The big picture of the universe reveals the family tree of galaxies

By Michael Cowley, Macquarie University

A small section from the ZFOURGE survey, which contains thousands of galaxies spanning billions of years. - Image Credit: ZFOURGE, Author provided

A new photo album made from hundreds of images of more than 70,000 galaxies represents one of the most detailed galaxy studies ever compiled, spanning a period of 12 billion years. The album, which my colleagues and I compiled, allows us to track the family tree of galaxies over more than 90% of the age of the universe.

We used the 6.5-metre Magellan Baade Telescope and FourStar infrared camera to snap these images over 45 nights. Taking inspiration from the camera, we named the study the FourStar Galaxy Evolution Survey, or ZFOURGE for short.

The Magellan Telescopes ( on the right) at the Las Campanas Observatory – Image Credit: KszuloFotki/WikimediaCommons

While any large telescope can snap photos of galaxies, the challenge is to accurately calculate their distance in an ever-expanding universe so we can build up the family tree and piece together their evolutionary history.

To do this, we need to account for galaxies moving away from us and observe how much their light is shifted to longer (redder) wavelengths. The greater the shift, the farther away a galaxy is from us.

Unfortunately, determining this shift often requires time-consuming observations with spectroscopic instruments. The advantage of ZFOURGE is its unique filter set, which overcomes this challenge and allows us to probe numerous galaxies at once, while also measuring accurate distances.

A comparison of visualising galaxies with and without ZFOURGE. – Credit: Texas A&M University

ZFOURGE not only measured the distances to these galaxies, but also how massive they are, how fast they’re forming new stars, and how much energy they’re emitting.

Such information allowed us to classify these galaxies according to their physical properties and thus construct a “family tree” to better investigate how they evolve through cosmic time.

The ZFOURGE survey catalogued the properties of thousands of galaxies to construct its ‘family tree’. Author provided

In general, we found the earliest galaxies (the bottom of the tree) to be much smaller and more irregular than their modern-day counterparts (top of the tree).

It is commonly believed that these small galaxies grew by way of hierarchical formation, a process in which they merged with their neighbours to form larger galaxies like our Milky Way.

Mature-aged baby boomers

ZFOURGE has produced a wealth of information, resulting in numerous scientific publications that continue to expand our understanding of galaxies and how they evolve.

One of the most exciting discoveries was the finding that the first galaxies, formed in the early universe, might have matured much sooner than expected.

A number of mature-aged galaxies were discovered at a distance of 12 billion light years, or 1.8 billion years after the Big Bang. This suggests that these galaxies rapidly formed their stars and then matured within a much shorter time than the typical galaxies of today.

Such findings pose new questions for astronomers and allow us to build on existing models of galaxy formation and evolution.

The Milky Way’s family album

Another exciting discovery came about by observing galaxies that resemble the ancestors of the Milky Way. It was discovered that 10 billion years ago, the Milky Way was potentially churning out newborn stars 30 times faster than the present.

Examples of Milky Way progenitors out to a redshift of 3 (~11.5 billion years ago). - Image Credit: Texas A&M University

But our own star was late to the party, arising 5 billion years later at a time when the rate of star formation had dropped to a mere trickle.

By examining these progenitor galaxies, we found that the Milky Way potentially began life as a small clump of stars, before building itself up into the grand spiral galaxy we know it as today.

Hunting for black holes

By combining the data from ZFOURGE with images from numerous other ground and space-based telescopes, we then set out in search of supermassive black holes.

While you may think this is a futile expedition, given black holes are “black”, those that are actively feeding on surrounding gas and dust are known to be among the brightest objects in the universe.

Such black holes have long been believed to be detrimental to a galaxy’s health, as they either heat the cold gas and dust required for birthing new stars, or simply blast it away with their immense amount of energy.

By cataloguing these black holes, we found contradictory results with no evidence of adverse effects for galaxies hosting the most active supermassive black holes. In fact, we found particular populations of these galaxies where the supermassive black holes may even have helped sparked new star formation, rather than quenching it.

While our survey is now complete and our catalogue of galaxies has been made public, this is only the beginning for the ZFOURGE survey.

With a family tree of more than 70,000 galaxies, spanning most of cosmic time, there’s plenty of data for astronomers around the world to dive into and make their own exciting discoveries.

Source: The Conversation