Constraining the Properties of Quasars and their Outflows Using Variability

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Imagine voyaging out of our solar system and beyond all the stars in our Milky Way galaxy. The faint dots of light visible to you then then wouldn't be stars, but entire distant galaxies. At the centre of any of those galaxies you'll find a swarm of stars and, if the galaxy is big enough, a black hole with a mass of millions or even billions of Suns. Watch as one of the clouds of hydrogen and helium gas and trace elements in that galaxy ventures too close to the black hole. Gravity stretches the gas cloud into a disk of gas bigger than our solar system, orbiting at incredible speeds but slowly spiralling inwards to accrete onto the black hole. In the inner disk, friction makes the gas hotter than the Sun over an area millions of times larger than the Sun's surface. That enormous, glowing disk can be seen all the way across the universe, and we call it a quasar. This turbulent accretion disk of hot, rotating gas often tosses up gas clumps which are pushed outward by the light from the inner disk. The clumps form spiralling streamers of gas which block the inner disk from view in some directions and which race out into the galaxy hosting the quasar. These fast-moving outflows can reverse the inflow of gas clouds supplying the accretion disk, shutting down the accretion process and turning off the quasar. They can even shock-heat and blow out almost all the gas in the galaxy, ending star formation in it and drastically changing its future appearance and environment. My research team and collaborators and I are surveying quasars for outflows, studying their time variability, and comparing them to computer simulations. We are learning how quasars and their outflows work so that we can refine the picture outlined above and improve our models of how galaxies form and develop. We also aim to help determine whether or not we can use the relationship between quasar size and luminosity to trace the history of dark energy farther back in time than we can with the exploding stars known as supernovae. If so, that would be a useful new test of dark energy, a mysterious force accelerating the expansion of our universe and one of the biggest mysteries in science today.