The age of galaxies from infrared stellar population analysis

Two techniques are commonly used when studying stellar populations in galaxies. When studying galaxies at distances of 15 Mpc or less, one can determine information from HR diagrams of their individual stars. For galaxies that are further away, which include all galaxies at high redshift, one can only analyse integrated spectra.

Methods to analyse integrated spectra have been improved significantly in recent years. A few years ago, it was difficult to measure more than the average age and metallicity of a galaxy. Nowadays, sophisticated fitting methods make it possible to fit reasonably complicated star formation histories (e.g. MacArthur et al. 2009, Koleva et al. 2008). Still, all results are based on the optical part of the spectrum. It would be great if the near-infrared could be used. In the near-infrared we don't have the problem of frosting (Trager et al. 2000), the effect that a small fraction of young stellar populations, which generally are very bright, make it very difficult to detect the oldest stellar populations. In the near-IR it is much easier to measure mass-weighted ages, i.e. the ages that theory predicts, than in the optical, where luminosity-weighted ages can be very different.

The problem in the near-IR is two-fold. Good data are scarce, and reliable models are not available. For the observations, the problem is that the sky brightness in the NIR is very high, and at the same time variable, making sky subtraction very difficult. The problem with the models is that in the NIR stars from the AGB are dominant. Observed libraries of stars, needed to make models, do not contain enough AGB stars at the moment, and also the theoretical isochrones are difficult to calculate.

A few years ago we have started a campaign to solve these problems. For bright ellipticals some good observations exist now (Silva et al. 2008, and Marmol 2009 (thesis Madrid, from the ESO-VLT, in a collaboration with Scott Trager and myself, still unpublished)). We have observed about 10 spiral galaxies at UKIRT and La Silla, which are still to be analyzed. The fact that these data come from large telescopes, and that the sky subtraction has been done carefully has ensured that these data are of a quality comparable to those in the optical.
We have also started to observe a stellar library, to link to our stellar population models (Vazdekis et al. 2003). Using telescopes at Hawaii, La Palma and La Silla (Chile) we now have obtained a library of 150 stars in the infrared H and K-band at a resolution of 2000, that will allow us to extend the stellar population models to this spectral region. The models will be particularly sensitive to AGB stars, with ages between 0.5 and 2 Gyr. Applying the models to nearby elliptical galaxies we find, purely based on observations, that galaxies in clusters show a different behavior than those in the field. This can be explained if cluster ellipticals are generally old (8 Gyr or older), while ellipticals in the field contain a significant fraction of AGB stars (Marmol et al. 2009, in preparation). At the moment, good stellar population models have not been made. In the near-future we will try to make them with our 150 stars, but it is very well possible that we need more stars, to cover the parameter range in metallicity and temperature of the stars in the galaxies.

Recently a new, very attractive possibility has appeared. A new instrument has appeared at the VLT, named X-shooter, which allow us to obtain spectra all the way from the blue to 2.5 micron at once, and with high efficiency. So, observing a library in the near-IR is much easier with X-shooter than before, gives much higher spectral resolution data (the resolution is about 9000), and gives the opportunity to make the optical and near-infrared models directly compatible. X-shooter is already offered from September 2009.

We are also working on improving the stellar isochrones, that are also needed for the stellar population models. In a collaborative effort with Scott Trager, Guy Worthey, Aaron Dotter, and Yanping Chen (Groningen) we are preparing isochrones with different abundance ratios.

We propose a PhD programme, which consists of the following tasks:

- Prepare the stars of the X-shooter stellar library
- Be responsible for the observations of this library
- Make new stellar population models in the NIR using improved isochrones and the new library
- Reduce the existing observations of the 10 spiral galaxies
- Analyse these observations, together with the literature observations of elliptical galaxies using the new models
- Analyse the resulting ages together with the results from the optical in the literature, trying to understand
  how and when galaxies formed, as a function of mass, and environment

The work will be done in collaboration with Peletier, Trager, and some of the PhD students in their groups. Suggestions for improving the programme are welcome.