The Star-Formation History of the Universe
Tuesday May 23, 2023 at noon
Dr Jim Condon, National Radio Astronomy Observatory
Stars have produced most of the radiation in the universe since the Big Bang, synthesized nearly all atoms heavier than Helium, and shaped the evolution of galaxies. The cosmic history of star formation can be traced via the radiation from short-lived stars in galaxies. Short-lived stars are very hot and emit primarily ultraviolet radiation, much of which is absorbed by interstellar dust and reradiated in the far infrared. However, the far-infrared is contaminated by radiation from dust heated by older stars and complicates calculations of the star-formation rate.
The radio continuum emission from star-forming galaxies is a clean tracer of current star formation unbiased by dust absorption and insensitive to older stellar populations. Although the radio radiation from distant star-forming galaxies is extremely weak, the new ultra-deep MeerKAT radio survey can detect galaxies like our own Milky Way up to redshift z ∼ 4, when the universe was only 1.5 billion years old. Evolving the local radio luminosity function to match the faint-source counts yields an independent radio estimate of the star-formation history since then. The star-formation rate peaked around “cosmic noon” when the universe was 3.5 billion years old and subsequently declined exponentially with a time constant τ ∼ 3 billion years. Consequently 90% of the stars in the universe are older than our Sun.