Things in the universe don’t stay still – they move about, they interact, they spin. Galaxies above all show off these motions, with their rotation a crucial aspect in their evolution. Astronomers have announced that they have measured such rotation in the earliest galaxy yet – the light of this object comes from just 550 million years after the Big Bang, at the very dawn of the Age of Galaxies.
The measurements of this galaxy – known as MACS1149-JD1 – have been published in The Astrophysical Journal Letters. The team used the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the redshift of the galaxy.
We live in an expanding universe, and the speed of light is finite. So light takes a certain time to travel between distant objects. As it travels, the universe expands, and the wavelengths of light from a distant galaxy are stretched, shifting the light we receive into the redder portion of the electromagnetic spectrum, hence the term "redshift".
However, motion has an effect on light too. Like the Doppler shift in the sound of a siren moving towards and away from us, when a luminous object moves towards us it becomes bluer, away from us redder. A rotating galaxy will have a bluer side (the part that appears to come towards us), and a redder side that rotates out of view.
With this measurement, the team estimated that JD1 spins at 50 kilometers (31 miles) per second, slower than the rotational speed of the Milky Way disk of 220 kilometers (136 miles) per second. However, JD1 is much smaller than our own galaxy. It has a diameter of only 3,000 light-years, compared to the Milky Way which is 100,000 light-years across.
“The rotation speed of JD1 is much slower than those found in galaxies in later epochs and our Galaxy and it is likely that JD1 is at an initial stage of developing a rotational motion,” co-author Professor Akio Inoue, from Waseda University, said in a statement.
As this galaxy grows, the rotation will create a disk of gas and stars. This initial motion, measured in the study, allows astronomers to begin to understand how galaxies like our own came to be. Together with measurements of other distant objects, a clearer picture of galaxy evolution has started to form.
“Beyond finding high-redshift, namely very distant, galaxies, studying their internal motion of gas and stars provides motivation for understanding the process of galaxy formation in the earliest possible universe,” added co-author Professor Richard Ellis from University College London.
The team also measured the age of the mature star population, which they believed had been shining in that galaxy for a whopping 300 million years, suggesting they formed when the universe was about two percent of its current age.