(credit: Lee Spitler, image taken with the Subaru telescope)

This is an image of the Sombrero galaxy, which is a spiral galaxy located
in a small galaxy group.  From our work described below, this galaxy probably emerged from the hydrogen fog of the dark ages early in the Universe's history.

Click on the images for the full-resolution versions

Reionization - the hydrogen fog burning off

Reionization is an event in the Universe that took place 13 billion years ago, immediately after the "dark ages" of the Universe.

After the Big Bang, the Universe cooled over hundreds of million years.  Hydrogen atoms were eventually able to condense into a thick "hydrogen fog" during the period known as the dark ages.  It is called the dark ages because there were very few sources of light and the fog obscured some of the light that was being produced by the first stars.

Eventually large numbers of stars, galaxies and black holes formed within the hydrogen fog.  These produced large quantities of ultraviolet light, which is efficent at reionizing or "burning off" the hydrogen fog. This process is called reionization.

The Dark Ages - hard to observe

The dark ages of the Universe are challenging to observe directly, since so little light is available to collect with telescopes. Nevertheless, it is a really interesting time period to investigate because it set the stage for the subsequent evolution of planets, stars and galaxies.  This is illustrated in this timeline of the Universe's history:

Dating reionization - images of nearby galaxies

In a recent work, our team approached this problem in an untraditional way:  instead of trying to observe the dark ages directly, we used characteristics of relatively nearby galaxies to understand when reionization took place.

More specifically we looked for an imprint of reionization on the globular star clusters within three galaxies. Using high-resolution computer simulations, we interpreted the spatial distribution of the star clusters and measured when they stopped forming during reionization.  In this way we obtain a measurement of when reionization took place around a galaxy.

Our own Milky Way, which is located in a relatively isolated part of the Universe, appears to have been one of the first galaxies to reionize its surrounding hydrogen fog: the Universe was only 375 million years old.  In contrast a galaxy named Messier 87, which is located in a dense region of the Universe, reionized much later, when the Universe was 650 million years old.

This work suggests dense regions in the Universe may have had thicker fog, which would have taken more time to burn off. This result provides an important clue about what was happening during the dark ages of the Universe.

To the left is an image of a large ellitpical galaxy, named NGC 1407.  NGC1407 is located at the centre of a large galaxy group called Eridanus. Based upon the spatial distribution of globular star clusters within this galaxy, we estimate reionization took place around this galaxy roughly 500 million years after the Big Bang.

(credit: Lee Spitler, image taken with the Subaru telescope)

On the right is an image of the small elliptical galaxy NGC 4494. The image was taken using the 8-metre Japanese telscope in Hawaii.  Based upon our work, we predict that because this low-mass galaxy is located in a small galaxy group, it is likely to have burned off its hydrogen fog early in the Universe's history.

(credit: Lee Spitler, image taken with the Subaru telescope)