Segue-1 May be One of First Galaxies Ever Formed in Universe, Say Astronomers

Apr 10, 2014 by

According to astronomers led by Dr Anna Frebel from Massachusetts Institute of Technology, an ultra-faint dwarf galaxy called Segue-1 may be one of first ever formed.

This image shows member stars of the ultra-faint dwarf galaxy Segue-1. Image credit: Marla Geha.

This image shows member stars of the ultra-faint dwarf galaxy Segue-1. Image credit: Marla Geha.

Discovered in 2007, Segue-1 is one of about two dozen small satellite galaxies orbiting our Milky Way Galaxy.

Segue-1 lies in the constellation Leo, about 75,000 light-years away from Earth. The galaxy is a billion times less bright than our Galaxy.

It was initially considered to be a globular cluster because of its small half-light radius – about 98 light-years.

But a 2009 study presented a strong case based on internal stellar kinematics that Segue-1 is highly dark matter-dominated and therefore a galaxy.

“I’m excited about this object. Segue-1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass,” said Yale University astronomer Dr Marla Geha, who discovered Segue-1.

In a new study accepted for publication in the Astrophysical Journal ( version), Dr Frebel and her colleagues have found that red giant stars in Segue-1 contain fewer heavy elements than those of any other galaxy known.

“Considering the detailed chemical abundances of the seven brightest stars in Segue-1 to describe the origin and evolution of this galaxy thus suggests that no significant chemical evolution, and hence star formation, has taken place in Segue1 since its formation. There is no indication of AGB star or supernova Ia enrichment prior to the birth of any of the observed stars,” the astronomers wrote in the paper.

They argue that Segue-1, together with four other objects – Ursa Major II, Coma Berenices, Boötes I, and Leo IV, is a candidate for one of first galaxies ever formed in the known Universe.


Anna Frebel et al. 2014. Segue 1: An Unevolved Fossil Galaxy from the Early Universe. ApJ, accepted for publication; arXiv: 1403.6116