Hidden Galaxies were Tom Morgan’s passion (and mine). We both fell under their spell when we were young and spent our lives, and other people’s too, searching for them. Were we mad, as many sensible astronomers thought, or were we lucky? After all, searching for a vast continent whose existence could only be inferred from coincidences and equations, seems close to insanity. But then Christopher Columbus had been driven to his own folly by finding tropical beans washed up on the wester shore of Ireland, and by scraps of manuscript written in Egypt but then left forgotten for a thousand years on a library shelf in the great dome of Byzantium — Agia Sofia.

The saga of of Morgan’s life-long obsession ( and mine) is the spine of my quartet of novels Written in the Stars, starting with Against the Fall of Night (AFN) and ending with Beyond the Western Stars (BWS), a sort of Sidereal Odyssey I won’t retell here. But what I can do for non-astronomers is add some scraps of the evidence, the tropical beans if you like and the pieces of parchment which kept Morgan and his comrades going when all the Odds looked to be against them.

The Wigwam diagram showing the Visibilty of any galaxy (upwards) plotted agains its dimness, plotted horizontally, dimmer to the right. It is the consequence of two plunging curves and so is very sharp and very thin, which surprised everybody. It is utterly unintuitive, yet entirely dominates our ability to see the extragalactic universe. It turns out that virtually all the galaxies we can measure lie right under the peak. That is either a miraculous coincidence or a warning that most galaxies are hidden out of sight.

Let’s begin with the calculation Morgan made back in 1975 in that caravan on the Teifi Estuary (AFN). Above we see it in the form of a graph. It shows the Visibility of a galaxy — that is to say how easy it will be to see, plotted upwards, against its dimness, plotted towards the right along the bottom. And what Morgan found, to general consternation and surprise, was an extremely sharp, narrow peak. The inference was that only galaxies of a very particular dimness (or ‘surface brightness’ in the jargon) would be visible to mankind. Those ones to the right (‘Icebergs’ Morgan called them) would be sunk too far below the night sky, whilst the ones to the left (‘Brilliants’) would be so small in apparent size as to be mistaken for background objects And here was the killer-coincidence: all the galaxies known to science at the time fitted exactly underneath Morgan’s peak. That is why the paper, with its implicit challenge, was published in the journal ‘Nature’ in 1976. What the diagram The ‘Wigwam diagram” as we came to call it, cannot convey is just how narrow the Wigwam really is. It is ten thousand times narrower than the total range over which the occasional galaxy has turned up by accident. Ten thousand times! Even Morgan sometimes couldn’t believe that. Apparently we are looking at the universe through a mere crack in the shutters. It was the Wigwam diagram which kept Morgan and his crew sailing on, through doldrum and tempest, for the next forty years.

Astronomy is beset by what are called “Selection Effects”. That is to say we build our picture of the cosmos selectively out of what we can observe down here, pretending that what we cannot observe, which might be much the greater portion, is not significant. What else could we do? Morgan’s wigwam was thus a rude shock, for it suggested, very directly, that Astronomy must be missing much of the extragalactic cosmos. What could be done about that? We had to try and devise alternative observing strategies which might enable us to see through one window, what could not be seen through another.


Using that approach Morgan and his colleagues decided to survey the sky in the radio band, and when they found a source, check what was there in the optical. The next figure shows some typical results, with a radio spectrum superposed on a negative image (easier to see) of the corresponding area of the visual sky..

Here are radio scans of the sky made with the Parkes Radio Telescope superposed on negatives of the optical sky behind. The receiver is tuned to the frequency of gaseous Hydrogen receding from the Earth at the velocities ( in Km/sec) shown at bottom. The two upper spectra corresponded to giant spiral galaxies, bottom left to a dwarfish Irregular galaxy, and bottom right to a dim galaxy barely visible above the sky. The area under each spectrum is a measure of the total amount of gas present while the width derives from the internal motions within the galaxy ,such as rotation. Much can be inferred from these measures. Copyright Monthly Notices of the Royal Astronomical Society.

