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Starts With A Bang

Sparkle-filled JWST galaxy solves a longstanding cosmic mystery

Most globular clusters appear to form their stars all at once, but there are exceptions. JWST just observed how "second formations" happen.
This sparkle-rich lensed galaxy located behind galaxy cluster SMACS 0723, known as the Sparkler, just happens to be catching this galaxy in the act of forming a second population of stars within some of its massive globular clusters. This could provide the solution to a longstanding mystery: how a second population of stars can come to exist within these objects.
(Credit: NASA, ESA, CSA, STScI; Annotation: E. Siegel)
Key Takeaways
  • The very first science image ever released by the JWST, of the gravitationally lensing cluster SMACS 0723, still represents one of our deepest views of the Universe.
  • One lensed galaxy appears three times: the Sparkler, whose light comes to us from 9.2 billion years ago. It's bright, magnified, and forming stars all throughout it.
  • Inside one of its images are details we've never seen before, which just might solve one of the longest-standing puzzles about globular clusters in the Universe.

Over half a year since its first science image was released, JWST’s data continues to educate us.

This side-by-side view of galaxy cluster SMACS 0723 shows the MIRI (left) and NIRCam (right) views of this region from JWST. Note that although there’s a bright galaxy cluster at the center of the image, the most interesting objects are gravitationally lensed, distorted, and magnified by the cluster itself, and are located far more distant than the cluster itself, including the “sparkler” galaxy highlighted in subsequent images.
(Credit: NASA, ESA, CSA, STScI)

Behind galaxy cluster SMACS 0723 lies a series of brightened, magnified, gravitationally lensed galaxies.

This almost-perfectly-aligned image composite shows the first JWST deep field’s view of the core of cluster SMACS 0723 and contrasts it with the older Hubble view. Looking at the image details that are absent from the Hubble data but present in the JWST data shows us just how much discovery potential is awaiting scientists working with JWST.
(Credit: NASA, ESA, CSA, and STScI; NASA/ESA/Hubble (STScI); composite by E. Siegel)

One such galaxy — the Sparkler — appears three independent times.

This NIRCam view of a selection of the graviationally lensed region surrounding galaxy cluster SMACS 0723 contains multiple lensed galaxies, including the thrice-appearing Sparkler galaxy, highlighted here. The “sparkles” have been identified as star-forming knots of gas appearing atop already-existing globular clusters.
(Credit: NASA, ESA, CSA, STScI; Annotation: E. Siegel)

Even the mid-infrared (MIRI) instrument captured it thrice.

This mid-infrared (MIRI) view of the central region of galaxy cluster SMACS 0723, as taken with JWST, reveals three separate images of the “Sparkler” galaxy. Although the sparkles themselves are not visible in mid-infrared light with current levels of exposure time, the MIRI data helps reveal general properties of the actively star-forming galaxy, whose light comes to us from 9.2 billion years ago.
(Credit: NASA, ESA, CSA, STScI; Annotation: E. Siegel)

The “sparkles” seen throughout it, though visually remarkable, are scientifically invaluable.

The scientific paper that first analyzed the light from the Sparkler galaxy and put forth the idea that these “sparkles” were globular clusters undergoing a second burst of star-formation, highlights the three images of the Sparkler galaxy in detail here.
(Credit: L. Mowla et al., ApJL, 2022)

These “knots” of star-formation correspond to globular clusters: where 100,000+ stars form locally, all at once.

This impressive-looking globular cluster doesn’t belong to the Milky Way, but rather to the dwarf galaxy WLM located ~3.04 million light-years away. It’s extremely metal-poor, but for some reason is the only known globular cluster that belongs to WLM. Most globular clusters are only visible as they are nearby: after not having formed any new stars in billions of years. But thanks to JWST and gravitational lensing, we have the opportunity to see globulars as they were when they were actively forming stars: not all for the first-and-only time, either.
(Credit: NASA, ESA/Hubble, and J. Schmidt (Geckzilla))

Only, when viewed in detail, these knots possess stellar populations already billions of years old.

