Puzzle of the “X” Galaxy Phenomenon

Mystery of the "X" Galaxy Phenomenon


The galaxy PKS 2014-55, located 800 million light years from Earth, is classified as ‘X-shaped’ because it appears to have four jets forming an ‘X’ as opposed to more common galaxies such as the Milky Way with their enormous twin jets of radio waves extending far into intergalactic space. Normally these go in opposite directions, powered by a massive black hole at the center of the galaxy.

New detail provided in the radio image shown above obtained with the MeerKAT telescope indicates PKS 2014-55 shape is best described as a ‘double boomerang’. Two powerful jets of radio waves, indicated in blue color, each extend 2.5 million light years into space (comparable to the distance between the Milky Way and the Andromeda galaxy, our nearest major neighbor). Eventually, they are ‘turned back’ by the pressure of tenuous intergalactic gas. As they flow back towards the central galaxy, they are deflected by its relatively high gas pressure into the shorter, horizontal, arms of the boomerang. The background image shows visible light from myriad galaxies in the distant universe. Adapted from W Cotton et al, MNRAS (2020). (NRAO/AUI/NSF; SARAO; DES)

Several possible explanations have been proposed to understand this phenomenon. These include changes in the direction of spin of the black hole at the center of the galaxy, and associated jets, over millions of years; two black holes each associated with a pair of jets; and material falling back into the galaxy being deflected into different directions forming the other two arms of the X.

Exquisite new MeerKAT observations of one such galaxy, PKS 2014-55, strongly favor the latter explanation as they show material “turning the corner” as it flows back towards the host galaxy; the results have just been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

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This work was carried out by a team from the South African Radio Astronomy Observatory (SARAO), the (US) National Radio Astronomy Observatory (NRAO), the University of Pretoria, and Rhodes University.

Previous studies of these unusual galaxies lacked the high quality imaging provided by the recently completed MeerKAT telescope. This telescope array consists of 64 radio dishes located in the Karoo semi-desert in the Northern Cape province of South Africa. Computers combined the data from these antennas into a telescope 8 km in diameter, and provided images in the radio band of unprecedented quality for PKS 2014-55 which enabled solving the mystery of its shape.


Mystery of the "X" Galaxy Phenomenon

Annotated image showing X-shaped giant radio galaxy PKS 2014-55, observed with the South African Radio Astronomy Observatory’s MeerKAT telescope, indicating the old X-shaped radio jets, the younger jets closer to the central black hole, and the region of influence dominated by the central galaxy’s stars and gas. The curved arrows denote the direction of the backflow that forms the horizontal components of the X. Credit: UP; NRAO/AUI/NSF; SARAO; DES

“MeerKAT is one of a new generation of instruments whose power solves old puzzles even as it finds new ones—this galaxy shows features never seen before in this detail which are not fully understood,” says lead author William Cotton of the NRAO, Further research into these open questions is already underway.

More information: Hydrodynamical Backflow in X-shaped Radio Galaxy PKS 2014-55. Monthly Notices of the Royal Astronomical Society. DOI: 10.10931/mnras/staa1240

The Daily Galaxy, Sam Cabot, via South African Radio Astronomy Observatory

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