ESO 137-001 is a perfect example of a spiral galaxy zipping through a crammed cluster of galaxies. Gas is being pulled from its disc in a process called ram pressure stripping. The galaxy appears to be losing gas as it plunges through the Norma galaxy cluster. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
Astronomers examined around 70,000 galaxies to address an important unsolved mystery in astrophysics. Galaxies come in 3 main shapes – elliptical, spiral (such as the Milky Way) and irregular. They can be massive or small. To add to this mix, galaxies can also be blue or red. Blue galaxies are still actively forming stars. Red ones mostly are not currently forming stars, and are considered passive.
The processes that cause galaxies to “quench,” ie cease star formation, are not well understood, however, and constitute an outstanding problem in the study of the evolution of galaxies. The team combed through available data from COSMOS UltraVISTA survey that give accurate distance estimates for galaxies over the past 11 billion years, and focused on effects of external and internal processes that influence star formation activity.
External mechanisms include drag generated from an infalling galaxy within a cluster of galaxies, which pulls gas away; multiple gravitational encounters with other galaxies and the dense surrounding environment, resulting in material being stripped away from the galaxy; and the halting of the supply of cold gas to the galaxy, thus strangling it of the material needed to produce new stars over a prolonged period of time.
Internal mechanisms include the presence of a black hole (in which jets, winds, or intense radiation heat up hydrogen gas in the galaxy or blow it out completely, thus preventing the gas from cooling and contracting to form stars) and “stellar outflow” (eg. high-velocity winds produced by massive young stars and supernovae that push the gas out of the host galaxy).
“By using the observable properties of the galaxies and sophisticated statistical methods, we show that, on average, external processes are only relevant to quenching galaxies during the last 8 billion years,” said Darvish. “On the other hand, internal processes are the dominant mechanism for shutting off star-formation before this time, and closer to the beginning of the universe.”
The finding gives astronomers an important clue towards understanding which process dominates quenching at various cosmic times. As astronomers detect quenched non-star-forming galaxies at different distances (and therefore times after the Big Bang), they now can more easily pinpoint what quenching mechanism was at work.
Prof Mobasher said, “We found that on average the external processes act in a relatively short time-scale, around 1 billion years, and can more efficiently quench galaxies that are more massive. Internal effects are more efficient in dense clusters of galaxies…A short time-scale suggests that we need to look for external physical processes that are fast in quenching. Another important result of the work is that internal and external processes do not act independently of each other in shutting-off the star formation.”
Next, the team will work on extending this study to the environment of galaxies on much larger scales (in the cosmic web). https://ucrtoday.ucr.edu/38460
Recent Comments