Systematic variation of the initial mass fonction (IMF) in early-type galaxies
1er avril 2012
For a better knowledge of the evolution of galaxies, and of the mass locked up into stars, it is essential to evaluate the initial mass function of stars (IMF), i.e what are the proportions of giant and dwarf stars formed during a starburst for example. The IMF is essential to deduce the Mass/Luminosity (M/L) ratio of a population of stars, because dwarf stars have an M/L ratio much higher than that of massive stars. The variation of the IMF from one galaxy to another was often proposed since nearly 50 years, but the data were always compatible with a universal IMF. In a recent study undertaken by an international team, including a researcher from Observatoire de Paris, the photometric and kinematic data of 260 galaxies of elliptical type (or early-type) were modelled in order to estimate the M/L ratio more precisely, and subsequently the IMF integrated during the whole life of a galaxy. The study concludes with a strong systematic IMF variation, according to the M/L ratio, being able to generate differences in mass of a factor three. These differences could come from the formation history of elliptical galaxies, thought to be formed more quickly and more violently than spiral galaxies.
The determination of the stellar mass of a galaxy is always subject to big uncertainties. However there is much stake to know it : first, knowing the luminosity of the stars, which is observed in various colors, one can derive the mass-to-light ratio (M/L), which depends on the nature of stars, their age, their metallicity, and on their distribution in mass. One can thus go back to the star formation history of the galaxy. In addition, knowing the dynamic mass of the galaxy, derived from the stellar velocity field, one can determine the missing mass, therefore the quantity of dark matter in the galaxy as a function of radius, by subtraction of the stellar mass from the total mass. In the elliptical or lenticular galaxies, which have very little interstellar gas, the dynamic mass is dominated by the stellar mass, the dark matter representing only a few percents. This is thus favorable to measure the stellar mass. However, these early-type galaxies are animated by random motions more than ordered rotation, and it is difficult to model the velocity field, dominated by dispersion. The determination of the dynamic mass then requires a more sophisticated model.
- Figure 1 : Modélisation des champs de vitesse de 5 des galaxies de l’échantillon. Chaque colonne correspond à une galaxie, dont le nom NGC est donné en haut. Les couleurs réprésentent les vitesses, avec la palette indiquée en bas. Les vitesses ont été symétrisées par rapport à l’axe vertical central. La rangée du haut sont les vitesses observées. La rangée du milieu représente les meilleurs modèles, ajustant le mieux les données, avec un M/L indiqué à droite pour chaque galaxie. La rangée du bas reprend ce modèle mais avec un M/L divisé par 1.5. On voit combien la variation de M/L détruit le bon ajustement avec les données. Cliquer sur l’image pour l’agrandir
The study just published by the international team, attempts to model in a very detailed and precise way a large sample of 260 galaxies of early-types, some examples of which are displayed in Figure 1. They have obtained the velocity fields on the telescope William Herschel in the Canary islands, and the homogeneous multi-color photometry from the SDSS (Sloan DIGITAL Sky Survey). They varied a great number of parameters in the modeling of the velocity fields, including the quantity and the distribution of dark matter, inclination, velocity anisotropy. The main result is that, not only the M/L ratio is different for each galaxy, but also the IMF function cannot be universal, and must vary from one galaxy to another, and in particular be different between elliptical and spiral galaxies. The spirals would have in proportion less stars of small mass, which have little luminosity, whereas the ellipticals on the contrary would have much more stars of small mass. Many studies had tried to demonstrate the non universality of the IMF, but there was not enough data to conclude in a convincing way.
Reference A systematic variation of the stellar initial mass function in early-type galaxies Michele Cappellari, Richard M. McDermid, Katherine Alatalo, Leo Blitz, Maxime Bois, Frederic Bournaud, M. Bureau, Alison F. Crocker, Roger L. Davies, Timothy A. Davis, P. T. de Zeeuw, Pierre-Alain Duc, Eric Emsellem, Sadegh Khochfar, Davor Krajnovic, Harald Kuntschner, Pierre-Yves Lablanche, Raffaella Morganti, Thorsten Naab, Tom Oosterloo, Marc Sarzi, Nicholas Scott, Paolo Serra, Anne-Marie Weijmans, Lisa M. Young Nature 26 April 2012 Contact Maxime Bois (Observatoire de Paris-LERMA, CNRS)
Dernière modification le 4 mars 2013