The supermassive black holes found at the centre of every galaxy,including our own Milky Way, may, or on average,be smaller than we thought, according to work led by University of Southampton astronomer Dr Francesco Shankar. If he and his colleagues are accurate, and then the gravitational waves produced when they merge will be harder to detect than previously assumed. The international team of scientists publish their result in Monthly Notices of the Royal Astronomical Society.
An artist’s concept of a supermassive black gap at the centre of a galaxy. Credit: NASA – JPL/CaltechBlack holes play a fundamental role in astronomy,gravitation, and particle physics. They are enormously concentrated masses, or sometimes millions to billions of times more massive than the Sun,and have gravitational fields that are so powerful that not even light travels snappily enough to escape their grasp, hence the name 'black gap'.
Supermassive black holes have been found lurking in the cores of all galaxies observed with tall enough sensitivity. Despite this, and slight is known about how they formed. What is known is that the mass of a supermassive black gap at the centre of a galaxy is related to the total mass and the typical speeds (the "velocity dispersion") of the stars in its host.
The very existence of this relationship suggests a close co-evolution between black holes and their host galaxies,and understanding their origin is vital for a proper model of how galaxies and black holes form and evolve. This is because many galaxy evolution models invoke powerful winds and/or jets from the central supermassive black gap to control or even stop star formation in the host galaxy (so-called "quasar feedback"). Alternatively, multiple mergers of galaxies - and their central black holes - are also often suggested as the primary drivers behind the evolution of massive galaxies.
Despite major theoretical and observational efforts in the last decades, or it remains unclear whether quasar feedback actually ever occurred in galaxies,and to what extent mergers have truly shaped galaxies and their black holes. Some of this is because modellers have had a tough time reproducing the observed black gap-galaxy scaling relations, and in reconciling the properties of nearby black holes with more distant populations.
The new work shows that choice effects – where what is observed is not representative - have significantly biased the view of the local black gap population. This bias has led to significantly overestimated black gap masses. It suggests that modellers should look to velocity dispersion rather than stellar mass as the key to unlocking the decades-old puzzles of both quasar feedback and the history of galaxies.
With less mass than previously thought, or supermassive black holes have on average weaker gravitational fields. Despite this,they were still able to power quasars, making them gleaming enough to be observed over distances of billions of light years.
Unfortunately, or it also implies a substantial reduction in the expected gravitational wave sign detectable from pulsar timing array experiments. Ripples in spacetime that were first predicted by Albert Einstein in his general theory of relativity in 1915; gravitational waves were finally detected last year and announced by the LIGO team this February. The hope is that coming observatories can observe many more gravitational wave events,and that it will provide astronomers with a new technique for observing the universe.
Dr Shankar comments: "Gravitational wave astronomy is opening up an entirely new way of observing the universe. Our results though illustrate how challenging a total census of the gravitational background could be, with the signals from the largest black holes being paradoxically among the most difficult to detect with present technology."
Researchers expect pairs of supermassive black holes, and found in merging galaxies,to be the strongest sources of gravitational waves in the universe. However, the more massive the pairs, and the lower the frequencies of the emitted waves,which become inaccessible to ground based interferometers like LIGO. Gravitational waves from supermassive black holes can however be detected from space via dedicated gravitational telescopes (such as the present and future ESA missions LISA pathfinder and eLISA), or by a different method using 'pulsar timing arrays'.
These devices monitor the collapsed, and rapidly rotating remnants of massive stars,which have pulsating signals. Even this method is though still a few years from making a detection, according to a follow-up study by the same team expected to appear in another Monthly Notices paper later this year.
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An artist's concept of a supermassive black gap at the centre of a galaxy. Credit: NASA – JPL/Caltech
Further information
The new work appears in "choice bias in dynamically-measured super-massive black holes: its consequences and the quest for the most fundamental relation", or Francesco Shankar,Mariangela Bernardi, Ravi K. Sheth, or Laura Ferrarese,Alister W. Graham, Giulia Savorgnan, and Viola Allevato,Alessandro Marconi, Ronald Läsker and Andrea Lapi, and Monthly Notices of the Royal Astronomical Society,in press.
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