Wednesday, 7 December 2011

Public Lecture : Galaxy Evolution

The Astronomy department of the University of Nottingham held another of their public lectures this week. Titled “Galaxy Formation and Transformation” it was presented by the wonderfully accented Dr. Alfonso Aragon-Salamanca - who will be referred to as Dr A for the rest of this post.

Dr A began by providing some background information of galaxy types, explaining that galaxies can be broken down into the following categories

Spiral Galaxies. With large numbers of young stars, these galaxies have the recognisable spiral arms, sometimes with a central bar.

Lenticular Galaxies. These are flat discs, as with the Elliptical galaxies, these contain few young stars and no gas or dust (which are the raw materials of star formation).

Elliptical Galaxies. These look like large balls of stars, with no particular structure. They contain few young stars and no gas or dust.

Keep the Lenticular and Spiral types in mind - we will be coming back to them later.

As well as these main classes, there are also irregular galaxies which have shapes that are, well, irregualar. Dr A explained that this was generally because these galaxies were either very small (and thus lacking in gravity) or the result of a car-crash between two separate galaxies.

These various types of galaxies can be shown on the Hubble Morphological Classification System

The Sc spiral? Just keep going and turn left at the lenticular galaxy. You can't miss it. If you see an Esso garage, you've gone too far

Dr A then moved on to the question of how galaxies formed. The detail was easy to explain - astronomers don’t really know !

However, researchers are able to explain galaxy formation in more general terms. To do so, we need to go back to the beginning. . .

Immediately after the Big Bang, the Universe was very hot, so hot that even atoms could not form, and was also opaque to light and other electromagnetic radiation. And it stayed this way for around 400,000yrs.

After this time, the universe had cooled sufficiently to allow the formation of the neutrons and protons, the building blocks of the elements. At the same time, the universe became transparent to light, and the light radiation that was released at this time has been travelling ever since, slowly getting cooler and cooler. It can now be seen by telescopes as the Cosmic Microwave Background Radiation.

Upon coming back from his holiday, Adam realised that he had forgotton to put the pizza in the freezer

It is the small differences in this radiation that gave rise, over time to stars and galaxies. You can see how this process might have worked in simulations by the Max Plank Institute (scroll down to the clip titled “The 4D Universe”). These simulations are believed to be more or less accurate because they describe the kind of distributions of galaxies that we actually see in space around us.

When astronomers look at galaxies the amount of matter they can see is a lot less than is required to explain the way they move. So they have proposed that there is another type of matter (known as “dark matter”) that provides the rest of the mass. Dark matter does not interact with light or with the ordinary matter we see around us - but does have mass and reacts to gravity. Weird eh?

Different telescopes are designed to operate at different wavelengths, from X-rays to radio waves, and these different frequencies of electromagnetic radiation are emitted by different types parts of a galaxy. For example :

UV shows massive, hot, young stars
Visible shows middle aged stars
Near Infra-red shows old stars
Mid Infra-red shows dust
Far Infra-red shows hydrogen

Thus galaxies that are emitting strongly in the UV, Mid and Far IR have young stars and the raw materials (dust and hydrogen) to form new start. (For example, M81)

Conversely, galaxies that are emitting weakly in these areas (but strongly in the Near IR) are composed of old stars and cannot make significant numbers of new stars. These galaxies are dying. (For example M84)

With the capabilities of telescopes such as Hubble, astronomers have been able to look at galaxies billions of light years away. As the light we are seeing from these galaxies left them billions of years ago we are essentially seeing them as they were then.

Studies on these images have shown that six billion years ago, the universe had a high proportion of spiral galaxies (often in clusters) and few lenticular galaxies.

Today, it is the lenticular galaxies that are found in clusters, with a few isolated spiral galaxies around them.

So what happened?

Research is suggesting many of the spiral galaxies that fall into clusters had the gas and dust stripped from them due to shock waves from interactions with other galaxies in the cluster.

These “stripped” galaxies can no longer produce new stars so, moving forward to today, we are left with the preponderance of (dying) lenticular galaxies that are observed around us.

You can see a lot of the slides presented in the talk here (although, to be fair, they are rather heavy going if you do not have an astronomer to guide you through them.

In short, a fascinating lecture, that provided a possible narrative for the evolution of the universe and contained some spectacular animations.

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