A cosmic romance written in the stars


An international team of astronomers has taken a step forward in understanding the evolution of galaxies, and in so doing, told a story written in the stars.
It has long been a mystery how some spiral galaxies obtained their central black hole. By combining visible and X-ray observations, astronomers have now discovered traces of what was probably once a small sphere-shaped galaxy, seen falling into a spiral galaxy and delivering what is thought to be the right-sized black hole.
The facts make for a cosmic romance, a similarity not lost on lead author of this new research, Professor Alister Graham, from Swinburne’s Centre for Astrophysics and Supercomputing and teaching into Swinburne Astronomy Online.
Galaxies can have mutual (gravitational) attraction for each other. The body of a smaller galaxy may fade over time, but its heart remains intact as it falls into and partners with a larger galaxy.
In this case, the heart is a million-strong cluster of stars, seen with the Hubble Space Telescope near the centre of the spiral galaxy NGC 4424.
The astronomers have informally named the star cluster ‘Nikhuli’. They turned to the Sumi tribe in the Indian State of Nagaland for the word, used for a festive period where the descendants of head-hunters celebrate and wish for a rich harvest and gathering. It seemed appropriate to the astronomers, who refer to space as ‘the field’ and whose discovery focuses on how a larger galaxy has harvested a smaller galaxy.
NGC 4424 was already known to display signs of activity from a past merger event. But the team has discovered the remnant central star cluster of an infalling galaxy with a black hole.
“The larger galaxy’s bar-like structure is excited and buckled. There was also a star-forming event less than 500 million years ago. One can think of this as a star party of sorts, associated with the announcement of the upcoming galaxy wedding.”
Professor Graham remarked with a smile that he has already been called a bit of a nerd, having written something which Sheldon from Big Bang Theory might have penned. However, Professor Graham is quick to add that, “This appears to be an important discovery for understanding the co-evolution of galaxies and black holes.”
Professor Roberto Soria, a co-author at the Chinese Academy of Sciences, obtained the Chandra X-ray Observatory image showing the high-energy X-ray source emanating from the stretched-out star cluster seen in the Hubble image. “We are likely seeing activity from around a black hole, located 1300 light-years from the centre of NGC 4424”, says Professor Soria.
Although 50 million light-years away from us, each square metre of Earth is bathed in an x-ray from this active black hole every 80 seconds.
The team’s best estimate for the mass of the black hole is 70,000 times the mass of our Sun. This mass makes it a candidate for the largely-missing population of ‘intermediate-mass’ black holes with masses greater than stars and smaller than the supermassive black holes known to reside at the centres of giant galaxies – like the famous first-ever image of a black hole taken by the Event Horizon Telescope.
This mass is on par with that expected at the centre of NGC 4424. “We may be witnessing a supply mechanism for black holes into spiral galaxies”, says Dr Ben Davis, a co-author at the New York University’s campus in Abu Dhabi. The planned Laser Interferometer Space Antenna, aka LISA, and the Chinese TianQin (天琴计划) space mission, will be able to detect the collisions of such massive black holes in the future.
This research has been published in The Astrophysical Journal and is available online
(Courtesy: Swinburne University of Technology)