Ancient faint galaxy is a heavy element factory

The ultra-faint Reticulum II galaxy with its nine brightest stars circled in red. The insets show the very strong presence of neutron capture element barium in three stars. – Dark Energy Survey / Fermilab / Alexander Ji / Anna Frebel / Anirudh Chiti / Josh Simon.

The chemical signature of a faint old galaxy shows gold, lead and other heavy elements are spat out by the rare collisions between the smallest, densest stars in the Universe.

American astrophysicists observed the brightest stars in a nearby dwarf galaxy and found them to be flush with heavy elements, likely scraps from merging neutron stars. The work was published in Nature.

Carl Sagan famously said that we're made of star stuff – and he's right. The lightest elements such as hydrogen were formed minutes after the Big Bang, and those a little heavier are created by nuclear fusion.

But what about the heaviest, such as gold and lead? The question has been debated for 60 years, with some astrophysicists claiming they're constantly pumped out by supernovae.

This new work suggests that instead of a constant dribble, they're produced in bursts by the rare events of neutron stars smashing into each other.

Iron and heavier elements are produced via "neutron capture", where an existing element acquires additional neutrons, one at a time, that eventually "decay" into protons turning the atom into a new element. 

So what does Reticulum II, an old ultra-faint galaxy, have to do with anything?

The galaxy was discovered last year. It's only 97,000 light-years away – close by cosmological standards – and is very, very old. It (and other ultra-faint dwarf galaxies) formed all their stars in the first one to three million years following the Big Bang.

So its proximity and age made it an attractive target for observation.

Gamma-rays emitted by ultra-faint dwarf galaxy Reticulum II. – Sergey E. Koposov et al / Dark Energy Survey

Similar ultra-faint dwarf galaxies have been found before, but they tend to be too faint to see. Reticulum II, though, was just bright enough to be observed.

So over five days in October, the Magellan Clay telescope at Las Campanas Observatory in Chile picked up the spectra of its nine brightest stars, which told astrophycisists about their chemical makeup. Seven of those showed extremely high levels of europium and other neutron-capture elements, the highest abundance found in a dwarf galaxy so far.

"These stars have up to 1,000 times more neutron-capture elements than any other stars observed in similar galaxies," said lead author Alexander Ji of the Massachusetts Institute of Technology.

The stars, the researchers reason, must have formed from gas already heavily infused with neutron-capture elements. Given that not all ultra-faint dwarf galaxies have such high amounts of neutron-capture metals, they must have been formed in a rare event – much rarer than supernovae.

They calculated the amount of europium in the stars perfectly matched what should be produced in a collision between two neutron stars. 

Astronomers looking at old stars in our galaxy, the Milky Way, have seen a similar pattern of neutron-capture elements as found in Reticulum II. This suggests that the heavy elements on Earth were produced in neutron star mergers too.

So the next time you admire gold jewellery, just think – those atoms were probably blasted out into space as dense stars destroyed each other.

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