IAUS241 Challenge 2

By A. Vazdekis, A.J. Cenarro, A. de Lorenzo-Caceres & M. Beasley (IAC, Tenerife, Spain)

Fitting the absorption line-strength indices

We have compared the line-strength indices measurements for the globular/star cluster spectra with those obtained for the single stellar population models of Vazdekis (2006) based upon the MILES library (Sánchez-Blázquez et al. 2006) using the Padova 2000 isochrones. The models were smoothed to match the FWHM and the instrumental configuration of the objects. Lick indices of the models and data were measured using the definitions given in Trager et al. (1998). Note that these indices were not corrected to the Lick system, but remain in the flux-calibrated system of MILES. Ages and metallicities where obtained from the [MgFe]-Hbeta grids. [CN/Fe] is obtained from the relative differences between the CN2-Hbeta and < Fe >-Hbeta grids. Similarly, [Mg/Fe] is obtained from the relative differences between the Mgb-Hbeta and < Fe >-Hbeta grids. Hence the abundance ratios are purely relative measures of these indices, rather than absolute values. The table below shows the ages, metallicities, [CN/Fe] ratios and [Mg/Fe] ratios obtained using our approach.

Dispersion (Å/pixel)
Age & [Fe/H]
47Tuc 3.1
Age: >18 Gy
[Fe/H]: -0.8 dex
[Mg/Fe]: 0.2
[CN/Fe]: 0.7
M67 2.7
Age: 3 Gy
[Fe/H]: -0.2 dex
[Mg/Fe]: 0.
[CN/Fe]: 0.
NGC1868 11
Age: 1 Gy
[Fe/H]: <-17. dex
[Mg/Fe]: 0.8
[CN/Fe]: 0.
NGC6528 3.1
Age: 18 Gy
[Fe/H]: -0.4 dex
[Mg/Fe]: 0.2
[CN/Fe]: 0.4

It is worth noting that the majority of the results obtained in this way are in good agreement with those from the test 1. Below the exceptions are remarked upon:

The predicted age for 47 Tuc is too old, and drops off the bottom of the grids. This problem has been noticed previously (Vazdekis et al. 2001, Gibson et al. 1999). If the age of this cluster is set to the oldest age of the models (18 Gy), then reasonable metallicities and abundance ratios are obtained.

In the case of NGC 1868, the low resolution and dispersion of the spectra complicate index measurement. We think that the age and metallicity from the CMD fit is ok and that the problem here is the low quality of the spectral data.

In the case of NGC 1805, our models do not reach these very low ages (0.06 Gy from test1) and therefore we have not been able to do this test.


Gibson, B.K., Madgwick, D.S., Jones, L.A., Da Costa, G. S., & Norris, J.E. 1999, AJ, 118, 1268
Sánchez-Blázquez et al. 2006, MNRAS, 371, 703
Vazdekis et al. 2006, (in preparation)
Vazdekis, A., Salaris, M., Arimoto, N., & Rose, J. A. 2001, ApJ, 549, 274