Youngest Milky Way-like galaxy rotates just like we do

The Milky Way, like all spiral galaxies, spins about its axis.

A galaxy that was governed by normal matter alone (left) would display much lower rotational speeds in the outskirts than toward the center, similar to how planets in the Solar System move. However, observations indicate that rotational speeds are largely independent of radius (right) from the galactic center, leading to the inference that a large amount of invisible, or dark, matter must be present. These types of observations were revolutionary in helping astronomers understand the necessity for dark matter in the Universe, and also explain the shapes and behavior of matter located within a galaxy’s spiral arms.

Stars and gas rotate in a disk, orbiting the galactic center.

The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. It is neither gravitationally interacting nor merging with any nearby neighbor galaxies, and so the star-formation occurring inside is driven primarily by the density waves occurring within the spiral arms in the galactic disk.

However, this state — in theory — can only be achieved after enough time has passed.

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.

Early on, cold streams of gas collide and collapse, forming stars.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions, and could lead to direct collapse black holes of an estimated ~40,000 solar masses. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the formation of stars and the growth of galactic structures.

This leads to asymmetrical shapes for the earliest proto-galaxies, as confirmed by JWST.

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.

Galactic disks only form once early irregularities are smoothed out by time’s passage.

This supercomputer simulation shows the emergence of a rotating disk after hundreds of millions of years of cosmic evolution from gas and dust; the simulation also includes stars and dark matter, which are not shown here. If the dark matter were visible, it would make an enormous halo much larger, in radius, than the entire size of the image shown here.

In the past, ALMA has revealed rotation in early galaxies.

This figure shows the detection of broad, powerful emission lines, specifically from singly ionized carbon, in two distant galaxies from 800-1000 million years after the Big Bang (top and bottom) with ALMA. By looking at how velocities differ across the objects themselves, ALMA can search for evidence of galactic rotation and the presence of a galactic disk as well.

In 2018, ionized carbon signatures confirmed rotation in galaxies as young as ~0.8-1.0 billion years old.

The two galaxies measured and detected with ALMA in 2018, shown overlaid atop the Hubble image containing them, exhibit clear signs of rotation and whirlpool-like behavior, remarkably consistent with what’s observed in modern spiral galaxies like the Milky Way.

Then, galaxy MACS1149-JD1 was found: just 530 million years after the Big Bang.

The distant galaxy MACS1149-JD1 is gravitationally lensed by a foreground cluster, allowing it to be imaged at high resolution and with multiple instruments, including Hubble and ALMA. Based on measurements of the stellar populations found inside, this object, whose light comes from when the Universe was just 530 million years old, contains stars that are at least 280 million years old within it. It shows strong evidence of differential motions inside of it, suggesting rotation, but it does not have an identifiable disk.

At the time, this galaxy, the second most distant known, showed signs of internal rotation, too.

A 2022 study with ALMA showed that galaxy JD1 does exhibit signs of rotation but was unable to confirm the presence of an evolved, settled-down structure like a galactic disk, even though one is suggested by the illustration here.

Its interior stellar populations indicated stars of at least 280 million years old.

This image shows a deep galaxy field containing a great many nearby, intermediate, and ultra-distant galaxies, along with an inset image with ALMA data that shows the morphology of the disk-like galaxy REBELS-25, as seen just ~700 million years after the Big Bang.

Although MACS1149-JD1 was not disk-like, newfound rotating galaxy REBELS-25 is.

Here, maps constructed with ALMA and ESO VISTA data of single-ionized carbon (left), dust (middle), and a composite that also includes rest-frame UV light (right) are all shown. This galaxy was determined to be actively forming about 200 solar masses worth of stars per year, with a gas reservoir of around 50 billion solar masses worth of neutral hydrogen.

At an age of just 700 million years, it’s the youngest mature, strongly rotating disc galaxy ever discovered.

This animation shows the transition between ESO VISTA data (orange) and ALMA data (blue, white, and red), where the latter shows the velocity profile of what VISTA clearly shows is a disk galaxy. This makes REBELS-25, the galaxy imaged here, the earliest, youngest rotating disk galaxy ever discovered.

Its disk is dynamically cold, and it contains nearly 10 billion solar masses worth of stars inside.

Compared to all other known disk galaxies with a measured rotation velocity (blue and orange points), REBELS-25 (red point) is the earliest, highest-redshift galaxy ever observed to robustly exhibit these features. How a mature disk formed when the Universe was just 5% of its current age is not easily explained.

Defying expectations, theorists must now explain how mature, rotating disks can form so quickly.

This infrared view of the Whirlpool Galaxy, Messier 51, reveals a plethora of active star formation and heated gas/dust lining the spiral arms. A gas bridge is being pulled from one of the extended spiral arms toward the interacting galactic companion, which itself is gas-poor and doesn’t show the same evidence of star-formation. Evolved spiral galaxies were thought to require billions of years of cosmic time to form, but the discovery of rotating disks from within the first ~1 billion years of cosmic history now challenges that view.

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