From Ethan Siegel: “What’s The Largest Galaxy In The Universe?”

Ethan Siegel
May 1, 2017

The SDSS view in the infrared – with APOGEE – of the Milky Way galaxy as viewed towards the center.

SDSS Telescope at Apache Point Observatory, NM, USA

Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

Our Milky Way contains some 400 billion stars, spanning 100,000 light years in diameter.

Galaxies come in a variety of types and sizes. While the Milky Way may be impressive from our location within it, it barely registers at all on a list of the largest galaxies. Los Cumbres Observatory

Yet compared to other galaxies, it’s not even especially large.

The Andromeda Galaxy resides in our local group, and is perhaps twice as large in diameter as our Milky Way. Adam Evans

In our own local group, the Andromeda galaxy is significantly larger, reaching 220,000 light years across.

Local Group. Andrew Z. Colvin 3 March 2011

Severely disrupted galaxies, like NGC 6872, can extend for many times farther than a quiet galaxy that hasn’t had a major gravitational interaction. ESO / VLT, Judy Schmidt

Interacting spiral galaxies can have their arms greatly extended and disrupted, with NGC 6872 spanning 522,000 light years from tip-to-tip.

The galaxy Malin 1 is one of the largest spiral galaxies ever discovered, at 650,000 light years (199 kpc) in diameter. Boissier/A&A/ESO/CFHT

Ultra-low surface brightness galaxies can see their stars extend even farther, with Malin 1 reaching 650,000 light years across.

The low-surface-brightness galaxy UGC 2885 is also severely gravitationally disrupted, making it arguably the largest known spiral galaxy. Kitt Peak / Zagursky & McGaugh, 2008

Unsurprisingly, a disrupted low surface brightness galaxy, UGC 2885, is the largest spiral known at 832,000 light years.

The galaxy NGC 262, at the image’s center (and in detail, inset), is only about the size of the Milky Way, but its hydrogen halo extends more than 10 times as far, as shown in blue in the main image. NRAO/AUI, with the VLA (main); SDSS (inset)

Hydrogen gas and dark matter halos can continue beyond the stars, like in NGC 262, extending over 1,000,000 light years.

The giant elliptical near the center of the Coma Cluster, NGC 4874, is typical of the largest, brightest galaxies found at the centers of the most massive galaxy clusters. NASA/ESA Hubble

But the largest and most massive galaxies aren’t spirals, but supergiant ellipticals, like NGC 4874 in the Coma Cluster.

The two bright, large galaxies at the center of the Coma Cluster, NGC 4889 (left) and the slightly smaller NGC 4874 (right), each exceed a million light years in size. Adam Block/Mount Lemmon SkyCenter/University of Arizona

Coma has two enormous core ellipticals, with the even larger NGC 4889 reaching 1.3 million light years in diameter.

The brightest cluster galaxy of the Phoenix cluster, shown at left from the South Pole Telescope and at right from Blanco/MOSAIC-II optical/infrared imagery, is one of the largest galaxies of all, still rapidly forming stars at hundreds of times the rate of our own Milky Way. R. Williamson et al., Astrophysical Journal 738(2):139 · August 2011

Even larger ones reside inside more massive galaxy clusters, like the brightest galaxy in the Phoenix cluster, at 2.2 million light years.

The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5 million light years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all. Digitized Sky Survey 2, NASA

Still, IC 1101 beats them all, extending for 5.5 million light years.

Early, intense star forming galaxies, like the Baby Boom galaxy shown in green/red here and imaged in the infrared, can form up to 4,000 new Sun-like stars every year. This behavior, from more than 12 billion years ago, can lead to the largest galaxies of all by present times. NASA/JPL-Caltech/P. Capak (Spitzer Science Center)

Periodic star formation, mergers, and gravitational growth over time inevitably lead to these cosmic behemoths.

See the full article here .

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan