Ghosts of quasars found by the Hubble Space Telescope

Images of eight galaxies containing green filaments that are the last effect of ancient quasars (Image NASA, ESA, and W. Keel (University of Alabama, Tuscaloosa))
Images of eight galaxies containing green filaments that are the last effect of ancient quasars (Image NASA, ESA, and W. Keel (University of Alabama, Tuscaloosa))

The Hubble Space Telescope photographed a series of ghosts of quasars that existed in the past. They are seen as ethereal green objects in various forms and are the last effects of ancient quasars. These phenomena are very interesting from a scientific standpoint because they can provide information about the past of those galaxies, which were once very active.

In the galaxies identified by the Hubble Space Telescope, in the past there was a quasar, a very active core in which materials were attracted by the supermassive black hole at its center and were heated so much as to create a very bright disk. Quasars are typical of young galaxies because there are many clouds of dust and gas that can be drawn by the supermassive black hole. When those materials are consumed the quasar fades away.

In certain cases, a quasar can leave a kind of ghost making filaments in deep space glow due to a process called photoionization. Elements such as oxygen, helium, nitrogen, sulfur and neon contained in the filaments absorb the light from quasars and release it at a relatively slow pace. We’re talking about astronomical times so the emissions continue for thousands of years.

The emerald green hue is a typical emission of ionized oxygen. These structures are so far from the core of their galaxies that it took tens of thousands of years for the light from quasars to reach them. This is another reason why they keep on glowing so long after the quasar exhausted.

This type of filaments was discovered for the first time in 2007 during the online project Galaxy Zoo. It’s an example of crowdsourcing in which the public was asked to collaborate in the classification of more than a million galaxies catalogued in the Sloan Digital Sky Survey (SDSS).

The Dutch schoolteacher Hanny van Arkel discovered green filaments near the galaxy known as IC 2497. The strange formation was named after her Hanny’s Voorwerp, the Dutch for Hanny’s object. It raised the interest of astronomers, particularly Bill Keel of the University of Alabama, Tuscaloosa, who started searching for others.

Using various telescopes, 20 galaxies were found with ionized gas from quasars. Eight of these gas clouds showed to contain more energy than expected. In Bill Keel’s opinion, it’s possible that it was originated by two massive blacks holes orbiting each other in the galaxy core. This can happen after two galaxies merge.

If this theory is right, in a few billion years, green filaments might also glow in the giant galaxy that will result from the merger between the Milky Way and Andromeda.

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