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Dwarf galaxy conundrum untangled

Electron energy puzzler resolved

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An ANU scientist has produced a model for electron energies in distant gas clouds, based on measurements taken within the solar system – and along the way may have resolved a long-standing controversy in astrophyics.

For many years, an inconsistency listed under “remaining to be explained” has existed in astrophysics: our measurements of the temperature of distant gas clouds didn’t quite gel with measurements of the element abundances in the same objects.

David Nicholls, a PhD candidate at ANU’s Research School of Astronomy and Astrophysics, told The Register the problem arises because of a long-standing assumption that electrons in the gas clouds would, over eons, had plenty of time to settle into a state of maximum entropy.

Nicholls explained to The Register that this led to the assumption that “the distribution of low, medium and high energy electrons has reached a stable state and can be described by a temperature.

“The problem that this assumption led to is that measuring temperatures and element abundances in distant gas clouds using different techniques gave conflicting answers,” he told The Register.

Closer to home, however, electron energy measurements turn up a different result: direct measurements taken around the solar system by space probes and satellites show more higher-energy electrons than exist at equilibrium. Nicholls explained that this is no surprise, since there are plenty of ways for energy to be added into the system.

The surprise, however, is this: the solar system’s energy distribution has turned out to offer a way to predict the electron energy distribution in remote gas clouds surrounding dwarf galaxies – helping to resolve the discrepancy that had puzzled astrophysicists since at least 1967, when the Mexican astronomer Manuel Peimbert noted an inconsistency between temperature measurements and spectrographic measurements of metalicity.

“I found that when I applied the solar system energy distribution to the distant hydrogen clouds, the results for element abundances and temperatures suddenly worked out consistently. This was not an obvious assumption, and flies in the face of orthodoxy,” Nicholls noted.

He noted, however, that the orthodox expectation was perfectly reasonable: in the absence of any means to directly measure the electron distributions (since the dwarf galaxies are too far away for probes), astrophysics took what we know of thermodynamics as the basis of their predictions.

Nicholls now believes the dwarf galaxies may never settle into equilibrium: “there are other forms of energy that get injected into the system, and these are very likely to keep the systems off-balance indefinitely,” he told El Reg.

As can so often be the case, electron energies weren’t the core focus of Nicholls’ work. His chief interest is in looking at what dwarf galaxies can tell us about the development of the universe since the Big Bang.

In particular, as noted in this ANU post, the slow-motion development of distant dwarf galaxies gives us a “laboratory” that helps us understand the formation of the heavier elements we see around us on later-generation systems like our own.

“I wasn't looking to solve all the problems the idea turns out to solve, I was just trying to identify the cause of a few ‘wrong answers’,” Nicholls said.

The research is published in the Astrophysical Journal, and for those who would like an overview, a conference presentation by Nichols is here. ®

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