[ExI] cosmological model gets it mostly right

[Damien Broderick]

Galaxies shaped by dark past
Friday, 22 January 2010

by Heather Catchpole
Cosmos Online


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 many 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.”

[[comment: obviously they've been harvested...]]