The role of interstellar gas in the evolution of galaxies

A paper by Brazilian and Spanish astrophysicists published in Monthly Notices of the Royal Astronomical Society sets out to detail gas infall.

By Peter Moon  |  Agência FAPESP – The evolution of galaxies is one of the most fascinating fields of cosmology, in which scientists strive to understand how primordial gas clouds in the newborn universe condensed to form stars and galaxies and how these then evolved into magnificent spirals like the Milky Way.

A paper by Brazilian and Spanish astrophysicists published in Monthly Notices of the Royal Astronomical Society sets out to detail gas infall, the process whereby interstellar gas falls for billions of years from the external regions of a spiral disk toward the galactic nucleus, attracted by its immense gravitational force. 

The rate at which this interstellar gas falls in time and space is key to understanding how stars are formed, since they are initially made up of gas, and the more gas falls through the disk, the more stars are formed and the brighter the galaxy becomes. 

But there is a problem. Astronomers study galactic evolution in observatories but, with rare exceptions, today’s technology does not permit the observation of galaxies from when the universe was young – about half its present age, which is approximately 13.8 billion years. 

“The image is weak and diffuse, with low resolution. This is a difficulty because we know that the first half of the universe’s life was the most dynamic period in the evolution of galaxies,” said Oscar Cavichia, a professor at the Federal University of Itajubá’s Physics & Chemistry Institute (IFQ-UNIFEI) in Minas Gerais, Brazil, and one of the authors of the paper. Cavichia received scholarships from FAPESP to support his master’s, PhD, and postdoctoral research.

To try to find out what galaxies were like when they were young, the researchers used the Alphacrucis supercomputer cluster at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP).

Alphacrucis is one of the largest computer clusters dedicated solely to research in astronomy, with 192 servers containing 2,304 processor cores. Unveiled in 2012, it was acquired with FAPESP’s support.

“We performed simulations with 144 different gas infall models, which varied according to galaxy mass and size, for example,” Cavichia said. “The computational power available from Alphacrucis enabled us to run all the simulations at the same time instead of separately, which saved a lot of time and got the job done relatively quickly.”

Spiral galaxies of three sizes were simulated: medium, like the nearby Triangulum galaxy (M33), which has 40 billion stars; large, like the Milky Way, with 400 billion stars; and gigantic, like the Andromeda galaxy (M31), the closest spiral galaxy to the Milky Way, with 1 trillion stars.

The simulations were designed to estimate gas infall rates in these three galaxy types at the time of their initial formation, when the universe was 1 billion years old (redshift 6), and again at 1.5 billion years (redshift 4), 3 billion years (redshift 2), 6 billion years (redshift 1), and 9 billion years (redshift 0.5). 

Light from other galaxies is shifted toward the red end of the spectrum as they move away from us, meaning that the wavelengths of the light get longer: this is called redshift. A distant galaxy’s redshift is measured by comparing its spectrum with a reference scale.

The researchers also analyzed variations in gas infall rate according to distance from the galactic nucleus, assuming that gravity causes gas to accelerate as it approaches the nucleus.

“Our hypothesis was that more massive galaxies formed faster than less massive galaxies because the greater a galaxy’s mass is, the greater is its gravitational force,” Cavichia said.

“Similarly, our hypothesis suggested that gas should fall faster in the inner regions of a galaxy than in its outer regions.” 

Chemical elements

The results of the simulations met the researchers’ expectations, albeit with one surprise. “Gas infall rates are more or less constant except in central regions,” Cavichia said. 

Matching the initial hypothesis, the closer the gas gets to the galactic nucleus, the faster it falls, and infall rates are slower in less massive galaxies.

But this does not mean that medium-size galaxies form more slowly than large ones or that large ones form less rapidly than huge ones. “The simulations showed that all galaxies, whether giants or not, capture gas at a very similar rate over time,” Cavichia said.

Most of the interstellar gas available for the formation of new stars must have already fallen once the universe reached 9 billion years of age, a finding that matches astronomical observations.

The researchers are currently studying the chemical abundance of elements such as oxygen in the disks of the simulated galaxies. They want to determine the amount of each chemical element in the gas present in the disks once they have formed and whether the observed similarity in gas infall rates for galaxies of different masses has any effect on the distribution of chemical elements in galaxies over time.

The article “The role of gas infall in the evolution of disc galaxies” (doi: 10.1093/mnras/stw1723) by Mercedes Mollá, Ángeles I. Díaz, Brad K. Gibson, Oscar Cavichia and Ángel-R. López-Sánchez can be read by subscribers at mnras.oxfordjournals.org/content/early/2016/07/18/mnras.stw1723?related-urls=yes&legid=mnras;stw1723v1.