For decades, passionate astronomers have been unravelling the unending mysteries of the Universe. Among these mysteries, there is one that has always stuck out like a sore thumb: galaxy death. But recent discoveries may have finally revealed what halts star formation: strangling.
According to a recent scientific article published in the Journal Nature, a process likening slow suffocation shuts down star formation. Astronomers from the University of Cambridge and the Royal Observatory Edinburgh attempted to approach the issue from a completely new perspective, so they searched for heavy metals in the stars they were studying.
Star Formation and Demise
Scientific literature points to two distinct classes of galaxies: ones that are star forming (containing large quantities of gas) and quiescent galaxies (where star formation is dulled down and gas quantities are poor).
“What kills galaxies is one of the most challenging questions in the past 20 years,” Yingjie Peng, Cambridge University astronomer explains.
The scientists had been looking at two possible hypotheses explaining star formation declines. One of the mechanisms involves an abrupt removal of gas through a process called stripping. The other mechanism is called strangulation. It involves a decrease in cold gas supply to the galaxy in question.
By carefully evaluating the metal metallicity of various galaxies in different spectra, the scientists postulated that strangulation represented the most likely mechanism involved in star formation declines.
In total, the researchers analyzed over 26,000 nearby galaxies and observed that, in the majority of cases, cold gas supply is suddenly choked off by external forces (possibly gravitational forces stemming from neighboring galaxies).
Fuel Requirements and Heavy Metal Buildup
The younger a star is, the higher its concentrations of hydrogen and helium, which it uses as main fuel sources throughout its lifespan. As young stars begin to consume this fuel, hydrogen and helium are fused together into increasingly heavier elements (such as iron or magnesium).
It’s precisely this metal concentration that the team of researchers investigated. As one would expect, “live” galaxies, where star formation processes are in full bloom, have low concentration of metals and an abundance of hydrogen and helium.
Similarly, quiescent (dead) galaxies, where star formation processes have halted, contain massive amounts of metals. Though unsurprising, this finding does correlate with the concept of strangulation and how it contributes to galaxy evolution.
The team used data from the Sloan Digital Sky Survey to capture sudden shifts in metal formation.
Even when a galaxy’s gas supply is interrupted, some quantities still remain which allow further star formation. But in the event that cold gas is suddenly removed from a galaxy, star formation comes to an abrupt halt, leading to a sudden decrease in metal formation.
The scientists therefore searched for those galaxies where metal formation had come to a quick halt, and the galaxy’s current metal concentrations matched those just leading up to its death.
Galaxy Death Timeline
The research team used complex computer models to understand exactly how long the strangulation process would take. According to their results, it takes approximately 4 billion years for star formation to be extinguished, a timeline consistent with the age difference between the two types of galaxies currently known (star-forming and quiescent).
And despite the fact that not all galaxies suffer the same fate, strangulation is now considered to be the most likely mechanism of star muffling. In the case of galaxies that are 100 billion times heavier than the sun (which comprise approximately 95 percent of all known galaxies), strangulation is the most likely explanation.
Peng and his collaborators note that further research is required for both the remaining 5 percent of galaxies (where a conclusive mechanism hasn’t been identified) and for the exact understanding of how strangulations occur.
Whether nearby galaxies enacting massive gravitational pulls are the main culprits behind the sudden star depletion remains to be seen.
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