What was the fate of the compact massive galaxies that populated the early Universe? This has been one of the biggest open questions in astronomy over the last decade. Giulia Savorgnan tells us how astronomers at Swinburne University of Technology believe they have the answer.
Because the speed of light is finite, when we look at distant objects in the Universe, we also look back in time. Their light, that we observe today, was emitted a long time ago, millions or even billions of years before. Thanks to this, we can get a snapshot of a young Universe and see how it compares with the snapshot of our old, present-day Universe. That’s how astronomers study galaxy evolution – i.e. how galaxies have evolved through the cosmic ages.
When the Universe was young, it was populated by a large number of massive elliptical-shaped galaxies. Some of these galaxies were very compact and dense, whereas other galaxies were more extended and less dense. However, in the present-day Universe, very few compact massive spheroids have been observed. i.e. almost all local massive elliptical-shaped galaxies are not compact. The lack of compact massive spheroids in the local Universe raises obvious questions: What happened to them? Where did they go? This has been one of the biggest open questions in astronomy over the last decade.
The most popular theory had been that, over time, the young compact spheroids have grown their size because of collisions and subsequent mergers with other galaxies. The problem with this theory is that both observations and simulations show that there have not been enough galaxy encounters to explain the size evolution of the compact spheroids. But, what if the compact massive spheroids were all still here, in our present-day Universe, and we had just been somehow not able to see them?
Most local massive galaxies are not simple elliptical-shaped objects. They are instead composite systems, primarily made of a spheroid encased within a thin disk of stars. If treated as single-component objects (spheroid+disk), the size of the whole galaxy will appear to be much larger than the size of its spheroidal component only. Disentangling the two components requires a careful modeling of the galaxy light distribution. This technique is dubbed galaxy decomposition.
Thanks to galaxy decomposition, a group of astronomers at Swinburne University of Technology, led by Prof Alister W. Graham, have now located the old present-day descendants of the compact massive spheroids populating the early Universe. Moreover, the number of local spheroids roughly matches the number of their distant progenitors. The interpretation proposed by the Swinburne astronomers is that the young compact massive spheroids have not significantly grown their size with time, but, instead, they have grown a disk of stars around them.