Data Reduction and Analysis Project

(This document is available on-line, with hyperlinks, at http://gila.la.asu.edu/observing/project.html.)

In this project, you will apply some of what you have learned in this class about data reduction and analysis.  You will also be introduced to one of the most important jobs of the scientist: communicating your results to others.

The data

The data with which you will work is a set of raw CCD images of M8, the Lagoon Nebula. These data files were taken through a variety of narrow- and intermediate-band filters. You will find the data in a gzipped (compressed) tar file in /usr/data. You can unpack the contents into the directory of your choice (it's easiest to make a subdirectory of your iraf directory) and typing: tar -xvzf /usr/data/m8.tar.gz . You may then browse the FITS files in the usual way from within IRAF. You can determine the contents of the files by looking at the FITS header. There is also a photocopy of the original telescope observing log from that night attached. The "filter" column of the log sheet gives both the central wavelength of the filter used for that exposure and its width (in Angstroms).

The science

Your scientific goal is to determine the nature of prominent dark lane in the middle of the nebula. To do this, you will make a map of the relative extinction at different points in the nebula. Does the dark lane stand out? What does this tell you? The technique used to determine the relative extinction at various points is to compare the ratio of H[alpha] emission to H[beta] emission at various points in the nebula. Departure from a canonical ratio expected for certain nebular conditions indicates a difference in extinction toward the emitting material. For more details, see Chapter 7 from Osterbrock, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei.

To determine this ratio, you will have to do some additional data reduction besides the standard CCD processing steps described in class. After processing all of the CCD images in the standard way, you will want to do the following things. Starting from the end result and working backward, we need:

  1. An image of the ratio of H to H[beta] . For this, we need:

  2. Images of the H and H[beta] line emission alone that do not include continuum emission. To make these, we need to:

  3. Subtract the appropriate continuum emission image from each line emission image. But in doing this, we need to be careful that the images are scaled properly with respect to one another. So we must:

  4. Do aperture photometry of several stars in each image. Plotting counts (not magnitudes!) for the stars in the continuum image vs. counts for the same stars in the line image should give a linear relation. Use this linear transformation to scale one image to the other before subtracting. The linear transformation takes care of both zero points offsets and overall bandwidth or extinction differences between the two filters.
While reducing the data, please be conscious of how much disk space you are using, and delete any files you no longer need. The unix command "du" (think disk usage) will tell you how much space the files in the current directory and its subdirectory are using. The command "df" (think disk full) will tell you how much is left on the disk. Look at the line for "/usr/data". I'll try to keep an eye on the disk usage. (For starters, please delete any old image files from the previous IRAF projects!)

The writeup

You will report your results in a scientific paper written in the style of the Astrophysical Journal or Astronomical Journal (your choice). The paper must be formatted in the proper style required for electronic submission to one of these journals. While several electronic formats are available, the recommended format is LaTeX (using AASTeX macros) with Postscript figures. An author's guide for using AASTeX is available. If you want to get an idea of how this works, many examples of astronomical papers written using these macros are available, for example by browsing the astro-ph preprint archive. (Search the archive index for "aastex".)

The paper should give a brief introduction to the problem being addressed, a discussion of the data reduction methods, and presentation of the results. It should have both an abstract and a conclusion that summarize the main findings. Again, look to published journal articles for guidance.

To submit the paper electronically, follow the guidelines given at the journal web sites, but use the ftp server gila.la.asu.edu instead. Log in as "anonymous" and leave your paper in the "incoming/ast598" directory. As with the actual journal ftp servers, you will be able to place a file in this directory, but will not be able to see its contents. This protects your fine scientific work from the prying eyes of your competitors!

The paper is due by 5:00 PM on Friday, May 8. If you wish to submit a draft earlier, I will gladly read it and give you comments within three days. Your grade for the project will be based on the version in my possession on the due date. (The syllabus states that you will be required to turn in two separate drafts. Because of the short time left in the semester, I am no longer requiring this. However, I strongly encourage you to give an earlier draft to me so I can give you comments on it. This will help your grade, and more importantly, more closely mimics the actual write-review-rewrite process of working on an actual journal article.)