|Department of Physics||University of Durham|| ||Level One|
|School of Physics||University of New South Wales|| ||General Education|
Measuring the Angular Sizes of Galaxies - Shown below is a deep exposure of a small region of sky taken with the Hubble Space Telescope. This exposure is the deepest optical image ever taken and thus shows some of the faintest, and most distant, galaxies known. This observation is called the Hubble Deep Field and was taken at the end of 1995. The observations consist images taken with a CCD camera through ultraviolet (U), blue (B), visual (V) and near-infrared (I) filters of a single field, roughly 2.5 x 2.5 arcminutes across. The image below is a small region of this field reproduced as a `true' colour image from the BVI exposures. This image is 800 x 800 pixels in size, with each pixel being 0.1 arcsec. When you click, drag and release the cursor the program will draw a line across the image and output the length of the line in arcseconds and the [X,Y] pixel coordinates for the start and end of the line. Use this facility to measure the diameters of the 15 galaxies marked on the image below using the mouse and cursor. The results are written both at the top of the image (for the current galaxy) and should also all be listed on the Java console of your browser, note down the diameter for each galaxy as you go along. In each case you should repeat the measurements for the different galaxies to gauge your measurement errors. When you are satisfied that you have enough measurements for each of the 15 galaxies then go to the next page of this exercise using the link at the bottom of the page.
Before you measure the sizes of the galaxies think about what you need - a robust estimate of the size of the galaxy. How might the apparent shape (and hence sizes of the major and minor axes) vary for a galaxy as you look at it from different angles? If galaxies are a uniform population of opaque disks, randomly orientated to our line of sight, what will be the most robust dimension of the galaxies to measure? In what follows you should measure the major axes of the galaxies.
Warning: the image below is a large one, and uses a java script. This can be slow to load, and java can also get itself in a mess if you try to type ahead too much. Be patient and work slowly if you want to achieve results. I'm afraid there is still some program development to go for these applications
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