The Formation and Evolution of Galaxies

  Professor N C Wickramasinghe

In the same way that a tiny acorn can grow into a majestic oak tree the origins of the first galaxies can be traced back to subatomic fluctuations at the very birth of our Universe. One scientist who studies the formation and evolution of galaxies is Dr Alfonso Aragón-Salamanca from Nottingham University. He talked to us about the different types of galaxies and how they relate to one another.

One way of classifying the various galactic shapes uses what is known commonly as the "Hubble tuning-fork" diagram. This resembles a tuning fork held horizontally with the two prongs on the right and different galaxy types dotted along it.

Along the single handle (from left to right) are the elliptical galaxies given the designations E0, E3, E5 and E7 with E0 being the most circular in shape and E7 more elongated. These reddish spherical blobs have little discernable structure with a bright centre that slowly fades to a hazy glow. The stars that they consist of are relatively old and move in random directions around the galactic centre. There are very few new stars born in this type of galaxy, which accounts for their reddish hue characteristic of older stars. The two prongs of the diagram contain spiral galaxies - well known for their iconic catherine-wheel shape with a central bright yellow bulge and sweeping bluish arms. The upper prong shows spirals Sa, Sb and Sc which have no bar emanating from their central bulge. The alphabetical order indicates how tightly wound the spiral arms are and the brightness of the central bulge i.e. a Sa galaxy would have tightly wound arms and a bright central bulge. The lower prong shows spirals with a barred central portion named SBa, SBb and SBc.

Where the prongs and handle meet lies a pivotal class known as lenticulars (S0). These have characteristics of both elliptical and spiral galaxies having a bright central bulge surrounded by a disc- like structure. These galaxies are of special interest to Dr Aragón-Salamanca as they are thought to be an intermediate stage between highly-flattened ellipticals and spirals.

It must be made clear that the Hubble tuning-fork diagram is not a timeline for galaxy evolution, going from ellipticals through lenticulars to spirals. In fact the early Universe appears to be dominated by spirals and an additional galaxy shape that fits nowhere on the diagram - irregulars. The scheme also ignores any effect that a galaxy's environment may have on its structure. Observations using the Hubble Space Telescope have shown that spiral galaxies falling into the central region of large galaxy clusters cease their star formation and evolve into lenticular galaxies.

One major factor that also needs to be taken into account is that most galaxies are thought to be surrounded by a halo of dark matter that extends well beyond their visible boundaries. It is not known what this dark matter is actually made of but it plays a major role in the birth of galaxies, their interactions and mergers.