This one prominent galaxy, known as the Big Wheel and located 22 billion light-years away at present, spans ~100,000 light-years in diameter: the largest disk galaxy known at these early cosmic epochs. The smaller galaxy to the Big Wheel’s upper right is located within the same galaxy cluster/protocluster. (Credit: NASA/ESA)
Large, massive, rotating galaxies like the Milky Way are common today. So how could one form a mere ~2 billion years after the Big Bang?
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. (Credit: CfA/M. Weiss)
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. (Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi, CC BY-SA 3.0 IGO)
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. (Credit: Ingo Berg/Wikimedia Commons; Acknowledgement: E. Siegel)
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. (Credit: Mario de Leo/Wikimedia Commons)
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. (Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute)
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. (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)
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. (Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023)
