There are still many things that are unknown about galaxies. One mystery which has yet to be solved is exactly how galaxies develop and age. The larger goal of my research this summer will be to contribute to the understanding of galaxy evolution.

    My mentor this summer is Greg Aldering. In 1995 he was observing a frequently studied cluster of galaxies called the Coma Cluster. The Coma Cluster is studied often because it is bright and relativley close to earth, and therefore easier to see than other clusters. Its location in the North Galactic Cap region of the sky also makes it easier to observe because it is not hidden behind the milky way. One of the goals of the observations made in 1995 was to discover what properties of galaxies were correlated with each other. Discovering the relations between properties of galaxies would supply astronomers with more information about galaxies for which not all of these properties can be directly observed. Previous research on galaxies led to the formation of the morphology-density relation. The morphology density relation basically states that galaxies with spiral morphology, which are young (often forming stars), and emit a blue spectra, will be less densely clustered than elliptical galaxies, which are older, do not form stars, and emit a redder spectra. Most of the galaxies in clusters are therefore red/elliptical, while the blue/spiral galaxies are spread throughout the sky. The reason for this relation is not yet fully understood. In order to study the galaxies in the Coma Cluster, observations of the concentration index, which measures how concentrated light is, were made. Measurements were also taken of the color of the galaxies. These measurements involve looking at galaxies through red (E) and blue (O) filters. If the galaxy is brighter through the red filter than it is through the blue, then it is known to be a red galaxy and vice versa. For the most part, the results of these observations agreed with the morphology-density relation. The more clustered galaxies were found to have higher concentration indices (CI), redder color (O-E) and more elliptical shapes, while the less clustered galaxies were found to have a lower CI, a bluer O-E and more spiral shapes. However, one exception to this relation was discovered. A group of blue spiral galaxies were found with a high concentration index. This blue cluster appeared to be located just slightly off from the center of the Coma Cluster itself.

    Greg Aldering has hopes that this cluster of blue galaxies, which he has named the Blue Blob, may provide the first good examples of a hypothesized phenomenon known as RAM pressure stripping. RAM pressure stripping occurs when blue galaxies, which may have originally been more dispersed, are pulled into a galaxy cluster (such as Coma) by its strong gravitational force. As the blue galaxies come into the cluster, the ionized hydrogen in the cluster will cause the neutral hydrogen in the galaxies to collapse and stars will begin to form at a very rapid rate. If there are young stars inside the galaxies the ultraviolet photons in the cluster will ionize the hydrogen in the galaxies Therefore, observing the amount of ionized hydrogen in a galaxy will provide a measurement of the young stars in that galaxy. However, this rapid increase in star formation will not last long. Soon, much of the hydrogen that was originally in the galaxy will be used up in the formation of stars. Then, the ionized hydrogen in the cluster will push the rest of the hydrogen out of the galaxy, and star formation will cease suddenly. It is possible that when this galaxy suddenly cools, it may become a red galaxy like the other galaxies in the cluster. In terms of the time scale of the universe, this all happens very quickly, and therefore has not previously been observed.

   My research this summer will involve investigation of whether or not the Blue Blob galaxies are an example of RAM pressure stripping. In order to prove that they are, it must be shown that: 1. They are actually in the Coma cluster. 2. They are forming new stars. If these two conditions are proven true, the Blue Blob galaxies will provide an example of a rare astronomical phenomenon which offers insight into the mystery of how galaxies evolve. The first task of my research this summer will be to confirm that the Blue Blob galaxies are actually located in the Coma Cluster. It is possible that the Blue Blob galaxies are behind or in front of the cluster. To figure out their distance, we need to know their redshifts. Redshifts are a measure of how much the spectrum of light was stretched as it travels the long distance from the galaxy to earth. Because the universe is expanding at a rate which is constant over all space, the distance to a galaxy is directly proportional to its recessional speed. (The rate of the universe's expansion is not constant over time, but calculations of the present rate have been made). The redshift will tell us the recessional speed of the galaxy, and from there, we can figure out its distance using the Hubble constant, which tells us the rate of the expansion of the universe. For the galaxies in the Coma Cluster there are actually two components to redshift. One component is the expansion of the universe. The other component is due to the gravitational force of the cluster itself. If the Blue Blob galaxies are in the Coma Cluster, they will have a redshift that is caused by both of these factors.

    In order to determine the redshifts of the Blue Blob galaxies, spectra of about 30 of the 44 Blue Blob galaxies have been recorded over the course of three nights. These observations were made at the CTIO telescope in Chile on January 19-21, 1996. The telescope makes an image called a CCD, which stands for Charged Coupled Device. The CCD uses electrons which react when hit by photons to form an image on a photographic plate. The images I am observing are spectra. This means that they are not direct pictures of the sky, but rather pictures of light which is separated by wavelength, somewhat like the separation that occurs using a prism, or in a rainbow. A spectrograph, which is used to make these spectra, works as follows:
a) light comes in from the sky and passes through a narrow slit.
b) light passes through the collimeter, which makes the light waves travel parallel to each other.
c) the light then travels through a prism or grating which separates the wavelengths of light.
d) the lens focuses the separated wavelengths onto the CCD plate. The images go into a computer file. Whenever an image is taken in this manner, there are many problems that interfere with the spectra of the galaxies being observed. In order to view the spectra, it is necessary to make reductions of them which subtract the interference and leave only the light which is actually from the galaxy.

