The Crowd and the Cosmos: Adventures in the Zooniverse

by Lintott, Chris

  the disc.

  So could such a jet, produced by activity in the core of IC2497,

  have excited the glowing gas in the Voorwerp? Such a scenario

  seemed more plausible after a close look at the neighbouring gal-

  axy itself. It seemed to be warped, its disc twisted and marked by

  thick dust lanes—features which suggest a recent interaction

  with a neighbour. Such an interaction, or even a merger, might

  have funnelled material down into the galaxy’s central region

  where the black hole lurks, piling on material and making the

  presence of a jet much more likely, and if such a jet existed at

  right angles to the galaxy’s disc, it looked plausible that it would

  hit the Voorwerp directly.

  196 Serendipity

  Mystery solved. Yet the details didn’t add up. If there was

  enough material falling into the black hole to produce a jet able

  to excite the Voorwerp, then the gas and the dust that immedi-

  ately surrounds the black hole should also be warm, glowing

  brightly in the infrared. The Voorwerp’s neighbour is bright in

  the infrared—it’s what astronomers call an LIRG, or luminous

  infrared galaxy—but it wasn’t bright enough to allow for the

  necessary activity. Despite the neatness of the explanation, it

  was clear that we couldn’t hide a powerful enough source of

  activity at the core of the galaxy to account for the Voorwerp’s

  brightness.

  This was confusing, but slowly, in discussions and emails and

  phone calls, those of us on the Galaxy Zoo team who were

  increasingly consumed with trying to answer Hanny’s simple

  question realized that we had to consider the possibility that the

  Voorwerp was an evolving object, capable of change. The dis-

  tance between the Voorwerp and the galaxy is about 50,000

  light years.* This means that light travelling from the centre of

  the galaxy to the Voorwerp, and then heading to Earth, would

  arrive 50,000 years after light taking the direct route. Put another

  way, the Voorwerp is a light echo—it reveals to us the state of its

  neighbour galaxy as it was 50,000 years ago.

  The fact that the Voorwerp is highly excited now tells us that,

  back in the stone age here on Earth, the black hole at the centre of

  IC2497 would have been feeding heartily, even though today it is

  relatively quiet. Had our Cro-Magnon ancestors had stone age

  binoculars, they would have found the galaxy—now too faint to

  be seen with small telescopes—easily visible, the nearest example

  of what we call a quasar. In the time that’s passed since then—

  * That’s the projected distance on the sky—the distance you get if you assume that the lines joining us to the Voorwerp and the Voorwerp to the galaxy make a right angle. If the Voorwerp is in front of or behind the galaxy, that distance might really be only 25,000 light years or as much as 75,000 light years.

  Serendipity 197

  not long in the 13.8 billion year long history of the Universe—the

  galaxy must have quietened down. Perhaps the black hole ran

  out of fuel, or the activity around the black hole might have

  heated the surrounding gas, preventing further collapse in a pro-

  cess known as feedback. In either case, as long as something dra-

  matic had changed in the galaxy in the last few tens of thousands

  of years, it would explain what we saw.

  This is an exciting idea. We can monitor what the black holes

  at the centres of galaxies are up to on human timescales, return-

  ing year after year to the same objects to see what, if anything,

  has changed. We can see too how the population of active galax-

  ies changes over cosmic time, by comparing the population of

  galaxies that we see at different epochs of the cosmos’s history.

  But being able to trace changes on a timescale of thousands of

  years, in-between these two more accessible timescales, is unique.

  Not that having had the idea was enough. Getting from there

  to a proper result was hard work, and a particularly hard slog for

  me; my PhD had made me an expert in using radio telescopes,

  not in handling data from anything with a mirror, and so I was

  learning what to look for in spectra and in the other data we

  gathered as we went along. With much help from Bill and others,

  eventually a short paper was ready, and we sent it off to our nor-

  mal research journal.

  We didn’t have to wait long for the referee’s report to appear in

  my inbox. Unfortunately, it was as critical as anything I’d seen.

  My analysis had, it said, basic problems, and the interpretation

  I’ve just spent paragraphs convincing you of was overblown. Our

  conclusion that the black hole couldn’t currently be active

  enough to account for the Voorwerp’s observed brightness was

  wrong, we were told.

  In some fields, such a bad report would have meant the end of

  the paper’s chances of publication. In astrophysics, there’s a

  more collaborative approach and it’s possible to go back and

  198 Serendipity

  forth with the referee until all is well, or until mutual exhaustion

  ends the process, at which point the editor as umpire steps in and

  ends the fight. This tangle with the referee was one of the most

  exhausting of my career, and we went back and forth more than

  a dozen times; what had started as a four page paper weighed in

  at thirty pages by the time we had satisfied every niggling doubt

  in their mind (or, I suppose, won through sheer exhaustion).

