The Hubble Space Telescope sees the ghost light of dead galaxies in Pandora’s Cluster

The galaxy cluster Abell 2744, also known as Pandora's Cluster. The starlight is colored blue (Image NASA/ESA/IAC/HFF Team, STScI)
The galaxy cluster Abell 2744, also known as Pandora’s Cluster. The starlight is colored blue (Image NASA/ESA/IAC/HFF Team, STScI)

The giant galaxy cluster Abell 2744, also known as Pandora’s Cluster, was already studied in the past, also for its interesting effect as a gravitational lensing. The various observations made using the Hubble Space Telescope allowed to see the “ghost light” of galaxies died after some clusters collided with each other.

The Pandora’s Cluster includes nearly five hundred galaxies for a total mass estimated to be about four trillion solar masses and it’s about four billion light years from Earth. When, in the course of some six billion years, some galaxy clusters collided, many stars were ejected from their galaxies by a combination of gravitational forces that invested them, so much that some galaxies were destroyed.

Models created with computers suggest that galaxies the size of the Milky Way are the likely candidates for the origin of the “orphan” stars. Some galaxies were torn apart when they ended up in the middle of a galaxy cluster, where the gravitational tidal forces are the strongest.

Astronomers have been speculated for some time that it should be possible to detect the light emitted by the “orphan” stars scattered here and there. However, this light is extremely dim so it’s difficult to detect. The cluster Abell 2744 has been studied using a variety of telescopes within the “Frontier Fields” for its gravitational lensing effect. This was a lucky case because the Hubble Space Telescope is extremely sensitive to even such a dim infrared light.

The results were very positive showing the combined light of about two hundred billion “orphan” stars. They account for approximately 10% of the brightness of the cluster Abell 2744 but their light is spread over a wide area. The light analysis determined that those stars are rich in heavy elements such as oxygen, carbon and nitrogen. This means that they are second- or third-generation stars that contain elements produced by first-generation stars.

The results of these observations confirm predictions about what happens inside massive galaxy clusters. It’s a good thing and the data collected will allow further progress in the understanding of the evolution of galaxy clusters.

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