Color-Magnitude Diagram of a Milky Way Satellite Galaxy

This is a small research project suitable for undergraduate Astronomy and Physics students. It was put together by Dr Helmut Jerjen from the Research School of Astronomy and Astrophysics (ANU). Please send me an e-mail if you are planning to do this project. I would be delighted to hear more about your own results. Enjoy and have fun!

MATERIAL: astronomical data visualization software like ds9, IRAF, and the two galaxy images in the V-band and I-band.


You received images in the V and I-band of an unknown, partially resolved Milky Way dwarf satellite galaxy. Your task is to generate a color-magnitude diagram (CMD) of this stellar system, find out what kind of stellar population it is made of (old, young?) and estimate its distance from the sun using well-known features identified in the CMD. Finally using these informations you shall guess what dwarf it is.

First we want to work out the characteristics of the images. For that purpose start IRAF in a xgterm (not xterm), display the images in ds9, run the imexam command and measure the FWHM of an isolated star on both images by pressing the r-key or a-key. Then measure the sky background level with the m-key having the cursor on an empty part of the image and the maximum peak flux below the saturation level by pressing the r-key again on the brightest star you can find. Load the package DAOPHOT and run a suitable command to search for stars in the images (e.g. daofind). Perform photometry on the output file that contains the coordinates of the detected stars. Find a command in the IRAF tables package that can match the two lists you just generated with daofind (e.g tmatch V.fits.coo.1 I.fits.coo.1 matchtable c1,c2 c1,c2 sphere+). Generate a new column in the matched table that represents the (V-I) color: tcalc matchtable color "c3_1-c3_2". Now, plot the color magnitude diagram, e.g. in IRAF with sgraph "matchtable color c3_2" wl=-0.5 wr=2 wb=-3 wt=-10 yflip+, or with Excel. After you successfully generated a CMD working out the different commands step by step you can try to write a small IRAF script that does that all automatically.

A colleague from the Observatory photometrically calibrated one star in the field. The star at position (284.6,784.2) has a I magnitude of 16.05 and a V-band magnitude of 16.45. Use this information to calibrate your CMD. Compare your x-y distribution of stars and the CMD you just derived for the unknown dwarf with the CMDs of the Ursa Minor (left) and Draco dwarfs (right) shown below, to decide which one of the two dwarfs is your object. Estimate its distance using the horizontal branch stars in your CMD and comparing the apparent magnitude with the absolute magnitude of a BHB star (wikipedia).

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Last update: January, 2008