There’s no place like home: Is the Milky Way unique?

Is our galaxy, the Milky Way, unique? Or is it typical of the more than a trillion galaxies spread throughout the observable universe? This question vexes astronomers. Whilst they can study the Milky Way in intricate detail, they worry their observations are telling us about its peculiarities rather than the overall cosmic evolution of galaxies. But now new observations presented by University of Sydney’s astronomers, Nic Scott and Jesse van de Sande, and international collaborators have found a twin of the Milky Way sharing the distinct properties that some felt made our cosmic home unique. 

From our vantage point deep within our Galaxy, we have been able to map out the entirety of the Milky Way. We now know that our Sun sits within a spiral disk orbiting the bulge of old stars at the Galactic centre, enveloped by a tenuous stellar halo that surrounds the Milky Way. In fact, it appears that the Milky Way is just like the myriad other spiral galaxies spied by telescopes.

This picture started to change in the early 1980s as astronomers began to measure the speeds and chemistry of large numbers of Galactic stars. To their surprise, they discovered the Milky Way possessed two disks of stars, with the Sun orbiting within a thin disk that sits inside a fatter structure known as the thick disk. Importantly, these two disks have distinct chemical signatures, with the thin disk being substantially more chemically enriched than the thick disk.

The formation of the thick disk remains uncertain. Some have suggested that it grew from a steady rain of gas onto the Galaxy, but others have concluded that it must have been created from a single accretion of a middle-sized galaxy in the early life of the Milky Way. 

However, detailed computer simulations of the evolution of galaxies have shown that the required merger at the right time in the life of a galaxy would be a rare event, with perhaps one in twenty spirals undergoing this violent accretion.

There are two main ideas on how galaxies take shape over time and come to be some of the largest structures in our Universe. Scientists are still attempting to resolve this question, with both arguments presenting lists of pros and cons as to which model it could be. 

The first focuses on the monolithic collapse of a large gas cloud that was generated in the early Universe. In this model, large clumps of interacting dark matter directed the normal, baryonic matter towards the centre of these gravitational wells, where it gained angular momentum, spinning into disc-like structures. From this large spinning structure, smaller clumps of matter started collapsing to form the first stars and the galaxy was born. 

The secondary model, a more bottom-up approach, proposes that smaller-sized chunks of matter were the first structures, which then accreted onto each other getting larger and larger until a galaxy-sized structure formed. Once again in this model, the addition of masses creates an increase in angular momentum, so the larger structure spins itself into a disc shape that we observe with spiral galaxies.

Of the two models, it is the latter, the hierarchical model is what most astronomers these days lean towards as it also leads to the Lambda-Cold Dark Matter model, which is the most accepted way that galaxies formed (despite this model also having gaps which are currently active areas of research). 

From there, galactic evolution takes over as the morphology of the structure changes - from irregular galaxies, spirals and barred-spirals, into more ecliptic shaped galaxies - as a result of mergers and collisions. 

Galaxies also tend to form into two colour classes - the redder galaxies, which represent the older generation of stars within them, exhibiting minimal new star birth, and those which are still rich in neutral hydrogen gas, creating young, bright blue stars and giving the galaxy its blue hues. When a galaxy halts its star formation rate, converting from blue colours to red, it is said to be ‘quenched’. There could be a number of reasons for this quenching, which include feedback provided by supernovae, active galactic nuclei, and ram stripping through cluster dynamics. 

When studying our own Milky Way galaxy (and from within it) it’s a little harder to watch the interactions from the distance between us and some of the satellite and larger neighbouring galaxies we have within the Local Group. We do however have other tools at our disposal, such as studying the colour/temperature of stars in our galaxy, detecting their metallicity, understanding asteroseismology and of course, measuring the large volumes of neutral hydrogen gas spread across our Galaxy’s spiral arm structures. It is from this data that we know the Milky Way has a thin disc and thick disc contained within it.

Astronomers have hunted for thick disks in external galaxies, and while their relative faintness can make them hard to detect, they have been found. But so far, observations to determine if they possess the chemical distinction seen in the Milky Way has been unfeasible. Until now!

Scott, van de Sande and their team targeted UGC 10738, a spiral galaxy with similar properties to the Milky Way. Located at about 320 million light-years away, this galaxy is seen edge-on and so offers an unparalleled opportunity to distinguish the thin and thick disks. Using MUSE, the Multi-Element Spectroscopic Explorer, on the European Southern Observatory’s 8.2m Very Large Telescope, the team mapped the chemical abundances of stars in detail. 

With the exquisite data from MUSE, the astronomers were able to determine the chemical signatures of the thin and thick disk, showing them to be distinct and similar to the Milky Way. Of course, the astronomers could have gotten lucky and targeted a rare galaxy with properties like the Milky Way. Or perhaps chemically segregated thin and thick disks are common features of spiral galaxies.

Rather than being formed in a dramatic collision, these new observations point to more peaceful coevolution of the thin and thick disks, although the precise nature of this process remains uncertain. But this also leaves astronomers scratching their heads, as there are clear signs of major accretion events in the early history of the Milky Way, including the existence of the extensive, and strangely named, Gaia-Enceladus-Sausage feature in the Galactic halo. This implies that the upbringing of our Milky Way was both peaceful and violent at the same time, something astronomers have yet to fully understand. 

Video credit: Francis Villatoro/M. Volonteri 2012.