The Hubble Deep Field (HDF) was first photographed in late December 1995 and again in 1997. The HDF is actually a combination of six separate images, with each image taken at a different wavelength. Because of this multi-passband nature and the image quality, the HDF has been very successful. Within the 2 square arcminute (comparable to a dime 75 feet away) field, there are 1,686 galaxies. Even though the HDF is extremely small, it is actually a good representation of the distribution of galaxies because we assume the universe is isotropic, i.e. the galaxy distribution is largely the same in all directions. The galaxies we see in the HDF are extremely young. This is because the light we see has taken billions of years to reach us here on earth, which allows us to see them as they were billions of years ago. We therefore are able to study galaxy evolution.

Since the galaxies are so far away, astronomers measure a quantity called redshift. Redshift is demonstrated to us on almost a daily basis. When an ambulance siren approaches, there is a noticeable increase in pitch. This is caused by the sound waves being compressed to your eardrum. The compression of waves is called a blueshift. When the siren passes, there is a decrease in pitch. The decrease is due to the sound waves being elongated. This is called a redshift. Now, this acoustic example is directly applied to the light of the galaxies in the Hubble Deep Field. Because space is expanding, the galaxies’ light is elongated. However, since the galaxies are so distant and faint, we are not able to measure the redshift directly. We have to measure what we call the photometric redshift. Astronomers do this by measuring the apparent brightnesses of the galaxies at different wavelengths. Redshift is an important quantity to astronomers because it is directly proportional to distance and because we can use it to speak of time.

The Hubble Deep Field image was taken electronically, much like taking a picture with a digital camera. The first part of the research project was to determine the photometric redshifts of the HDF galaxies by using a computer program that was developed by my research advisor, Dr. Andrew Connolly. We measured the redshift on a pixel-by-pixel basis. We then determined the actual redshift by making a plot of the number of pixels versus the redshift. Wherever the most pixels with a particular redshift occurred was what the galaxies’ redshift was measured to be. This technique also allowed us to determine the distribution of young and old sections of the galaxies. This set us up for the other part of the project. By having the distribution of young and old sections, we are able to deduce whether there is star formation in the galaxies. The star formation within a galaxy is an indicator of the age of the galaxy, or how much the galaxy has evolved. So the last part of the project was to obtain a plot of the percentage of the star formation of a galaxy versus redshift.

So far, we’ve analyzed about 50 galaxies. These galaxies have measured redshifts, which allows us to check our photometric redshifts. Our photometric redshifts have been in very good agreement with the measured redshifts. We do not expect to have exact agreement because the photometric redshifts are only an approximation to the true measured redshifts. We are now in the process of automating the procedure. Once that is done, we will finish analyzing the rest of the galaxies.