Rotation Curve of the Spiral Galaxy NGC 7331 from the Luminosity and Mass Profiles

Abstract

The present work consists in the obtention of the rotation curve of the spiral galaxy NGC 7331 from its luminosity profile. In order to do that, 14 images of different spectral bands (from the far ultraviolet to the far infrared) were used, dividing the galaxy in concentric elliptical annuli to obtain the flux for the different distances to the galactic center, and for each spectral band. The luminosity profiles were then corrected for dust absorption. These spectra were then adjusted to stellar population models, using a two-burst function to represent the stellar formation events. With these stellar populations, the stellar mass profile is calculated. The mass profile for the gas in the galaxy is calculated as well, and added to the stellar mass to obtain the total mass profile. This result is used to calculate the rotation curve, with the hypothesis of spherical mass distribution for the bulge and circular planar mass distribution for the galactic disk. This curve was then compared with the rotation curve obtained from spectroscopy, and some theories (i.e. underestimation of the distance, dark matter halo) were formulated in order to explain the differences between the two curves. Some other hypothesis, like the MOND theory, strong magnetic fields in the outer disk or undetected mass clouds, were also introduced, although not developed.