M51, also known as the Whirlpool Galaxy, is one of the brightest spiral galaxies in the night sky. The finely detailed spiral structure is thought to be the result of an interactions with its close galactic neighbor, NGC 5195.
Credit: NASA/ESA
A FEW YEARS AGO Avishai Dekel gave up chess in favour of mud wrestling. Dekel is a cosmologist and he isn’t known to frequent strip clubs.
But there are two types of cosmologists: those who study fundamentals, like the initial conditions and content of the early universe, and those who immerse themselves in the messier problem of the evolution of galaxies, replete with gas and stars that heat and cool, form jets, make black holes, and sometimes explode.
Martin Rees of the University of Cambridge in England calls the two classes of cosmologists chess players and mud wrestlers. Cosmology is “a fundamental science just as particle physics is,” says Rees. “The first million years [of the universe] is described by a few parameters ... but the cosmic environment of galaxies and clusters is now messy and complex.”
Now that the chess players have established those basic parameters – such as the relative amounts of invisible dark matter, the even-more mysterious dark energy, and ordinary matter – more cosmologists are turning their minds to mud. Recent surveys of the shapes, colours, and masses of galaxies have put a new focus on the nitty-gritty of galaxy formation.
“Now that we know the cosmological parameters, it’s really time to understand how galaxies form,” says Dekel, of Israel’s Hebrew University of Jerusalem. To do that, “we have to trace the gas”, not dark matter, because it’s the gas that forms stars. “That’s where the action is.”
The physics governing large-scale gas interactions, or ‘gastrophysics’, is much more complicated than that of dark matter. Gas molecules respond to a host of forces, while dark matter is simple to model because it responds predominantly to just one force: gravity. Nonetheless, says Dekel, he is a recent convert to gastrophysics.
Through the 1980s and 1990s, Dekel spent most of his time trying to estimate the density of matter in the universe by mapping the velocities at which galaxies and matter move through the vast, invisible reaches of dark matter. Although no one knows what dark matter is made of, it appears to constitute 85% of the mass of the universe.
And simply because there’s so much of it, the stuff provides the gravitational scaffolding that pulls together ordinary gas – electrons, protons, atoms, etc. – to make stars and galaxies.
The behaviour of dark matter has thus been considered a reliable map for the path of galaxy formation. Every galaxy is nestled within a halo of cold dark matter, composed of exotic particles that move much slower than the speed of light (this relatively slow pace is why this dark matter is dubbed ‘cold’).
“The universe started with lots of dark matter everywhere, so gravity caused bits to fall together. Bigger clumps attracted even more dark matter,” says Joss Bland-Hawthorn of the Sydney Institute for Astronomy at the University of Sydney. “The fraction of the mass in the form of gas is crucial [for galaxies to form]. Some halos may have little gas, others have a great deal more. We think the maximum gas fraction is [about] 17% of the dark halo mass.”
The halos start out small but continually merge to grow bigger, dictating that all structure in the universe should evolve in the same way – from little to big. The growing clumps of dark matter form the backbone of a cosmic web, with clusters and superclusters of galaxies falling into place along the densest filaments, like paint onto a dark canvas.
There is some debate about whether galaxies grow largely by merging with one another, or if fresh gas is primarily drawn to galaxies along the filaments of this vast web of dark matter (see “Big babies in a cosmic web”).
