Astronomers have announced that they have measured such a rotation in the earliest galaxy ever – this object’s light comes from the dawn of the Age of Galaxies, just 550 million years after the Big Bang.
Objects in the Universe don’t stand still – they move, interact, spin. Galaxies in particular show these motions, and their rotation is a crucial element in their evolution.The measurements of this galaxy, known as MACS1149-JD1, were published in The Astrophysical Journal Letters. The team used the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the galaxy’s redshift.
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We live in an expanding universe and the speed of light is limited. Therefore, it takes a certain amount of time for light to travel between distant objects. As it travels, the universe expands and the wavelengths of light from a distant galaxy get longer, which shifts the light we receive to the redder part of the electromagnetic spectrum, hence the term “redshift”.
But motion also has an effect on light. Like the Doppler shift in the sound of a siren moving towards and away from us, a bright object becomes bluer as it moves towards us and redder as it moves away from us. A rotating galaxy will also have a bluer side (the part that appears to be coming towards us) and a redder side that rotates out of view.
With this measurement, the team estimated that JD1 is spinning at 50 kilometers per second, slower than the Milky Way disk’s rotation speed of 220 kilometers per second. But JD1 is much smaller than our own galaxy. It is only 3,000 light-years in diameter, compared to the 100,000-light-year-wide Milky Way.
“The rotation speed of JD1 is much slower than that of galaxies of later epochs and our Galaxy, and it is likely that JD1 is in the early stage of developing a rotational motion,” co-author Professor Akio Inoue of Waseda University said in a statement.
As this galaxy grows, the rotation will create a disk of gas and stars. This first motion measured in the study allows astronomers to begin to understand how galaxies like ours form. Together with measurements of other distant objects, a clearer picture of galaxy evolution is beginning to emerge.
“Beyond finding galaxies at high redshift, i.e. very distant, studying the internal motions of gas and stars provides the motivation to understand the galaxy formation process in the earliest possible universe,” said co-author Professor Richard Ellis from University College London.
The team also measured the age of the population of mature stars believed to have been shining in this galaxy for 300 million years, suggesting that they formed when the universe was about two percent of its current age.