Results

Unfortunately, the calculated colour images of the two model galaxies do not look very different. The Salpeter galaxy forms a bulge a bit earlier, at an age of 4 Gyr

Figure 25.1. Top panel: absolute r magnitude evolution of the Salpeter (thin) and Kroupa (thick) models, both including (solid) and omitting (dashed) gas absorption. Bottom panel: differences in absolute r magnitude between the Salpeter and Kroupa models, both including (solid) and omitting (dashed) gas absorption. The thin dotted line shows the zero level.

Figure 25.1. Top panel: absolute r magnitude evolution of the Salpeter (thin) and Kroupa (thick) models, both including (solid) and omitting (dashed) gas absorption. Bottom panel: differences in absolute r magnitude between the Salpeter and Kroupa models, both including (solid) and omitting (dashed) gas absorption. The thin dotted line shows the zero level.

(z = 1-59) versus 6 Gyr (z = 0.94) for the Kroupa galaxy, but, apart from that, the two galaxies look disappointingly similar throughout their evolution.

This is confirmed on comparing the integrated magnitudes and colours of the two models. Owing to the higher SFR, the stellar light (which is calculated in the absorptionless models) is brighter (by up to ~ 0.5 mag, depending on the filter band used) and bluer (by up to ~ 0.2 mag, depending on the colour) in the Kroupa model from 5 Gyr onwards. Unfortunately, when one includes the gas absorption in the colour analysis, this effect is counter-balanced by the higher gas absorption due to the higher gas and 'dust' content, thereby reducing the differences in magnitude and

Figure 25.2. Apparent SLOAN u magnitudes of the Salpeter and Kroupa models as seen face-on (dashed) and edge-on (dotted) as a function of redshift z (not to be confused with the SLOAN z passband). Overlaid are the SDSS DR4 data with their observational errors of galaxies that have similar sizes, concentration parameters and orientation to the models.

Figure 25.2. Apparent SLOAN u magnitudes of the Salpeter and Kroupa models as seen face-on (dashed) and edge-on (dotted) as a function of redshift z (not to be confused with the SLOAN z passband). Overlaid are the SDSS DR4 data with their observational errors of galaxies that have similar sizes, concentration parameters and orientation to the models.

colour between the two models almost to zero. We call this unfortunate conspiracy the 'IMF degeneracy'. It is illustrated in Figure 25.1 for the r-magnitude.

The magnitudes and colours which offer the best chance of breaking this 'degeneracy' lie in the infrared (JHK, etc.), but even in this wavelength range, the differences between the two models are too small to be measured.

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