Galaxy clusters are graveyards for Milky Way-like galaxies
If our Milky Way were located in the Virgo cluster instead of the Local Group, chances are we'd already be a "red and dead" galaxy.
This view of the Perseus cluster of galaxies 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. In the centers of large galaxy clusters, Milky Way-like galaxies with spiral structures, new stars, and gas-rich disks are rare, as cluster centers are instead dominated by gas-free, red-and-dead giant elliptical galaxies
ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi, CC BY-SA 3.0 IGO / Big Think
Key Takeaways
- In isolation or in small galaxy groups like our own, relatively large, massive galaxies, like the Milky Way and Andromeda, can persist and still form stars for many billions of years: longer than the age of the Universe at present.
- But in rich galaxy clusters, particularly toward the centers of these clusters, gas-rich spiral galaxies, the ones that are still forming stars actively, are rare, and instead are outnumbered by giant, red-and-dead ellipticals.
- Instead of gas-rich spirals like our own galaxy, galaxy clusters are populated with former spirals that expelled their gas, merged, and formed giant ellipticals long ago. These grand clusters are, in some sense, the ultimate cosmic graveyard.
What makes a galaxy dead or alive is simple: internal stores of gas.
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 the nearby, more massive M81. As 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. The young stars within it have formed from gas reservoirs still present within this galaxy, indicating an “alive” galaxy.
Inside living galaxies, gas is required to enable the formation of new stars.
The enormous bar at the core of galaxy NGC 1300 spans many tens of thousands of light-years, nearly the full width of the galaxy. While many spiral galaxies contain large, prominent bars such as this one, our Milky Way’s central bar is far more modest, extending only about a third of the way out to the Sun’s position. The pink regions found along the spiral arms are evidence of new star formation, triggered by the interaction of internal gas and the density waves of the internal structure.
When massive gas clouds gravitationally collapse, new stars inevitably arise.
Star-forming regions, like this one in the Carina Nebula, can form a huge variety of stellar masses if they can collapse quickly enough. Inside the ‘caterpillar’ is a proto-star, but it is in the final stages of formation, as external radiation evaporates the gas away more quickly than the newly-forming star can accrue it. Within the first ~2 million years of this star’s birth, protoplanets should already begin arising within the accompanying protoplanetary disk.
As matter fragments, the various clumps grow rapidly, forming new stars and massive star clusters.
The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image, although they have the shortest lifetime, living for between 1-10 million years only. Within a cloud of gas, the process of core fragmentation leads to enormous populations of large numbers of stars.
Many events trigger galactic star-formation, including:
The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, and has a similar diameter (118,000 light-years) to our own Milky Way.
- internal dynamics,
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
- external gravitational tugs,
The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth.
- or even galactic mergers.
Zw II 96 in the constellation of Delphinus, the Dolphin, is an example of a galaxy merger located some 500 million light-years away. Star formation is triggered by these classes of events, and can use up large amounts of gas within each of the progenitor galaxies, rather than a steady stream of low-level star formation found in isolated galaxies. Note the streams of stars between the interacting galaxies, which can either become part of a population of stars in the post-merger galaxy’s stellar halo, or could get expelled from the post-merger galaxy entirely, roaming the intergalactic medium.
Isolated galaxies are more likely to form stars quiescently: at slow, steady rates over long timescales.
The isolated galaxy MCG+01-02-015, all by its lonesome for over 100,000,000 light years in all directions, is presently thought to be the loneliest galaxy in the Universe. The features seen in this galaxy are consistent with it being a massive spiral that formed from a long series of minor mergers, but that has never experienced a major merger, and where star-forming activity has been relatively quiet for the past several billion years. A galaxy such as this may continue forming new stars in an ongoing fashion for much longer than the present age of the Universe.
However, once a galaxy becomes gas-depleted or even gas-free, star-formation ceases within it.
This nearby galaxy, NGC 1277, although it may appear similar to other typical galaxies found in the Universe, is remarkable for being composed primarily of older stars. Both its intrinsic stellar population and its globular clusters are all very red in color, indicating that it hasn’t formed new stars in ~10 billion years. When all of the gas within a galaxy is expelled and no new gas enters, that galaxy becomes permanently “red and dead,” as no new populations of stars can form within it.
Without gaseous material, there’s no “fuel” for future generations of stars.
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Over time, overdense clumps of matter grow richer and more massive, growing into galaxies, groups, and clusters of galaxies, while the less dense regions than average preferentially give up their matter to the denser surrounding areas.
This is often the fate of even initially Milky Way-like galaxies as they fall into galaxy clusters.
Galaxy clusters, like Abell S740, are the largest bound structures in the Universe. When spirals merge, for example, a large number of new stars form, but either post-merger or by speeding through the intra-cluster medium, gas can be stripped away, leading to the end of star formation in that galaxy and, eventually, a red-and-dead final structure. It is possible that red-and-dead galaxies can form much earlier in the Universe’s history than current observations indicate, and JWST will be the tool that determines the answer.
Inside rich clusters, galactic mergers are common, with major mergers often leading to galaxy-wide starbursts.
The galaxy NGC 7727 shows extended spiral arms: likely the aftermath of a recent major merger between two comparably massive galaxies. The presence of two supermassive black holes inside this galaxy, as well as the extended streams of gas and stars, show one possible outcome of a major merger of two similar-mass, initially gas-rich galaxies.
These violent episodes of star-formation generate incredible winds, expelling large reservoirs of gas.
This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds. The lower in mass the wind-driven starburst galaxy is, the more of its normal matter it loses during an event like this.
Furthermore, galaxy clusters contain a gas-rich intracluster medium.
Located within the Norma cluster of galaxies, ESO 137-001 speeds through the intracluster medium, where interactions between the matter in the space between galaxies and the rapidly-moving galaxy itself cause ram pressure-stripping, leading to a new population of tidal streams and intergalactic stars. Sustained interactions such as this can eventually remove all of the gas from within a galaxy, eliminating its ability to form new stars. Phenomena such as this allow us to conclude that the galaxy, the cluster, and the gas within it are all made of matter, not antimatter, while the tidal streams of new stars will contain practically no dark matter at all.
Rapidly speeding through it strips a galaxy’s gas away.
When galaxies like the spiral galaxy at right, D100, speed through a rich environment (like the Coma Cluster, which D100 is a member of), the friction with the environment can cause gas stripping, leading to the formation of stars and increasing the dark matter-to-normal matter ratio of the host galaxy. A few of these stripped star clusters that form, trailing the galaxy, could later re-form into a dark matter-free galaxy of their own.
The end state of a gas-free galaxy is a giant elliptical where only old stars survive.
A map of neutral hydrogen (in red) overlaid on the galaxy D100 in the Coma Cluster shows how much gas is being quickly stripped from this galaxy as it travels through the cluster. Galaxies found in environments like this one become ‘red-and-dead’ far more quickly than galaxies in less dense regions of space.
With many mergers and constant gas stripping, galaxy clusters are graveyards for Milky Way-like galaxies.
The Coma Cluster of galaxies, as seen with a composite of modern space and ground-based telescopes. The infrared data comes from the Spitzer Space telescope, while ground-based data comes from the Sloan Digital Sky Survey. The Coma Cluster is dominated by two giant elliptical galaxies, with over 1000 other spirals and ellipticals inside. Gas-free, red-and-dead elliptical galaxies are very common, especially toward the cluster center, in large galaxy clusters such as this one. The speed of galaxies within the cluster can be used to help determine the cluster’s total mass.
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