A new image produced by the James Webb Space Telescope (JWST, or Webb) sheds light on how early stars formed during "cosmic noon" more than 10 billion years ago.
About 200,000 light-years away from Earth, the Small Magellanic Cloud, a satellite galaxy and one of the Milky Way's closest neighbors, is notable for lacking metals heavier than hydrogen and helium.
The levels of these metals in the galaxy reflect a uniquely nearby environment to study what galaxies were like during the universe's early history, when it was only two to three billion years old and star formation was at its peak. Although various telescope missions have studied the Small Magellanic Cloud in the past, much remained to be understood.
Now, thanks to Webb's high-resolution imaging, astronomers have discovered more than 33,000 young stars embedded in the NGC 346 nebula, the galaxy's brightest and largest star formation region.
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"This is sort of minutes on a field to find these things," said Margaret Meixner, an astronomer at the Universities Space Research Association and one of the authors of the study. "That's how powerful James Webb is."
Meixner shared the early imaging results of NGC 346 using Webb on Wednesday (Jan. 11) at the 241st conference of the American Astronomical Society in Seattle.
"Ribbons of gas and dust"
Cosmic dust is formed when stars and planets are being created, and it hovers in the space between them — the interstellar medium — once their formation process ceases. Being a spinoff of these processes, the dust is made of similar heavy metals as stars and planets. So far, astronomers thought that such heavy elements like carbon, oxygen and iron — all of which are in low concentrations in the Small Magellanic Cloud — are needed to form the dust from which terrestrial planets eventually materialize.
So when astronomers used Webb's powerful near-infrared camera (NIRCam) instrument to study NGC 347, they were surprised to discover "ribbons of gas and dust" in the interstellar medium. This finding is early proof that despite being low on metals, NGC 347 is dusty and has the building blocks necessary to form rocky planetary systems.
Using six of Webb's NIRCam filters, the team found more than 33,000 sources with what they call "IR [infrared] excess." In stars with low masses, dust in surrounding debris disks absorbs starlight and re-emits it at infrared wavelengths. So when astronomers detect infrared emissions, it usually means they have detected dust.
"We were so excited to see the dust around these things," Meixner told reporters at the press conference.
Telescope missions have studied NGC 346 in the past, but this is the first time that astronomers have imaged the dust. For example, the now-retired Spitzer spacecraft found 87 massive young stars in the same region, but it wasn't powerful enough to identify the smaller ones. The Hubble Space Telescope, too, had spotted thousands of pre-main sequence stars that were sprinkled throughout the nebula and connected to each other through gas and dust filaments. However, a lot of these stars went undetected because Hubble wasn't sensitive to the dust surrounding these stars.
JWST is now helping astronomers unveil stars enveloped in dust, and search for the youngest stars by reaching 10 magnitudes below what Spitzer could detect, and two magnitudes fainter than what Hubble is capable of for pre-main sequence stars. Searching for star-forming regions in infrared wavelengths has allowed astronomers to discover many stars that are not visible, or are incorrectly identified at optical wavelengths.
In the coming months, astronomers hope to learn how the star formation process in the Small Magellanic Cloud is similar or different from what we have learnt from our own. They also plan to continue observing protostars in the region. These stars grow in size and composition by sucking surrounding dust, so it remains to be seen how much of the dust spotted now by Webb survives the star formation process, and ultimately if what remains is sufficient to form rocky planets.
For now, JWST has already unveiled a mammoth number of new young stars, and astronomers say they are just getting started.
"We have just scratched the surface of this data," said Meixner. "We are going to go back and push it down to [almost] brown dwarf limits to see what we can find."
The research is also described in a paper published Jan. 10 in the preprint server arXiv.
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ref - Discovery of dusty sub-solar mass young stellar objects in NGC 346 with JWST/NIRCam, https://arxiv.org/pdf/2301.03932.pdf, 11-Jan-2023.
My observation. From the 17-page PDF report: “1 Introduction Located in the Small Magellanic Cloud (SMC) at a distance of ~ 62 kpc , NGC 346 is a prominent young cluster (~ 3 Myr; ) actively forming stars. It is the brightest and largest star-formation region in this metal-poor galaxy (~ 1/5 Zsun; ) which has a comparable metallicity to galaxies at the epoch of peak star formation . Below these levels of chemical enrichment, the dust content of the interstellar medium (ISM) drops precipitously, altering the environment in which stars form (e.g., ) … “NGC 346 is one of the most active star-forming regions in the Local Group. Its proximity, size (~ 100 x 100 pc^2), low foreground extinction, and an abundance of wide-field, high-resolution panchromatic data make it an ideal system for the study of both low- and high-mass star formation, the effects of this star formation on the surrounding medium, and the potential triggers of star formation in an environment vastly different from our local Galactic surroundings, and akin to galaxies at cosmic noon.”
My note. NGC 346 is close with a distance of about 62 kpc or about 202,000 light-years. The cluster compared to star formation at z ~ 3.0 (cosmic noon) in the BB model, the universe could be some 2 Gyr old after BB event (and comoving radial distance indicates 4D space expanding faster than c velocity) compared to present. During cosmic noon in the BB model, much higher and faster star formation rates than the present we see in our galaxy. NGC 346 is about 3 Myr too. Considerable interpretation is needed and extrapolations when making star formation comparisons using “cosmic noon” and NGC 346.
But the surrounding clouds and filaments seem to have a higher level of metals. So, am I getting this wrong, or is there a disconnect between pristine stars and clouds?
I suppose new dust could have been injected by other encounters with dwarf galaxies, or perhaps the MW. Is this a consideration?
It's great the JWST is improving what could not be seen before!