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The field of stellar populations is very much alive, and will stay alive for a long time to go, since it is by far the most accessible way to study galaxies in the very distant Universe. Led by the Hubble Space Telescope, we know much more about the Star Formation History of the Universe than 10 years ago. However, as far as the interpretation of observations of galaxies at very high redshifts is concerned, stellar population modelling lies far behind observations, and it is very important that we make a huge effort together to catch up. This mainly involves the modelling in the NUV and FUV, where just a few stars dominate the integrated stellar populations. Also other areas need work: a lot of effort is needed in the near-IR, to understand better how stars evolve, so that we can use near-IR features to study the evolution of galaxies. For a few other challenges, I refer to the excellent list by Brinchmann (2010). To these I add the understanding of abundance ratios in galaxies. We are slowly getting ready to apply the techniques that have been so powerful for resolved galaxies to integrated spectra. Interpreting abundance ratios will give us another dimension that will help us to solve the puzzle of galaxy evolution. Abundance ratios affect everything. The fact that they also affect broadband colors (Ricciardelli & Vazdekis 2012) shows that also photometric redshifts will suffer from systematic errors. Another issue is the IMF, which at the moment is popular again. Understanding the dark mater in galaxies, and therefore also the stellar mass, is fundamental. It is still very difficult to derive accurate stellar masses of galaxies, but we are making progress. The main difference with the past is we now have new observations from gravitational lensing which independently can constrain the IMF slope.

In the future there will be many new facilities useful for stellar population studies. A few examples, ranked by increasing telescope size, are X-shooter on the VLT, the JWST and the E-ELT. They will give the high spectral resolution and the high signal-to-noise needed to derive accurate star formation histories which can be combined with high resolution LOSVDs. They will open new fields, such as the redshifted optical and near-IR. At the same time GAIA will determine the stellar populations in our Galaxy in much more detail as before, so that we can use the Milky Way as template for studies of other galaxies.

The fact that more and more data is available of always higher quality is also reflected in an increasing popularity of the field of stellar populations within the astronomical community (see Fig. 1.34). Brinchmann (2010) shows that many astronomers feel interested in helping to solve the problems in the field of stellar populations. I am sure that a lot of progress will come from you, the attendants of the Winter School.

Figure 34

Figure 1.34. The fraction of papers in A&A, AJ, ApJ and MNRAS each year that mention Stellar populations (solid line) or Population synthesis (dashed line, scaled up by a factor of 8) in their abstracts. Currently about 12% of all papers mention Stellar populations. (from Brinchmann 2010).

I would like to finish this review with a quote of Renzini (2006): Baryon physics, including star formation, black hole formation and their feedbacks, is highly nonlinear, and it is no surprise if modeling of galaxy evolution relies heavily on many heuristic algorithms, their parameterization, and trials and errors. Dark matter physics, on the contrary, is extremely simple by comparison. Thus, the vindication of the CDM paradigm should be found in observations demonstrating that the biggest, most massive galaxies are the first to disappear when going to higher and higher redshifts. This is indeed what has not been seen yet, and actually there may be hints for the contrary. This statement is probably supported by Tolstoy et al. (2009): The hierarchical theory of galaxy formation contains at its heart the concept of smaller systems continuously merging to form larger ones. This leads to the general expectation that the properties of the smaller systems will be reflected in the larger. (...) From recent abundance studies of low-metallicity stars in dSphs, it seems likely that there exist only narrow windows of opportunity when the merging of dwarf galaxies to form larger systems would not lead to inconsistencies. The biggest challenge for the future is to connect baryon and dark matter physics. But finding the solution here obviously is not so easy.


I thank Johan Knapen and Jesús Falcón-Barroso for having organized a very interesting and pleasant meeting, and the IAC secretaries for making sure that everything ran smoothly.

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