Sunday, February 21, 2010

Dwarf galaxies start making sense

Cosmology has, for a decade, had its "standard model", which largely explains most of the cosmological phenomena that astronomers are able to observe. Except for a relatively small number of things that don't seem to make sense in the model. Prominent among the latter are dwarf galaxies – by one definition, galaxies having less than 10% of the total mass of the Milky Way.

The standard model of cosmology is known officially as the Λ-cold-dark-matter model – ΛCDM. (This theory has no particular relation to the Standard Model of particle physics.) Cold dark matter (CDM) refers to the hypothesis that a large part of the detectable mass content of the universe consists of particles that are not accounted for by the Standard Model of particle physics. The dark matter is said to be "cold", because it appears to consist mostly of "non-relativistic" particles, meaning particles moving at speeds not close to the speed of light. That excludes, for example, neutrinos.

As weird as the idea of dark matter might seem, there is abundant evidence for it, which can't easily be better explained in other possible ways. (Although, many other possibilities have been proposed.) I haven't written a lot about this recently, since the evidence for CDM just keeps piling up, but here's one important study. Dark matter is "observed" indirectly through its gravitational effects on ordinary visible matter. For instance, the motions of stars in the Milky Way have recently been analyzed closely enough to show that the dark matter in which the Milky Way is embedded has the shape of a squashed beach ball. (See here, here, here.)

Λ is the conventional symbol used for the "cosmological constant", which is a concept from Einstein's general theory of relativity. It is supposed to account for the observed phenomenon of "dark energy". This too is controversial, but there is much evidence for it, from a variety of different studies that are not all based on the same kinds of observations. I last wrote at length on the evidence here.

I need to write a lot more about recent evidence for dark energy, but I'll be very brief about it here. There is very recent evidence involving the motion of galaxies quite near our own (see here). Other than that, the evidence for dark energy is based on observations of distant Type Ia supernovae (about which there's a lot of recent news), "weak lensing" (see here), and "baryon acoustic oscillations" (a large topic).

In spite of all this evidence, ΛCDM isn't without its problems. As already suggested, one set of problems involves dwarf galaxies. There are at least two (somewhat related) parts to this problem. The larger part of the problem is simply that not enough very small dwarf galaxies (masses less than a percent of the Milky Way's) have been detected. This is often known as the "missing satellite problem".

Dwarf galaxies, being very small, are also intrinsically dim, and thus difficult to observe at all unless they're very nearby. However, only about 11 dwarf galaxies are known to be satellites of the Milky Way – and such satellites should be the easiest of dwarf galaxies to detect. This is a serious problem, since simluations of expected galaxies sizes based on the way that dark matter should be expected to clump together predict as many as 500 dwarf satellites of the Milky Way.

The other problem is known as the cuspy halo problem. "Halo" refers to the cloud of cold dark matter in which all visible galaxies are expected to be embedded. Simulations indicate that the dark matter should be concentrated in the center of the halo instead of being evenly distributed throughout. This is intuitively reasonable – after all, most of the ordinary matter in our solar system is concentrated right in the middle, in the Sun itself.

This problem exists somewhat even for large galaxies like the Milky Way, but it is much more severe for dwarf galaxies. In fact, it seems as though the smaller the galaxy is, the greater the tendency for the dark matter (as indicated by orbital motion of stars within the galaxy) to be distributed fairly smoothly, with little or no density cusp in the center.

Related to this is a recent finding (see here) that smaller galaxies seem to have a smaller proportion of ordinary baryonic matter to dark matter than does the universe as a whole. And, in fact, the smaller the galaxy, the smaller the proportion of ordinary matter. In the universe as a whole, there is much evidence, based on detected abundances of light elements and observations of the cosmic microwave background, that there should be about 5 times as much mass in the form of dark matter as there is of ordinary matter. One might expect this proportion to be about the same in galaxies. Yet instead, in the smallest galaxies, astronomers can detect less than 1% as much ordinary matter (in the form of visible stars) as one would expect to find.

This would suggest that an important reason we can't detect very many small galaxies is that they simply have too few stars and are too dim to see. But it still doesn't explain why this should be the case.

