Faint galaxies are the worst 'polluters'

24 October 2017

The chemical elements that compose everything we interact with (our bodies, the planets and every object observed with our telescopes) were created by nuclear reactions inside stars and powerful events such as supernovae (exploding stars). The Big Bang created only hydrogen, helium and a bit of lithium, but when the first stars formed they began ‘cooking up’ other elements. Part of the research we are carrying out focuses on the chemical enrichment of the intergalactic medium (IGM) – the thin gas in the space between galaxies – at a very early stage of the Universe.

Using the spectra of distant quasars (some of the most brilliant objects in the Universe), we can determine the nature of objects lying between us and the quasars, and how the intergalactic medium has changed over time.

Between leaving the quasar and falling into our telescopes, a quasar’s light travels through a lot of material in space. This material selectively absorbs certain frequencies of the light, leaving a collection of absorption line features imprinted on the quasar spectra. Simulations have been able to reproduce these observed spectra very accurately.

How material in space modifies light from a distant quasar

Light from a distant quasar (left) passes through gas in the intergalactic medium before reaching us in our own galaxy (right). The intergalactic gas selectively absorbs certain frequencies from the quasar light, creating absorption lines in the quasar’s spectrum (inset). Credit: James Josephides and Michael Murphy, Swinburne University of Technology

A team led by CAASTRO PhD student Luz Angela Garcia has used this technique to explore the intergalactic medium right back to an early stage of the Universe (about a billion years after the Big Bang, redshift z~6). At this time, two simultaneous processes were transforming the primordial gas (which was mainly hydrogen and helium). The first process was the chemical enrichment of the intergalactic medium, discussed above. The second was ‘neutral’ (uncharged) hydrogen gas in the intergalactic medium being ionised (having its electron stripped off) by ultraviolet radiation from the first stars and quasars. This ionisation event is called the Epoch of Reionization (EoR). It gave us the Universe we have today, where light can travel for long distances without being absorbed by a fog of hydrogen gas.

Garcia's team recreated the physical conditions of the intergalactic gas with simulations and evolved them for redshifts between 4 and 8. This has produced hundreds of synthetic quasar spectra and their absorption features. These synthetic spectra match well the different ionisation states of carbon, oxygen and silicon seen by observations of different epochs of the Universe’s history.

A key result of the work is that low-luminosity (that is, faint) galaxies are the ones most responsible for the chemical pollution of the intergalactic medium. These galaxies are located near the regions that show chemical enrichment. Other studies claim that such galaxies are also responsible for completing the process of reionising hydrogen.


L. A. García, E. Tescari, E. V. Ryan-Weber and J. S. B. Wyithe, “Simulated metal and HI absorption lines at the conclusion of reionization”, MRNAS, 470, 2494 (2017)

L. A. García, E. Tescari, E. V. Ryan-Weber and J. S. B. Wyithe, “Theoretical study of an LAE–CIV absorption pair at z = 5.7”, MNRAS Letters, 469, L53 (2017)