Distant galaxies are strangling each other to death

Research from the University of Cambridge suggests that galaxies can use up other galaxies gas supplies, effectively strangling them to death.
By Tracy Williams | May 14, 2015
Just like everything else in the universe, galaxies have a distinct life cycle. They are born out of converging gas clouds and star clusters, brought together by massive gravitational attractive forces. These masses continue to converge and interact as they form stars, eventually reaching more or less of a state of equilibrium. New research from the University of Cambridge and the Institute for Astronomy, Royal Observatory was published this week in the journal Nature. The research describes telltale signs of a dying galaxy. The study described the phenomenon of galactic strangulation the buildup of metals as a result of star formulation typically signals a downturn in galactic growth.

Famed astronomer Edwin Hubble classified galaxies into three morphological types nearly 90 years ago. The three types of galaxies described were spirals, ellipticals, and lenticulars. In spiral galaxies, stars form a disk-like shape and spin around in circles, not unlike a carousel. Elliptical galaxies are the result of galactic collisions, with stars moving haphazardly around each other in many different directions. Lenticular galaxies are somewhere in the middle of the other two, likely the result of a near miss or partial collision. Typically, spiral galaxies are made up of blue stars, which are young and exhibit extremely high temperatures. Elliptical and lenticular galaxies contain little fuel for star formation, so the remaining stars give off red light, signifying a later phase in their life cycle.

Further research demonstrated that blue, star-forming galaxies and red, passive galaxies composed distinct populations. The first hypothesis for explaining this phenomenon comes from the idea that the gas in elliptical galaxies and their surroundings is often too hot to form stars, and cools inefficiently. The second hypothesis posits that the gas that would potentially cool and form stars is kept at a high temperature by phenomena linked to the supermassive black holes that form at the center of all known elliptical galaxies. The second hypothesis is formed from computer simulations of elliptical galaxy formation as a result of converging galaxies. If there were no mechanism to get rid of gas, blue stars would form. Blue stars are extremely rare in ellipticals, so the fuel source has to have gone somewhere else.

Dr. Peng and his team presented evidence that stars forming in the majority of passive galaxies ended through a slow strangulation process. The secret to their discovery lies in the chemical composition of the galaxies, which was detected using high-powered telescopes.

In galaxies that were no longer producing stars, astronomers detected high levels of metals present. According to Dr. Yingjie Peng, the study's lead author, "Metals are a powerful tracer of the history of star formation; the more stars that are formed by a galaxy, the more metal content you'll see. So looking at levels of metals in dead galaxies should be able to tell us how they died." The relative concentrations of specific metals present in a dead galaxy can offer further insight into the forces that culled star growth and began the decay.

In a rapid and violent galactic death, the cool gas that typically feeds star formation is stripped away by internal or external forces. Star formation stops immediately, and the metals present in the galaxy would more or less be the same as they were before the gas source was removed. If the source is removed from a galaxy, but enough gas to form more stars remains, different concentrations of metals would continue to build up until the gas is exhausted and the galaxy "suffocates."

Other astronomers weighed in on the findings of the research, supporting the assertions made by the team at the University of Cambridge. Andrea Cattaneo, from the Observatoire de Paris in France likened galactic strangulation to patterns observed in human suffocation, particularly the buildup of carbon dioxide that results from asphyxiation. It is important to distinguish between strangulation and suffocation, however. The term "strangulation" is used in this study to describe the active stealing of gas sources between galaxies. In large clusters, the gas simply doesn't go searching for a new place to live; nearby galaxies actively compete for the fuel source, eventually grabbing enough gas to shut down new star formation altogether. Large galaxies lose their gas from tidal forces or down black holes, but the evidence presented clearly supports strangulation as the cause of death for most small galaxies.

"During strangulation, the victim uses up oxygen in the lungs but keeps producing carbon dioxide, which remains trapped in the body," Cattaneo wrote in her commentary on the study. Similarly, suffocating galaxies accumulate metals and other elements heavier than helium, which is the primary fuel source for the nuclear fusion that powers suns. The buildup of heavy metals in a galaxy is primary indicator that star-forming activity has drastically slowed or stopped completely.

According to Dr. Peng's research, metals were accumulating in 23,000 red, passive galaxies, and 4,000 blue star-forming galaxies. The team used data from the Sloan Digital Sky Survey, an expansive collection of detailed, multicolored images of up to a third of the nearby sky that has been used to make a precise map of the known universe.

Galaxies with a lower mass tended to shut down star formation much more slowly than large elliptical galaxies. Ellipticals' low iron content, which is increased by supernova explosions, explains this difference in strangulation speed.

The team of Cambridge astronomers found that star-forming galaxies were an average of four billion years younger than dead galaxies. This figure falls in line with estimates of how long it would take for a galaxy to burn up its remaining gas supply once its source has been cut off. Four billion years is much longer than it would take to strangle a human being, but the analogy between the two remains accurate.

Dr. Peng was pleased to find evidence of galactic strangulation in faraway galaxies, but he is still unsure of what causes these deaths. Though researchers are not completely sure what leads to the exhaustion of gas supplies, they have come up with a few hypotheses. One likely explanation is overcrowding. If a galaxy is in a large group of cluster, surrounded by other galaxies, it is sharing the same source of gas used to form stars with all of the other galaxies in its vicinity. Its source of gas might be interrupted or stolen completely by a competing nearby galaxy, leading to the beginning of the strangulation process. This hypothesis was supported by the detection of higher levels of heavy metal buildup in galaxies that were a part of a group or cluster. Large galaxies are also at risk of running through their gas reserves more quickly, and are expected to experience a sudden and violent death by strangulation.


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