Astronomers can use the form and dimension of their disk to foretell when a galaxy’s star formation will finish

The main business of a galaxy is star formation. And when they are young, like youth everywhere, they deal with it. But galaxies age, evolve, and slow down their star formation rate. After all, galaxies no longer form new stars, and astronomers call this erasure. You’ve studied extinguishing for decades, but much of it remains a mystery.

A new study based on the IllustrisTNG simulations has found a link between the erasure of a galaxy and its star size.

About 10 billion years ago, the universe was in what cosmologists call “cosmic noon”. At this point in time, star formation in galaxies reached its peak. How and why galaxies have not formed stars since then is a mystery.

In a new article entitled “MOSEL and IllustrisTNG: Delete massive extended galaxies at z = 2 later than normal sized galaxies”, a research team wanted to investigate the deletion. The lead author of the study is Dr. Anshu Gupta from the Australian ARC competence center for all-sky astrophysics in 3 dimensions (ASTRO 3D). The paper is published in the Astrophysical Journal.

“There is a time in the life of the universe known as ‘cosmic noon’ that occurred about 10 billion years ago,” said Dr. Gupta in a press release. “At that time, star formation in massive galaxies was at its peak. After that, the gas in most of these galaxies got hot – in part because of the black holes in the center – and they stopped forming stars. “

Photo credit: Gupta et al., 2021.

At cosmic noon, galaxies also developed the properties we see today: for example, regularly rotating disks and bulges. That was also when a population of dead galaxies or extinguished galaxies appeared. Something was going on.

Cosmic noon was not just a time of star formation. It was also a time of the black hole’s highest accretion. As the black holes in the center of the galaxy became massive, they pulled the galaxy’s gas towards them, compressing and heating the gas. But stars need cold gas to form. hot gas refuses to merge and collapse into a star.

However, this compression and warming effect did not dominate all galaxies. For a swollen, less dense galaxy with more spacing between the stars, the black holes didn’t have the same effect. They couldn’t get enough gas to quench star formation.

“However, in galaxies that are really, really big, what we found was that things didn’t heat up as much and the black holes weren’t that big of an impact that stars kept forming over a long period of time.”

The research team focused on what is known as the galactic disk. The galactic disk is a flattened circular region that surrounds the core and contains stars, gas, and dust. If this disc is not compact but spread out, star formation persists and quenching is delayed.

“Where the stars are widespread in the disk – you could call them ‘puffy’ – the gas stays cooler, so it continues to merge under gravity and form new stars,” said Dr. Gupta. “In galaxies with more compact disks, the gas heats up fairly quickly and is soon too energetic to crush together, so that star formation ends shortly after cosmic noon. Puffy disks run much longer, for example until cosmic afternoon tea. “

That number from the study helps explain the results. On the left are normal massive galaxies, on the right the expanded or “swollen” galaxies in the TNG simulation. As indicated in the upper bar, normal massive galaxies only changed their mean star size by z ~ 2.5. However, sprawling massive galaxies saw a steady increase in size between z ~ 2 and 4. Photo credits: Gupta et al., 2021.

Their study found that z = 1 erased only 36% of the extended massive galaxies, while 69% of the more normal massive galaxies were erased. With z = 2 to 4 they found that “… massive galaxies of normal size build their central stellar mass without their star size increasing significantly.” In the case of large, massive galaxies, their stellar mass almost doubled.

This research was based on both observations and simulations.

The IllustrisTNG simulations were an ambitious effort involving mostly German and American scientists. The IllustrisTNG website best describes the efforts: “Each simulation in IllustrisTNG develops a large part of a pseudo universe from shortly after the Big Bang to the present day, taking into account a multitude of physical processes that drive galaxy formation. The simulations can be used to study a wide range of topics dealing with the evolution of the universe – and the galaxies it contains – over time. “

What do these results mean? “Based on the results, we were able to establish a connection between hard drive size and star formation for the first time. Now astronomers can look at every galaxy in the universe and predict exactly when it will stop forming stars – shortly after lunch or later in the cosmic afternoon. “

The Milky Way is the home of humanity. Where does our galaxy fit into all of this? As it turns out, the Milky Way is a late bloomer. It was cosmic noon here, but it was still very small and decidedly not massive. At that point it was only a tenth of the stellar mass it now has. Thanks to mergers, it’s gotten more massive over time. Now it’s a huge galaxy, but it’s still making stars.

Stand next to the Milky Way. Photo credit: P. Horálek / ESO

Where are we now in the cosmic-galactic daily timer? “Cosmic noon was a long time ago,” said Dr. Gupta. “I would say that the universe has now reached the cosmic evening. It’s not night yet, but things have definitely slowed down. “

The team behind this study integrated the IllustrisTNG simulations with observations from the MOSEL (Multi-Object Spectroscopic Emission Line) survey. MOSEL relied on the Hubble Space Telescope and the WM Keck Observatory. The team consisted of scientists from Great Britain, Germany, Mexico, the USA and Australia.

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