The Tumultuous History of Gas in Distant Galaxies

FCAD Scholars Spotlight

The Tumultuous History of Gas in Distant Galaxies

The early universe may operate under different rules than the modern-day universe. In her research that has been submitted to the Astrophysical Journal, Yingjie Cheng, a graduate student from UMass Amherst, investigates why we see correlations between a galaxy’s evolutionary stage and its’ distribution of gas and metals in galaxies around the Milky Way but not for very distant galaxies.

By Moiz Khalil | March 8, 2024

During Cosmic Noon, when the universe was only a mere “child”, at the age of 2-3 billion years old, galaxies were extremely active and formed roughly half of their current stellar mass. This time period is thus essential for studying the mechanisms of star formation. Gas forms stars, and gas is constantly being shifted around in space due to inflows/outflows, caused by stellar winds, active black holes, and the interactions between galaxies merging into one another. There are two scenarios for the location of gas/metals in a galaxy: (1) gas/metals are centrally located, with less material towards the outskirts of the galaxy (negative gas phase metallicity gradient), or (2) the gas/metals are shaped like a donut, with the majority residing towards the edges of a galaxy and with noticeably less at the center (positive gas phase metallicity gradient). A negative gas phase metallicity gradient implies that the galaxy formed “inside-out” as the gas that forms stars starts at the center of the galaxy and then spreads outward. Since star formation leads to metal enrichment, the core of a galaxy is enriched before its edges which leads to this metallicity gradient.

Gas phase metallicity gradients have been studied extensively in the local universe and it is known that disk-like galaxies in the local universe tend to favor the first scenario with negative gradients. The gas phase metallicity gradient is also known to have strong correlations with other basic properties of galaxies such as mass and morphology in the local universe. Naturally, astronomers expect to see the same correlations for very distant galaxies during Cosmic Noon, however UMass Amherst graduate student Yingjie Cheng finds that this is not the case.

Under the guidance of Prof. Mauro Giavalisco (UMass Amherst), Yingjie Cheng, a 5th year PhD candidate at UMass Amherst, and her collaborators are investigating how the gas phase metallicity gradient correlates with other galactic properties in the distant universe. Using a sample of ~240 distant galaxies observed by the Hubble Space Telescope, Yingjie and collaborators calculated the star formation rates of each of these galaxies at different epochs in the universe which allowed them to reconstruct the past history of star formation for each galaxy. A star formation history essentially shows how actively a galaxy forms stars throughout its lifetime. Researchers have previously calculated the gas phase metallicity gradients of these galaxies using observations from the CLEAR survey, a sky survey imaging extragalactic objects, and when Yingjie and collaborators compared the star formation history to other basic properties of galaxies, like mass and color, they found very weak correlations.

Yingjie and collaborators decided to introduce a new parameter to describe how evolved a galaxy is, dubbed evolutionary time (t_e in technical terms), which would serve as a proxy for the poorly constrained star formation history, as evolutionary time requires far less assumptions to be made. Yingjie recalls the pivotal moment when she found a correlation between evolutionary time and gas phase metallicity gradient, “we find a strong correlation with t_e [and gas phase metallicity gradient] which is quite encouraging” as she finally found a substantial correlation after so many null results. When looking for correlations between evolutionary time and metallicity gradient, they find that as a galaxy evolves, its metallicity gradient goes from positive to negative. In other words, as a galaxy evolves, gas from the outskirts of a galaxy travels inwards towards the galaxy's center. They believe that this is due to gaseous interactions such as winds, accretion, and merging events, which could easily pull gas from the outskirts, where it is not as strongly gravitationally bound, towards the center of a galaxy (see Figure 1).

Illustration depicting how gas is constantly being recycled and transported throughout a galaxy due to inflows and outflows
Figure 1: Pictured above is an illustration depicting how gas is constantly being recycled and transported throughout a galaxy due to inflows and outflows. This gas comes from stars that have exploded and expelled their material into the surrounding intergalactic medium which will create the next generation of stars in a continuous cycle. (Credit: Tsinghua University)

Many of the galaxies in their sample have also been observed by the James Webb Space Telescope and Yingjie plans to use these observations to better constrain the correlations she found. Even without the Webb data, Yingjie’s observed correlations show that in the early universe the intricate dynamics of gas disturbs the distribution of metals in galaxies such that the metallicity gradients may only reflect the state of the galaxy at the time it was observed. These correlations and lack thereof will also help improve our models of galaxies and how they evolve with time.

Yingjie Cheng
Figure 2: Pictured above is Yingjie Cheng, a 5th year graduate student at UMass Amherst who studies the histories and evolution of galaxies.

About the Author:

Moiz Khalil is a senior undergraduate studying astronomy at UMass Amherst. He currently studies the atmospheres and colors of brown dwarfs and wants to pursue a career in science communication. Outside of science, he enjoys listening to music and reading novels.