    My first job this summer is to reduce the spectra of the Blue Blob galaxies observed in 1996. There are several steps for reducing the spectra. I will outline them briefly in this paper. All of the reduction steps are conducted using a computer program called IRAF. The first step is simply to categorize each spectrum by what type of image it is. It could be Blue Blob galaxy, but it could also be a bias spectrum, a dark spectrum, a twilight sky spectrum, a dome flat spectrum, an arc lamp wavelength calibration spectrum, or a spectrophotometric standard spectrum. Each of these type of spectra are used for various stages of the reduction process.

    The next step is to subtract the bias spectra from all of the other spectra. The bias is taken with the telescope shut and is exposed for zero seconds. It will tell me what effect the telescope itself, with no light coming in, has on the other spectra. All of the bias spectra are combined into one bias, and that is subtracted from the other images.

    The following step is to subtract the dark spectra. The dark spectra serve to identify what is called the "dark current." Because the telescope is not at a temperature of absolute zero (it cannot function at absolute zero) there will be some stray electrons inside it. Sometimes the CCD will misinterpret these electrons as a signal, and that will interfere with the data. Dark frames are exposed for about an hour, with the telescope shut. They too are combined and subtracted from the other images.

    The next step is called flat fielding. This process has several steps. The flats I am using are called dome flats. To make the flats, a uniform light is placed inside the telescope dome. The spectrum from that light is then recorded. The result of this is a spectrum which will tell you how the different pixels of the CCD vary. Because they are so tiny, there is some uncertainty in the absorption levels of the pixels, and therefore they do not all absorb light evenly. Since the light used to make the flat is a uniform light, all the pixels should be of the same brightness. Thus, the unevenness of the dome flat spectra will tell you how the pixels differ from each other. The dome flats are combined into groups depending on the night they were taken and the type of filter used. These combined images are then fit to a smooth curve, and this curve is divided into the appropriate spectra.

    The next step involves combining the twilight flats. The twilight flats are spectra taken in the evening or early morning, just before or after the sky becomes dark. At twilight, the light in the sky is very even, not dispersed into stars or galaxies. Thus, the slit in the spectrograph absorbs light very evenly at this time. The twilight flats serve to fix up things the dome flats did not quite fix. They are combined and used to perform what is known as an "illumination correction." After this step, the arc lamp wavelength calibration spectra are used. The arc spectra were taken with lamps of known wavelength. They are compared to the galaxy spectra and used to identify certain wavelengths in these spectra.

    Next, it is necessary to perform a wavelength calibration. This process assigns wavelength for each pixel of the spectra in the x-direction. Then background subtraction is performed. This process involves looking at the brightness of a sky where there is no object located, and then subtracting that intrinsic brightness of the sky from the spectra which need to be analyzed. The sky itself has a lot of brightness, due in part to cosmic rays, and if that brightness is not subtracted it will interfere with the analysis of objects.

    The next step is fringe subtraction. This step is not always necessary, and hopefully will not need to be performed because it can be very complicated. Fringes are formed when some very red light makes it through the CCD and then bounces off the back and goes through again. The CCD, because it is so thin (about 15 microns) can vary slightly in thickness. Thus, these rays that bounce off the back will interfere with each other and cause a fringing pattern on the spectra. After all of these subtractions are performed, spectra of the same object are added together (if multiple observations of that object were made) and the spectra are ready to be extracted. The extraction process changes the spectra from a two-dimensional slice of the sky to a one dimensional view of just the light from the galaxy itself.

    There are still several corrections which need to be made. The Spectrophotometric standards are spectra of stars for which redshifts are are already known. They are compared to the galaxies being observed to correct for the variation in the efficiency of light detection as a function of wavelength. Several corrections then need to be made to correct for the atmospheric absorption of light as it enters our atmosphere. After making some final preparations, the spectra are finally suitable for determining redshift. The determined redshift will tell me the distance of the Blue Blob galaxies. This distance will be compared with that of the Coma cluster galaxies, allowing me to finally determine where the Blue Blob lies in relation to Coma. This is the major task for my research this summer.

    The other task, which I will work on if time allows, involves using the redshifts I have reduced and extracted to determine information about the formation of stars in the Blue Blob galaxies. The blue color of the Blue Blob galaxies could be an error in observation or it could be due to gas which has already been used up. The absorption and emission lines in the redshifts will tell me about the chemicals in the galaxies. If a certain amount of ionized hydrogen is being emitted from the galaxies, then it will be clear that they are in fact producing new stars.

    To conclude, my three research goals for this summer are as follows:
1. Reduce and analyze the spectra of the Blue Blob galaxies using IRAF.
2. Confirm the location of the Blue Blob galaxies in the Coma Cluster by measuring their redshifts.
3. Measurement of star formation indices in these galaxies in order to determine if these galaxies are an example of RAM pressure stripping.

This research is very significant to the study of the universe because it may reveal information about how galaxies form and evolve.