Where did all the mini-galaxies go?

Space

Galaxies 10 times smaller than the Milky Way were common 3 billion years after the big bang, but their absence today is a puzzle

23 April 2008

The dense objects imaged by Hubble weigh as much as our own galaxy, but are less than 1/10th as large (Illustration: NASA/ESA/A Field/STScI/P van Dokkum/Yale U)

The stars in the ultra-compact galaxies are packed together 50 to 100 times more densely than in the Sun's neighbourhood. If there are any planets in these galaxies, their skies would be filled with 200 times as many stars as Earth's sky — The stars in the ultra-compact galaxies are packed together 50 to 100 times more densely than in the Sun鈥檚 neighbourhood. If there are any planets in these galaxies, their skies would be filled with 200 times as many stars as Earth鈥檚 sky
(Image: NASA/ESA/P van Dokkum/Yale/M Franx/Leiden U/G Illingworth/UCSC/Lick Obs)

Tiny galaxies littered the early universe 鈥� but where are they now?

A team led by Pieter van Dokkum at Yale University measured the light of distant galaxies from around 3 billion years after the big bang. They had the same mass as the Milky Way, but were 10 times smaller (The Astrophysical Journal, vol 677, p L5).

Peering at younger regions of the sky shows that galaxies this size are no longer around, but it鈥檚 not clear what happened to them.

鈥淭his is a very puzzling result,鈥� says Simon White of the Max Planck Institute for Astrophysics in Garching, Germany. 鈥淕alaxies cannot disappear.鈥�

Team member Marijn Franx of Leiden Observatory, the Netherlands, suspects they have since merged with extremely massive galaxies.

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