BACON · The baryonic and chemical content of disc galaxies: A key to galaxy formation

Final Activity Report Summary - BACON (The baryonic and chemical content of disc galaxies: A key to galaxy formation)

The term 'baryons' indicates ordinary matter, detectable by the radiation it emits, at odds with 'dark matter' that reveals itself just by its gravitational effects. Disc galaxies, like our Milky Way, rotate with a velocity that depends on their total mass, both dark and baryonic; therefore, if we understood the mass distribution of the 'visible' baryons, which mainly consist of stars and gas, we could infer the distribution of dark matter. Thus, our project was devoted to measuring the baryonic content of disc galaxies, which was possible by associating the underlying mass to the light that was emitted by stellar populations. As the nuclear reactions transforming hydrogen and helium into heavier elements, i.e. metals, were the source of stellar luminosity, the chemical composition, luminosity and colours of stellar populations were intimately related and evolved side by side in time in terms of chemo-photometric evolution. Hence, the project was named 'The baryonic and chemical content of disc galaxies'. We used the detailed information of our local galactic region collected by Hipparcos satellite to reconstruct what our Galaxy would look like if seen from outside. We found that the Milky Way was not a very typical disc galaxy, since it had a somewhat lower luminosity and baryonic content than the ones expected based on its rotation velocity, i.e. its total mass. In this sense, the Milky Way was similar to 'simulated' galaxies, which were recreated in the computer starting from first principles, from Big Bang conditions. If baryonic matter were the only matter in a disc galaxy, its rotation velocity should decline beyond the luminous regions, where the baryons are concentrated; on the contrary, rotation curves remain 'flat' much further out, because of the surrounding dark matter 'haloes'. For decades astronomers used the difference between the observed flat rotation curves and the 'declining' rotation curves which were expected from the baryonic component in order to study the mass and shape of dark haloes. A basic assumption was that the light profile traced perfectly the mass distribution of the baryons. However, the chemical composition and the colours in disc galaxies changed with radius, which implied that the mixture of stellar populations and its mass-to-light ratio changed as well. With models of chemo-photometric evolution, we demonstrated that the stellar mass-to-light ratio significantly decreased with radius, so that the mass profiles of the stellar disc were more concentrated and the rotation curve which was expected from it declined even more rapidly than traditionally thought. Stellar discs were less massive and the sizes of dark halo 'cores' were smaller than previously thought. Yet the existence of such constant density cores remained a puzzle for cosmology. Disc galaxies show a tight relation between luminosity, used for tracing baryonic mass, and rotation velocity, utilised for tracing the total mass, i.e. both dark and baryonic. This relation is formulated by the Tully-Fisher (TF) relation. We used simulated galaxies to predict the evolution of the TF relation in time (redshift). The TF relation was more luminous in the past, at high redshift, because stellar populations were younger and brighter; however the underlying stellar mass, for a given rotation velocity, was the same as today. Hence, the proportion between baryonic and dark matter for a given total mass did not evolve with redshift. We also studied the chemical evolution of helium, which is, after hydrogen, the second most common element in the Universe. Helium was produced really early in space history by Big Bang nucleosynthesis (BBNS) and nuclear reactions in stars increased its abundance ever since. We determined the luminosity and temperature of about 100 nearby stars and derived their helium content by comparison with stellar models. This was the largest sample ever analysed for this purpose. For stars of metallicity close to that of the Sun, the helium content was in good agreement with chemical evolution models; nevertheless, for stars of low metallicity the analysis yielded unreasonably low helium contents, which were even lower than the initial BBNS level. This revealed inadequacies in the stellar models at low metallicity that ought to be improved in the future. This helium project was part of a PhD thesis which I co-supervised.

Data: CORDIS, © European Union