Usually there is indeed a galaxy to be seen there. But of course the team were hoping to find cases where the optical counterparts were invisible — i.e. true ‘Hidden Galaxies’

A montage of galaxies found at Parkes and then observed in several colours with the Sloan Survey Telescope in New Mexico. The six bottom right are all colossal giants more massive than our Milky Way. Nevertheless, as you can see, some are very dim. This all ties in with the Wigwam diagram and indicates just how treacherous a purely optical survey of the Universe might be. Courtesy of Professor Julianne Dalcanton, University of Washington, Seattle

The figure above shows that, from time to time they came close. Each postage stamp in the montage shows the optical image corresponding to a radio signal found in a blind survey of the sky made with the Multibeam Receiver fitted to the Parkes Radio Telescope in Australia. As you can see some are almost invisible, lying in the very wings of the Wigwam diagram. It is important to emphasise that the Luminosity of a galaxy (which corresponds to the number of stars it contains — generally billions) and its surface-brightness (dimness) are entirely different concepts, the latter depending on how its Luminosity is spread out across the sky. Although the six galaxies bottom right are all luminous giants, some are nevertheless, extremely dim.

There is another trick though in astronomy for finding something invisible in Space: observe an object behind it and look for tell-tale gaps (‘spectral ghosts’) in its spectrum where specific atomic species in the invisible object have absorbed out the light coming from behind. That is what Frank Cotteridge and his like found, albeit by accident, when they observed the spectra of very distant Quasars — lots and lots of inexplicable absorption lines (‘spectral ghosts’). “What else could they be”, Morgan argued, “If not my Hidden Galaxies?” Thus the bitter battle over QSOALs or ‘Quasi Stellar Object Absorption Lines’ began (see especially “Crouching Giant“).

The spectra of Quasars showing the many absorption lines (spectral ghosts) etched into them. Measurements show they are caused by clouds of atoms like Hydrogen and Nitrogen lying in the foreground along the line of sight to the quasar. But what form could those clouds take? Morgan claims they are the numerous Hidden Galaxies you would expect. Opponents who don’t like that idea are forced to postulate that visible galaxies must have absolutely vast gaseous halos surrounding them. Controversy continues [see Whispering Sky and Crouching Giant in particular]. As you go down the montage one is looking at higher and higher redshift quasars. Out there, back in time, the absorbing clouds appear to have been crowded closer and closer together. The humps are features in the spectra of the Quasars themselves. Copyright The European Southern Observatory (eso.org).

In 1987 the whole field was electrified by a paper written by Greg Bothun, Chris Impey and colleagues who were then based in California. Quite by accident, while observing dwarf galaxies in the nearby Virgo Cluster, they noticed that one wasn’t a dwarf, but the nucleus of a “Crouching Giant”, that is to say of an absolute monster of a spiral galaxy 25 times further away than the cluster but too dim to show much of itself above the sky. Here was unequivocal evidence that Hidden Galaxies of the most dramatic kind actually existed.

The Crouching Giant found by Greg Bothun, Chris Impey and co. by accident in 1987. The bright nucleus (this is a negative) was thought to be a dwarf galaxy in the nearby Virgo Cluster. But some very smart detective work revealed that it was instead the core of an absolutely colossal but dim giant spiral 25 times further away, whose spiral arms you can just pick out. It is no less than half a million light years across, ten times the extent of our own colossal Milky Way. Because of the accidental way it was found, finding others like it would be infernally difficult. Copyright Astronomical Journal 1987

That might have been that — except that nobody could find another such. The sceptics could, and did, write it off as a freak. If Hidden Galaxies were to become ‘significant’ they needed to make up a healthy fraction of the cosmic light and mass. In other words astronomers needed to find lots of Crouching Giants.

And how we all tried! But even when Jon Davies & co. did find one at Jodrell Bank (below) the opposition was fanatical.

Theoreticians who’d ‘proved’ that Hidden Galaxies couldn’t exist were furious; observers with even bigger telescopes than Jodrell were adamant that if they hadn’t found one then certainly we could not. And then there were the computer modellers who, at the drop of a hat, could prove or disprove anything, often without acknowledging the manifold frailties of their craft.

The putative Dark Galaxy VirgoHI-21 in the Virgo cluster. Left shows the radio contours superposed on a negative optical image. (Data obtained with the Westerbork Array in Holland) The giant spiral NGC4254 has obviously been disturbed by an encounter with a massive object which could only be Virgo HI-21, which is Dark, but note the bridge of gas between them. But the dynamical map (Right) shows it is spinning rapidly which can only mean that it is indeed massive. Massive, dark, spinning; what else could it be but a Dark galaxy? Copyright The Astrophysical Journal, 2007.

So although , after titanic refereeing battles, Virgo HI-21 did eventually get published in the prestigious Astrophysical Journal, most of the self appointed ‘experts’ stubbornly refused to acknowledge it as the first Dark Galaxy. But, in my opinion, if you read all the arguments carefully enough, it cannot be anything else.

