THEORY AND PRAXIS

SPECIAL ISSUE 1999

Course: Stellar population synthesis models at low and high redshift

Prof. Gustavo Bruzual
CIDA
Venezuela

Theoretical models and observation are combined to explain the evolution of galaxies. When it comes to studying the stellar population, uncertainties become degeneracies (age-metallicity, extintion by dust) that determine our interpretation of the galaxy spectrum. The final result depends therefore on the cosmological model on which our estimates are based. Gustavo Bruzual, from the CIDA (Venezuela), thinks that real progress will come when we have a clearer understanding of the process of galaxy formation on cosmological timescales, as well as of the prevalent cosmology. "This has to come from the theoretical side, which seems to be lagging behind observations in this area", comments Bruzual.

"On the contrary. HST results have been very spectacular, as well as the follow up from the ground. However, a small number of objects and a very small volume of the universe has been investigated. I think that the large telescopes from the ground will provide excellent data on larger samples that will allow us to establish how general the HST findings are. Astronomy needs large samples to derive statistically significant results. Besides that, there are many fields of astronomy that will benefit from the large telescopes, like galactic structure studies, which will help us understand more deeply many properties of galaxies in general. The interferometric capabilities of the new telescopes have not been exploited yet. For sure they will provide enormous amounts of valuable new information."

"Photometric redshifts depend on the comparison of an observed set of colours (or low resolution spectrum) with similar quantities computed either from a set of evolutionary models or from the K-corrected spectrum of local galaxies. Thus, the photometric redshifts themselves are not very sensitive to the uncertainties of the models; the empirical K-corrected spectra produce essentially the same answers as the models. The uncertainties arise once we want to interpret the spectrum of a given galaxy in terms of its stellar population content. There are multiple degeneracies. The age-metallicity degeneracy is mentioned most often. Old metal-poor galaxies have, in general, similar spectra to younger metal-richer galaxies. Unless we know either the age or the metallicity, our interpretation of a galaxy spectrum will be ambiguous. Similar degeneracies are introduced by our choice of one of the many possible star formation initial mass functions, or by ignoring or including the effects of extinction by dust in the galaxy models. Relating evolution in the galaxy rest frame to redshift requires knowledge about cosmology (Ho, Wo, L). Hence, until we do not know which cosmological model is the correct one, our interpretations will have an extra degree of freedom.

Differences, not yet well understood, between different sets of evolutionary models, add to the confusion. The physics of galaxy formation and evolution is most likely to be the same for a large fraction of the visible universe. The problem is that we still do not know well the role played by all actors. Big galaxies seem to form from smaller units in a process dominated by dark matter, which seems to be a real physical entity, but whose nature we ignore. In summary, I think that real progress will come when we have a clearer understanding of the process of galaxy formation on cosmological time scales, as well as of the prevalent cosmology. This has to come from the theoretical side, which seems to be lagging behind observations in this area."

"Your statement is relative and should be quantified. A few years ago it was a dream to think that we would have at hand such an enormous amount of data on distant galaxies as has become available in the last 5 years, or so. Just finding out that there are galaxies already formed at high redshift (4, 5, 7 or even 10) is a remarkable piece of information. It is true that the spectra of the most distant galaxies are noisy when compared to those of their local counterparts. However, the large 8 and 10 m telescopes will surely change this situation. The rest frame UV spectrum of the galaxy LBDS 53W091 (z=1.55) obtained with Keck by Spinrad and collaborators is of comparable quality to most of the UV spectra of local stars obtained by IUE. Obviously, we need not only better quality data of a few objects, but data of the best quality possible on a large number of objects to establish more constrains on the large scale picture of galaxy formation and evolution. A few galaxies here and there is not enough."

"The current status of Venezuela as a whole is quite uncertain. The role of science in society is unclear. Astronomy is considered much less of a priority and less necessary than other sciences. We do not produce either food or money, only knowledge, and that is not very attractive to the government, nor to the population. Modern astronomy is quite young in Venezuela. It started less than 25 years ago. There are few people working on astronomy in the country, but many of us have made a real effort to increase this number. We have trained many excellent students. Some of them have left the country, some of them are being trained outside and will come back, others will finish their doctoral degree in Venezuela. There is no doubt that a new generation of young Venezuelan astronomers is being formed and that they will make sure that astronomy remains important as a scientific discipline. Despite the unfavorable meteorological conditions of our country, we have managed to modernize our astronomical instrumentation through collaborative agreements with other institutions. We expect that in the near future significant results will start coming out of this effort."

PROFILE

GUSTAVO BRUZUAL was born in Caracas (Venezuela), in 1949.

He studied Physics at the UNAM (México), where he was awarded his BSc in 1973.

He specialised in Astrophysics and continued his post-grad studies at the University of California in Bekeley. In 1981 Bruzual got his PhD with a thesis on the spectral evolution of galaxies, a field to which he has devoted most of his scientific carreer.

With a wide teaching experience, Bruzual has taught Astronomy at the University of California in Berkeley(1977-80), at the UNAM (México, 1981-82), at the Central University of Venezuela (since 1987) and, since 1981, at the CIDA Foundation (Venezuela), an institution currently under his direction.

As one of the most outstanding scientists in his country, Bruzual has been awarded with several distinctions; he was appointed "Associate of the Royal Astronomical Society in regodnition of his Leadership in Astronomy,(1991) and the Government of Venezuela awarded him with the "Orden al Mérito en el Trabajo, primera clase" (1992).

He has been part of the Organizing Committees of a number of internacional conferences in Latin America, and was appointed Coordinator of the Latin American Astronomical Network (1998).

Gustavo Bruzual has acted as referee for international scientific journals such as The Astrophysical Journal, Nature, Astronomy & Astrophysics, Tribuna de Astronomía and the Revista Mexicana de Astronomía y Astrofísica. Bruzual has published more than 40 articles in scientific journals; is co-author of a book on Astronomy ("Astronomía: una visión del cosmos", 1987) and has written a series of articles on Astronomy for the general public.