New Research Uncovers a Cosmic Romance Written in the Stars

The joint discovery sheds light on the mystery of how some spiral galaxies obtained their central black hole.

Research Associate Benjamin Davis along with an international team of astronomers have taken a step forward in understanding the evolution of galaxies, and in so doing,  told a story written in the stars. 

Working with Professor Roberto Soria at the Chinese Academy of Sciences, Davis, from NYU Abu Dhabi’s  from the University’s Center for Astro, Particle, and Planetary Physics, has been working on explaining the long-held mystery of 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 in 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 center of the spiral galaxy NGC 4424.

The astronomers have informally named the star cluster ‘Nikhuli’, inspired from the word used by the Sumi tribe in the Indian State of Nagaland to describe a festive period where the descendants of head-hunters celebrate and wish for a rich harvest and gathering. The connection was made since astronomers refer to space as ‘the field’ and their 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. According to Professor Graham 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. This appears to be an important discovery for understanding the coevolution of galaxies and black holes”, he added.

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 center of NGC 4424,” said Soria.  

Although 50 million light-years away from us, each square meter 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 seventy thousand 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 centers 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 center of NGC 4424. Research Associate at NYUAD Ben Davis said: "We may be witnessing a supply mechanism for black holes into spiral galaxies.”

Professor Graham and Ben Davis are also members of the LISA Consortium, whose Laser Interferometer Space Antenna, aka LISA, space mission is working towards discovering events involving the collision of bigger black holes.