Observations from the NASA/ESA Hubble Space Telescope have hinted that spiral galaxies might transform into jellyfish galaxies before they evolve into elliptical galaxies. The new theory proposes that the interaction with a companion galaxy in the early Universe could set the stage for this odd transformation.

Elliptical galaxies are among the most common types of galaxies in today’s Universe, and are characterised by their smooth, featureless appearance. They seem to have used up all the gas and dust from which new stars could be formed, and now only host old stars.

Spiral galaxies, on the other hand, are characterised by their beautiful spiral arms and actively form stars along these arms, giving them a bright and blue appearance.

As the Universe evolves, astronomers think that many spiral galaxies will eventually transform into elliptical galaxies. The most favoured theory proposes that this happens as the result of a process known as “galaxy harassment”. This refers to a continual gravitational jostling between two galaxies that do not merge, but pass each other at high speed. This ongoing process gradually strips the spiral galaxy of all its gas, which fuels star formation, and eventually transforms it into a featureless elliptical galaxy.

Now, a team of astronomers led by Francesco Ferraro from the University of Bologna, Italy, have imaged jellyfish galaxies from the early Universe. These are a rare and poorly understood type of galaxy that look like jellyfish: a bright galaxy centre with long gaseous filaments trailing behind it.

Using images from Hubble’s Advanced Camera for Surveys and Wide Field Camera 3, the team investigated the properties of four jellyfish galaxies. The analysis revealed that these galaxies are mostly found in regions of the early Universe where galaxy mergers and interactions are common, and that almost all jellyfish galaxies have a massive elliptical companion.

The astronomers propose that an encounter with a massive elliptical galaxy could heat up the gas in a spiral galaxy, causing it to suddenly expand or even flow out of the galaxy. This expulsion could lead to the characteristic shape of a jellyfish galaxy and would stop star formation in the spiral arms, eventually leading to the transformation into an elliptical galaxy.

Although this theory needs further investigation and data to be fully confirmed, it is the first attempt to explain the birth and evolution of jellyfish galaxies.