In fact, I wrote 2½ years ago about a study that reported finding many small galaxies consisting of 99% or more of dark matter (here). The authors of the study even speculated that the reason such galaxies were mostly composed of dark matter was that "the fierce ultraviolet radiation given off by the first stars, which formed just a few hundred million years after the Big Bang, may have blown all of the hydrogen gas out of the dwarf galaxies forming at that time." And they added, "The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or in many cases completely dark. When this effect is included in theoretical models, the numbers of expected and observed dwarf galaxies agree."

Kind of makes sense, doesn't it? In fact, even for galaxies that began to form later, a large number of supernovae early in the life of a galaxy might be enough to blow away most of the hydrogen from which additional stars could form. And indeed, a recent much more detailed simulation of galaxy formation supports precisely this idea.

Why is it that previous simulations had not caught this? The reason is very simple: detailed simulations of galaxy formation and evolution are exceedingly demanding of computer resources. In order to make such simulations even possible – up until now – astrophysicists considered only the effect of gravitational collapse of a mixture of ordinary and dark matter. The effects resulting from star formation and subsequent supernovae were omitted entirely.

Duh.

Actually, this simplification is pretty understandable. The simulation that is the subject of the research under discussion here, that did take into account stellar formation processes, consumed an almost incredible amount of computing time. According to one report, "The simulation was carried out using about 250 processors running for about two months." That's more than 40 processor-years.

And that's just for one simulation, involving a single set of initial conditions.

Here's the abstract:

Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows
For almost two decades the properties of ‘dwarf’ galaxies have challenged the cold dark matter (CDM) model of galaxy formation. Most observed dwarf galaxies consist of a rotating stellar disk embedded in a massive dark-matter halo with a near-constant-density core. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate. Here we report hydrodynamical simulations (in a framework assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies—bulgeless and with shallow central dark-matter profiles—arise naturally in these simulations.

Basically what the simulation has to do is to incorporate a level of granularity that reflects the size of a typical star-forming region: "Baryonic processes are included, as gas cooling, heating from the cosmic ultraviolet field, star formation and supernova-driven gas heating. The resolution is such that dense gas clumps as small as 105 M are resolved, similar to real star-forming regions."

It certainly wasn't possible to do a simulation where the granularity was on the order of the size of a single star – that could take 105 times as long. Yet the results are very reasonable. The simulation produced a galaxy that closely resembles dwarf galaxies actually observed. In particular, the simulated galaxy has no "cusp" of dark matter density at the center, and no central bulge of visible stars in the center either.

And so the simulation adequately accounts for properties of real dwarf galaxies, which no previous simulation has done. The intense outflowing "winds" from supernovae that result from the heaviest initially-formed stars sweep all ordinary baryonic matter out of the central region. These winds are simply high-energy photons, which interact only with ordinary matter, not dark matter. However, the ordinary matter does interact gravitationally with the dark matter, which also then gets pulled away from the center.

The simulation does not directly settle the question of why so few very small dwarf galaxies are observed. Presumably, many small dwarfs actually do form. They just have so little ordinary matter that is able to coalesce into stars that the galaxies are too dim to detect at any great distance. This is in accord with other studies that show that the smallest galaxies have only a very small proportion of visible ordinary matter.



ResearchBlogging.org
Governato, F., Brook, C., Mayer, L., Brooks, A., Rhee, G., Wadsley, J., Jonsson, P., Willman, B., Stinson, G., Quinn, T., & Madau, P. (2010). Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows Nature, 463 (7278), 203-206 DOI: 10.1038/nature08640


Further reading:

Supernova winds blow galaxies into shape (1/13/10)

Supernovae put dark matter in the right place (1/13/10)

New research resolves conflict in theory of how galaxies form (1/13/10)

Astrophysicists unwind 'Cold Dark Matter Catastrophe' conundrum (1/14/10)

Puzzling Dwarf Galaxies Finally Make Sense (1/13/10)

Galaxy formation: Gone with the wind? (1/13/10)

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