There's a zoo of galaxies out there. Understanding exactly how they formed has been a notoriously difficult problem. But including details of 'dark energy' and each galaxy's dark matter halo in calculations now seems to do the trick.
Credit: NASA
SYDNEY: Using a detailed cosmological model that includes dark energy and dark matter, two American astrophysicists have been able to correctly predict the shapes and proportions of the different types of galaxies in the universe and discover the Milky Way’s past.
The shape of galaxies depends on their turbulent history, and understanding how they evolve is a major task.
Astrophysicists Nick Devereux of Embry-Riddle University in Arizona and Andrew Benson of the California Institute of Technology used a sophisticated computer model called GALFORM, combined with data from the infrared Two Micron All Sky Survey, which scanned 70% of the sky between 1997 and 2001.
Researchers were ”completely astonished”
GALFORM simulates galaxy formation in a universe dominated by the enigmatic dark energy and dark matter. It’s based on a cosmological model of the universe called the Lambda Cold Dark Matter (LCDM) that predicts how matter flows and lumps together. The Lambda component is represents ‘dark energy’, which drives the expansion of the universe.
The model was able to reproduce the evolutionary history of the universe over its 13.7 billion years. Moreover it not only got the shapes but also the numbers of various galaxies right and the rate at which galaxy mergers occur.
“We were completely astonished that our model predicted both the abundance and diversity of galaxy types so precisely,” said astrophysicist Nick Devereux of Embry-Riddle University in Arizona.
“It really boosts my confidence in the model,” said Benson.
Shapely galaxies
If galaxies are close enough together, then gravity can cause them to merge, with spiral galaxies morphing to elliptical galaxies. The Milky Way and its neighbour Andromeda are close enough that this will happen,.
Benson and Devereaux said that their model, published in the Monthly Notices of the Royal Astronomical Society shows that the Milky Way has a complex past but so far has only undergone minor collisions and the gravitational collapse of its inner disk to form the central bar.
A galaxy’s shape depends on how it formed – and can vary from elliptical and lens shapes to spirals. Our own Milky Way galaxy is classified as a barred spiral.
American astronomer Edwin Hubble defined these ranges of galactic shapes as the ‘Hubble sequence’. They appear as elliptical blobs, or spiral disks with circles or bars at the centre. Our own Milky Way is classified as a barred spiral.
But the story of how the shapes arise is incredibly complex, so much so that it stretches the limit of current computing capacity. To understand it, astrophysicists use analytical models that can give an approximation of the physics involved in everything from the evolution of stars to the merging of entire galaxies.
Benson and Devereaux were able to predict the shapes and proportions of galaxies with buldges, bulges and discs or just discs.
Model predicts too may dwarves
Australian astrophysicist Geraint Lewis from the University of Sydney says while the results are very encouraging, there’s still a few holes in the LDCM model, which the authors acknowledge.
“These guys have refined the recipe – what’s coming out is not only elliptical and spiral galaxies but the right proportion of these galaxies – which is very encouraging, but it’s not the end of the answer,” Lewis said.
While the model is working well large scales, it predicts a greater number of dwarf galaxies (small galaxies like the nearby Magellanic Clouds) than we actually observe, says Lewis.
“The [dwarf galaxies] are inconsequential in some way but the number of them is important. If your recipe was right you should get dark blobs with nothing in, but if these are not there, it’s a problem for the LCDM.”
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