For the 45 galaxies in the HydraI sample we have the color
and for 19 of these galaxies we also have the colors
All the colors correlate strongly with
We find the following color-
Franx & Illingworth (1990) found the relations , and . It is not specified which photometric system the R-magnitudes are on. By comparing with our data, we find that it could well be Johnson R and most likely not Kron-Cousins R. The difference between the two is , based on (Davis et al. 1985) and a typical E/S0 color of . It is seen from Fig. (a-b) that the dot-dashed lines matches our data reasonably well, while a line above would match our data less well, especially for (B-r). We conclude that the R-magnitudes that Franx & Illingworth used for their color- relations are probably close to Johnson R, but that the zero point could be a bit off.
Under the assumption that the color- relations from Franx & Illingworth refer to Johnson R, we can transform their (B-R) relation by means of Eq. (, p. ) to the relation . This relation is shown as the dot-dashed line in Fig. (a). To transform their (U-R) relation, we combine the relation from Davis et al. (1985) and the relation from Jørgensen (1994) to give the relation . Then the Franx & Illingworth (U-R)- relation becomes . This relation is shown as the dot-dashed line in Fig. (b). Finally, we subtract their two color- relations to give . This relation is shown as the dot-dashed line in Fig. (c).
The coefficients in the Franx & Illingworth color- relations are determined as the geometrical mean of the coefficients from two least squares fits, one in each direction. When we fit our data in the same way, we get , and . These slopes do not differ much from the slopes obtained from our normal fitting method, cf. Eq. (). The slopes from Franx & Illingworth are in rough agreement with the slopes that we find. The slope differences (`our'-`their') are for (B-r) and for (U-r).
Bender et al. (1993) established the relation for their DHG sample. They found that the relation matched their DHG sample well. This relation was established by Burstein et al. (1988) for 276 bright ellipticals. Bender et al. combined the two relations to . By means of the relation (B-V)= 0.673 (B-r)+ 0.184 from Jørgensen (1994), this can be transformed to . This is shown as the dotted line in Fig. (a). The slope is in agreement with our data, but not the zero point, we find a mean difference of (with the differences calculated as .
The colors are also well correlated with
We find the following relations
Burstein et al. (1988) established the relation for 276 bright ellipticals, as mentioned above. This can be transformed into . This is shown as the dotted line in Fig. (d). The slope is in agreement with the slope that we find. The zero point is not in agreement with our data, we find a mean residual from their relation of .
We would like to use stellar population models to estimate the variation in age and metallicity needed to reproduce the three observed color- relations. Unfortunately, the Vazdekis et al. (1996) models (with a bi-modal IMF with high mass slope ) are not able to reproduce any of them - galaxies with large values (say ) are predicted to have much redder colors than what is actually observed. The failure to reproduce the color- relations could be due to problems reproducing the colors and/or problems reproducing . Several models cannot reproduce the -Mgb relation, which could indicate a problem in reproducing . Some models do not get the colors right. Worthey (1994) noted that his (B-V) colors were too red by when compared to globular clusters. This might be related to the problems we observe with the Vazdekis et al. models. However, we also observe problems for the (U-r) color, which is independent of B. Systematic differences between the different models are known to exist. For example, Borges et al. (1995) find that their (B-V) colors are - more blue than those of Worthey (1994).
Finally, we note that is also weakly correlated with the , as expected from the weak correlations with and . For the HydraI sample we find = 7.0%. No significant correlations can be seen with and , but this is most likely due to the small sample size (N = 17).
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (email@example.com)