Cosmic Clue: What's Killing the Massive Galaxies?

When I was a kid, my favorite board game was Clue. I loved being handed random, scattered pieces of evidence, and having to deduce through sheer logic and process of elimination, who, where, and how Mr. Boddy was killed. So it's not surprising that years later, as a galaxy formation theorist on the CANDELS team, I am drawn to a murder mystery on a cosmic scale, a mystery that is responsible for the most fundamental dichotomy in the galaxy population yet has puzzled astronomers since galaxies were first discovered: What kills the biggest galaxies in the Universe?

The existence of dead galaxies has been known since the time of Edwin Hubble. Dead galaxies are easy to spot as they are among the largest in the Universe, containing up to a hundred times more stars than the Milky Way. They are also very red, since they have not formed any new stars for a long time. This happens because stars in the sky are much like stars in Hollywood: The biggest ones are the hottest and shine the brightest, but they also die out the quickest (and most spectacularly). Since young, hot stars are blue (a consequence of their black body temperature), after they die out, the galaxy is left with nothing but plebian longer-lived redder stars like our Sun. The galaxy then becomes red and dead.

But why? What serial killer is responsible for systematically killing these erstwhile happily star-forming systems?

Red and dead galaxies, being large and bright, are well studied in the nearby Universe. They tend to be elliptical in shape, devoid of any cold gas, and reside in the densest regions of the cosmos such as groups and clusters of galaxies. These are valuable clues, but today's red and dead galaxies died long ago. In a "cold case", it's always tough to sort out the cause from the effects. Does losing a galaxy's gas cause it to become an elliptical? Or does becoming an elliptical cause it to lose its gas? And why does all this happen preferentially to the most massive galaxies, living in the densest regions? There are many clues, but no clear answer.

Elliptical galaxies often have strong X-ray emission associated with gas at many millions of
degrees. These images show a sample of elliptical galaxies, with the optical image showing
the stars on the right, and the X-ray image showing the hot gas on the left. The X-ray gas
often shows a lot of structure, indicating that it has been disturbed putatively by jets
from the galaxies' supermassive black holes. From the Chandra image archive.

For a long time, the answer seemed fairly obvious: Galaxies begin with a reservoir of cold gas, and once they use that up, they can't form any new stars, and they die. But upon closer scrutiny, this explanation doesn't hold water (or in this case, gas). Firstly, non star-forming galaxies form a tight red sequence in color-magnitude space, distinct from star-forming galaxies in the blue cloud, and the region in between (known as the green valley) is conspicuously devoid of galaxies. This means that whatever turns galaxies red happens quickly; if it was gradual, there would be a continuous distribution in color towards the red sequence, with no green valley gap. Hence galaxies don't die of "natural causes" by just gradually running out of gas. They are actively being murdered.

Even more damning for the "running out of gas" explanation is that red and dead galaxies actually are surrounded by a halo of hot gas, typically at a few million degrees. We can see this gas via its X-ray emission with telescopes such as the Chandra X-ray Observatory. And here's the rub: This gas should be cooling! By any reasonable estimate, the energy loss rate of this gas implies that red and dead galaxies should be acquiring tens to hundreds of solar masses per year of fresh cold gas to fuel star formation. But this is clearly not happening -- we see no cold gas in these galaxies, and no star formation. Something is keeping this gas hot, and not allowing a dead galaxy to revive itself.

So it appears that we may need two killers: One to quickly quench a galaxy's star formation by removing the cold gas, and another to prevent any new cold gas from falling on the galaxy. The plot thickens!

Do astronomers have any suspects? Well, one simple rule in astronomy, analogous to "follow the money" in a terrestrial investigation, is to "follow the energy". Removing cold gas from a galaxy takes a lot of energy. So does keeping an entire halo of hot gas hot. What agent has the ability to provide such enormous amounts of energy?

When viewed this way, the list of suspects narrows dramatically, basically down to one single agent. It is an improbable agent, one already shrouded in mystery as the ultimate harbinger of doom in the Universe:  The Supermassive Black Hole.

It was only around 15 years ago that it was realized that most sizable galaxies contain a supermassive black hole in their center, with masses that can exceed a billion Suns. But the radius of a black hole is miniscule when compared to that of the galaxy. And the vast majority of black holes seem to be fairly inert, like the one in our Milky Way, with only a small fraction emitting any significant amount of energy in a so-called active galactic nucleus, or AGN, phase. So how can these black holes, as dark and malevolent as they may seem, be responsible for killing an entire galaxy whose mass is a thousand times larger? The poor, persecuted black hole pleads innocence!

