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Battle of the bulges: Have astronomers been wrong about spiral galaxies all this time?
Some big news from Galaxy Zoo: It's looking very much like a classic classification scheme for galaxies — first dreamed up by Edwin Hubble himself! — may be, um, wrong. The consequences of this are as dramatic as they are cosmic: It implies very strongly that the way we've been thinking about how galaxies form and maintain spiral arms is also wrong. Instead of being semi-permanent features of galaxies, they may instead actually wind themselves up, disappear, and reform again!
Wow. This is a very big deal.
Classification is a natural first step when you have a collection of objects and you're trying to figure them out. If you can sort them in some way, you can start to gain insight into them. Once astronomers started mounting cameras on big telescopes in the late 19th and early 20th centuries, they found that galaxies — which for the most part look like small fuzzy patches to the eye — have a staggering variety of shapes. But they fell into four broad categories: elliptical, spiral, irregular, and peculiar.
Spiral galaxies (the ones with spiral arms) had two major components: the arms themselves, and a central bulge of stars from which the arms appeared to emanate. Moreover, there appeared to be a spectrum of these galaxies' shapes, from small bulges to big ones. Galaxies with big central bulges had tightly wound spiral arms, while ones with smaller bulges had more open arms. Hubble and his team classified them this way, too, with the former called Sa and the latter Sc galaxies, with Sb galaxies intermediate between the two. Also, Sa galaxies tended to have well-formed continuous arms, and Sc ones have clumpier, less well-defined arms.
There was also a third structure, called a bar, which looks like a rectangular or lozenge-shaped feature across the middle of the galaxy. These are called barred spirals, and they appeared to have the same structures as regular spirals otherwise, so they were called Sba to Sbc.
The Hubble Tuning Fork diagram classifies galaxies according to shape and structure. Spiral galaxies go from big bulges and tight arms to small bulges with widely flung arms (with a parallel row for barred spirals). But new work shows this diagram may be obsolete. Credit: Karen Masters, Sloan Digital Sky Survey
Hubble created what was called the tuning fork diagram to describe all this. On one side were elliptical galaxies, which ranged from circular (called E0) to more stretched-out (up to E7). Those became the stem of the fork, with the two tines being the spirals from Sa to Sc on one and the barred spirals on the other.
There have been other ways to do this, but Hubble's tuning fork has been more or less the standard for a century now. It's done a decent job, but there have been some galaxies that don't fit the pattern. For example, one might have a small bulge but tight arms, or another with a large bulge and fairly open arms. If there's a galaxy that didn't quite fit, astronomers usually used the arm structure to place it in the diagram.
Now, though, new observations have thrown a monkey in the wrench with all this. Those galaxies aren't the exception. They're the rule.
NGC 1398, a barred spiral galaxy that is, quite simply, gorgeous. Credit: ESO
This comes from Galaxy Zoo, a "citizen science" project that lets the public make simple analyses of real data. For example, modern surveys of the sky return so much data it’s impossible for just a few astronomers to sit down and grind through it. However, it turns out that non-astronomers can do a pretty reliable job on certain analysis tasks, like, for example, looking at a picture of a spiral galaxy and determining whether the arms are tightly wound or more open. Studies have shown that if enough people go over the data, overall the results (when checked by professionals) are quite accurate. The work is actually fun, and only involves a simple web interface.
In this case, Galaxy Zoo presented people with data from the Sloan Digital Sky Survey. More than 250,000 large, bright galaxies were looked over by 160,000 people, who answered straightforward questions about them. Even the simplest results are important; for example, they found that 92% of the galaxies classified are spiral or elliptical, while the remaining 8% are irregular or peculiar. Right away that tells us something about big, bright galaxies!
But it’s the followups where things got interesting. When a subset of spiral galaxies (ones that had characteristics easier to identify) were classified according to their bulge size and spiral arm windiness (what astronomers call the pitch angle), they found almost no correlation between the two!
There's a weak correlation in that galaxies with big bulges tend to favor having more tightly wound arms, but that was by no means the rule. And galaxies with smaller bulges were found to have arms across the spectrum, from tightly wound to far-flung.
This flies in the face of the Hubble tuning fork diagram. And it gets better: They also found that galaxies with bars tend to have more open arms. This is telling us something important about how spiral galaxies form and evolve over time. But what?
The spectacular spiral galaxy M61, observed by the 8.2-meter Very Large Telescope. Credit: ESO
You might think spiral arms form because, due to gravity, stars close in to the center of the galaxy orbit faster than stars farther out, so a spiral pattern naturally appears. But there's a problem with this: Over time, this would wind up the arms, destroying the structure. The thinking was that we see so many galaxies with arms that this can't be the way things work.
So astronomers came up with an idea called the density wave hypothesis. Spiral arms, according to this, are more like traffic jams of stars and gas in a galaxy. A traffic jam on a road can persist for a long time, even though individual cars move in and out of them. Spiral arms were thought to be the cosmic equivalent, where spiral density waves form due to the complicated physics of the gravity of a galactic disk. Stars move in and out of them, but the wave itself persists. This way, arms don't wind up over time.
This is the basic paradigm astronomers use, and countless papers have been published on it … but these new results show this may not be correct. This density wave idea is based on the tuning fork diagram, where arms and bulges are strongly related; the physics of the density wave connects the spiral arms in some ways with the bulge size. But the Galaxy Zoo findings show something else is going on, since the arms don't seem to care overly much in general about the bulge size.
What the astronomers in charge of the project postulate is that spirals arms form due to some sort of perturbation in the galaxy disk (similar to how the density wave hypothesis says they form, too), but the pattern is not necessarily persistent. Instead, the arms can wind up, getting tighter over time. At some point the pattern gets destroyed, the arms disappear, and then the process starts up all over again.
This would explain the behavior they see in the data. Although they don't dive into this, they also postulate that a central bar somehow slows this process, since barred galaxies tend to have more open arms. The connection there isn't understood yet, though.
So how did Hubble miss all this? Simple: He and his team had a very limited sample of galaxies to look at. They simply didn't have enough samples to make strong conclusions. Modern surveys have thousands or even millions of galaxies in them, allowing far better statistical analysis.
It's funny to me; we've been using Hubble's diagram for all these decades, and the reason this has been missed for that time is, ironically, so much data! It took Galaxy Zoo and its huge throng of participants to be able to sift through that data, allocating small amounts to each person. Even computers don't do a perfect job, since they tend to be programmed with our own preconceived notions for classification. Citizen scientists don't labor under that burden, and are free to find new things.
Mind you, this is only the first step in a new direction. The idea that spiral arms wind up has been around in the astronomical community for a while, gaining popularity, but this is a sizable boost for it. Now the pros have to come in and work out the physics, and explain the observations in ways they hadn't done before.
That's exciting. What new things will they discover, processes that hadn't yet been considered that work on scales of tens and hundreds of thousands of light years?
As the authors point out, even after well over a century of studying spiral arms in galaxies, there are still basic things about them we don't understand. Hopefully this is a big step toward fixing that.
