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One tremendous question puzzling astronomers is, “how did the Universe grow up?”
At the start of the hot Big Bang, the Universe was rapidly expanding and filled with high-energy, very densely packed, ultra-relativistic quanta. An early stage of radiation domination gave way to several later stages where radiation was sub-dominant, but never went away completely, while matter then clumped into gas clouds, stars, star clusters, galaxies, and even richer structures over time, all while the Universe continues expanding. The time after the relic radiation has faded away but before stars have ignited marks the cosmic dark ages.
Today, most of our Universe’s stars are found within large spiral and elliptical galaxies.
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 a critical tool for mapping out the dark Universe.
Spirals — containing gas, disks, and newly forming stars — exhibit a fascinating overall rotation.
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.
Large stellar velocities near the galactic outskirts suggest an enormous dark matter halo surrounding it.
The extended rotation curve of M33, the Triangulum galaxy. These rotation curves of spiral galaxies ushered in the modern astrophysics concept of dark matter to the general field. The dashed curve would correspond to a galaxy without dark matter, which represents less than 1% of galaxies. Vera Rubin’s work throughout the 1970s was essential in demonstrating that galaxies practically universally require an explanation for this unexpected but robustly observed behavior.
Meanwhile, dense galaxy clusters possess mostly ellipticals, with only a few spirals.
Most of the largest known galaxies in the Universe are found at the hearts of massive galaxy clusters, like the Hercules galaxy cluster shown here. Over time, galaxies within these clusters collide and merge, leading to bursts of new star-formation but making the galaxies more gas-poor, overall. After enough time has passed, most galaxies within such a cluster will become giant ellipticals, rather than disk-containing spirals.
Eventually, major mergers will “use up” and expel the galactic gas.
This view of galaxy NGC 1275, at the core of the Perseus cluster of galaxies, is one of the closest modern giant ellipticals known, located merely 230 million light-years away. Although the center of the galaxy is gas-poor, the circumgalactic medium surrounding it still possesses gas. Highlighted with Hubble imagery here, the red filaments are composed of cool gas being suspended by a magnetic field, with ~50,000,000+ K hot gas located internally.
However, many questions remain about how these galaxies form, evolve, and grow up.
Regions born with a typical, or “normal” overdensity, will grow to have rich structures in them, while underdense “void” regions will have less structure. However, early, small-scale structure is dominated by the most highly peaked regions in density (labeled “rarepeak” here), which grow the largest the fastest, and are only visible in detail to the highest resolution simulations.
It’s the initially most massive overdense regions that lead to large, rotating spirals.
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: just 700 million years after the Big Bang.
With JWST’s unprecedented power, we’ve just found the youngest super-Milky Way ever.
The blown-up galaxy shown here, serendipitously captured as JWST observed the field of the quasar shown at right, is the largest disk galaxy ever observed within the first 2 billion years of cosmic history. Located at a redshift of z=3.25, when the Universe was 1.95 billion years old, it spans 100,000 light-years across and contains more than six times the stellar mass of the modern Milky Way galaxy.
Located at a redshift of z=3.25, the “Big Wheel” galaxy has a stellar mass of ~400 billion Suns.
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, but a few that possess them are still found very early on. Over time, galaxies merge together and accrete matter: growing in their typical mass but decreasing in overall number.
Credit: NASA, ESA, P. van Dokkum (Yale U.), S. Patel (Leiden U.), and the 3-D-HST Team
This is more than six times the Milky Way’s stellar mass, but nearly 12 billion years ago.
Although local, irregular motions are found within various features inside the Big Wheel galaxy, overall it is observed to rotate at similar speeds to the stars in the Milky Way: orbiting the center at ~200 km/s. The bulk, overall rotation is consistent with the presence of large quantities of dark matter.
It rotates just like modern spirals: indicative of dark matter.
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.
It follows the Tully-Fisher relation perfectly.
Compared to other disk galaxies observed at such early times (top left), the Big Wheel galaxy is larger and more massive, by far, than other such galaxies. However, this galaxy, despite its size, remains consistent with known star-formation rates (lower left) and the relationship between rotational speed and stellar mass (lower right) previously established for galaxies of this class.
The disk itself is huge for such early times: spanning ~100,000 light-years.
Although most disk galaxies found this early have a half-light radius of only about 10,000 light-years, the Big Wheel’s half-light radius is more than triple that figure, with the overall galaxy spanning ~100,000 light-years across. Its features are prominently revealed by JWST, with only a small set of features visible at Hubble’s shorter-wavelength sensitivities.
Measuring beyond these edges could test dark matter’s predictions.
The dark matter models of today (top curves) fails to match the exact rotation curves, of older galaxies, as (black curve) does the no dark matter model. However, there are models that allow the dark matter distribution to evolve with time (magenta, blue, and cyan dotted lines), and they do match observations. The ability to conduct precision measurements of even more distant galaxies, such as the Big Wheel, could test models of dark matter’s evolution in massive galaxy/cluster environments.
By today, this gas-rich ancient predecessor will have evolved into a giant elliptical.
This view showcases galaxy IC 1101 inside the galaxy cluster Abell 2029: the single largest giant elliptical galaxy known in the modern Universe. Most of the other galaxies visible in the picture are comparable to (or larger than) the Milky Way, but they appear tiny next to this evolved, modern behemoth. After many major mergers, the centers of most modern galaxy clusters are richly littered with gas-free giant elliptical galaxies.
Credit: Pan-STARRS
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.
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Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