  This effort mattered. One reason for publishing in a journal is

  to show publicly that a result stands up, that it has at least passed the minimum standard required for peer review. A second might

  be to spread the word, communicating a result to one’s col-

  leagues. Another goal is to build up a public track record of

  work—even though there are well-documented problems with

  the approach, we use someone’s publication record when con-

  sidering them for jobs or for promotions. The fourth reason,

  though, is to stake a claim for posterity. Despite all the wrangling

  and the back and forth, we needed the paper out so that Hanny—

  as a co-author alongside the rest of us—could get the proper sci-

  entific credit for her discovery.

  The trouble was, the Voorwerp was big news. Hanny had been

  on Dutch television, who were reporting on the mysterious blob,

  and we’d been blogging to share our progress and our excite-

  ment with the Galaxy Zoo volunteers. As a result, a Dutch team

  of astronomers had decided to take a look at the new object

  themselves, using a network of radio telescopes to get a very dif-

  ferent view of the system, one more detailed than had been

  obtained at similar wavelengths before. Before our referee was

  happy and our paper could be accepted, this rival group pub-

  lished their own paper which told a different story.*

  * This Dutch group were kind enough to add Hanny and a few of the Galaxy Zoo team to the author list for the paper, but it was still annoying not to get there first.

  Serendipity 199

  A quick glance at their data showed that the Voorwerp was

  bigger th
an we thought. It turns out that only part of it has been

  excited enough to glow brightly when viewed in visible light;

  there exists a much longer streamer of cold gas wrapped round

  the galaxy, exactly as one might expect in the aftermath of a

  merger. The Milky Way’s satellites, the large and small Magellanic

  Clouds, have been disrupted by the encounter with our larger

  galaxy, and trail gas and stars behind them as a result. The conse-

  quences of a more dramatic merger should be even more spec-

  tacular, producing long ‘tidal tails’, streamers of gas that here

  happen to be in the right place to be excited. Was the Voorwerp

  just part of such a tidal tail, illuminated by a still-active jet?

  The cause of that excitation seemed to be visible for the first

  time in the Dutch radio data. Deep in the heart of IC2497, it turns

  out there is indeed a jet of material, moving fast and heading

  straight for the Voorwerp. This is exactly what we’d originally

  expected, and completely consistent with a currently active gal-

  axy. By the time our paper was published we had to argue that

  there were two possibilities—we were either right, and the galaxy

  was now quiet, or our calculations were wrong and the jet discovered

  by the Dutch team was evidence that we’d made a mistake.

  Luckily, there was a clear test available. Kevin—he of the original

  classifications that inspired Galaxy Zoo—and friends applied for

  and won time to use two telescopes in space. XMM-Newton and

  Suzaku are European and Japanese telescopes that look for x-rays, high-energy radiation emitted most commonly by very hot gas

  such as that swirling around an active black hole which is still

  growing. Using x-rays also allows us to peer through the dust

  clouds that surround the centre of a galaxy like IC2497, getting

  directly to the heart of the action.

  When we used these telescopes to look at the centre of the

  Voorwerp’s neighbour we saw precisely nothing. There were a

  200 Serendipity

  few stray photons recorded by the detectors, but nothing worth

  writing about. In other words, the galaxy was dead, just as we

  had predicted. And the implications are startling. The black hole

  at the centre of IC2497 probably weighs in at a few million solar

  masses, and yet a system dominated by an object that large has

  managed to change its behaviour completely in the small matter

  of a few tens of thousands of years.

  In the past, the Voorwerp must have been the brightest quasar

  in the sky, and as it would be too much of a coincidence to have

  the nearest such object behave oddly, such behaviour must be

  relatively common. Much better to assume that such things hap-

  pen all the time than to argue that the nearest such galaxy just

  happened to misbehave shortly before we came along to watch.

  The lesson of the Voorwerp is that galaxies switch from active to

  passive—and presumably back the other way—all the time.

  Classifying a galaxy as either active or quiescent becomes not a

  property of the galaxy, like its mass, but a statement about what

  stage of its life a galaxy is in.

  Even the story of our own Milky Way changes when you start

  thinking like this. The centre of our galaxy is a quiet place today,

  containing a supermassive black hole but one that is quiet and

  devoid of any substantial accretion disc. There was great excite-

  ment a few years ago when what appeared to be a gas cloud of

  about the same mass as Jupiter, named G2, appeared likely to fall

  in, and coffin-chasing astronomers were keen to watch. As it

  turned out, G2 survived its close passage around the black hole

  (it didn’t come close enough to pass the event horizon, the point

  of no return) and may well be something more substantial than

  just a cloud of gas.* Just the fact that everyone got so excited

  * The most favoured hypothesis seems to be a pair of stars embedded in a cloud of gas, for a variety of complicated reasons.

