Galaxy Clusters – Our Local Group

Merging Galaxies Polar Ring Galaxy NGC 4650 Antennae Galaxy NGC 4038 And if you still aren’t convinced that galaxies collide – then these, rather graphic, examples can surely change your mind. Polar Ring Galaxy NGC 4650 Antennae Galaxy NGC 4038

The Cartwheel Galaxy   On the topic of merging galaxies the Hubble Space Telescope produced this famous image of the Cartwheel galaxy. A very strange galaxy indeed, apparently a spiral galaxy which has been obliterated by another galaxy passing straight through it - like a stone hitting water. Well the HST image shows two candidate galaxies that could have been responsible, but which one was it? Some astronomers favoured the top galaxy, because its yellow colour reveals that it is only comprised of old stars as its star creating gas was stripped away during the collision. Other astronomers favoured the bottom galaxy which is blue due to its vast number of young newly created stars, which they explain as having formed due to the impact with the Cartwheel galaxy.

The Latest HST Image The HST has recently been installed with a new camera – called ACS – Advanced Camera for Surveys. This is the first image taken by the ACS and is truly spectacular. Here we have two merging galaxies, with evidence of intense star formation being triggered. Thanks to ACS, we should expect to see quite a few more spectacular images like this before Hubble is finally retired.

Advanced Camera for Surveys …in detail Now the really amazing thing about the ACS is its ability to see so much more than the target we are interested in. Designed for survey work the camera is the most sensitive and accurate to date, and as a result when taking photograph of this peculiar galaxy it managed to not only record a spectacular view of the Tadpole galaxy, but also reveals many hundereds of tiny (read distant) background galaxies, all with an unprecedented level of detail surpassing that of even the HDF which was shown in Julie’s talk last week.

NGC 3079 – A Starburst Galaxy Starburst Galaxies Here we have another image of a spiral galaxy, where the Hydrogen light has been exaggerated. In this case we can see an enormous bubble of gas expanding out from the centre of the the galaxy. This is a galaxy which formed part of my research as an undergraduate student at Birmingham university. NGC 3079 is a starburst galaxy, in its nucleus hundreds of stars are constantly exploding in supernovae. The pressure of such explosions blows a galactic superbubble of gas out of centre of the galaxy. Again this is an enhanced colour image. Here the colours have been chosen so that red represents hydrogen gas, showing us the nebulae and the galactic supperbubble. I simulated the formation of this superbubble which then gave us information on the age of the bubble and hence the history of the galaxy’s starburst period, and information regarding the galaxy’s gas structure.

Hubble's Law – And the Expanding Universe As well as classifying galaxies into different types, Edwin Hubble observed the spectral lines of many different galaxies. He discovered that almost every galaxy is moving away from us! What's more there is a relationship between the recessional speed (or redshift) of a galaxy and distance it lies away from us! Galaxies that are further away are moving more quickly! So a galaxy with a large redshift is not only moving away from us very quickly but is in fact a very long way away from us! redshift (z) = v / c = (l – l0)/l V = H0 D H0 Hubble's Constant gives the age of the universe! It is believed to be 71 km/s/Mpc

/Quasi-stellar radio sources Quasars /Quasi-stellar radio sources The most distant objects in the known universe have extremely redshifted spectral lines Images only 2D need spectra for 3D The previous picture pointed out the problem of perspective in astronomy. Quite often we cannot tell how far away an astronomical object is, and so when we see two apparently related objects such as these two galaxies we cannot be sure as to whether they truly are related or are just chance alignments. The small galaxy to the right of this barred spiral galaxy was once thought to be a companion or satellite galaxy which orbits around the spiral galaxy. However a spectral observation of the companion galaxy revealed that its features were strongly redshifted indicating that the galaxy was moving away from us at a speed much greater than would be expected for a galaxy in a stable orbit. So either this galaxy has been somehow flung out of the spiral galaxy, or it is quite unrelated, and is in fact a distant quasar at the edge of universe explaining its fast speed as just being the expansion speed of the universe at that distance.

Active Galactic Nuclei