ScienceWise - Autumn 2013

Stardust

Studying dwarf galaxies leads to unexpected discovery

David Nicholls is a graduate student at the ANU Research School of Astronomy and Astrophysics who’s interested in galaxies and the way they change with time. “When we look at a big galaxy like our own Milky Way we’re seeing something that has been hugely mixed up and perturbed by everything from supernovae to collisions with other galaxies. So it’s hard to learn much about the early history of the Universe from it. You could imagine it being like a big city such as Beijing. If you walk through the centre you’d have no idea what it was like 1000 years ago because everything has been dug up and rebuilt so many times.”

Such changes in galaxies are not just in the position of their stars; the very nature of those stars depends on the gas they form from.  “The gas clouds form into stars rapidly in a big galaxy. This is due, among other things, to interactions with nearby galaxies, and the capture and digestion of smaller galaxies.  These processes lead to vast amounts of star formation, which stirs up additional hydrogen, initiating further star formation.” David says, “Added to that you have supernova explosions spreading heavier elements throughout the interstellar space which mixes into the nearby hydrogen clouds, from which the next generations of stars and planets form.”

This is just the situation with our own solar system. All that iron that makes up the core of the Earth was created by a long dead giant star that exploded and spread its ashes through the clouds of gas that would ultimately condense into the Sun and planets we know today.

But what would have happened if our sun had formed not in the midst of a huge spiral galaxy, but out in the boondocks of space?

“One of my particular areas of interest is isolated Dwarf galaxies.” David explains, “You can think of them like a little village. Things have changed over the years but most of the old buildings are still there so you can get a much better idea of what it used to be like in the past. If our sun had formed in such an ancient isolated dwarf galaxy then chances are there’d be very few heavy elements in the solar system which means no iron cored Earth and no humans!”

Astronomers use the term metallicity to describe the amount of elements heavier than helium in a gas cloud, even though not all of those elements are actually metals. From looking at the spectra of dwarf galaxies using the WiFeS spectroscope on the ANU 2.3m telescope at Siding Spring s David was able to see that it wasn’t just metallicity that was different in dwarf galaxies.

“We were looking at the spectra of the oxygen, nitrogen and sulphur atoms in the hydrogen clouds and trying to match our data to computer models”, David says, “We found that the fit between theory and observations was poor.” 

Having checked the instruments, the computer programs and the theory David concluded that one aspect of the models that we’ve been using for decades to describe the behaviour of hydrogen in galaxies was probably wrong!

“The models all assumed that the free electrons within these hydrogen clouds followed a Maxwellian distribution.” David says.

A Maxwellian distribution is a kind of bell curve of velocities that particles in motion settle into after several collisions. The air molecules in a room are endlessly colliding with each other, which leads their velocities to follow a Maxwellian distribution. Same is true for electrons in a solid. It’s what you would normally expect to happen. You’d also expect from the gas densities that electrons in hydrogen clouds of dwarf galaxies would follow those Maxwellian distributions too. But it looks like they don’t!

“Although these clouds of hydrogen should have come to equilibrium, you have to remember that they’re not just sitting quietly minding their own business,” David says, “They are constantly stirred up by high energetic electrons, some accelerated by magnetic fields and some expelled from interstellar dust grains by X-ray and far-ultraviolet photons. There are lots of ways the equilibrium can be upset by the processes going on inside the hydrogen clouds.” 

When a UV photon ionizes a hydrogen atom, much of its energy goes into getting the electron away from the electrostatic attraction of the positively charged nucleus so the resulting free electron isn’t nearly as energetic as it might be. This is the normal process, and is what makes the hydrogen clouds glow in the first place.  If this was all that was happening, you’d expect the electrons to settle into equilibrium, like the theory predicts.  It’s these other mechanisms that upset the equilibrium. And it’s these mechanisms that the conventional theory doesn’t take into account. 

“When you plug the numbers in, it turns out that you really don’t need a lot of very energetic electrons being injected into a hydrogen cloud to perturb the Maxwellian distribution,” David explains, “And you also don’t need that perturbation to be much to shift the spectra to what we’re observing.”

“One of the exciting things about science is that when you research one thing, you just never know what you might discover about another.”

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