Not so fast, says the prosecution. Black holes act like a cosmic drain, drawing in any unsuspecting mass near the galaxy's center to be devoured into its space-time singularity. But it turns out that black holes are sloppy eaters. A significant portion of the mass that approaches the black hole, by being accelerated close to the speed of light, is converted into pure energy, and is released back out into the galaxy instead of being swallowed. This follows Einstein's famous relation, the energy released is the mass times the speed of light squared. Since the speed of light is a large number, a little bit of mass going in can mean a lot of energy coming out, perhaps enough to kill a galaxy!

This movie shows a numerical simulation of how a galaxy merger might trigger an AGN that removes the gas from a galaxy. The gas distribution is shown for two spiral galaxies, color-coded by temperature. As the two galaxies collide owing to their mutual gravitational attraction, a black hole is fed, which injects energy into the surrounding gas and evaporates it away. Mergers can also transform the galaxy's morphology from spiral to elliptical, which can explain why dead galaxies are usually ellipticals. Movie by T. Di Matteo, V. Springel, and L. Hernquist from Nature, 433, 604 (2005).

Energy released from black holes is called AGN feedback, and it's been observed. It is seen to be strong during galaxy mergers, when the disorder of the collision results in a sort of feeding frenzy for the central black hole, which can eject large amounts of gas. It is also seen to happen intermittently in galaxy clusters, where the cluster gas shows bubbles in X-ray gas that astronomers suspect have been inflated by powerful jets from the central black hole that are only active about 10% of the time. Crude estimates suggest that the amount of energy released in these events could plausibly provide enough energy to kill a galaxy. Heuristic models that include these effects such as the semi-analytic models of CANDELS theorist Rachel Somerville are able to predict a population of red and dead galaxies mostly as observed, which is encouraging. So it seems to be a plausible scenario.

But plausibility wouldn't yield a conviction in a court of law, and it doesn't hold up in a court of science, either. The energetics are a necessary but not sufficient condition for black holes to kill galaxies. Closer examination reveals many puzzling aspects in this story, such as: How does the energy released by the black hole get distributed on such large scales? How does that energy know to go into exactly the gas needed to kill a galaxy, and not other gas? Why does the black hole start putting out all this murderous energy only once the galaxy is massive and living in a dense environment? Theorists have struggled to come up with well-justified answers to these questions, so the case against black holes remains full of, well, holes.

This is where CANDELS comes in. CANDELS will provide us many more clues than we had before.  For the first time, we will be able to probe back to when the first red and dead galaxies began to appear -- when the murders were fresh -- about 2-3 billion years after the Big Bang. Since red and dead galaxies are relatively rare, one needs a wide survey area to be able to find them, and since the first red and deads appeared long ago, one needs very deep imaging to see so far away. This combination of wide and deep is exactly what CANDELS is designed to provide. At the same time, CANDELS can also be used to track and identify black holes over much of cosmic time, with a little help from its survey friends. Using this data, we hope to directly associate the killing of galaxies with black holes, in essence try to catch the culprit red-handed.

But early results from CANDELS have only deepened the mystery. Work by Dale Kocevski and collaborators has shown that, at higher redshifts, the correlation between galaxy mergers and AGN is not nearly so clear as it is today. David Rosario led a study that showed that galaxies with AGN are not obviously distinguished from galaxies that don't, a puzzling result if AGN are supposed to be a harbinger of turning galaxies red. Jen Donley found that AGN at earlier epochs are increasingly surrounded by lots of obscuring gas and dust, which is odd if AGN are supposed to be removing all the gas. CANDELS has seen the first red and dead galaxies appear long ago, but they are strangely compact, unlike anything we see nearby, adding a new puzzling twist to the saga.

So far, we have yet to find any smoking gun evidence pinning the murder of galaxies on supermassive black holes, all while the black holes sit smugly smirking at our hard detective work.  Or perhaps we are in fact falsely accusing the poor black hole, and there is some other murderous agent responsible.  The end of this game of Clue still appears to be far away, which means that for CANDELS astronomers, the fun is just beginning!