Active Galactic Nuclei
Black holes are some of the most interesting objects in the Universe. Astronomers now know that at the center of most galaxies there lives a giant black hole, which may play a very important role in the evolution of the galaxy it lives in. Here we explain how black holes form, how astronomers identify them, and what techniques are used to study them in detail.
Deep in the heart of nearly every large galaxy there is a gigantic black hole that is roughly a million to a billion times more massive than our Sun. In a black hole, the gravitational force is so strong that not even light can escape its pull. As no light is allowed to escape, it can no longer be observed, and this why scientists call these objects “black holes”.
Black holes exist over a wide range of masses. While a small black hole can be only 5 times more massive than our Sun, the most gigantic black holes can be up to a billion times more massive than our Sun. These so-called “supermassive black holes” are seen to only exist in the very central regions of galaxies. How these supermassive black holes formed is not entirely clear, and astronomers are actively working on theories to explain their existence. Sometimes the strong gravitational force from a supermassive black hole will drag nearby gas and even stars into the very center of the galaxy, where the black hole lies (It does not pull in material from farther out in the galaxy). The gas and stars that are pulled in towards the black hole form a rotating disk, is called an accretion disk. All supermassive black holes are thought to have such accretion disks surrounding them. Additionally, some powerful black holes also create jets of gas that eject material out of the center of the galaxy. The central regions of such galaxies, which include the black hole, the accretion disk, and in some cases a powerful jet, are called Active Galactic Nuclei or AGN. Figure 1 shows an artist’s rendition of a supermassive black hole surrounded by an accretion disk and jets at the center of a galaxy.
Although supermassive black holes are very small, compact objects, they could potentially be very important for the evolution of the galaxies they live in. During the past couple of the decades astronomers have been using different techniques to understand the possible effects that these supermassive black holes can have on different properties of their host galaxies, and in turn how the galaxies they live in can affect the black holes. For example, astronomers are trying to understand how much of the gas in galaxies is accreted by the black holes as opposed to being used to form new stars. Additionally, how do the powerful jets created by the black holes impact the gas farther out in the galaxy? Do these jets sweep gas out of the galaxy, or do they eject energy into the gas and keep it from forming new stars?
AGN can emit light over a wide range of wavelengths through different processes that happen near the black hole. The collision between the low mass and fast-moving electrons near the black hole can produce X-ray light. Swirling hot gas both in the accretion disk and near the black hole can produce optical and ultraviolet light. Some of this optical and ultraviolet light can be absorbed by dust grains near the black hole, which heat up and re-emit that light at longer, mid-infrared wavelengths. The emitted light from an AGN is very useful in our quest to understand these objects and how they are created, evolve over time, and how they impact the galaxies they live in.
The next question is how can astronomers detect these objects. There are different telescopes in the space and on the ground that take high quality images from the sky. Some of these telescopes are designed to be very sensitive to a particular range of the wavelengths. For example, the Chandra X-ray Observatory is a satellite telescope that orbits the Earth and can detect X-ray light. The X-ray light produced around supermassive black holes makes AGN brighter than normal galaxies at X-ray wavelengths. Therefore Chandra or other X-ray telescopes can detect AGN. There are other telescopes that take images of the sky at longer mid-infrared wavelengths. The mid-infrared light coming from the dust around supermassive black holes enables these telescopes to detect AGN. In addition, there are telescopes that are sensitive to the ultraviolet and optical light that can detect the UV-optical light coming from the hot gas near the supermassive black hole. However, not all AGN are bright at all of these wavelengths. Therefore, when we observe AGN in distant galaxies, billions of light years away, if we identify AGN using just one of these wavelengths we will likely miss many of the AGN that exist. Therefore, in the MOSDEF survey we combine all of these methods to identify as many AGN as possible.
Generally, astronomers analyze the light emitted from objects in the sky to determine their properties, such as their distance from us or their age, mass, composition, or motion. Two techniques that astronomers commonly use to measure and analyse light are called photometry and spectroscopy.
Photometry is a technique for measuring the brightness of objects in the sky. Here the light coming from an object, e.g. an AGN, is allowed to pass through a limited number of filters when it reaches the telescope. These filters are special pieces of glass which provide information about a specific range of wavelengths. Figure 2 shows some of the filters that astronomers often use in photometry measurements. Each filter allows light in only that wavelength range to pass through the filter to the detector. The U filter allows only ultraviolet light to pass through, while the B, V, R, and I filters allow blue, visible, red, and infrared light respectively to pass through.
Figure 3 illustrates two examples photometry obtained in multiple filters for an AGN (left panel) and a normal galaxy (right panel) in the MOSDEF survey. The horizontal axis shows the wavelength and the vertical axis shows the amount of light detected in each filter. As wavelength increases towards longer wavelengths (right side of the panel), and in the mid-infrared specifically, the amount of radiation detected from the AGN continues to increase. In a normal galaxy (where the supermassive black hole is not actively accreting gas) there is no such an increase at mid-infrared wavelengths. This photometric information can be used to 1) identify the presence of an AGN (from the “excess” light at long wavelengths, near ~5-10 microns), and 2) determine the mass of stars and the rate of formation of new stars in the galaxy (from the light at other wavelengths).
The other common technique used to extract information from astronomical objects is spectroscopy. Spectroscopy allows for detailed measurements about various physical processes occurring in a galaxy and can be used to determine information about the gas, stars, and black holes in galaxies. In this method, light from a galaxy passes through a dispersive element, such as a prism. The prism makes a single beam of light spread into different colors like a rainbow(Figure 4); this allows astronomers to more finely measure how much light is being emitted at different wavelengths.
The gas in a galaxy and near the black hole is composed of different elements from different atoms. Each atoms emits light at specific wavelengths, such that light at a given wavelength indicates the relative amount of atoms of a given element. The light from AGN is powerful enough to ionize nitrogen in particular, such that AGN often show elevated amounts of nitrogen compared to the bulk of the gas, which is hydrogen. In a normal galaxy without an AGN (where the supermassive black is not actively accreting gas) there is less light emitted from nitrogen atoms compared to hydrogen atoms.
Figure 5 shows results from spectroscopic data in the MOSDEF survey for an AGN (left panel) and a normal galaxy (right panel). The horizontal axis is wavelength and the vertical axis is the amount of light detected at a given wavelength. In a normal galaxy the amount of light emitted by nitrogen atoms is less than that from hydrogen atoms, while in an AGN there is a greater amount of light emitted from nitrogen atoms relative to hydrogen atoms in gas near the AGN.
Overall, the supermassive black holes that lie at the centers of galaxies are very powerful objects that can play important roles in the evolution of the galaxies they live in. Astronomers identify these powerful black holes with X-ray, mid-infrared or optical telescopes in space or on the Earth. They use different techniques such as photometry and spectroscopy to analyze the light coming from these objects. Using photometry, astronomers can identify AGN through their excess light at mid-infrared wavelengths and also estimate the mass in stars and the rate of formation of stars in galaxies that host AGN. Spectroscopy provides more detailed information about the gas near the supermassive black hole. Combining observations at X-ray, mid-infrared, and optical wavelengths allows astronomers to identify more AGN than can be detected at any one wavelengths, providing a more complete census of how many AGN exist and how they impact their host galaxies.