The galaxy NGC 4654 has a clearly lopsided silhouette. A recent image from the Hubble Space Telescope shows a rounded edge, a star-rich arm and a long gas trail extending beyond the observed field. This shape reveals the combined action of its environment and an ancient galactic encounter.
Located about 72 million light-years away, in the constellation Virgo, NGC 4654 belongs to the Virgo Cluster. This densely populated cluster also contains hot, low-density gas between its galaxies. NGC 4654 is moving through it at high speed.
This image from the NASA/ESA Hubble Space Telescope shows the spiral galaxy NGC 4654, located 72 million light-years away in the constellation Virgo.
Credit: NASA, ESA/Hubble, D. Thilker, J. Lee and the PHANGS-HST team
This motion produces ram pressure. The intergalactic gas compresses the front edge of the galaxy and then drags part of its own gas toward the rear. This mechanism, called ram-pressure stripping, explains the long gas tail observed on the opposite side.
This pressure alone, however, is not enough to account for the whole distortion. The stars are also distributed unevenly. The front-facing arm contains a great deal of gas and stars, while the opposite arm clearly lacks them.
Astronomers attribute this second asymmetry to a gravitational interaction with NGC 4639, another galaxy in the cluster. The two objects are now far apart. According to the interpretation presented by NASA and the European Space Agency, their close passage would have taken place about 500 million years ago.
This encounter would have stripped gas from one side of NGC 4654. Star formation there would then have decreased, leaving a less populated arm. The current shape would thus preserve the trace of an ancient event, added to the ongoing effects of the environment through which the galaxy is moving.
NGC 4654 still produces nearly two solar masses of stars per year. This rate remains comparable to that of galaxies of similar size. The pink regions visible in the image correspond to excited gas clouds, associated with young stars distributed in the disk and around the faint central bar.
The data come from two observation programs devoted to the links between gas and star birth. By comparing several nearby galaxies, researchers seek to better track the movements of gas, its collapse into stellar clouds and the effect of young stars on their surroundings. NGC 4654 offers a case where several forces have left signatures still visible today.