Recipe for how to kill a galaxy revealed at last

Most observed galaxies, astronomically, are alive.

While there are many instances of numerous galaxies in the same region of space, they normally occur either between two galaxies only or in very dense regions of space, like at the centers of galaxy clusters. Seeing 5 galaxies interacting within a space of less than 1 million light-years is an extreme rarity, captured in gorgeous detail by Hubble here. As all of these galaxies are still forming new stars, they’re all classified as “alive” by astronomers.

Alive, to a galaxy, means “actively forming stars.”

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.

Several large star-forming regions line our Milky Way’s spiral arms.

This ground-based, wide-field image of the Eagle Nebula shows the star-forming region in all its glory, with new stars, the blue glow of reflected starlight, and the red glow of ionized atoms all present. Dusty, light-blocking features are also prominent. The reddish glow at the gaseous outskirts is a result of hydrogen atoms recombining, and a photon of precisely 656.3 nanometers being emitted every time an electron transitions from the n=3 to the n=2 energy state.

When new stars form, they arrive with various masses and colors.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star, along with about 5-10% of total stars. However, in the early Universe, almost all of the stars were O- or B-class stars, with an average mass 25 times greater than average stars today.

Although they all form simultaneously, the hottest, bluest, shortest-lived stars evolve and die first.

The image shows the central region of the Tarantula Nebula in the Large Magellanic Cloud. The young and dense star cluster R136 can be seen near the center of the image. The tidal forces exerted on the Large Magellanic Cloud by the Milky Way are triggering a wave of star-formation in there, which happens to be the largest star-forming region known in the Local Group. R136a1, at the cluster’s center, is the most massive single star known, with approximately 260 times the mass of our Sun.

A few galaxies — mostly ellipticals within clusters — ceased forming stars long ago.

This chain of large galaxies is found near the center of the Perseus cluster of galaxies, with several of these galaxies being typical of the large, bright, evolved galaxies found at the centers of most massive galaxy clusters. For many of these galaxies, the stars found inside of them are primarily older and redder, with only small populations of bluer stars found inside.

Over time, the heavier, bluer stars die off.

By mapping out the colors and magnitudes of stars that were all born at the same time, like members of a star cluster or globular cluster, you can determine the age of the cluster by identifying where the main sequence ends and the heavier, more massive stars have “turned off” and begun evolving into subgiants. Measuring the subgiant population very well is one key to understanding a stellar population’s age.

Since redder stars survive, galaxies lacking new stellar populations are called “red-and-dead” by astronomers.

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.

Measuring the intrinsic color of a galaxy’s starlight determines whether a galaxy is dead or alive.

In this image, a massive set of galaxies at the center causes many strong lensing features to appear. Background galaxies have their light bent, stretched, and otherwise distorted into rings and arcs, where it gets magnified by the lens as well. This gravitational lens system is complex, but informative for learning more about Einstein’s relativity in action.

Mainstream astronomy suggests that molecular gas reserves form new stars within galaxies.

This image features data from 10 different JWST filters: 6 from the near-infrared and 4 from the mid-infrared. As a result, features that include stars, gas, dust, and various molecular signatures can all be revealed at once, showcasing where star formation is occurring and will occur in the future. Molecular gas, present in great abundance, is the key.

If a galaxy possesses no new stars, it must be gas-free.

The ‘red-and-dead’ galaxy NGC 1277 is found inside the Perseus cluster. While the other galaxies contain a mix of red-and-blue stars, this galaxy hasn’t formed new stars in approximately 10 billion years. Foreground, closer objects, like stars, as well as more distant galaxies, are all ubiquitous throughout this image.

Gas can be removed by intense star-formation periods, frequently triggered by mergers and interactions.

Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red, covering the whole galaxy, thanks to hydrogen emissions. This particular galaxy, M82, the Cigar Galaxy, is gravitationally interacting with its neighbor, M81, causing this burst of activity. Although the winds and ejecta are copious, this episode is not expected to completely “kill” the galaxy, as some gas will still persist after this episode completes.

Rapid journeys through a galaxy cluster’s intergalactic medium also strip interior gas away.

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.

In 2018, the first red-and-dead galaxy in our cosmic backyard was identified: NGC 1277.

Speeding at 900 km/s through the Perseus cluster, it hasn’t formed new stars in ~10 billion years.

The galaxy NGC 1277, speeding through the Perseus cluster, not only contains predominantly red stars, but red (and not blue) globular clusters, as well as an abnormally large supermassive black hole to go along with its rapid speed through this rich galaxy cluster.

Its stars and globular clusters are exclusively red-colored.

This is a blink comparison that plots the location of the red stars and blue stars that dominate the globular clusters in galaxies NGC 1277 and NGC 1278. It shows that NGC 1277 is dominated by ancient red globular clusters. This is evidence that galaxy NGC 1277 stopped making new stars many billions of years ago, compared to NGC 1278, which has more young blue star clusters.

Unless gas reserves arrive anew, no new stars should form within it.

This graph shows the distribution of globular clusters as sorted by the color of the stars inside, for the field of view of the Perseus Cluster that includes the neighboring galaxies NGC 1277 and NGC 1278. While NGC 1278 and background globular clusters are predominantly blue, NGC 1277 shows itself to be red-and-dead. Some studies suggest that no new stars have been formed within it for approximately 10 billion years.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.