Why Galaxies Delay Star Birth

Two teams have independently pinpointed the same key player in postponing galaxy growth.

You might think that the smaller the galaxy, the earlier it grows to its full size. Some computer simulations have suggested as much: a small galaxy, like the Large Magellanic Cloud or even the Milky Way itself, beefs up its stellar population within the universe’s first few billion years.

The LMC
The Large Magellanic Cloud, about 165,000 light-years distant, is dimly visible to the naked eye in the south circumpolar sky. It contains several sites of furiously ongoing star formation.
Akira Fujii
But observations show that that's not true: galactic runts grow slowly. The less massive the galaxy, the later it assembles the bulk of its stellar mass — meaning that today, the less massive galaxies are more actively forming stars than the more massive ones.

The earliest attempts to recreate galaxy growth in a computer put some dark matter and gas together, allowed gravity to collapse the stuff into a galaxy, and turned on star formation when the gas cooled and condensed to a certain point. (Stars can only form from cool, dense gas.)

“Those models are basically a disaster,” says Philip Hopkins (Caltech and University of California, Berkeley). “They just tell you that everything should turn into stars.”

The simulations revealed that star formation is far less “efficient” in real life, meaning galaxies don’t form stars willy-nilly. Something keeps galaxies from eating through their gas reservoirs like a child gobbling down his Halloween candy in one sitting. So astronomers added supernovae, which heat and blow out vast amounts of gas from galaxies. But these simulations still suggest less massive galaxies should form most of their stars in the universe’s first 3 billion years or so. Observations show that more than half of the stars form after 6 billion years, during the latter half of the universe’s history.

Work by both Hopkins’ team and by Sebastian Trujillo-Gómez (New Mexico State University) and colleagues shows that the secret behind this delayed growth lies with a galaxy’s youngest, most massive stars. Massive stars (tens to hundreds of solar masses) pump out a lot of radiation. This radiation heats and expels gas, much as supernovae do. The lack of cold, dense gas limits the number of stars created.

galaxy and its gas
This still image from simulations by Philip Hopkins and his colleagues show the various types of gas in their computerized galaxy: pink is cold gas, which forms stars; green is warm, ionized gas (most of which cools and builds up the galaxy); and red is hot gas in the galactic halo.
Philip Hopkins / Caltech
But these massive stars only live a few million years. Once they’re dead, the gas they’ve blown out into the galaxy’s halo can cool and rain back into the inner galaxy (this takes a few hundred million to a couple of billion years), fueling a new round of star formation. Then the whole process — young stars, heating, stellar death, cooling — repeats itself. As a result, star formation in less massive galaxies proceeds in fits and spurts, stretching the gas consumption out much longer than would otherwise be possible.

Trujillo-Gómez’s team came to this conclusion by zooming in on the growth of a single dwarf galaxy and spiral galaxy in cosmological simulations. They basically started with assumptions of how supernovae and radiation pressure would affect star formation, then added these effects together in different ways to see what would happen. They found that radiation pressure kept gas in a warm, diffuse state, reducing the star-formation rate at the 3-billion-year age mark to a hundredth of what supernovae-only calculations predicted. With radiation pressure, the galaxies formed more than 80% of their stellar mass in the last 7 or 8 billion years.

Hopkins’ team took a more nuts-and-bolts approach, starting from the ground up. Instead of fiddling with different proportions, they started from the fundamental physics involved in various types of stellar feedback — including supernovae and radiation pressure — and threw these into the computational pot, stirred it all together, and “baked” it (i.e. ran the simulations) to see what came out. The result also showed that radiative feedback from young, massive stars was key in reproducing what observers see in the cosmos.

“It’s a pretty incredible achievement to understand the role of stellar feedback, which seems to explain why all galaxies equal to or less massive than the Milky Way are at least an order of magnitude less massive than our most naïve models (ignoring stellar feedback) would predict,” Hopkins sums up.

The results don’t apply to the most massive galaxies (with 10 times or more the mass of our galaxy). With so much mass, these leviathans have a stronger gravitational grip on their gas, making radiation pressure less effective at moving it. Instead, Hopkins suspects radiation from material being swallowed by the galaxies’ central supermassive black holes is the main controller of stellar growth in such cases.

Trujillo-Gómez notes that other work by his colleagues explores what the gas around a dwarf galaxy would look like if radiation pressure shoved it there. The results will hopefully help astronomers understand how galaxies wind up with the distribution of materials that they do.

References:

Sebastian Trujillo-Gómez et al. "Low-mass galaxy assembly in simulations: regulation of early star formation by radiation from massive stars." Also presented January 8th at the American Astronomical Society meeting, abstract #310.06D.

Philip F. Hopkins et al. "Galaxies on FIRE (Feedback In Realistic Environments): Stellar Feedback Explains Cosmologically Inefficient Star Formation."

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