The dwarf galaxy that reveals the history of the universe

Institute of Astrophysics of Andalusia Press Release

Image credit: NASA, ESA Y. Izotov (MAO, Kyiv, UA) and T. Thuan (University of Virginia)

“Dwarf galaxy I Zw 18 is the least abundant in metals (in astrophysics, elements heavier than hydrogen and helium) in the nearby universe, and one of the most akin to the primeval galaxies. Its study therefore allows us to catch glimpses of the conditions that prevailed in the primordial universe,” says Carolina Kehrig, researcher at the Institute of Astrophysics of Andalusia in charge of the study that analyses the properties of I Zw 18.

The study has found a very large region in this small galaxy of ionised helium, which tends to be more frequent in very distant galaxies with low presence of metals. The ionisation of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms. “In this study we propose a new interpretation of the origin of this radiation in galaxy I Zw 18, a subject which is still enigmatic,” Kehrig says.

Using the PMAS integral field spectrograph of the 3.5-metre telescope at the Calar Alto Observatory (CAHA), researchers have obtained the first detailed map of this region of I Zw 18 and have analysed possible ionising sources.

Conventional sources of ionisation, such as Wolf-Rayet stars — very massive and with very violent stellar winds — or shocks generated by remnants of supernovae, cannot provide the energy necessary to explain the halo of ionised helium present on I Zw 18, so researchers considered other possibilities.

“Our data point to the fact that extremely hot stars, such as supermassive stars with low metal content or massive stars practically devoid of metals may hold the key to the enigma of the excitement of helium on I Zw 18, even though the existence of these stars has not yet been confirmed by observations on any galaxy,” says Carolina Kehrig (IAA-CSIC).

We would be talking about very hot stars analogous to first generation stars (known as Population III stars) which, according to theoretical models, would be composed only of hydrogen and helium and could be hundreds of times more massive than the Sun. These stars are believed to have played a decisive role in the “reionisation” of the universe, during which period the first stars and galaxies became visible and which is still little known.

This study shows how it is possible to extract information about the history of the universe within our own galactic vicinity.