Seeing the light
DOI: 10.1063/PT.5.010308
The United Nations has declared 2015 to be the International Year of Light . As a former astronomer, I appreciate the gesture. Although astronomers observe cosmic rays and neutrinos and although space probes have landed on planets and brought back samples of asteroids, astronomy remains a light-based science.
Astronomers know that to justify the expense of building new telescopes—especially space-based ones—each generation of instruments has to be significantly more capable than the previous one. When observing technology makes great leaps forward, the discoveries are often exciting and unanticipated.
But because of their reliance on photons, astronomers can sometimes fall into the natural and reasonable habit of regarding celestial bodies wholly as emitters of light of one form or another. When we look at a spectacular image of a spiral galaxy, we should remind ourselves that about 90% of the matter that constitutes the galaxy is not only dark, it is also of unknown origin.
Then there’s the astronomers’ habit of classifying objects based on their appearance. Broadly speaking, galaxies whose central black holes power luminous cores are classified in two types, Seyfert I and Seyfert II, depending on their spectral lines and whether their cores emit copious amounts of UV and x rays (Seyfert I) or not (Seyfert II).
But in 1984 Robert Antonucci and Joe Miller discovered that the archetypal Seyfert II galaxy, NGC 1068, when viewed through a polarizer, has emission lines that are characteristic of a Seyfert I. Ordinarily, that emission would not reach an observer because it would be absorbed by the thick torus of dust and gas that surrounds the central black hole and stifles the UV and x-ray emission. But some of the emission misses the torus altogether. If it bounces off free electrons above and below the torus, it not only becomes polarized but can also be reflected around the torus and into the observer’s line of sight. Antonucci and Miller proposed that the principal difference between a Seyfert I and a Seyfert II arises from viewing angle—that is, whether the central emission region is viewed through the torus (Seyfert II) or not (Seyfert I).
Thinking first of a celestial body’s light-emitting properties is not necessarily a bad habit. It reminds us that everything we know about certain classes of object, such as quasars, comes only from light. But the light-first approach can be confusing to physicists who are not astronomers.
I made that realization myself when I wrote a news story for Physics Today‘s July 2008 issue. The paper I covered reported the results of observing distant quasars through vast filaments of warm gas that lie between the observer and the quasar and that imprint absorption lines on the quasar spectra. The authors’ goal was to account for the baryons that were left behind when dark matter and baryonic matter condensed to form galaxies.
I could have written the story, astronomer-style, by starting with the observations and working toward their interpretation. That approach would have worked for my astronomer readers, who’d have some idea in advance of how the story would unfold. But I worried that it risked confusing my non-astronomer readers, who might not know much about quasars, gaseous filaments, and missing baryons.
So I resolved instead to devote the story’s first paragraphs to describing what astronomers understood to be going on physically without regard to the emission or absorption of radiation:
By the time the universe was three minutes old, all the baryonic building blocks of normal matter had formed. As the young universe aged, a minority of those primordial baryons—a few percent—joined clumps of the far more abundant dark matter to condense and form the first galaxies.
Those galaxies grew by merging with each other. They grouped together in clusters. New generations of stars sprang from the gas left by their predecessors’ explosive demise. Throughout those processes, which are still going on, the overall distribution of baryons more or less persisted: Most baryons remain outside galaxy clusters.
Astronomers have confidence in their predictions of how much baryonic matter formed in the Big Bang. And their observations of luminous matter indicate how much baryonic matter lies in stars, galaxies, and galaxy clusters. What’s been harder to determine is the baryonic content of the intergalactic medium.
In principle, accounting observationally for all the IGM baryons is straightforward. Both the density fluctuations that led to the first galaxies and the mergers that formed their successors have squeezed dark and baryonic matter into a foamy network of widely spaced nodes, the galaxy clusters, connected by wispy filaments, the IGM.
Having set the physical scene, I felt ready by the fifth paragraph to write about what astronomers could infer about the filaments from their observations.
Computer simulations like this one show that galaxies and clusters of galaxies form at the nodes of a foamy, filamentary web. The area of sky in the image is about 100 000 light-years across. Stars appear in yellow. The colors from violet through blue and green to white correspond to gas of increasing density.
CREDIT: Sergey Mashchenko, McMaster University and SHARCNET
Looking back at that story, I realized that I had also adopted what might be called a physics-first approach to my choice of figure 1, which I’ve reproduced here. The figure shows a dwarf galaxy in an early stage of its formation, as simulated by Sergey Mashchenko of McMaster University and his colleagues. Although ionizing radiation was a key ingredient in the simulation, the visualization simply shows where the gas and stars are.