Giant radio galaxy Alcyoneus is now the largest known galaxy in the Universe

Ever since other galaxies were discovered, we’ve wondered, “which is the largest?“

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. Because galaxies assemble and grow over cosmic time, it’s the closest galaxies, on average, that are most likely to be the largest ones we observe today. (Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute)

Most galaxies are smaller than ours, with under 1% of the Milky Way’s stars.

The low-mass, dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to M81. One of 34 galaxies in the M81 Group, it’s an example of the most common type of galaxy in the Universe: much smaller and lower in mass, but far more numerous, than galaxies like our Milky Way. (Credit: ESA/Hubble and NASA)

In physical extent, the Milky Way’s stars span a diameter of ~130,000 light-years.

Often touted as a galaxy similar to the Milky Way, the Sunflower Galaxy, Messier 63, also displays stellar streams and debris that’s evidence for a recent, and perhaps even an ongoing, minor merger: an example of galactic cannibalism. Although we’d love to have a picture of our Milky Way from outside of it to know what our true galactic extent is, the sheer magnitude of cosmic distances make that an impossible task. (Credit: Tony and Daphne Hallas/Astrophoto.com)

Andromeda, just next door, is almost twice the size: ~220,000 light-years.

The Andromeda galaxy (M31), as imaged from a ground-based telescope with multiple filters and reconstructed to show a colorized portrait. Compared to the Milky Way, Andromeda is significantly larger in extent, with a diameter that’s approximately 220,000 light-years: comparable to double the Milky Way’s size. If the Milky Way were shown superimposed atop Andromeda, its stellar disk would end roughly where Andromeda’s dust lanes appear darkest. (Credit: Adam Evans/flickr)

Tidally interacting galaxies, however, occupy much grander scales.

The Tadpole Galaxy, shown here, has an enormous tail: evidence of tidal interactions. The gas that’s stripped out of one galaxy gets stretched into a long, thin strand, which contracts under its own gravity to form stars. The main galactic element itself is comparable to the scale of the Milky Way, but the tidal stream alone is some ~280,000 light-years long: more than twice as large as our Milky Way’s estimated size. (Credit: NASA, H. Ford (JHU), G. Illingsworth (USCS/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS science team, and ESA)

The largest known spiral is UGC 2885: 832,000 light-years across.

This galaxy, UGC 2885, also known as Rubin’s galaxy, is the largest spiral galaxy ever discovered, and possess about 10 times as many stars as the Milky Way. UGC 2885 is severely gravitationally disrupted. At an estimated 832,000 light-years across, it is arguably the largest known spiral galaxy, although its tidal arms and distorted shape are likely temporary on cosmic timescales. (Credit: NASA, ESA, and B. Holwerda (University of Louisville))

Elliptical galaxies, particularly in cluster cores, achieve superior sizes.

The massive galaxy cluster SDSS J1004+4112, like many galaxy clusters, is richer than most environments in space in terms of elliptical galaxies. Although spirals are present, they are few in number, particularly towards the cluster center. At the very core of the cluster, the largest galaxy in terms of its stellar extent can be found. Hints of a jet can also be seen, to the left of the central, largest galaxy. (Credit: ESA, NASA, K. Sharon (Tel Aviv University) and E. Ofek (Caltech))

Messier 87, the Virgo Supercluster’s largest, spans 980,000 light-years across.

Located approximately 55 million light-years from Earth, the galaxy M87 contains an enormous relativistic jet, as well as outflows that show up in both the radio and X-ray. This optical image showcases a jet; we now know, from the Event Horizon Telescope, that the rotation axis of the black hole points away from Earth, tilted at about 17 degrees. (Credit: ESO)

The Phoenix Cluster’s brightest central galaxy measures 2,200,000 light-years in size.

This optical/radio composite of the Phoenix Cluster shows the enormous, bright galaxy at its core. Spanning 2.2 million light-years across for its stellar extent, it’s even larger when measured by its radio emissions. Also, not shown, are copious levels of X-rays, including filaments and cavities, created by the powerful jets of high-energy particles originating from supermassive black holes within the cluster. (Credit: Optical: NASA/STScI; Radio: TIFR/GMRT)

But IC 1101, at cluster Abell 2029‘s center, has the largest stellar extent.

The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5-to-6.0 million light-years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all by many metrics. It’s unfortunately difficult for the Universe to make something significantly larger owing to its finite age and the presence of dark energy. (Credit: Digitized Sky Survey 2; NASA)

With a 6,000,000 light-year diameter, no galaxy’s stars cover greater lengths.

Composite of galaxies from the smallest to the largest, shown (approximately) actual size. The giant elliptical galaxy at the heart of cluster Abell 2029, IC 1101, is the largest known galaxy in the Universe, at least in terms of stellar extent. It is much, much larger than the Milky Way or Andromeda (or any spiral galaxy), but also towers over even other typical giant ellipticals. (Credit: E. Siegel)

Beyond the stars, however, galaxies possess matter-rich halos.

Even the Milky Way, a relatively quiet galaxy with a relatively small central supermassive black hole, exhibits giant geysers of charged particles emanating from the galactic center. They can be revealed by radio telescopes, such as this image constructed with data from the Parked radio telescope, a.k.a. The Dish. (Credit: A. Mellinger (C. Michigan), E. Carretti (CSIRO), S-PASS Team, E. Bressert (CSIRO))

Although non-luminous in optical light, they can shine in the radio.

The supermassive black hole at the center of giant radio galaxy J021659-044920 is active, and produces radio lobes on large scales, spanning millions of light years, as shown in yellow and red contours. This is massive and impressive, but also typical for giant radio galaxies. (Credit: P. Tamhane et al., MNRAS, 2015)

Active black holes create jets, which excite gas and trigger emissions.

This illustration of a radio-loud quasar that is embedded within a star-forming galaxy gives a close-up look of how giant radio galaxies are expected to emerge. At the center of an active galaxy with a supermassive black hole, jets are emitted that slam into the larger galactic halo, energizing the gas and plasma and causing radio emissions in the form of jets close by the black hole, and then plumes and/or lobes farther away. (Credit: ESA/C. Carreau)

Giant radio galaxies possess lobes: the largest galactic structures of all.

A combination of optical data from the Hubble Space Telescope and radio data from the Very Large Array reveals the full structure of the giant radio galaxy Hercules A. The radio jets and lobes create a structure that absolutely outclasses the stellar extent of the galaxy in question. (Credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA))

In 2022, astronomers identified lobes emitted from giant radio galaxy Alcyoneus.

The radio feature of the galaxy Alcyoneus includes a central, active black hole, collimated jets, and two giant radio lobes at either end. The Milky Way is shown at the bottom for scale, as well as “10x the Milky Way” for perspective. (Credit: M.S.S.L. Oei et al., Astronomy & Astrophysics, 2022; animation: E. Siegel)

They span 16,000,000 light-years in extent, breaking all prior records.

This image, which shows radio data overlaid atop WISE (infrared) data, displays the full physical extent of the giant radio galaxy Alcyoneus, now identified, at a scale of 16 million light-years (5 Megaparsecs), as presently the largest known galaxy in the Universe. (Credit: M.S.S.L. Oei et al., Astronomy & Astrophysics, 2022)

No other galaxy, even IC 1101, can compare: Alcyoneus is the largest known galaxy of all.

In a first-of-its-kind image, the scale of galaxies, including the Milky Way, Andromeda, the largest spiral (UGC 2885), the largest elliptical (IC 1101), and the largest radio galaxy, Alcyoneus, are all shown together and, accurately, to scale. (Credit: E. Siegel)

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