The most important lesson from JWST’s “baby Milky Way”

How did our Universe become the way it is today?

A portion of the dwarf galaxy Wolf–Lundmark–Melotte (WLM) captured by the James Webb Space Telescope’s Near-Infrared Camera. This region showcases some of the stars located within WLM, some ~3 million light-years away, along with many background galaxies of various sizes and distances. The Universe, even when we look within a nearby galaxy, can’t help but reveal itself when we look with JWST’s eyes.

Nearby, modern galaxies only give us a single snapshot.

This view of the Perseus cluster of galaxies, from ESA’s Euclid mission, shows over 1000 galaxies all clustered together some 240 million light-years away, with many tens of thousands more identifiable in the background portion of the image. While optically, the image is dominated by the most massive, star-rich galaxies, they are vastly outnumbered by smaller, fainter, low-mass galaxies that are exceedingly difficult to detect, even nearby. Euclid’s capabilities are critical for mapping out the dark Universe.

To understand cosmic history, we must look far away: back in time.

This selection of 55 galaxies from the JWST’s GLASS Early Release Science program spans a variety of ranges in redshift and mass. This helps teach us what shapes galaxies take on over a range of masses and stages in cosmic time/evolution, revealing a number of very massive, very early, yet very evolved-looking galaxies. If we can see them now, they’ll always be visible, a contrast to the myth of the disappearing Universe.

JWST has shown us the earliest galaxies ever discovered.

Even from this zoomed-in view of the JADES field, it’s very difficult to pick out the most distant galaxy ever found, JADES-GS-z14-0, by eye. This animation shows its location with a green circle: overlapping with a brighter, bluer, closer galaxy.

But there’s a problem: it can only see the brightest ones.

JADES-GS-z14-0, in the top inset box, is found behind (and just to the right of) a closer, brighter, bluer galaxy. It was only through the power of spectroscopy with incredible resolution, capable of separating the two sources, that the nature of this record-breakingly distant object could be determined. Its light comes to us from when the Universe was only 285-290 million years old: just 2.1% of its current age. JADES-GS-z14-1, just below it, comes from when the Universe was ~300 million years old. Compared to large, modern-day galaxies, all early galaxies contain a paucity of stars and have irregular, ill-defined shapes.

These galaxies are curiously abundant so early on.

When the light from not just stars, but also from the central, supermassive black hole is also included, the additional brightness over what’s expected from these early galaxies can finally be explained. The question of why they have the abundances they do, which is still slightly mismatched from theoretical models, still remains.

But they’re the early analogues of the biggest, brightest galaxies of all.

At a level of 36x zoom, Euclid’s first mosaic contains the distant but abundant galaxy cluster Abell 3381, which features a line of bright galaxies similar to Markarian’s chain in the Virgo cluster. The most massive, luminous galaxy in a modern galaxy cluster is analogous to the majority of ultra-distant galaxies spotted by JWST; more modest Milky Way-like analogues are much harder to find.

Milky Way-like galaxies are much fainter — and harder to spot — early on.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than the galaxies we see today. Fewer galaxies have disks and spiral shapes as we look farther back in time. Over time, many smaller galaxies become gravitationally bound together, resulting in mergers, but also in groups and clusters containing large numbers of galaxies overall.

Only with the natural enhancement of gravitational lensing can early Milky Way analogues be seen.

These eight very faint, low-mass galaxies would be invisible to even JWST at these great distances under normal circumstances. Only from gravitational lensing’s severe brightness enhancement, an effect of Einstein’s general relativity, can these galaxies be revealed at all. Unlike the more common, brighter galaxies found at great distances by JWST, these smaller but more common objects are closer early analogues of our modern Milky Way.

Earlier in 2024, the Cosmic Gems arc was spotted: with only a few million stars total.

This image shows the Cosmic Gems arc, a bright and highly magnified early galaxy from just 460 million years after the Big Bang, in a JWST color composite image. The galaxy is resolved into five young star clusters located within just a ~240 light-year span.

Now, the even more detailed Firefly Sparkle galaxy appears in JWST data.

Within the field of galaxy cluster MACS J1423, a few stretched-out arcs can be seen: examples of lensed background galaxies. One such arc corresponds to the Firefly Sparkle galaxy, with ten component star clusters inside of it. Nearby, two other lensed galaxies, only 6500 and 42,000 light-years away, respectively, indicate young proto-galaxies in the process of assembling into a larger, more modern galaxy.

Inside, ten independent star clusters are bursting into existence right now.

Shown along with contoured lines that indicate lensing magnification within the cluster, the Firefly Sparkle galaxy is shown in a central box with two nearby companion galaxies also highlighted. Within the arc, ten individual bright spots corresponding to star clusters of 100,000+ solar masses apiece appear.

Their stars range from 2-to-8 million years old: incredibly young in a ~600 million year old Universe.

Compared to the formation history that’s been reconstructed for the modern Milky Way (black points/line), the Firefly Sparkle galaxy represents the earliest, lowest-mass analogue ever discovered. Within it, ten newly formed star clusters dominate its light output, showcasing the effectiveness of bursty star-formation. The stellar densities here are large, but much smaller than those inferred for the galaxy found in the Cosmic Gems arc, discovered earlier in 2024.

Stellar densities exceed even those of modern globular clusters.

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. The bright spots within the unrelated Firefly Sparkle galaxy are likely highlighting a similar example of bursty star-formation.

Multiple other galaxies exist within ~100,000 light-years.

The Firefly Sparkle galaxy, alongside its two companions (BF and NBF), indicates a history of bursty star-formation, and points to a formation history of the first stars of all within the first 50-150 million years of cosmic history.

They’ll all someday merge to form a grown-up, modern, Milky Way-like galaxy.

Small, early, baby galaxies such as this provide the majority (80+%) of ultraviolet photons for reionizing the Universe.

For the first ~550 million years of the Universe, neutral, light-blocking atoms persist in the space between galaxies, continuing what’s known as the cosmic dark ages. While that material persists, starlight is largely absorbed, and cannot penetrate through this “fog.” Once the last of that neutral matter becomes reionized, largely due to the ultraviolet light emitted from the large number of small star clusters and early galaxies, starlight can propagate freely through the Universe, marking the end of the reionization epoch.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.