In Big Science the problem is very often Lack of Breadth, rather than Lack of Depth. The clues are here and there but who has the breadth to spot them all, and assemble a coherent picture? Often we fail because no one individual in the field has the required breadth. And then there are the Systematic Errors that can bedevil any ambitious undertaking, errors held on to fanatically, especially by those who do not appreciate the frailty of The Scientific Method, and the need for caution in applying it (See my book Thinking for Ourselves) . This is highlighted in the following image based on observations we made with the Jansky Telescope in New Mexico, much the most powerful radio telescope on Earth at present. It reveals a huge cloud of hydrogen, the signature you would expect of a Dark Galaxy, but with a giant but optically visible galaxy to the South, receding away from us at the exactly the same speed as the Hydrogen. Previously the Parkes team, to which I then belonged, had mistakenly identified that as the source of the Hydrogen, and so overlooked what appears to be a true dark galaxy. Galaxies, Dark or Light, cluster so gregariously together that one needs a very powerful beast like the Jansky, to distinguish between them. None of us fully appreciated that, certainly not the Quasar observers with their spectral ghosts, who could always postulate, around visible galaxies, ‘gaseous haloes’ of unlimited size, to discount the invisible ones, which is what most of them choose to do.

What a Dark Galaxy ‘looks like’. Parkes 0039+03 was first discovered as a massive Hydrogen source out at 5,300 km/sec recession-velocity by Morgan and co. using the Parkes dish. They mistakenly associated it with the luminous optical galaxy (marked ‘cont’ here ) which happened to have an almost identical radial velocity, even though it is rather far away on the sky. But much later these more precise observations with the colossal Jansky array revealed that the Hydrogen and the bright galaxy are unassociated, as you can see. Even later a much deeper optical observations of the cloud made with the William Herschel 4.2 metre telescope in the Canary Islands revealed that it has tiny patches of light in it, but that is all. The strong clustering of galaxies together, both in space and in velocity, makes the search for Dark galaxies far harder than anyone had imagined. But if this isn’t a dark galaxy then what is? We found more like this out there.


Thus far I have spoken entirely of Icebergs, hidden below the sky on the right hand (dim) side of the Wigam; what about the ‘Brilliants’ on the other? They would be even harder to find so Morgan and co almost forgot them altogether. Apart from anything else, being compact, they would be largely shrouded in their own smoke, disguising their true brilliance, appear like ordinary galaxies, but far far in the background, and therefore of no particular interest.

It was only after WFC-3 was operating on Hubble (2009) that Morgan began to worry about the extremely high redshift galaxies dotted all over the background in deep Hubble images (see below). If the universe were really expanding they oughtn’t to have been there — dimmed out of visibility by the so called ‘Tolman Effect’. And they turned out to be very small physically, much smaller than galaxies of the same luminosity situated close by to us in Space. Then the penny dropped with a clang for Morgan. Here were his Brilliants but at very high redshift, dimmed just enough by expansion to place them in the Visibility Wigwam where they became possible for us to see. The implications though were startling: Space must be inhabited by vast numbers of Brilliants , just as it probably was by Icebergs. And together all their extra radiation would have sufficed to re-ionise the Universe — otherwise a major problem for Cosmology. So it all fitted together: Hidden Galaxies, Expansion, Brilliants, the Wigwam diagram, Reionisation…….if Morgan was right. If…….. This was the theory which obsessed him towards the end of Beyond the Western Stars.

The Hubble Ultra Deep Field, the deepest image ever taken. In an expanding universe distant galaxies ought to be dimmed to the point of invisibility by straightforward physical effects. Yet here they are, dotted all over the place. Either the universe isn’t expanding or these are normally invisible Brilliants, shifted into the Wigwam by redshift so as to be visible. Courtesy ESA/NASA

Who was right, and who was wrong can only be decided by posterity . But in my story of Hidden Galaxies I have tried to convey, above all, just how engrossing it all was: the tournament of ideas, the clashes of personality and ambition, the conflicts of evidence, the camaraderie, the bravery and the cowardice, the wild misunderstandings and the hazards of fortune……. They make science such an exciting career; though not one for the faint-hearted.

PS. I have actually left out the biggest reason for mystery here, because it has a post of its own entitled HOW DARK IS THE NIGHT?

Professionals who would like to see a fairly up-to-date review of this subject can look at my opening address to the International Astronomical Union symposium No.355 held at the IAC in Tenerife in 2019 entitled “The Realm of the Low Surface Brightness Universe” (Procs. edited by David Valls- Gabaud to appear soon in CUP) at:


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