An international team of Chinese and foreign astronomers has taken a step forward in understanding the evolution of galaxies, and in so doing, told a story written in the heavens.
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.
Departing from typical articles, it appears to have been written with an eye to romance.
The report starts by acknowledging the mutual (gravitational) attraction galaxies can have for each other. It then tells how 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.
NGC 4424 was already known to display signs of activity from a past merger event. “The galaxy’s bar-like structure is excited and buckled. There was also a star-forming event less than 500 million years ago,” says the lead author, Professor Alister Graham, at Swinburne Astronomy Online (Melbourne, Australia). “One can think of this as a star party of sorts, associated with the announcement of the upcoming galaxy wedding,” he adds.
Professor Roberto Soria, a co-author at the University of the Chinese Academy of Sciences (National Astronomical Observatories, NAOC), has obtained a Chandra X-ray Observatory image showing a 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 within what was the centrally-located star cluster of the infalling galaxy,” says Soria.
Although 50 million light-years away, each square metre of Earth is bathed in an x-ray from this active black hole roughly every 80 seconds. The X-ray hotspot is just 1300 light-years from the centre of NGC 4424, a galaxy some sixty thousand light-years across.
The star cluster has informally been named Nikhuli, after a festive period in the Indian State of Nagaland, where the descendants of head-hunters celebrate and wish for a rich harvest and gathering.
The main body of the smaller galaxy - which once housed the resilient star cluster - is now contributing to an inner `bulge’ of stars above and below the spiral galaxy’s disc, which contains the spiral pattern.
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 centres of giant galaxies, like M87. “This in itself is exciting,” says Graham. “Moreover, 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.
“Furthermore, potential collisions with other black holes make this an ideal setting for the emission of long-wavelength gravitational waves rippling across space,” says Davis.
A number of future laser interferometer space missions, such as China's Tianqin and Taiji missions, the European Space Agency and NASA's Laser Interferometer Space Antenna (LISA), and The Japan Aerospace Exploration Agency's DECIGO, are working towards discovernig such events involving bigger black holes, which results in longer waves.
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, he is quick to add that "Nikhuli appears to be an important discovery for understanding the coevolution of black holes and galaxies."
Soria added that the initial sparkle of discovery happened during a visit to NAOC by Prof Graham and Dr Davis. This shows the importance of personal interactions and international exchanges for scientific research. The collaborative environment at NAOC is the catalyst for ground-breaking progress.
The article has been published in The Astrophysical Journal and is available online.
https://www.doi.org/10.3847/1538-4357/ac235b
https://arxiv.org/abs/2112.05318
Image credits：NASA/ESA, Or Graur (University of Portsmouth), Adam Riess (Johns Hopkins University), Lisa Frattare (Space Telescope Science Institute).
19 Dec 2021.
