Pedagoguery

You have no doubt heard that normal matter is only a very small constituent of the universe. What may come as a surprise to you, however, is how small a proportion of normal matter resides in galaxies. If you add up all of the matter in the galaxies we can detect, you end up with only one tenth of the total matter in the universe (dark matter excepted). Given that there are probably well over 200 billion galaxies in the visible universe, where is all of the other matter?

This is not a case of dark matter or dark energy. Those items, which compose 96% of the universe, are mysterious but which we know are there from a number of other observations. The issue here is with the 4% of the universe which we supposedly understand. Why does so little of this matter reside in galaxies, and where is the rest of it? This all points to the fact that galaxy formation must be very inefficient.

We know with fairly good certainty how many baryons (protons and neutrons) exist in the visible universe. There are a couple of different ways. First, the baryon density is encoded on the microwave background radiation in the pattern of fluctuations, and their sizes relative to each other. We get an independent confirmation of this number by looking at the initial abundances of helium, deuterium, and lithium created during the first minutes of the unniverse.

After the recombination era, we can still track the baryon abundance. When you look at the spectra of quasars, a number of features stand out. One of the strongest emission lines of the quasar is the Lyman alpha line, which represents the transition from the first excited state to the ground state in hydrogen. Just blueward from this line, is what is called the Lyman alpha forest. Literally hundreds of sharp absorption lines, each one representing an atomic cloud that the quasar's light passed through on its way here. The strength of the line is determined by the size of the cloud, and by taking a careful census, we can account for all the baryons in the early universe.

Once baryons join up with a galaxy, however, they take on many different forms. We can still take a census of the baryons, but it requires more work. Using visible and near-infrared light, we can catalog stars, infrared shows us dust, in radio, we can see cold molecular clouds, and so on. However, doing so only results in 10% of the mass of baryons that we know are there. Therefore, we must conclude that that matter lies elsewhere. The question is where. A second question is how.

Some of the baryons can be found in galaxy clusters, which are filled with diffuse gas, but this only accounts for another 4%. However, a clue to the location of the rest can be found in the evolution of the structure of dark matter..

According to simulations, dark matter has evolved into a filamentary structure. Because there is so much more dark matter than baryonic matter, the baryons tend to follow the dark matter distribution. Galaxies, it seems, are only the highest density nodes in this structure, so the rest of the baryons should be out there. However, they must have evolved into a form that is very hard to detect.

That form is what is called the WHIM, or Warm-Hot Intergalactic Medium. As the gas falls into the dark matter filaments, it gets heated by shock waves to from 100,000 to tens of millions of Kelvins. While that sounds quite hot, it is cool in comparison to the intergalactic gas in galaxy clusters; too cool to shine out in X-rays. So how can the WHIM be detected? A number of studies using a variety of orbiting telescopes have caught tantalizing glimpses. By viewing the absorption lines of strongly ionized oxygen in the far ultraviolet, astronomers have been able to detect traces of the coolest portion of the WHIM. Observations using Chandra and XMM-Newton of even more strongly ionized oxygen (retaining only one or two electrons) has seen even more; potentially enough to account for the missing baryons. However, there are drawbacks to these observations. The ionized oxygen is being used as a tracer – we are not seeing the bulk of the WHIM, and we have to make assumptions about the ratio of oxygen to other constituents – assumptions which may not be correct. This technique also relies on quasars, which have to be placed in such a way that one or more of the filaments lies between us and them. Since quasars, especially bright ones, are rare, this makes it a rather hit-or-miss proposition.

Why does the WHIM exist? By knowing the general nature of the WHIM, we can get clues about where it comes from, and why galaxy formation is so inefficient. The key involves what is termed galactic feedback. It turns out that galaxy formation is a balance between inflowing gas, and ejection of gas through several mechanisms: supernovae, stellar winds, and supermassive black holes. Each of these mechanisms adds energy to the interstellar medium, and can choke off the inflow of gas from the intergalactic medium. In large galaxies, the black hole is probably the dominance mechanism, while for smaller galaxies, it is the supernovae and stellar winds. This is probably why we see fewer small galaxies than simple models would suggest – the first few supernovae literally eject most of the gas from the shallow gravitational well of the galaxy. However, in a larger galaxy, it takes the highly energetic jets from a black hole to accomplish the same thing. Thus, it appears that there is a constant exchange of matter between galaxies and the intergalactic medium. In fact, it is possible, if not likely, that some of the atoms in our bodies spent some time in intergalactic before settleing into our solar system. You may be far more widely traveled than you know.

Next time, spinning black holes.