Why Supermassive Black Holes Are Hard to See and How to See Them

by Mislav Baloković

Many different observations over the past few decades have shown that a very massive black hole, weighing between a million and a billion times more than a typical star such as our Sun, lies at the center of virtually every normal galaxy. To distinguish them from stellar black holes, which are typically ten times more massive than the Sun, they are usually called “supermassive” black holes. Our galaxy, the Milky Way, also has a supermassive black hole at its center, located roughly 27,000 light-years away from the Sun in the direction of the constellation Sagittarius. While our neighboring supermassive black hole is on the lower side of the mass range, the most massive ones contain as much mass as a small galaxy!

The Milky Way is a normal spiral galaxy. This artistic rendering highlights its main parts, including the supermassive black hole at the center. Image credit: Lynette Cook, published in Nature

The Milky Way is a normal spiral galaxy. This artistic rendering highlights its main parts, including the supermassive black hole at the center. Image credit: Lynette Cook, published in Nature

The majority of supermassive black holes, including the one in the Milky Way, inactively sit at the galactic center being orbited by stars. However, when large quantities of gas and dust find their way into the center, the black hole will greedily pull them in to increase its own mass, producing a phenomenon called active galactic nucleus (AGN). As gas and dust spiral towards the black hole to be sucked in, a range of physical processes shed their gravitational energy over the full range of the electromagnetic spectrum from radio frequencies to gamma rays. By observing AGN in different parts of the spectrum, we are trying to piece together a self-consistent picture of how have these black holes grown to their monstrous masses.

Left: The main parts of an active galactic nucleus (AGN) in the current paradigm called the Unified Model. Right: Types of electromagnetic radiation that different parts of the structure emit most, from radio (RF) to gamma rays. Image credit: Brooks/Cole Thomson Learning via AstronomyOnline.org

Left: The main parts of an active galactic nucleus (AGN) in the current paradigm called the Unified Model. Right: Types of electromagnetic radiation that different parts of the structure emit most, from radio (RF) to gamma rays. Image credit: Brooks/Cole Thomson Learning via AstronomyOnline.org

The presence of such a massive black hole in the center of a young galaxy can have a profound effect on the evolution of the galaxy itself. If the AGN is powerful enough, it can effectively stop formation of stars in its host galaxy. Formation of stars requires cold gas, and an active supermassive black hole can either steal away part of that gas, heat it with intense radiation, or push it out of the galaxy in high-velocity jet streams. Which of these processes is key to understanding how galaxies are shaped? How does the interaction between the supermassive black holes and their host galaxies play out over time? Observing it directly is difficult, but, for example, it is sometimes possible to measure the velocity of gas outflowing from the galactic center, or observe bubbles blown through the galactic gas by a powerful AGN. The nature of this interaction is one of the hot topics in modern astrophysics.

The clearest insight into the world of AGN is provided by observations in X-ray light. Stars do not emit much X-ray light at all, so it is usually only the hot plasma in the host galaxy creating a haze of X-ray light in addition to the bright AGN emission. Not unlike medical X-rays, using an X-ray telescope allows us to look through otherwise opaque, thick layers of material that absorb almost all of the AGN light. Most AGN are in fact hidden from direct view, enshrouded by gas and dust in the host galaxy as well as surrounding the black hole itself. Even the most powerful X-ray telescopes available today, like Chandra and XMM-Newton, are unable to directly observe these obscured, hidden AGN that are buried deeply in their dusty cocoons. To find them and examine their activity one can either indirectly observe the heat that the warm cocoon gives off in the infrared, or use a specialized telescope that catches the higher-energy X-rays punching through the thick cocoon wall, which we usually call the dusty torus.

The Cosmic X-ray Background (cyan spectrum) is composed of combined light of all AGN: those less obscured (yellow and purple lines), as well as those heavily obscured by gas and dust (orange line), which are most numerous. Image credit: NASA/Goddard Space Flight Center

The Cosmic X-ray Background (cyan spectrum) is composed of combined light of all AGN: those less obscured (yellow and purple lines), as well as those heavily obscured by gas and dust (orange line), which are most numerous. Image credit: NASA/Goddard Space Flight Center

NASA’s newest orbital X-ray observatory NuSTAR (Nuclear Spectroscopic Telescope Array) is a specialized instrument ideal for studies of obscured AGN. One advantage it has over previously available telescopes is its spectral band, which reaches higher-energy X-rays; equivalent of adding ultra-violet light sensitivity to the human eye. Another advantage is the sharpness of its images: it can pinpoint the source of emission 10 times better than any other telescope operating in the same range of the electromagnetic spectrum. These qualities are the result of new technology developed specifically to open up a new window into the Universe.

One of the main goals of the NuSTAR mission is to provide the first complete census of supermassive black holes in the Universe, including the hidden ones that the previous telescopes could not have detected. According to the most recent predictions, there could be as many as four hidden AGN for every directly observable one. This is based on the previously known Cosmic X-ray Background, which can be easily understood as being the faint, hazy glow of all AGN combined. It provides us with a handle on the total number of supermassive black holes and the amount of radiation they have generated over the age of the Universe, interacting with their host galaxies as they evolve.

This game of hide and seek is not at all a trivial task. The first NuSTAR results on supermassive black hole demographics will become available in mid-2015, so stay tuned! However, this is only the beginning: knowing how many supermassive black holes there are does not by itself reveal much about the physical processes that shape their emission and the structures that form in their vicinity as gas and dust are pulled in. Using NuSTAR and other X-ray telescopes, as well as optical and radio telescopes on the ground, astrophysicists aim to better characterize the observable properties of AGN and understand the physics behind these exotic phenomena.

For example, you might have noticed that this article did not even mention where the X-rays are coming from. This is because it has not been well established yet. They are certainly emitted from the region very close to the black hole (nearly touching its “surface”, the so-called event horizon), but are they coming from the gas plunging into the black hole at its equator, or from the plasma diverted by the magnetic field towards the poles, perhaps being pushed away from the black hole in a narrow high-speed jet stream? NuSTAR has already provided a fresh and unique perspective on this important question, and many more insights are expected over its orbital lifetime of about 10 years. Exciting times lie ahead for any black hole fan!

Mislav Baloković is a 2011 fellow of the Fulbright Science & Technology Award program from Croatia. He is working on his PhD thesis in astrophysics at the California Institute of Technology.

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