  Serendipity 201

  about it, though, tells you that not much happens in the centre of

  the Milky Way.

  Look away from the disc of the galaxy with the right eyes and

  the picture changes. NASA’s satellite Fermi has been mapping the sky in light that is even more energetic than the x-rays that

  betrayed the Voorwerp’s secrets. The gamma-ray sky is marked

  by two large bubbles of hot gas, extending symmetrically for tens

  of thousands of light years and centred on the galaxy’s heart. A

  good explanation for these structures, now known as the ‘Fermi

  bubbles’, is that they’re a pair of shock waves exciting the thin gas that exists beyond our disc, the echoes of a time not that long ago

  when the centre of our galaxy was home to an active black hole.

  As material fell on the Milky Way’s central engine, it shone so

  brightly that as its radiation travels out into space it can still

  excite its surroundings.

  So even the Milky Way can be active, and the Voorwerp helps

  us understand this behaviour. We also managed to get Hubble

  Space Telescope images and data, which made the Voorwerp yet

  more famous. (It made a brief appearance in one of David

  Letterman’s monologues, being revealed at the end as nothing

  more than a smear on a camera lens, easily wiped away.) In the

  meantime, Galaxy Zoo volunteers in search of their own discov-

  eries quickly assembled a set of ‘Voorwerpjes’ (Figure 24).* Each

  of them is a glowing blob of hot gas, excited by activity around a

  galaxy’s central black hole.

  The variety of shapes seen in the Voorwerpje sample, many of

  which have now been imaged by Hubble too, is remarkable. There are long braids of gas, which seem to twist round each other.

  * Voorwerpen would be the Dutch plural, but these are smaller versions, so we use Voorwerpjes—the diminutive. It’s amazing what you end up having to know in this job.

  202 Serendipity

  Figure 24 Example Voorwerpjes imaged with the Hubble Space Telescope.

  The emission is from excited oxygen and shows a remarkable range of

  shapes, which are difficult to explain.

  There are dense clouds, in front of, behind, and surrounding

  their host galaxies. There are a surprising number of rings, and

  features that look rather like the still-mysterious ‘hole’ in the

  Voorwerp. The complexities of these shapes are either caused by

  Serendipity 203

  an uneven distribution of gas or rapid changes in the luminosity

  of material falling down into the black hole, and they don’t make

  understanding these systems easy. Nonetheless, after a lot of

  work it seems clear that about a third of galaxies with visible

  Voorwerpen* have faded dramatically, just like the original.

  Hanny’s find is still inspiring new scientific discoveries a dec-

  ade after it was found, but it can seem like an sideshow compared

  to the main Galaxy Zoo story. Galaxy Zoo wasn’t set up to find

  giant glowing gas clouds, nor to reward volunteers who got inter-

  ested in curiosities they came across while classifying. The vast

  majority of the scientific papers which have been published as a

  re
sult of the project use the results produced by volunteers click-

  ing away on the main interface, few of whom get the recognition

  and dose of fame meted out to Hanny, and pay no attention to

  random discoveries made and discussed on the forums.

  Its importance is rather that it turned out to be the first of

  many similarly serendipitous discoveries, and in many cases the

  volunteers went far beyond just pointing at the presence of an

  object. The idea that citizen scientists could do more than just

  spotting odd stuff first became obvious when we came across

  the Green Pea galaxies—a good few months after our volunteers

  had started thinking about them.

  As the name suggests, these are small, round, and green

  objects which appear in the background of some of the images

  from the Sloan Digital Sky Survey that populated the original

  Galaxy Zoo. A small group of volunteers, calling themselves the

  Peas Corps,† set out to collect them but also, importantly, to find

  out what they were. First they noticed that the peas were all the

  * Got the plural in too!

  † It took me months to realize this was a joke. The revelation finally came when I was on stage, giving a lecture, and said the name out loud.

  204 Serendipity

  Figure 25 A ‘pea’ as found by Galaxy Zoo volunteers. This tiny galaxy is undergoing a dramatic burst of star formation; most of the light we see is due to emission from excited oxygen.

  same colour, a blueish-green; and then that the only other things

  this colour were small, irregular galaxies. Remarkably, the Peas

  Corp organized their own version of Galaxy Zoo to sort through

  the 15,000 or so objects that shared this hue, emerging with a set

  of a few thousand peas (Figure 25). Sloan also provided spectra

  for these objects, and the volunteers went and grabbed this extra

  data. When they inspected their haul, they noticed that their

  light is dominated by emission from oxygen.

  The significance of that discovery wasn’t immediately appar-

  ent, but they quickly decided that having a strong oxygen line

  was a good test of whether something was a ‘pea’ or not. Having

  tested their sample, they sent an email announcing to the Galaxy

  Zoo team—their professional counterparts—that they’d discovered

  not an odd object or a new galaxy, but a new type of galaxy.