4 mins read 23 Mar 2022

GAMA Survey Helping Astronomers Understand the Structure of the Universe

The Galaxy and Mass Assembly Survey has just released their complete data set of spectroscopic and redshift data that will help astronomers to refine theories of structure formation in the Universe.

The large-scale structure of the Universe as it is today, modelled using the Illustris simulation. The GAMA survey aims to test the cold dark matter paradigm of structure formation. Credit: Illustris Collaboration

An international team of astronomers studying around 300,000 galaxies in the nearby Universe has just released their latest data to the public. A remarkably complete map of galaxy spectra and redshifts across a large part of the sky has given scientists the opportunity to study the formation and evolution of the structure of the Universe.  

The project, which has been called the largest multi-wavelength survey ever put together, is known as GAMA, the Galaxy and Mass Assembly Survey. It has brought together data from many of the world’s most powerful telescopes, but at the heart of the project is the AAOmega multi-object spectrograph on the Anglo-Australian Telescope.

A spectrograph is an instrument that splits incoming light into its component colours or wavelengths, and records the resulting spectrum for further analysis. The AAOmega can observe 392 simultaneous spectra, making the data collection less arduous for astronomers and allowing them to observe greater volumes of space faster than had previously been possible.

The current data release is the fourth, known as DR4, and unlike previous releases of static data, the GAMA team is this time planning to update DR4 with new data as it becomes available. Anyone can access the data via the GAMA DR4 webpages, but, be warned, you are probably going to need a supercomputer running some highly specialised astronomy software to process it.

Regardless, the data will be of interest to anyone who is wanting to test the cold dark matter (CDM) paradigm of structure formation, and will no doubt provide the basis for studies testing theories of gravity, analysing galaxy evolution and mergers, and measuring the dependence of star formation on a galaxy’s local environment. Previous data releases have generated a wealth of scientific knowledge, with a paper reporting the first core data release having been cited over 1,000 times.

And the conclusions are fascinating. For starters, it appears as though at the age of 13.8-billion years the Universe is slowly dying. Astronomers using GAMA data have confirmed that the energy produced in the Universe today is only about half of what it was 2-billion years ago, and that this is happening at all wavelengths of light. The Universe is in the equivalent of human middle-age.

But that’s not all. Using the full data set from the DR4 release, astronomers calculated that the Universe has converted just about 5% of its ordinary, or baryonic, matter into stars. The rest is mostly gases like hydrogen and helium in interstellar space.

The other kind of matter, dark matter, is a whole other story. Scientists are still trying to understand exactly what dark matter is, but the GAMA data can help with that too. One of the goals of the survey is to understand the dark matter halos surrounding galaxies and galaxy clusters.

A Model of the Universe

A sample of galaxy postage-stamp images by type and redshift. Credit: Driver et al. 2022.

Dark matter is often referred to as being either hot or cold, depending on how quickly it moves. Cold dark matter moves slowly compared to the speed of light (which can still be very, very fast compared to our human experiences), and being dark it interacts only very weakly with ordinary matter and light.

Despite this, it is very important in understanding the formation of structure within the Universe. The theory of cold dark matter was first published in 1982 and suggests that things grow hierarchically. Small objects form first and then gravitate together and merge into increasingly larger structures. Astronomers use cold dark matter to explain how the Universe went from being smooth and uniform at its birth to clumpy and structured as we see it today.

Fast-moving hot dark matter on the other hand would require a top-down approach. Large superclusters of galaxies fragment into smaller pieces until eventually, you end up with galaxies like our Milky Way.

Cold dark matter is the favoured theory as its predictions do seem to be in general agreement with what we actually see. But there are some worrying discrepancies that astronomers are working hard to understand. For example, models predict that there should be many more dwarf galaxies around the Milky Way than we have been able to find.  

What will be interesting to see is whether problems like these can be solved without abandoning the cold dark matter paradigm. The goal of GAMA is to help astronomers investigate these matters, and DR4 is an integral part of that.

The article is available on the arXiv preprint server