A Photometric Study of the Galactic Disks based on Strömgren Photometry

University dissertation from Lund Observatory

Abstract: One of the key questions in todays galaxy research is the formation and evolution of disk galaxies such as the Milky Way. Using observations in the intermediate band Stromgren uvby filter system we have investigated how both density and metallicity varies within the Galactic disks in order to learn more about the Galaxy and its past. As a first step we test the abilities of the Stromgren photometric system, and find that it can accurately reproduce metallicities determined from high resolution spectroscopy. We trace standard sequences for dwarf stars in the c1 vs. (b-y) plane and the c1 vs. (v-y) plane and find them to be metallicity dependent. The possibility of developing a log(g) calibration from Stromgren uvby photometry is explored, but we find such calibrations unable to separate dwarf stars from sub-giant stars. Five lines of sight have been observed with the Stromgren photometric system, all at intermediate galactic latitudes in the northern Galactic hemisphere. Stromgren photometry enables us to identify dwarf stars, dominantly located in the thin and the thick disks of the Galaxy, and to determine their metallicities. With accurate metallicity determinations we can infer absolute magnitudes for the dwarf stars using isochrones and thereby get stellar distances. We calculate the density along two of our lines of sight, and fitting a density model to the observed stellar number density we determine the scale-height of the thick and the thin disk along with the local normalisation of the two. The metallicity distribution function along our observed lines of sight is explored and we find it to be peaked at M/H of about -0.7 dex with a tail towards the metal poor. We study how the metallicity distribution function varies with distance from the Galactic centre and find no radial metallicity gradient in the thick disk. How the metallicity distribution function varies with height above the Galactic plane is also explored and we find a vertical metallicity gradient of -0.23 dex/kpc for G-type dwarf stars. We have some partial success at disentangling the thick disk metallicity distribution function from the thin disk metallicity distribution function and find no vertical metallicity gradient among thick disk stars.

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