IN THE early universe, there were constantly random quantum fluctuations, particles flickering in and out of existence. Some of them stuck around, and today we live with what those remainders have become, including galaxies.
All quantum fluctuations are essentially alike, and those that randomly occurred everywhere in the early universe were impossible to distinguish from one another. Yet, 14 billion years later, each galaxy is a structure with its own unique features and it is very easy to tell the difference between our home – the Milky Way – and, say, an elliptical galaxy. In other words, all galaxies have a shared origin in quantum fluctuations that occurred in the early universe, but the galaxies we see in the sky are incredibly diverse in the forms that they take.
Categorising galaxies based on their shape is a practice known as galaxy morphology. In 1926, Edwin Hubble (yes, of the Hubble constant!) introduced a classification scheme based on galaxies’ appearances.
In his system, Hubble identified three kinds of galaxies. The two better known types are spirals like the Milky Way – which have a central bulge and spirals orbiting it – and ellipticals, which look like an ellipsoid or, more colloquially, a three-dimensional oval. The ones that you are perhaps least likely to have heard of are lenticular galaxies. These have a spheroidal bulge at their centre, with a visible disc around it.
To this day, astronomers still use a morphology classification scheme that was based on Hubble’s: a system developed in the 1950s by Gérard de Vaucouleurs. One extension that de Vaucouleurs introduced was the “irregular” class of galaxies.
He also deepened the spiral category by identifying subcategories: bars, rings and those with different types of spiral arms. By looking for patterns and differences, astronomers have come to understand that galaxies are quite diverse, despite sharing humble origins in small fluctuations.
Of course, being able to see that galaxies have lots of different structures is one thing. Being able to explain why is something completely different.
Today, understanding how galaxies became so diverse is an active field of research. In the years since de Vaucouleurs came up with his system, Vera Rubin and Kent Ford provided the first substantial evidence for the existence of dark matter, adding another ingredient to the equation.
“Knowing how galaxies got to be the way they are means understanding what dark matter is and how it behaves”
Measurements of cosmic microwave background radiation are the strongest signs we have of the existence of dark matter, and indicate that there is so much of it in the universe that it must play a major role in the formation of large-scale structures, such as galaxies. Therefore, knowing how galaxies got to be the way they are requires understanding what dark matter is and how it behaves.
As regular readers know, this particular question has captured my attention and it is one that I am currently devoting my career to answering. Yet, as elusive and fascinating as dark matter is, it is only one part of the conversation about structure formation – and the fact is, there is still much we don’t know about luminous matter.
When we talk about luminous matter in the context of galaxies, we mean stars for the most part, since, at the end of the day, the visible part of a galaxy is a collection of stars and dust. To understand the evolution of a galaxy, therefore, is to understand, in part, the histories of its stellar populations. Our understanding of galaxy evolution is dependent on our models of stars.
One example of how the two types of work are entwined is a paper led by Lauren Porter that appeared in the December 2014 issue of the Monthly Notices of the Royal Astronomical Society. Porter – then a graduate student at the University of California, Santa Cruz – and her team of collaborators used computer simulations to study the formation history of elliptical galaxies by looking at the age and metallicity of their stellar populations.
Ellipticals, like lenticulars, tend to have older, redder stars, indicating that these galaxies are from an earlier time in the universe. Because it takes a generation or two of stars to make heavier elements (as discussed in an earlier iteration of this column), elliptical galaxies’ stars are also likely to have lower metallicity – elements present that are heavier than hydrogen or helium.
In their paper, Porter and her colleagues find that there is a correlation between how fast stars are moving inside a galaxy and how long it took that galaxy to form. In other words, although galaxies have shared origins, where they end up depends greatly on the constituents that they begin with.