From SRON: “Astronomers make composition drawing of elusive wandering black holes”


17 June 2020


When two galaxies collide, their central black holes merge emitting gravitational waves. Astronomers theorize that a recoil effect sometimes kicks the merged black hole out of the galaxy while dragging nearby stars along for the ride. Researchers from SRON and Radboud University have now made a prediction of what these clusters will look like in a database, so that we can identify them and prove their existence. Publication in MNRAS.

Astronomers think that all massive galaxies harbor a black hole in their center, weighing millions to billions of solar masses. Smaller black holes might be present in the nuclei of dwarf galaxies. The most famous central black hole is the one inside the Messier 87 galaxy, which became the first ever photographed black hole in 2019.

Mesier 87*, The first image of a black hole. This is the supermassive black hole at the center of the galaxy Messier 87. Image via JPL/ Event Horizon Telescope Collaboration.

When two galaxies merge, their stars will mostly just mingle without colliding, but the two central black holes will merge. The merger produces gravitational waves carrying off extreme amounts of energy, comparable to an atomic bomb with the mass of several suns. You can imagine that if this energy is radiated even slightly asymmetrically, there will be a recoil in the other direction, similar to an astronaut firing a gun in space. If the recoil is strong enough, the resulting merged black hole is ejected out of its own galaxy. Any stars that were gravitationally bound to it, will be tugged along for the ride. This is how hypercompact stellar clusters (HCSCs) arise. At least according to theory; they have not yet been spotted in real-life.

A group of astronomers from SRON Netherlands Institute for Space Research and Radboud University figured that HCSCs might be hidden in existing databases, including those from the Gaia telescope and the Sloan Digital Sky Survey.

ESA/GAIA satellite

SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude2,788 meters (9,147 ft)

But they quickly realized that no-one has made any detailed-enough predictions about what they would look like in the database. So, as a first step in their quest, they have now made their own predictions and published them in the Monthly Notices of the Royal Astronomical Society [above]. The team—including first author Davide Lena and group leader Peter Jonker—predict the colors, images and spectra of HCSCs specifically tailored for each database. They also calculated how a cluster would appear on a two-dimensional telescope image.

If the researchers manage to identify the first real-life HCSC, they can derive the kick velocity that it received from the recoil following the merger of the two galaxies it emerged from. Lena: ‘That has already been calculated from simulations of gravitational waves, but those are based on theories that need to be confirmed by observations.’

Any ejected black holes in the Milky Way’s outskirts will have been the result of mergers between a dwarf galaxy and a young Milky Way that had recently started from scratch with building a massive black hole in its center. So those merged black holes should be of no more than intermediate mass; between hundreds and hundreds of thousands of solar masses. ‘The existence of intermediate mass black holes is debated,’ says Lena. ‘If we indeed find HCSCs, we will at the same time show the existence of intermediate mass black holes. We can then confirm this by measuring the mass of the black holes through spectroscopic observations of the HCSC.’

Jonker: ‘We think that mergers play an important part in forming massive black holes. ESA’s LISA satellite, to be launched in 2034, will be able to detect their gravitational waves. Among others SRON and Radboud University are set to contribute to building this fantastic satellite.’

Rendition of a hypercompact stellar cluster made for the NISP instrument on board the forthcoming Euclid telescope. The color bar represents photon count.

See the full article here .


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SRON’s mission is to bring about breakthroughs in international space research.

Therefore the institute develops pioneering technology and advanced space instruments, and uses them to pursue fundamental astrophysical research, Earth science and exoplanetary research. As national expertise institute SRON gives counsel to the Dutch government and coordinates – from a science standpoint – national contributions to international space missions. SRON stimulates the implementation of space science in our society.

SRON is the Dutch national expertise institute for scientific space research. It is part of NWO. Since the foundation of the institute by university groups, in the early 1960s, we have, often in a leading role, provided key contributions to instruments of missions of the major space agencies, ESA, NASA, and JAXA. These contributions have enabled the national and international space-research communities to explore the universe and to investigate the Earth’s atmosphere and climate. As a national expertise institute, we stimulate collaboration between the science community, technological institutes, and industry.

Our vision is to continue to belong to the international forefront in search for answers to some of the most fundamental existential and societal questions of mankind: What is the origin of the universe and what is it made of? Is there life elsewhere in the universe? What is the future of the Earth’s climate? What are the atmospheric processes that govern changes in the Earth’s climate and air quality. What role does human activity play?

Our strategy is to develop science cases, key enabling technologies, prototypes/demonstrators, space-qualified instrumentation, and data-analysis tools that will define the next generation of space missions, to be launched in the 2020s and 2030s. This enables us to lead major contributions to answering the fundamental questions of our time. The institute has made sharp choices in its programme based on its strengths, the priorities of the national science community, and the opportunities in international space research. Driven by the Netherlands commitment to the ESA charter, it is our strategy to be principal investigator (PI) or co-PI institute for major instruments on ESA missions.

How did the Earth and life on it evolve? How do stars and planets evolve? How did the universe evolve? What is the position of the Earth and humankind in that immense universe? These are fundamental questions that have always intrigued humankind. Moreover, people have always possessed an urge to explore and push back the boundaries of science and technology.

How did the Earth and life on it evolve? How do stars and planets evolve? How did the universe evolve? What is the position of the Earth and humankind in that immense universe? These are fundamental questions that have always intrigued humankind. Moreover, people have always possessed an urge to explore and push back the boundaries of science and technology.


Since the launch of Sputnik in 1957, Dutch astronomers have seen the added value of space missions for science. Reaching beyond the Earth’s atmosphere would open up new windows on the universe and provide fantastic views of our home planet. It would at last be possible to pick up cosmic radiation that never normally reached the Earth’s surface, such as X-rays, ultraviolet and infrared radiation. A wealth of scientific information from every corner of the universe would then become available.

The first Dutch scientific rocket experiments and contributions to European and American satellites in the early 1960s, formed the start of an activity in which a small country would develop an enviable reputation: scientific space research.

Groundbreaking technology

Nowadays we take for granted images of the Earth from space, beautiful photos from the Hubble Space Telescope or landings of space vehicles on nearby planets. Yet sometimes we all too easily forget that none of these scientific successes would have been possible without the people who developed groundbreaking technology. Technology that sooner or later will also prove useful to life on Earth.