New keys for the mystery of superdense elliptical galaxies
The relation between the size and density of the elliptical galaxies in the distant universe is one of the most pressing current problems in astrophysics. A research by astronomers at Universidad Autónoma de Madrid, University of Florida, Universidad Complutense de Madrid, and the Instituto de Astrofísica de Canarias (IAC) reveals important data — related to the dispersion of velocities of its stars — that could decrypt the keys to solve it.
Elliptical galaxies are the most massive of the near-Earth universe. These galaxies are oval shaped and regular, and do not have a disk like the spirals of the Milky Way. Using large telescopes, astronomers have identified elliptical galaxies 10 times more massive than our own galaxy, the Milky Way, and are approximately 10 billion light years from Earth. These galaxies are twice smaller than the elliptical galaxies today, but contain almost the same number of stars. Understanding how these galaxies have been able to grow so much, is one of the most pressing problems in astrophysics.
Let us imagine that we received the news of the birth of a baby that is 50 cm and weighs 80 kg. Most of us would think that it is a printing error. Well, that is what happened to astronomers when they found a type of galaxies in the distant universe with a very small size, but with a mass approximately 200 billion times the mass of our Sun. This mass can be compared to the mass of the most massive galaxies we observe around us.
Measurements of the superdense elliptical galaxies
If measurements are true, these galaxies would have densities between 10 to 100 times bigger than the density of the galaxies at maturity. Since this discovery, astronomers have tried to understand how these compact galaxies have been able to expand their sizes to reach the size of the galaxies that we observe in the universe.
As in the case of the baby, many researchers hesitated at the beginning of the measurements of the masses and sizes, made from deep images in different colors, therefore astronomers repeated these measurements to detect if it was an error in the determination.
However, an international team of astronomers from the Universidad Autónoma de Madrid, University of Florida, Universidad Complutense de Madrid, and the Instituto de Astrofísica de Canarias (IAC), decided to get a different measure: the velocity dispersion of its stars to distinguish the state of evolution of the compact galaxies. The velocity dispersion is a measurement of the speed with which stars move in relation to other stars, and it is a way to measure the density of galaxies. The smaller is the size of a galaxy with a given mass, faster have to move some stars in respect to other stars to compensate the effect of gravity and do not collapse.
The velocity dispersion measured by the team reveals not much higher values than the values measured in nearby galaxies. To understand this, it is necessary to consider that the velocity dispersion that we observe is caused by two components: the visible matter, and the invisible (dark matter). Both contribute to the total mass of the galaxy. In the past, the influence of the visible matter at the velocity dispersion was higher, because the light was more concentrated, but not the dark matter, and therefore the velocity dispersion has not changed much.
These results are fundamental to understand the causes of growth of these galaxies and favor a scenario in which these galaxies captured dwarf galaxies which are moved to occupy the outer part of the same, with little changing its central configuration. In this way, the size may increase by a factor of 2 while the velocity dispersion only increases by a factor of 0.8.
To perform these measurements, the research team has used the largest optical telescope in the world, the Gran Telescopio Canarias (GTC), meaning “Canaries Great Telescope“, with a diameter of 10 meters.