We’re only capable of identifying individual stars within the nearest globular clusters, such as Messier 71, shown here as imaged by the Hubble Space Telescope, located only ~13,000 light-years away. However, by performing population analyses of the light emitted from the stars inside, we can determine what the age(s) of the multiple bursts of stars inside a globular were, and can tell whether the stars were all formed at once or over multiple “bursts” separated by billions of years.
(Credit: ESA/Hubble and NASA)

This current star-formation episode appears to represent a second burst of stellar creation within them.

This view of the Sparkler galaxy, the most clear image of the galaxy lensed by galaxy cluster SMACS 0723, clearly shows a series of bright sparkles throughout it. As this is a gas rich, star-forming galaxy, these knotted clumps of new star-formation might be our first hint of how globular clusters can obtain a second, later population of stars within them.
(Credit: NASA, ESA, CSA, STScI)

We’ve never seen globular clusters this distant before: at a redshift of 1.378, or ~9.2 billion years ago.

Here in the heart of Omega Centauri, one of the largest, richest globular clusters visible from Earth’s location within the Milky Way, lots of stars of various colors have been imaged. Despite the long exposure times devoted to Omega Centauri and the millions of stars inside, no transit events have been observed. Omega Centauri is an example of a globular cluster with (at least) two independent populations of stars inside, formed at timescales separated by billions of years.
(Credit: NASA, ESA, and the Hubble SM4 ERO Team)

In fact, only one globular cluster is presently forming nearby: R136 within the Tarantula Nebula.

The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses, making it the heaviest known star. All told, this is the largest star-forming region within our Local Group, and it will likely form hundreds of thousands of new stars that will bind together in a globular cluster, while the brightest stars within it shine several millions of times as bright as our Sun.
(Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team)

Many nearby globulars contain a longstanding mystery: two separately-aged populations of stars.

13.8 billion
The life cycles of stars can be understood in the context of the color/magnitude diagram shown here. As the population of stars age, they ‘turn off’ the diagram, allowing us to date the age of the cluster in question. The oldest globular star clusters, such as the very old cluster shown at right, have an age of over 13 billion years, but many globulars also exhibit a second, more youthful population of stars alongside the older one.
(Credit: Richard Powell (L), R.J. Hall (R))

It’s mysterious because the initial burst of star-formation should expel all remaining star-forming gas.

The near-infrared view of the Tarantula Nebula taken with JWST is higher in resolution and broader in wavelength coverage than any previous view. Expanding on what Hubble taught us, we can now study star-formation within our Local Group in greater detail than ever, including the expelled/pushed-away gas and matter that’s being blown of, completely, by the radiation and winds from the cluster R136.
(Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team)

But “the Sparkler” provides a way out: a second galaxy-wide wave of star-formation, repopulating already existing globular clusters.

This Hubble view of globular cluster Terzan 5, just 22,000 light-years away in our own Milky Way, reveals its brilliant core and stars of a wide variety of colors and masses. The stars within Terzan 5, like within many (but not all) globular clusters, indicate two independent populations of stars inside, one formed several billion years earlier than the other. At last, we might finally have a pathway that explains these properties thanks to the Sparkler galaxy seen by JWST.
(Credit: ESA/Hubble & NASA, R. Cohen)

With the power of gravitational lensing, numerous similarly longstanding puzzles could fall to JWST.

This annotated, rotated image of the JADES survey, the JWST Advanced Deep Extragalactic Survey, shows off the new cosmic record-holder for most distant galaxy: JADES-GS-z13-0, whose light comes to us from a redshift of z=13.2 and a time when the Universe was only 320 million years old. Although we’re seeing galaxies farther than ever, these records will likely be broken when more serendipitously-aligned gravitaitonal lenses are discovered.
(Credit: NASA, ESA, CSA, M. Zamani (ESA/Webb); Science credits: Brant Robertson (UC Santa Cruz), S. Tacchella (Cambridge), E. Curtis-Lake (UOH), S. Carniani (Scuola Normale Superiore), JADES Collaboration; Annotation: E. Siegel)

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