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(The Red Sequence in the Galaxy Cluster Abell 2390)
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GOAL: This tutorial shows you how to use the MAST Discovery Portal to create a Color-Magnitude diagram of extended sources.
SCIENCE CASE: The science case is to isolate the red sequence - a color-magnitude relation for elliptical and lenticular galaxies in clusters of galaxies - in a cluster well observed by HST (i.e. Abell 2390 at z=0.2; see Gladders and Yee 2000, AJ, 120, 2148). Aperture corrections and extinction corrections are also performed.
Other potential use cases could include testing cluster evolution via the change in the slope of the red sequence, and identifying other clusters with pronounced red sequences.
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Step 1 - Go to the MAST Discovery Portal.
Use the pull down menu under Select Collection to choose the HSC (blue). Enter the name of the cluster (or if you prefer the coordinates) and search radius (i.e. Abell 2390 r=0.2d) in the Search box (green). Perform the search by just hitting a carriage return. The results are displayed in the List of Objects, while the AstroView window shows the objects against the DSS image. The left column is a series of Filters that can be used to refine the data selected.
Step 2 - Refine the sample.
The HSC includes both compact (point) and extended sources, as well as a few residual cosmic rays and image artifacts. Since we are looking for galaxies (i.e. extended sources), we want to remove as many point sources as possible. Scroll down the Filters section (blue) to the CI (Concentration Index) area and set the lower limit to be 1.3 by either moving the slider or typing in the value (green). Note that about 1/3 of the objects have been rejected (orange).
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Extinction correction is best estimated using an absorption calculator, like Doug's Excellent Absorption Law Calculator. The HSC table includes a column labeled Extinction, which is the Schlegel, Finkbeiner, & Davis 1998 (ApJ, 500, 525, 1998) E(B-V)=0.114 map value at that position. Using the absorption law calculator, with R_V = 3.1, and A_V = E(B-V) x R_V = 0.353, and assuming target wavelength equal to the pivot wavelength for each filter, or:
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W2_F555W_Cor = W2_F555W - 0.53
W2_F814W_Cor = W2_F814W - 0.37.
Step 4 - Create new Magnitude and Color columns
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Do the same thing to make the corrected W2_F814W (I) magnitude. Finally, we need to create the VC-IC color by selecting "VC", "-", "IC", and "VC-IC".
Step 5 - Make the Color-Magnitude Diagram
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