The supermassive black holes that lurk at the heart of most galaxies have enormous appetites. They’ve already consumed millions of times the mass of our own Sun, and they’re not done yet. Everything’s on the menu: mostly gas, dust, planets and stars, but the occasional exotic delicacy gets consumed too. “Compact objects”, such as stellar mass black holes, neutron stars, and white dwarfs occasionally fall into their grasp too. But these objects don’t go with a whimper; they make screams we’ll soon hear across intergalactic space.
Astronomers are now convinced that there are supermassive black holes at the centre of nearly every galaxy. Yes, even our own Milky Way (don’t worry, we’re at a nice, safe distance). These monsters can range from million to billions the mass of our Sun. Matter from their neighbourhood is constantly spiraling in, like water going down a drain. If the black hole consumes matter too quickly, it clogs up, glowing brightly in various wavelengths of radiation. We can spot these “active nucleus” galaxies from millions, and even billions of light years away.
Most of the matter falling into these black holes is run-of-the-mill material, like dust, gas, stars and planets, but astronomers reason that exotic objects like black holes, neutron stars and white dwarfs must get consumed as well – astronomers call them “compact objects”. Because of the tremendous energies involved, these compact objects should give off blasts of radiation, and generate gravity waves that we can detect here on Earth.
The detections will be made using the Laser Interferometer Space Antenna (LISA). This mission, scheduled for launch in 2015, will comprise three identical spacecraft, flying in formation and forming an equalateral triangle five million kilometers (about three million miles) apart. As the compact objects are consumed by the supermassive black hole, the enormous energies involved will send powerful ripples across space called gravity waves. As the gravity waves wash across the LISA spacecraft, they expand and contract the distances between them ever so slightly. LISA should be able to detect these changes, and even be able to detect the direction from where they originated.
According to new estimates by Jose Antonio de Freitas Pacheco, Charline Filloux, and Tania Regimbau from the Observatoire de la Cote d’Azur, in Nice, France, gravity wave detectors like LISA should be able to detect 9 of these gravity wave events a year. But this number depends on how dense the environment around these supermassive black holes can get.
The larger supermassive black holes have more gravity, so they cleared out the regions around their local environments millions of years ago. All their compact objects would be long gone. But for less massive black holes, they could still be surrounded by black holes, neutron stars and white dwarfs, and we should able to detect their collisions more frequently.
The Cote d’Azur team predicts:
if the mass distribution extends down to 200,000 stellar masses, then the total number of expected events increases up to 579 (corresponding to 274 stellar black hole captures, 194 neutron star captures and 111 white dwarf captures).
Scientists using LISA to watch for these events could be very busy. We only have to wait another 9 years or so to find out.