|Artist impression of a supermassive black hole |
surrounded by an accretion disk of infalling gas
and twin, highly-collimated plasma jets.
Credit: Mark Garlick (University of Warwick)
It has recently become clear that at the center of most, if not all, galaxies lies a black hole of epic proportions. These so-called supermassive black holes are typically a billion times more massive than the stellar-mass black holes formed when stars die and collapse in on themselves. Their event horizons are also significantly larger, with radii on the order of billions of kilometers (roughly the radius of our solar system). Once inside this horizon, nothing in the Universe, not even light, can escape the gravitational pull of the singularity at the center of the black hole.
In most galaxies, including the Milky Way, supermassive black holes lie dormant and are not readily detectable. Astronomers have inferred the presence of a black hole at the heart of our Galaxy due to the orbital motion of stars as they circle a massive, yet invisible object near the Galactic center (see the movie below). In about 10% of galaxies, though, supermassive black holes are actively growing through the accretion of gas and sometimes stars. When gas spirals into a black hole, it forms an accretion disk and rapidly heats up. As it does so, it emits immense amounts of energy and the gas becomes visible at a variety of wavelengths. When this happens, astronomers refer to the black hole and its surrounding gas as an active galactic nucleus, or AGN. These accretion events can be so energetic that even moderate amounts of gas infall (like the mass of our Sun accreted over the course of a year) will result in an AGN that outshines the starlight of its entire host galaxy.
The animation above shows the motion of stars orbiting the black hole at the center of the Milky Way. One of these stars has been observed over its complete 15.8-year-long orbit. The star approached the black hole to within one light day, which is only about five times the distance between the planet Neptune and the Sun. Credit: ESO
Finding Active Galactic Nuclei
There are several observational signatures that astronomers use to detect AGN out to cosmological distances. Primary among these is the radiation produced by the gas in or near the accretion disk surrounding the central black hole. As this gas heats up, it emits radiation at optical, ultraviolet and X-ray wavelengths. If the accretion disk is obscured by interstellar gas and dust, some of this radiation will be absorbed and re-radiated at infrared wavelengths. The net result is excess emission near the center of a galaxy with a unique spectral energy distribution that can be detected. X-rays observations are a particularly powerful way to find AGN as galaxies themselves do not produce strong X-ray emission. In addition, the radiation from the accretion disk can excite cold atomic gas close to the black hole which then produces unique emission lines that are visible in the spectrum of the galaxy. On occasion, AGN will also produce powerful plasma jets, twin highly-collimated outflows that emerge on opposite sides of the accretion disk, which are detectable at radio wavelengths. Many of these signatures, especially those at optical and radio wavelengths, can be extremely luminous. As a result, many of the most distant objects ever detected are galaxies with a central AGN. The most distant AGN found thus far is at a redshift of z=7.1, an era when the Universe was only 760 million years old (5% of its current age).
The Role of AGN in Galaxy Evolution
Although AGN have been studied for more than half a century, their potential importance to the evolution of galaxies has only recently become evident. Observations over the past decade have revealed that a tight correlation exists between the mass of a galaxy’s spheroid component, or central ‘bulge’, and the black hole at its center. This correlation suggests that the formation and growth of supermassive black holes is intimately linked to the growth of their host galaxies. In fact, when astronomers measure the rate at which black holes are growing as a function of cosmic time, we find this to be directly proportional to the rate at which galaxies grow as measured by their star formation rates. Understanding how this connection between galaxies and their central black holes is established and maintained is one of the key goals of modern astronomy and the CANDELS survey.
Current theories propose that this link is forged, in part, by the energy released during an AGN phase. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that halt the accretion of gas onto the central black hole, while simultaneously acting to suppress the surrounding galaxy's star formation activity. In this way, AGN can self-regulate the growth of both supermassive black holes and their host galaxies. This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. In fact, without the energy input from AGN, many models fail to reproduce fundamental properties of galaxies, such as their color bimodality and space densities. The dramatic effects of AGN feedback can be seen in the movie below, which shows the simulated collision of two spiral galaxies with supermassive black holes at their centers. As the merger progresses, outflows driven by the central black holes eventually sweep away the cold gas within the two galaxies, effectively terminating their star formation activity.
The Future with CANDELS
Despite this emerging picture, several major questions remain about AGN and their potential impact on galaxy evolution. What mechanisms fuel black hole growth and turn a dormant black hole into an AGN? What is the precise nature of AGN feedback? What role do AGN play in giving rise to the first generation of quenched galaxies. Using the latest imaging from the Hubble Space Telescope, the CANDELS survey is now providing insight into many of these questions by allowing us to characterize the stellar populations and structural properties of AGN host galaxies over a redshift range than encompasses 2/3 of cosmic time. In the coming years, CANDELS will help us construct an integrated model for the triggering of AGN activity, the quenching of star formation, and the structural evolution of galaxies in the early Universe.