Most of the matter in any given galaxy is dark matter. But the rest of the matter—called baryonic matter—can be significant to our understanding of the Universe as well, despite it being the vast minority of stuff out there. Baryonic matter might be familiar to you—you’re made of it, as are the Earth, the planets, and the stars. Indeed, baryonic matter makes up everything in the Universe we can directly see.

Predictions hold that most of the baryonic matter in the Universe should be in the form of stars, close to the centers of their galaxies. But observations show a lot of molecular gas near many galaxies. How did it get there? According to some models, the gas should have remained inside the galaxies and formed into stars.

A new observation of the distant galaxy SDSS J0905157 reveals that a huge amount of gas is being expelled from the galaxy—up to a third of its total is being shed. This is a shocking contrast with local galaxies, where outflowing gas makes up only a few percent of their reservoir. J0905‘s gas flow, racing at a whopping 2,500 kilometers per second, is also one of the fastest known of any star-forming galaxy (more mundane galaxies typically reach 200-500km/s). As a result, SDSS J0905157 is shooting the gas farther out into space than has ever been observed.

The team’s measurements are more precise than previous ones of similar galactic outflows, allowing many characteristics of the outflow to be determined, providing insight into the mechanism that could be producing it.

Feedback

Theorists have long speculated that there is some sort of feedback mechanism, in which the creation of stars somehow limits the formation of further ones. In smaller galaxies, one such feedback mechanism is supernova explosions. When a massive star explodes, it can give the surrounding gas enough energy to overcome the galaxy’s gravitational attraction and escape.

But that doesn’t seem to be the case with J0905—for one thing, there just wouldn't be enough powerful supernovas to eject gas at such a high rate. For another thing, supernova explosions are energetic events that would break down the molecular gas found in these clouds.

Another possible explanation for the incredible outflow is the supermassive black hole at the center of every galaxy. As matter accretes onto (falls into) the supermassive black hole, it releases a huge amount of energy, which could be accelerating gas enough to eject it from its galaxy. But based on the new observations, J0905 doesn’t seem to have a central black hole massive enough to create these outflows.

Radiation pressure

If supernovae and supermassive black holes don’t seem to be responsible for the incredible outflow, what is? A hint comes from a key correlation: the gas leaving the galaxy in a single year has a total mass of a hundred times that of the Sun. That roughly corresponds to the rate at which the galaxy is forming stars.

J0905 is very compact, and its star-forming regions are also compact. Half of the galaxy’s star formation rate comes from a region only 100 parsecs across (the Milky Way’s diameter is 20,000 parsecs).

Because so many stars are forming in such a tight space, they’re producing an incredible amount of radiation. And while it may not feel like it when you turn on your desk lamp, light puts pressure on the matter it strikes. This radiation pressure becomes an intensely powerful force when so much radiation is emitted in such a tight area. So if, say, it were to meet some molecular gas, it could impart a significant acceleration to the gas.

This mechanism has the advantage that, unlike supernovae, it wouldn’t break apart the molecules in the molecular gas. While radiation pressure has been considered before as the mechanism driving these outflows, it has only begun to gain serious consideration and to be included in models. The researchers behind the new work, however, calculated that star formation is sufficient to generate enough radiation pressure from star formation in J0905 to account for the extreme ejection of gas.

Limits

This mechanism puts a strict limit on the rate of star formation. As gas condenses, it begins to form stars, so the rate of star formation tends to rise—until it reaches what's known as the Eddington limit. As the star-formation rate increases, so does the radiation pressure from the star-forming regions. Ultimately, at the Eddington limit, the star formation rate and the radiation pressure balance out.

Black holes also experience an Eddington limit because the hot, accreting matter creates radiation pressure that pushes against the inflating material. The more infalling matter, the more radiation, which prevents extra matter from falling in.

In the case of J0905, the radiation pressure means that forming new stars comes at a significant price. If it continues expelling its gas at the same rate, it will be completely depleted in just 10 million years—and without gas, the galaxy by then will have seen its last stars form.

Of course, it’s still possible that J0905‘s black hole had more influence in the past than now, in which case it might be responsible for some of the intense outflow. Nonetheless, future work could test this by building a statistical sample of galaxies like J0905, to see if radiation pressure is the dominant mechanism most of the time. And until then, the radiation pressure is the simplest, most elegant explanation.

Nature, 2014. DOI: 10.1038/nature14012 (About DOIs)