Scientists observe galaxy ‘stealing material’ to feed black hole

An artist's rendition of W2246-0526, the brightest-known galaxy that is cannibalising three neighbouring galaxies(Credit: (NRAO/AUI/NSF) S. Dagnello)

Interactions with companion galaxies recorded for the first time

For the first time, a distant, hugely-luminous galaxy has been observed stealing material from its neighbours to feed the black hole at its centre.

An international team of astronomers, including Professor Andrew Blain from the University of Leicester’s Department of Physics and Astronomy, announced the discovery in a paper published this week in Science.

The galaxy studied is designated W2246-0526, and holds the record as the most luminous galaxy known. At its centre is a super-massive black hole (SMBH), obscured from view by massive amounts of interstellar gas and dust, which is why this type of galaxy is known as a host, dust-obscured galaxy or Hot DOG.

Hot DOGs emit mostly infrared radiation, drawing energy from the accretion of material around the SMBH at their centre, but until now it was not known where that material originated. One hypothesis was that it was drawn from neighbouring galaxies, merging with the Hot DOG – but there was no direct evidence.

The team behind this new paper identified three small, nearby companion galaxies around W2246-0526, referred to as C1, C2 and C3, using data from the Atacama Large Millimeter/Sub-millimeter Array (ALMA) in Chile, the Karl G Jansky Very Large Array (VLA) in New Mexico and the Hubble Space Telescope. Furthermore, they found ‘bridges’ of dust-rich material connecting the central galaxy to C1 and C3 and a ‘tidal tail’ of material stretching to the more-distant C2. This system of galaxies, bridges and tail contains as much gas as W2246-0526 itself.

An image of the 20-arcsec square region around W2246 - about 1/100 of the diameter of the full moon. The black contour lines show the ALMA data, while the orange greyscale shows the Hubble Space Telescope image. C1, C2 and C3 are the companions to W2246 at the same distance, linked to it by the dusty bridges and tails, shown by the contours. K1, U1, U2 and U3 are other galaxies detected in the image, but which are not yet confirmed to be at the same distance as W2246. In the top left is the edge of a much closer galaxy. The scale is 10 kpc or about 33,000 light years, which is similar to the diameter of the Milky Way.

This evidence suggests that W2246-0526 is accreting its neighbours – or at least stripping a large proportion of their gas. Measurements indicate that this influx of material is sufficient to simultaneously feed and obscure the black hole at the centre of the galaxy, and to provide enough fuel for the formation of new stars at the currently observed rate.

Professor Blain said: “The ALMA images are important because after trying to understand extremely powerful galaxies like W2246 since we discovered them with the WISE satellite in 2010, we can now see some real details about how they get to boost their power to about 10,000 times that of our Milky Way.”

The paper, The Multiple Merger Assembly of a Hyper-Luminous Obscured Quasar at redshift 4.6, is a collaboration between astronomers in Leicester, Chile (Universidad Diego Portales), the USA, China, South Korea, France and Cambridge.

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