The limits to galactic growth

In the system NGC 253, astronomers observe outflows of matter curtailing the birth of stars

Astronomers have long assumed that when a galaxy produces too many stars too quickly, it greatly reduces its capacity for producing stars in the future. Now, a group of astronomers that includes Fabian Walter from the Max Planck Institute for Astronomy were able to obtain the first detailed images of this type of self-limiting galactic behaviour: an outflow of molecular gas, the raw material needed for star formation that is coming from star-forming regions in the Sculptor Galaxy (NGC 253). The observations were made with the newly commissioned telescope array ALMA in Chile.

Galaxies – systems that contain up to hundreds of billions of stars, like our own Milky Way galaxy – are the basic building blocks of the cosmos. One ambitious goal of contemporary astronomy is to understand the way that galaxies evolve from the first proto-galaxies shortly after the big bang to the present. A key question concerns star formation: what determines the number of new stars that will form in a galaxy?

A key ingredient of current models of galaxy evolution are mechanisms by which ongoing star formation can actually inhibit future star formation: When new stars are formed, a certain fraction of them are very massive. Massive stars shine brightly, and their intense radiation drives "stellar winds", outflows of gas and plasma that can be sufficiently strong to push gas out of the galaxy altogether. Also, massive stars end their comparatively brief lives in spectacular explosions (supernovae), flinging their outer shells – and any additional material that might be in their way – out into space. Consequently intensive star formation, known as a "starburst", and the resulting formation of many massive stars, can hamper the growth of future generations of stars. After all, molecular gas that has been flung out of a galaxy cannot serve as raw material from which to fashion that galaxy's new stars. There is a limit to galactic growth.

So far, so good – but what was missing was direct observational evidence for starbursts producing outflows of molecular gas. Until now, that is, when a team of astronomers led by Alberto Bolatto from the University of Maryland at College Park observed the starburst galaxy NGC 253.

NGC 253, also known as the "Sculptor Galaxy", is a spiral galaxy located in the constellation Sculptor in the Southern sky. With a distance of 11 million light-years it is one of our closer intergalactic neighbours and the closest starburst galaxy visible from the southern hemisphere. Using the compound telescope ALMA the astronomers targeted the central regions of NGC 253, where the most intense production of new stars takes place, and found a telltale outflow of molecular gas at right angles to the galactic disk.

Bolatto, who is the lead author of the study now appearing in the journal Nature, concludes: "The amount of gas we measure gives us very good evidence that some growing galaxies spew out more gas than they take in." Indeed, the astronomers estimate that each year the galaxy ejects gas with a total mass of nine times that of our Sun. This ejected mass is about three times larger than the total mass of all stars produced by NGC 253 each year (which, in turn, is several times larger than the mass of all stars produced in our home galaxy, the Milky Way, each year).

Fabian Walter from the Max Planck Institute for Astronomy, a co-author of the study, adds: "For me, this is a prime example of how new instruments shape the future of astronomy. We have been studying the starburst region of NGC 253 and other nearby starburst galaxies for almost ten years. But before ALMA, we had no chance to see such details." The study used an early configuration of ALMA with only 16 antennas. "It's exciting to think what the complete ALMA with 66 antennas will show for this kind of outflow!" adds Walter.

MP / HOR