Introduction to Astrophysics Lecture 15: The formation and evolution of galaxies

This class’s questions: How are galaxies located in the Universe? How do galaxies evolve? How did galaxies form in the first place?

Galaxy evolution Galaxy collisions are not rare events. We see many galaxies which are either in the process of collision, or show evidence of recent collisions, such as intense star formation. Conclusion: the Universe is not a static place, looking more or less the same now as it always did. Rather, it evolves.

The faint sky In 1995, the Hubble Space telescope looked at a single region of sky for many orbits. It imaged objects, all distant galaxies, which were fainter than anything which had been seen before.

A single image captures the different stages of galaxy evolution.

Conclusions from the Hubble Deep Field M Galaxies have existed from when the Universe was very young. Remember, if the light has travelled a long way, it was emitted when the Universe was much younger than it is now. M When the Universe was younger, there are many more signs of evolution and irregular galaxies than there are now.

What do we know about the galaxy distribution? A vital tool for understanding galaxies is large maps of the local Universe, showing the locations of galaxies in as large a region possible. The tool used to derive the distances of galaxies is their redshift. According to cosmology (see later lectures), the Universe is expanding, and the more distant an object is, the more rapidly it is receding from us. So the recession velocity can be used a distance estimator.

The speed with which an object is moving away from us can be determined by the shift in its characteristic spectral lines. For nearby objects, if they recede with velocity v, then the wavelength of the spectral lines changes wavelength by where c is the speed of light. Redshifts  / = v/c

Galaxy surveys To make a map of galaxy locations, it is necessary to painstakingly measure the spectrum of each galaxy to determine the redshift. This may require around an hour of observing per galaxy. The first large effort to do this was the CfA galaxy survey in the mid nineteen-eighties. This contained a few thousand galaxies, going out to distances of several hundred million light years.

The CfA galaxy survey. Each dot is a galaxy! We are here

The local galaxy distribution Galaxies are not located either uniformly or randomly. Instead, the galaxy distribution shows considerable clustering, with galaxies concentrated into clusters in some places and into walls and filaments elsewhere. There are also large voids where there are no galaxies.

Modern redshift surveys Now a survey the size of the CfA survey can be done in a single night’s observing! There have been two recent large efforts to make the biggest maps ever of the Universe: 2dF : aimed to get 250,000 redshifts (UK/Australia): achieved about 220,000. Sloan Digital Sky Survey : aims for a million redshifts (USA), though probably won’t quite manage that. Simulated 2dF catalogue

Evolving Structures: Gravity Does It It is believed that the main force at play in the evolution of structure is simply gravity. Initially overdense regions of space preferentially attract more material. Density Position As time goes by, material accumulates in the higher density regions, eventually forming galaxies and other structures. Although gravity is not the only force acting, it is the most important one. Simulation of a galaxy cluster forming

Understanding structure formation Dark matter plays a crucial role in providing the gravitational attraction which causes structures to evolve. To follow the development of structure in detail, computer simulations are needed. At Sussex we are involved in some of the world’s largest simulations, using supercomputers in Durham, Nottingham and Munich.

Movie credit: Ben Moore (Zurich)

All matterGalaxies

What do we not understand? We know how gravity brings material together to form galaxies, but we do not know how the stars form within galaxies. We do not know for sure what starts the whole process off, though there are some ideas. We do not understand how events in the first galaxies to form, such as supernovae, might affect the environment in which other galaxies form.