Inside JWST’s first view of the Local Group’s edge

How and when did the stars in the Universe form?

The cluster Terzan 5 has many older, lower-mass stars present within (faint, and in red), but also hotter, younger, higher-mass stars, some of which will generate iron and even heavier elements. It contains a mix of Population I and Population II stars, indicating that this cluster underwent multiple episodes of star formation. The different properties of different generations can lead us to draw conclusions about the initial abundances of the light elements and holds clues as to the star-formation history of our cosmos.

To answer, we must look back across cosmic time.

Galaxies comparable to the present-day Milky Way are numerous throughout cosmic time, having grown in mass and with more evolved structure at present. Younger galaxies are inherently smaller, bluer, more chaotic, richer in gas, and have lower densities of heavy elements than their modern-day counterparts, and their star-formation histories evolve over time. This was not discovered or well-known until the 1960s, when we began to see large numbers of galaxies from much earlier in our cosmic history.

But individual stars are only resolvable in nearby galaxies.

This image, perhaps surprisingly, showcases stars in the Andromeda Galaxy’s halo. The bright star with diffraction spikes is from within our Milky Way, while the individual points of light seen are mostly stars in our neighboring galaxy: Andromeda. Beyond that, however, a wide variety of faint smudges, galaxies in their own right, lie beyond. Individual stars can be resolved in galaxies up to tens of millions of light-years away, but that represents only one-in-a-billion galaxies overall. This image showcases both the power and limitations of Hubble.

Big, Milky Way-like galaxies form stars all throughout their history.

The grand spiral galaxy Messier 51, also known as the Whirlpool galaxy, has sweeping, extended spiral arms, most probably owing to its gravitational interactions with the nearby neighboring galaxy shown tugging on it. Although it’s now common knowledge that these cosmic spirals are galaxies all unto themselves, the evidence necessary to draw such a conclusion didn’t arrive until 1923: a full 100 years ago.

But smaller galaxies formed stars all-at-once, long ago, within our Local Group.

Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red, covering the whole galaxy, thanks to hydrogen emissions. This particular galaxy, M82, the Cigar Galaxy, is gravitationally interacting with its neighbor, M81, causing this burst of activity.

One such galaxy is Wolf-Lundmark-Melotte: WLM, merely 3.04 million light-years away.

This wide-field view shows the sky around the dwarf galaxy WLM in the constellation of Cetus (The Sea Monster). This picture was created from images forming part of the Digitized Sky Survey 2. The bluish clump in the center of the image is galaxy WLM; the bright, colored, spikey points, including the red and yellow ones, are simply foreground stars within our own Milky Way.

WLM, in the constellation of Cetus, is gravitationally bound to us, moving toward us at 122 km/s.

This map of many of the galaxies within the Local Group highlights the three biggest members: Andromeda, the Milky Way, and Triangulum. Galaxy WLM, shown at the bottom of the image, lies about 3 million light-years from the Milky Way and is extremely isolated. It contains some of the oldest, most pristine stars within our cosmic backyard, close enough to be resolved by observatories such as JWST.

A large fraction of its internal stars formed suddenly: 13 billion years ago.

This image, captured by ESO’s OmegaCAM on the VLT Survey Telescope, shows a lonely galaxy known as Wolf-Lundmark-Melotte (WLM). Although considered part of our Local Group of dozens of galaxies, WLM stands alone at the group’s outer edges as one of its most remote members. Its isolation from all other Local Group members is remarkable, and helps provide a unique window into our cosmic past.

Those stars are extremely pristine, with just 0.6% of the heavy elements found in the Sun.

Here on the outskirts of dwarf galaxy Wolf-Lundmark-Melotte (WLM), stars of various colors and brightnesses can be seen as revealed by ESO’s OmegaCAM on the VLT Survey Telescope. The galaxy is so isolated that it may never have interacted or merged with any other galaxy since its formation more than 13 billion years ago, and the most metal-poor stars within it, highlighted here, support that picture.

New stars still form sporadically inside, but those “old” stars represent a relic, ancient population.

WLM’s only known globular cluster is similarly old and metal-poor.

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.

But JWST’s new view provides astounding new insights.

This view represents the full field of JWST’s NIRCam view of dwarf galaxy WLM, located on the outskirts of the Local Group. The dust within this galaxy is distributed asymmetrically, and so are the stars. The left regions of this image are located closer to the galactic center, while the right side represents regions farther away, and hence more pristine.

It’s a great improvement over Spitzer’s prior infrared view.

A portion of the dwarf galaxy Wolf–Lundmark–Melotte (WLM) captured by the Spitzer Space Telescope’s Infrared Array Camera (left) and the James Webb Space Telescope’s Near-Infrared Camera (right). The images demonstrate Webb’s remarkable ability to resolve faint stars outside the Milky Way. The incredible side-by-side improvement in resolution, light-gathering power, and number of filters can all be seen immediately by even an untrained eye with these images.

Even its faint, dim component stars are easily resolved.

Located several million light-years away from the Milky Way, Andromeda, and Triangulum galaxies, dwarf galaxy Wolf-Lundmark-Melotte (WLM) is extremely isolated within our Local Group. The stars that are revealed inside largely formed all at once and long ago, meaning that we’re effectively looking back at a relic from the early Universe when we examine this galaxy with sufficient detail, which JWST’s NIRCam instrument provides.

JWST’s NIRCam reveals many thousands of individual objects.

This high-density region of stars from within dwarf galaxy Wolf-Lundmark-Melotte (WLM) contains a few bright, higher-luminosity stars, but most of the stars present here are very old and very poor in metal content, enabling astronomers who hone in on these populations to discover many facts about how such stars formed and evolved when the Universe was only a few hundred million years old.

Low-density regions showcase more pristine stellar populations.

The regions of low stellar and dust density within dwarf galaxy WLM are found close to the outskirts and have undergone very little star-formation since a big burst, all-at-once, 13 billion years ago. Studying these ancient stars can help us understand how stars formed in the early Universe, when less than 1 billion years had passed since the hot Big Bang.

The dustiest regions suggest ram-pressure stripping.

The dustiest portions of dwarf galaxy Wolf-Lundmark-Melotte (WLM) show evidence of small amounts of quiescent, ongoing star-formation, as well as some evidence that this gas is being ram-pressure stripped away. Perhaps, even though mergers and interactions have been rare for WLM, there are clumps of gaseous, intergalactic matter within the Local Group that it regularly encounters.

Occasionally, background galaxies peek through.

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.

Scientific insights will reveal how stars formed, long ago, in the early Universe’s pristine environment.

An artist’s impression of the environment in the early Universe after the first few trillion stars have formed, lived, and died. While there are sources of light in the early Universe, the light is very rapidly absorbed by the interstellar/intergalactic matter until reionization is complete. While JWST is working to reveal evidence for these early stars, it’s only able to reveal those galaxies whose light isn’t completely extincted by the intervening neutral matter.

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