Mr. Willy Kranz

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Education:
2003-present Physics Major, Universitaet Zuerich

Research History:
Fall 2007-present Bachelor's Thesis (supervised by Prof. Dr. Kim-Vy Tran)

Project:

Finding the Oldest Galaxies in the Local Universe using Spectral Indices

Given a population of galaxies, one can usually find the oldest ones by looking at their properties, e.g. color and brightness in the optical. Using the measured magnitudes of the optical bandpasses g' and r' from the SDSS 6th Data Release (Sloan Digital Sky Survey), and calculating the color (g'- r'), one gets a typical color-magnitude diagram of galaxies. In such a plot, the oldest galaxies can be found in a narrow band of (g'-r') color; this feature is called the Red Sequence.

Another way of finding the oldest galaxies is by looking at their spectral indices, e.g. measuring the equivalent width (EW) of Halpha (Balmer line) or calculating the value of DN(4000). The oldest galaxies should have a high value of DN(4000) and clear absorption characteristic for the Halpha EW. For high redshift galaxies, one can measure the EWs of Hdelta and OII instead.

For my Bachelor's Thesis I compare the two methods of measuring EWs of spectral lines and select galaxies by using a combination of Halpha, Hdelta, and OII.

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Figure 1 shows a color-magnitude diagram of a random selection from the SDSS galaxy database. The red points represent galaxies with Halpha absorption, the green points represent galaxies with small emission in Halpha, and the blue points represent clear and strong emission in Halpha. The solid line is a fit to all points and the dash-dot line is an arbitrary color cut-off above which the oldest galaxies usually can be found.

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Figure 2 shows the color-magnitude diagram of Fig. 1, but now only galaxies with absorption in Halpha are plotted (red points). One can clearly see how selecting only by Halpha identifies the Red Sequence. The solid line is as in Fig. 1; the dotted line is fit to the color-selected Red Sequence (see Fig. 1) while the dashed line is fit to the Halpha selected galaxies.