Australian Sky & Telescope - May 2018

22 AUSTRALIAN SKY & TELESCOPE May | June 2018

Cosmic dawn

Besides the obvious prize of discovering the most distant

object, why do we really care about finding the first galaxies?

Because they are the missing piece of our picture of galaxy

formation. We don’t know how or exactly when the first

galaxies ignited, or what their properties were, and that’s

frustrating. To rectify the situation, we must explore the

earliest frontier of galaxy formation.

For the first few hundred thousand years of cosmic history,

there were no atoms in the universe, just a hot broth of

particles and photons. But about 370,000 years after the

Big Bang, in what is known as the Epoch of Recombination,

the universe cooled sufficiently for free electrons to bind

to protons, transitioning the cosmos from a totally ionised

phase to one that was electrically neutral. At this point most

of the normal matter in the universe was in the form of

neutral hydrogen atoms.

This vast medium wasn’t smooth. Gravity had teased the

subtle density fluctuations present at the start of the universe

into thicker filaments and clumps, where dark matter and gas

had gradually pooled and coagulated to form the seeds of the

first galaxies.

At some point, a few hundred million years after the

Epoch of Recombination, the first stars in these proto-

galaxies flashed into life and streamed ionising starlight into

What did these reionising

galaxies look like?

Observations show that

when they were bursting

into life, the majority of

these galaxies were small,

just a few hundred light-

years across. These young

galaxies also had quite

low masses, perhaps

less than 1% of the Milky

Way’s mass.

COSMIC DAWN