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Galaxy collision
fires up quasar

Posted: 25 June 2010

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Using the Very Large Telescope (VLT) in Chile and the Gran Telescopio Canarias (GTC) on La Palma, a team of astronomers have found evidence for a high luminosity quasar powered by the collision of two galaxies.

In this artist impression a black hole resides at the centre with a disc of hot material surrounding it (white, green and blue). An outer torus (red) is seen edge-on, blocking the bright emission from the central disc but allowing the quasar environment to be studied in more detail. Image: NASA/CXC/M.Weiss.

Quasars are the most luminous objects known in the Universe, and reside in the centre of so-called active galaxies. Emitting vast quantities of energy from their central regions, it is widely believed that these galaxies contain a supermassive black hole. The black hole's overpowering gravity drags material into an accretion disc where it becomes extremely hot and emits the energy responsible for the quasar's brightness. Around this central engine is a thick torus through which visible light emitted from the accretion disc cannot be seen. However, from our terrestrial viewpoint, if the torus is face-on we will see the radiation, but if the torus is edge-on the radiation is obscured.

Astronomers call the face-on examples type 1 quasars, and the edge-on occurrences type 2. “Type 2 quasars are a family of still rather unknown galaxies, which so far have been investigated mostly from a statistical point of view,” explains team leader Montserrat Villar-Martin. “The goal of our work is to study their individual characteristics in detail. In our study we have obtained some surprising results. For example, we have observed a giant nebula of ionized gas associated with SDSS J0123+00, and signs of an interaction with a nearby galaxy.”

Using the GTC astronomers obtained this image revealing a bridge of material that connects the quasar with a neighbouring galaxy. Red shows regions that emit light produced by stars, green represents emission from hot, ionized gas and yellow a mixture of both. Image: Montserrat Villar Martin (IAA-CSIC).

Even though type 2 quasars are more difficult to detect, the dimming of the central radiation by the torus allows the quasar environment to be studied in greater detail. In the case of SDSS J0123+00, the discovery of the faint nebula of ionized gas around the galaxy, which covers an area six times larger than our own Milky Way Galaxy and in one location bridges a gap to a neighbouring galaxy, strengthens the idea that activity in galaxies is partly driven by the exchange of material between active galaxies. The accumulation of gas at the hub of the interaction would provide enough material to feed a black hole while perhaps also triggering the formation of new stars.

The study is the first based on images derived with the tuneable filter of the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) on the GTC and the results will be published in the Monthly Notices of the Royal Astronomical Society.

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