Using the Hubble Space Telescope and the Spitzer Space Telescope, a team of astronomers studied the first few billion years of the universe to determine the distribution of stellar masses and the star formation rates in galaxies at epochs as early as about 800 million years after the big bang.
In the last decade, the unprecedented sensitivity of astronomical observations has powered a revolution in our understanding of galaxies in the young universe. It is now possible to study directly the mechanisms and processes which formed the diverse array of galaxies we find in the local universe today. The results not only provide insight into the processes of galaxy formation, they also help us understand the role those first galaxies played in the cosmos: in particular, the ultraviolet light from hot, young stars ionized the intergalactic medium, which had contained neutral atoms ever since they formed during an earlier epoch that also produced the cosmic microwave background radiation.
One of the key questions in cosmology is the stellar composition of young galaxies that formed about two billion years after the big bang. Surveys find that a significant contribution to the reionization must have come not from sources that are seen but from faint galaxies below the current detection limits. Although faint, these galaxies and their stars also contribute to the cumulative growth of the stellar population in the cosmos. Successful models of galaxy evolution need to include all sources of star formation.
Measuring the stellar masses of galaxies at cosmological distances is hard. CfA astronomer Matt Ashby joined with a team of astronomers to use the Hubble Space Telescope and the Spitzer Space Telescope to study over 30,000 galaxies in a survey of the sky specifically targeting objects in the early universe. They studied the first few billion years of the universe to determine the distribution of stellar masses and the star formation rates in galaxies at epochs as early as about 800 million years after the big bang. In an impressive piece of observational and analytic skill, the team improved on earlier measurements and found (among other things) that the star formation rate per volume of space peaked about 3 billion years after the big bang and then declined, and that as the universe expanded and aged the star formation rate per unit of available mass also steadily decreased.
Publication: K. Duncan, et al., “The mass evolution of the first galaxies: stellar mass functions and star formation rates at 4 < z < 7 in the CANDELS GOODS-South field,” MNRAS, (November 1, 2014) 444 (3): 2960-2984; doi: 10.1093/mnras/stu1622