A 100-year-old mystery surrounding the "shape-shifting" nature of some galaxies has finally been solved, uncovering the fact that our Milky Way galaxy did not always have its familiar spiral appearance. Astronomer Alister Graham has used a combination of old and new observations to shed light on the process of galactic speciation, where galaxies evolve from one shape to another through clashes and mergers. This process, akin to "natural selection," drives cosmic evolution and reveals that the Milky Way’s history of cosmic violence is not unique. It seems that survival of the fittest is just as prevalent in the vast expanse of space as it is on Earth.
For decades, astronomers have classified galaxies based on the Hubble sequence, a sequence of varying galaxy anatomy. Spiral galaxies like the Milky Way sit at one end of the sequence, while elliptical galaxies like Messier 87 (M87) sit at the other. Bridging the gap between the two are elongated sphere-shaped galaxies known as lenticular galaxies. However, what has been missing from this classification system are the evolutionary paths that connect one galaxy shape to another. Astronomer Alister Graham’s research has now filled this gap by examining 100 galaxies near the Milky Way, comparing their mass and the mass of their central supermassive black holes. This analysis has revealed the existence of two types of bridging lenticular galaxies: old, dust-lacking ones, and young, dust-rich ones. The former are created when gas and dust accrete onto dust-poor galaxies, disrupting their central region and creating spiral arms. The latter, on the other hand, are formed through the collision and merger of spiral galaxies. This research not only redraws the galaxy sequence but also provides insights into the history and future of the Milky Way, including its upcoming merger with the Andromeda galaxy and the potential formation of a daughter galaxy.
The Shape-Shifting Mystery of Galaxies Solved
Astronomer Alister Graham has finally solved a century-old mystery surrounding the "shape-shifting" nature of some galaxies. His research reveals that clashes and mergers between galaxies drive the process of galactic evolution, a process known as galactic speciation. This means that the Milky Way galaxy did not always possess its familiar spiral shape and that its history of cosmic violence is not unique.
The Hubble Sequence and Galaxy Anatomy
Since the 1920s, astronomers have classified galaxies based on a sequence known as the Hubble sequence. Spiral galaxies like the Milky Way are at one end of this sequence, while elliptical galaxies like Messier 87 (M87) are at the other end. Lenticular galaxies, elongated sphere-shaped galaxies lacking spiral arms, bridge the gap between the two. However, what has been lacking until now are the evolutionary paths that connect one galaxy shape to another.
Reshaping Galactic Evolution
To understand the evolutionary paths on the Hubble sequence, Graham compared optical light images from the Hubble Space Telescope to infrared images from the Spitzer Space Telescope. By comparing the mass of stars in each galaxy to the mass of their central supermassive black holes, Graham identified two types of bridging lenticular galaxies: one old and dust-poor, and the other young and dust-rich.
Dust-poor lenticular galaxies develop spiral arms when they accrete gas, dust, and other matter, disrupting the disk that surrounds their central region. Dust-rich lenticular galaxies, on the other hand, are created when spiral galaxies collide and merge. The surprising conclusion is that spiral galaxies like the Milky Way lie between dust-rich and dust-poor lenticular galaxies on the Hubble sequence.
A History of Cosmic Acquisitions and Mergers
The Milky Way has a history of "cannibalistic" events in which it devoured smaller satellite galaxies to grow. Graham’s research suggests that the Milky Way also accreted other material, gradually transforming from a dust-poor lenticular galaxy to the spiral galaxy we see today. In the future, the Milky Way will collide and merge with the Andromeda galaxy, erasing the spiral arm pattern of both galaxies and likely creating a dust-rich lenticular galaxy.
If this daughter galaxy encounters another dust-rich lenticular galaxy and merges with it, the disk-like aspects of both galaxies will be wiped clean, creating an elliptical-shaped galaxy without the ability to harbor cold gas and dust clouds. This research sheds light on the processes that transformed disk-dominated galaxies in the early universe and may explain the existence of some of the universe’s most massive galaxies.
Alister Graham’s research provides a new understanding of galactic evolution and the interconnectedness of different galaxy shapes. It highlights the role of cosmic mergers and acquisitions in shaping the Milky Way and other galaxies. This research has important implications for our understanding of galaxy formation and the future evolution of our own galaxy.
