The formation and evolution of the Galactic bulge and its relationship with the other Galactic populations is still poorly understood.
To establish the chemical differences and similarities between the bulge and other stellar populations, the astronomers have performed elemental abundance analysis of oxygen, magnesium, silicon, calcium, titanium, sodium and aluminium of red giant stars in the bulge as well as of local thin disk, thick disk and halo giants (see figure).
The stellar populations of the Galaxy can be distinguished as being part of the bulge, of thin disk, , of thick disk and halo giants.
1) The image on the left, of the galaxy NGC 4526, gives a visual perception of galactic structure. Credit: NASA.
2) The image on the right presents schematically the galactic structure.
The authors used high-resolution optical spectra of 25 bulge giants in Baade’s window and made a comparison with 55 giants (4 halo, 29 thin disk and 22 thick disk giants) in the solar neighbourhood.
Baade's Window is named after the German astronomer Walter Baade and lies towards the constellation of Sagittarius. This region has relatively low amounts of interstellar "dust" along our line of sight and is a "window" because in this direction we are able to see all the way to the Milky Way Galactic Center and beyond. Therefore it is used to inspect distant stars and to determine the internal geometry of the Milky Way. Using bulge giants and giants in our neighbourhood the authors could compare the inner structure of the Galaxy with our vicinity.
The authors have found that all stars have similar stellar parameters but cover a broad range in metallicity. In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Since stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers, for convenience's sake, use the blanket term "metal" to describe all other elements collectively.
The team of astronomers was able to confirm the well-established differences for [α /Fe] at a given metallicity between the local thin and thick disks. For all the elements investigated, they found no chemical distinction between the bulge and the local thick disk.
Their findings lead to the conclusion that the bulge and local thick disk stars experienced similar formation timescales, star formation rates and initial mass functions. The scientific team thinks that the identical types of stars that can be found on the thick disk and bulge stars may reflect a rapid chemical evolution taking place before the bulge and thick disk structures we see today were formed, or it may reflect Galactic orbital migration of inner disk/bulge stars resulting in stars in the solar neighbourhood with thick-disk kinematics.
This reasearch was accepted for publication byAstronomy & Astrophysics.(read more)
Original paper:Alves-Brito,A., Melendez,J., Asplund,M., Ramirez,I., Yong,D. (2010) Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca and Ti, Astronomy & Astrophysics, in press.