2.2. The colour-luminosity plane
The next most obvious key global parameter, after luminosity and surface brightness (size) is colour, and in particular the rest (u - r) which straddles the 4000Å-break and hence a crude indicator of the current star-formation rate. Baldry et al. 2003 and Hogg et al. 2004 have recently studied this plane extensively with SDSS data and demonstrate clear bimodality of the colour distribution. Fig. 4 shows this trend for the 10k galaxies of the MGC (using SDSS colours). Fig. 4 also shows this trend for the bulge and disks separately. To obtain the bulge colour we use the SDSS PSF magnitudes and to obtain the disk colour and we remove the bulge colour component from the global colour to reveal the disk colour 5. We now see that the bimodal distribution can readily be explained in terms of predominantly red bulges and blue disks. This component segregation implies distinct stellar populations with distinct evolutionary paths. Bulges must contain old stellar population and disks intermediate or young populations. Again this follows conventional wisdom but highlights yet further the important of bulge-disk decomposition and the need to study the component properties of galaxies rather than the global properties.
Figure 4. (upper) The bimodal distribution of rest-(u - r) colour (white) subdivided into bulge (red) or disk (blue) components. It is clear that the bimodal distribution is is really a bulge-disk dichotomy. (lower) the same data shown according to rest-(u - r) colour and absolute (B) magnitude. The sample is not volume corrected but nevertheless shows that bluer systems are typically of lower luminosity.
5 i.e., (u - r)D = - 2.5 log[10(-0.4uT) - 10(-0.4(u-r)B)] - rT -2.5 log[10(1 - B/T)] where the filter subscript refers to total Petrosian magnitude (T), disk (D) or bulge PSF magnitude (B). Back.