Supermassive Black Holes

In the 1960s, long before the Milky Way was known to have a supermassive black hole, violent activity was seen in the centers of some distant galaxies. The manifestations were gas motions a hun­dred times faster than would be seen in a normal stellar system, intense radio emission concentrated in a tiny region of the galactic nucleus, and radiation across the electromagnetic spectrum from radio waves to gamma rays. Some of the strongest X-ray sources in the sky are galaxies hundreds of millions of light-years away, which means they have an X-r ay emission thousands of times stronger than a galaxy like the Milky Way. Rapid variability of the radio, optical, and X-ray emission localized the intense activity to regions light-days across, only ten times bigger than the Solar Sys­tem. No star cluster can pack so much radiation into such a small volume; the only plausible explanation was a gravitational engine: a black hole.60

Imagine a city like Los Angeles viewed from high above in a helicopter, where every streetlight, every house light, and every car light is standing in for a star. There are probably 100 million lights in the greater Los Angeles area, so in this analogy each light rep­resents a thousand stars. Los Angeles is like the disk of a spiral galaxy. Now imagine a light one inch across in the city center that emits a hundred times more radiation than all the city lights put together. That’s the intensity of an active galactic nucleus. It would be clearly visible from an immense height, long after the individual lights in the city had faded from view. We can see an incredibly bright point of light but the surrounding galaxy is too faint to see. That’s a quasar.

Through the 1990s astronomers realized that every galaxy con­tains a black hole, with a mass that scales with the mass of the old stars in the center of the galaxy.61 Moreover, these black holes are not active most of the time, so are not feeding and spewing out the energy that makes them easy to detect. The Milky Way is a medium-size galaxy with a 4-million-solar-mass black hole that’s currently inactive. The brightest quasars harbor black holes a thou­sand times larger, or a few billion times the mass of the Sun. These extraordinary gravitational engines form and grow quite naturally as galaxies form and grow but steadily consume matter, and by occasional mergers. It’s remarkable that nature makes black holes spanning a factor of a billion in mass, from a few times the mass of the Sun to the mass of a small galaxy. Their sizes range by the same factor of a billion, but they all share the properties of having an event horizon and (it is supposed) a central singularity. Strange as it might seem, massive black holes do not represent an extremely dense state of matter. The size of the event horizon is proportional to mass, but the volume increases by the cube of the size, so the more massive a black hole is the less dense it is within the event horizon. The 3-billion-solar-mass beast at the heart of the elliptical galaxy M87 is not much denser than water! Modest density does not, however, reduce their intrinsic strangeness (figure 10.4).

Chandra has played a vital role in telling the story of supermas­sive black holes and their behavior. In nearby galaxies it has de­tected vast bubble-like cavities and wavelike ripples in hot gas near the center, signs of “blowback” from the central engine.62 X-rays provide evidence of repetitive explosive activity, where mass falls onto the black hole, sparking it into activity and triggering jets of high-energy particles, which then evacuate the nearby volume and starve the black hole until new material accumulates. In this way, a massive black hole can be inactive most of the time. Chandra has also seen binary black holes in the centers of merging galaxies.

In one case, two huge black holes are only 3,000 light-years apart and will merge in the next hundred million years, provoking a cataclysm of gravity waves and new activity and yielding a single galaxy with a larger black hole.63 This is direct evidence for the manner in which galaxies and their embedded black holes have grown by mergers and acquisitions over billions of years.

Black holes and galaxies evolve on cosmic timescales of billions of years, so to figure out how they change the strategy is to make deep surveys that capture the census of active and inactive black holes at a range of distances or redshifts. The Chandra Deep Field was stared at for 2 million seconds, over three weeks, to create the deepest X-ray image of the sky ever made. This deep field, covering a patch of sky smaller than a postage stamp held at arm’s length, is combined with a shallower but wider survey to tell the full story.64 One of us (CDI) has participated in this research, with a survey of active galaxies in a two square degree region that netted more than two thousand supermassive black holes, and showed their evo­lution in cosmic time. This survey was sensitive enough to catch feeble accretors and black holes similar in size to the one in the Milky Way.

The broad conclusion of these surveys is that star formation and black hole growth in a galaxy are tightly linked. The heavi­est black holes, a hundred million times the Sun’s mass or larger, ate voraciously in the first few billion years after the big bang, and have been grazing or fasting since then. Black holes ten to a hundred million times the Sun’s mass had a more well-regulated diet, and because they consume smaller proportions of their gas and dust early on, they continue to grow even today. X-rays are crucial to telling this story, since they can penetrate regions that would extinguish optical and ultraviolet radiation.65 The typical size of a galaxy forming a supermassive black hole of a particular mass reduces with cosmic time, so the biggest black holes formed first. Massive galaxies in the far-flung universe misspent their youths by building monstrous black holes. This result is appar­ently at odds with the hierarchical mode of formation of structure in the universe, where small objects form first and gradually grow into larger objects. This unexpected phenomenon has been called “downsizing.”66

A by-product of this research is the solution to an old mystery of the X-ray sky. When the first X-ray satellites took their data in the 1960s and 1970s they saw a faint diffuse glow that spanned the entire sky. This X-ray “background” was unexplained. Surveys with Chandra and XMM-Newton show that the background is ac­tually composed of a myriad of pinpoints of X-ray emission from active galaxies 3-8 billion light-years away. Also, a majority of the emission is from galaxies where the nuclear activity is shrouded by dust and would not have been visible to an optical telescope. These surveys have also shown that black holes in the most mas­sive galaxies are “green.” By comparing the available fuel with the energy required to evacuate cavities in the central regions, the ef­ficiency can be calculated. It’s high because the mass is accreted slowly and smoothly and the energy is extracted very close to the event horizon of the black hole. If a family car was as efficient as one of these supermassive black holes, it would get a billion miles to a gallon of gas.