Galaxy in JWST Early Release Science Data,"
published Feb. 17 in the Astrophysical Journal Letters, with eight
co-authors who are current or former members of the Department of Astronomy
in the College of Arts and Sciences.
Earlier images of the same Einstein ring captured by the Atacama Large
Millimeter/submillimeter Array (ALMA) in Chile contained hints of the
companion resolved clearly by JSWT, but they couldn't be interpreted as
anything more than random noise, said Amit Vishwas, a research associate at
the Cornell Center for Astrophysics and Planetary Sciences (CCAPS) and the
paper's second author.
Investigating spectral data embedded in each pixel of images from JWST's
NIRSpec instrument, Peng identified a second new light source inside the
ring. He determined that the two new sources were the images of a new galaxy
being gravitationally lensed by the same foreground galaxy responsible for
creating the ring, although they were eight to 16 times fainter—a testament
to the power of JWST's infrared vision.
Further analysis of the light's chemical composition confirmed that strong
emission lines from hydrogen, nitrogen and sulfur atoms displayed similar
redshifts—a measure of how much light from a galaxy stretches into longer,
redder wavelengths as it grows more distant. That placed the two galaxies
roughly the same distance from Earth—calculated as a redshift of about 4.2,
or about 10% of the universe's age—and in the same neighborhood.
To verify their discovery, the researchers returned to earlier ALMA
observations. They found an emission line of ionized carbon closely matched
the redshifts observed by JWST.
"That really nailed it down," Vishwas said. "Because we have several
emission lines shifted by exactly the same amount, there's no doubt that
this new galaxy is where we think it is."
The team estimated the companion galaxy, which they labeled SPT0418-SE, was
within 5 kiloparsecs of the ring. (The Magellanic Clouds, satellites of the
Milky Way, are about 50 kiloparsecs away.) That proximity suggests the
galaxies are bound to interact with each other and potentially even merge,
an observation that adds to the understanding of how early galaxies may have
evolved into larger ones.
The two galaxies are modest in mass as galaxies in the early universe go,
with "SE" relatively smaller and less dusty, making it appear bluer than the
extremely dust-obscured ring. Based on images of nearby galaxies with
similar colors, the researchers suggest that they may reside "in a massive
dark-matter halo with yet-to-be-discovered neighbors."
Most surprising about the companion galaxy, considering its age and mass,
was its mature metallicity—amounts of elements heavier than helium and
hydrogen, such as carbon, oxygen and nitrogen. The team estimated that as
comparable to our sun, which is more than 4 billion years old and inherited
most of its metals from previous generations of stars that had 8 billion
years to build them up.
"We are seeing the leftovers of at least a couple of generations of stars
having lived and died within the first billion years of the universe's
existence, which is not what we typically see," Vishwas said. "We speculate
that the process of forming stars in these galaxies must have been very
efficient and started very early in the universe, particularly to explain
the measured abundance of nitrogen relative to oxygen, as this ratio is a
reliable measure of how many generations of stars have lived and died."
The researchers have submitted a proposal for JWST observing time to
continue study of the ring and its companions and reconcile potential
differences observed between the optical and far-infrared spectrum.
"We're still working on this galaxy," Peng said. "There's more to explore in
this data."
The team thanked the early release science program that made the JWST data
immediately available to the public, called TEMPLATES: Targeting Extremely
Magnified Panchromatic Lensed Arcs and Their Extended Star formation, led by
NASA astrophysicist Jane Rigby, the observatory's operations project
scientist.
Reference:
Bo Peng et al, Discovery of a Dusty, Chemically Mature Companion to a z ∼ 4
Starburst Galaxy in JWST ERS Data, The Astrophysical Journal Letters (2023).
DOI: 10.3847/2041-8213/acb59c
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Space & Astrophysics