Caption: A figure illustrating the Hubble sequence. Image: Ville Koistinen
One look at a Hubble Deep Field image reveals that galaxies come in all sorts of shapes and sizes. But why? Astronomers have been at a loss to explain the diversity of galaxy shapes seen in the Universe. But now, two astronomers have tracked the evolution of galaxies over thirteen billion years from the early Universe to the present day, helping to clarify the “Hubble Sequence,” a classification of galaxies developed by Edwin Hubble. Keys to their model include galaxy mergers and dark energy.
Dr. Andrew Benson of Caltech and Dr. Nick Devereux of Embry-Riddle University in Arizona Benson and Devereux combined data from the infrared Two Micron All Sky Survey (2MASS) with sophisticated computer model they developed, called GALFORM. The model reproduced the evolutionary history of the Universe over thirteen billion years. To their surprise, their computations reproduced not only the different galaxy shapes but also their relative numbers.
Caption: The image shows some of the galaxies generated by the computer model. The yellow objects are most distant and therefore appear as they were 13 billion years ago, whilst those closer are seen as they looked more recently. Image: A. Benson (University of Durham), NASA / STScI
“We were completely astonished that our model predicted both the abundance and diversity of galaxy types so precisely,” said Devereux. “It really boosts my confidence in the model,” Benson said.
The astronomers’ model is underpinned by and endorses the ‘Lambda Cold Dark Matter’ model of the Universe. Here ‘Lambda’ is the mysterious ‘dark energy’ component believed to make up about 72% of the cosmos, with cold dark matter making up another 23%. Just 4% of the Universe consists of the familiar visible or ‘baryonic’ matter that makes up the stars and planets of which galaxies are comprised.
Galaxies are thought to be embedded in very large haloes of dark matter and Benson and Devereux believe these to be crucial to their evolution. Their model suggests that the number of mergers between these haloes and their galaxies drives the final outcome – elliptical galaxies result from multiple mergers whereas disk galaxies have seen none at all. Our Milky Way galaxy’s barred spiral shape suggests it has seen a complex evolutionary history, with only a few minor collisions and at least one episode where the inner disk collapsed to form the large central bar.
In Hubble’s classification, there are three basic shapes: spiral, where arms of material wind out in a disk from a small central bulge; barred spiral, where the arms wind out in a disk from a larger bar of material; and elliptical, where the galaxy’s stars are distributed more evenly in a bulge without arms or disk. The different types clearly result from different evolutionary paths, which Benson and Devereux’s model now explains.
“These new findings set a clear direction for future research. Our goal now is to compare the model predictions with observations of more distant galaxies seen in images obtained with the Hubble and those of the soon to be launched James Webb Space Telescope (JWST)”, said Devereux.
Their results appear in the journal Monthly Notices of the Royal Astronomical Society.
Benson and Devereux’s paper.
Lead image complete caption: A figure illustrating the Hubble sequence. On the left are elliptical galaxies, with their shapes ranging from spherical (E0) to elongated (E7). Type S0 is intermediate between elliptical and spiral galaxies. The upper right line of objects stretch from Sa (tightly wound spiral) to Sc (loosely wound spiral). The lower right line shows the barred spirals that range from the tightly wound SBa to loosely wound SBc types. Image: Ville Koistinen