Late-type Giants in the Inner Galaxy

Abstract

The Milky Way is the cornerstone of our understanding of galaxies. The structure

and kinematics of its gas and stars can be studied in unique detail due to

their relative proximity. However, being located well within the Galactic disk

and thereby observing the Milky Way in non–linear projection makes it difcult to properly map its large-scale morphology. One of the interesting ndings has been that observations of molecular line emission (CO, HI) and stellar motions show signatures of a Galactic Bar in the inner Galaxy. However, its characteristics such as elongation, thickness and viewing angle are still poorly constrained. One of the main obstacles has been the strong obscuration by interstellar dust toward the inner Galaxy, which makes optical studies of the stellar population in that region almost impossible. The extinction is less severe at near- and mid-infrared wavelengths. To characterise the structure and formation history of the Milky Way, several infrared surveys were conducted during the past decade: ISOGAL, MSX, DENIS, 2MASS. These data contain a wealth of information on the structure of the stellar populations that has yet to be fully analysed. Having entered a golden age for Galactic astronomy, soon even more detailed imaging and spectroscopy will be provided by the Spitzer Space Telescope, while the GAIA satellite will provide unprecedented astrometry. My thesis research has focused on the structure and stellar population of the inner 4 kpc of the Milky Way. I have analysed data from recent infrared surveys and obtained SiO radio maser line observations of late-type giants to study the star formation history and the gravitational potential of the inner Galaxy. With ages ranging from less than 1 to 15 Gyr, the infrared-luminous late-type giant stars are representative of the bulk of the Galactic stellar population, and hence trace its star formation history. Their spatial abundance variation maps the stellar mass distribution, and thereby probes the Galactic gravitational potential. The reddening of their spectral energy distribution can be used to map the interstellar extinction. Their envelopes often emit strong molecular masers (OH, SiO) that can be detected throughout the Galaxy, and through the precise measurement of the maser line velocity they reveal the stars' line-of-sight velocities. Therefore they are ideal tracers of the Galactic kinematics and gravitational potential.