Wednesday, February 24, 2010

Where have all the protons gone?

Astronomers have long known that there is a rather close relationship between the intrinsic luminosity of a spiral galaxy and the rotational velocity of stars (around the galactic center) in the outer portions of the galaxy. This relationship even has a name: the Tully-Fisher relation.

It has also been known that small, nearby dwarf galaxies, which are irregular in shape, are not nearly as bright as they "should" be, according to the Tully-Fisher relation, given the measured average velocities of their stars.

Recent research shows that, nevertheless, the Tully-Fisher relation can actually be extended, with slight modification, to very large structures: entire clusters of galaxies. In that case, the intrinsic brightness of a cluster is mostly in the X-ray part of the spectrum (because it's due to very hot intergalactic gas), yet the correlation of cluster brightness to the average velocities of galaxies in the cluster is still quite good.

There's actually a very good explanation for the correlation, in that intrinsic brightness and average velocity of constituents are both closely tied to the total mass of the object.

And this is where things get very interesting. One has to consider the mass of ordinary "baryonic" matter separately from the mass of non-luminous dark matter. Many different kinds of independent observations point to the existence of almost 5 times as much mass of the universe in the form of dark matter as there is in the form of ordinary matter. Stated differently, ordinary matter makes up only 17% (a bit more than 1 part in 6) of the total mass of matter in the universe.

As long as the intrinsic luminosity of an object is proportional to its total mass, then mass can be taken as a proxy for luminosity, and a relationship between total mass and average constituent velocity is to be expected. This relationship is in fact predicted even by Newtonian mechanics – total mass should be proportional to the 4th power of velocity (M ∝ V4).

If one could further assume that the ratio of mass in the form of ordinary matter to mass in the form of dark matter in a galaxy or cluster is the same as the ratio in the universe as a whole (1 : 5), then the Tully-Fisher relation makes perfect sense. And this is so even though luminosity is entirely produced by ordinary matter, not the invisible dark matter. Indeed, this holds up very well – for spiral galaxies.

Surprisingly, there is also a fairly good relationship between luminosity and average velocity even in galaxy clusters – but there's a slight difference in the exponent: M ∝ V3. Again, this holds regardless of whether one considers total mass (including dark matter), or just visible ordinary matter. (The mass of a large cluster can be determined independently by techniques such as gravitational lensing.)

It's customary to plot mass vs. velocity (on vertical and horizontal axes, respectively) with logarithmic scales on both axes. When this is done, one gets straight lines that have slopes of approximately 4 (for spiral galaxies) and 3 (for galaxy clusters).

However, when plotting visible mass vs. velocity, the relationship breaks down almost completely for nearby dwarf galaxies. The smallest and dimmest dwarf galaxies are far below the curve. Their visible mass and luminosity – not counting dark matter – is far too small. On a log-log plot, such galaxies fall, with quite a large scatter, around a straight line having a slope of 5 or more.

There's a simple way to restate this observation: dwarf galaxies have far less visible ordinary matter than predicted by a traditional Tully-Fisher relation, and even much less than that if the ratio of ordinary matter to dark matter in the dwarf galaxies were close to what it is in the universe as a whole. In most dwarf galaxies, the ratio is less than 1% of what it "should" be.

In other words, there's an awful lot of ordinary matter missing and unaccounted for in dwarf galaxies. Hence the question (since ordinary matter is mostly hydrogen (protons)): where have all the protons gone?

Observations of nearby dwarf galaxies are pretty reliable – these are our closest neighbors. Assuming Newtonian gravity, we know the masses of these objects very reliably from the velocities of the stars (which we can see individually) within them. There's no hot hydrogen in these galaxies, as there is in distant galaxy clusters, since we see no signal of it in any part of the spectrum down to the infrared. Astronomers are also pretty certain that there's not a lot of cold hydrogen, which should emit strongly at radio frequencies – the famous HI 21-centimeter line.

So where are all the protons? Quite possibly they've been blown outside of the dwarf galaxy entirely, by supernova winds. Escape velocity from a dwarf galaxy is a lot less than what it is for a typical spiral, yet supernovae have just as much bang as they do anywhere else. Very recent detailed simulations have supported this idea, as I discussed here.

An alternative, and rather more radical, possibility is that Newtonian gravity is wrong – the protons still aren't there (why?) but neither is any "dark matter". Instead, the total mass of visible stars – as surprisingly small as it seems to be – is still enough to account for observed stellar velocities, using some form of "modified Newtonian dynamics" (MOND).

Unfortunately, for believers in MOND, the theory was concocted as an alternative to dark matter for explaining rotational velocities in spiral galaxies. MOND theories are typically adjusted carefully to fit the spiral galaxy data. They would need to work differently in dwarf galaxies. And they are already known not to work right for large galaxy clusters either.

Lots of intriguing questions here...

Original abstract:

The Baryon Content of Cosmic Structures
We make an inventory of the baryonic and gravitating mass in structures ranging from the smallest galaxies to rich clusters of galaxies. We find that the fraction of baryons converted to stars reaches a maximum between M500 = 1012 and 1013 M, suggesting that star formation is most efficient in bright galaxies in groups. The fraction of baryons detected in all forms deviates monotonically from the cosmic baryon fraction as a function of mass. On the largest scales of clusters, most of the expected baryons are detected, while in the smallest dwarf galaxies, fewer than 1% are detected. Where these missing baryons reside is unclear.
McGaugh, S., Schombert, J., de Blok, W., & Zagursky, M. (2010). THE BARYON CONTENT OF COSMIC STRUCTURES The Astrophysical Journal, 708 (1) DOI: 10.1088/2041-8205/708/1/L14

Further reading:

The Baryon Content of Cosmic Structures – preprint at arXiv

Team Shines Cosmic Light on Missing Ordinary Matter (1/7/10)

Dark Matter and Dark Energy Update (1/9/10)

Inventory Asks: Where Is All the Non-Dark Matter Hiding? (1/15/10)

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