The vertical velocity dispersion of the stars in the disks of two spiral galaxies

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Galactic Rotation, Galactic Structure, Power Spectra, Spiral Galaxies, Star Distribution, Velocity Distribution, Astronomical Photometry, Disk Galaxies, Stellar Spectrophotometry

Scientific paper

The velocity dispersion in the interarm regions in the disks of two face-on spiral galaxies has been observed. At approximately one luminosity scale length from the center, the Z-velocity dispersion of the old disk population is (60 + or 20) km/s in NGC 628 and (80 + or - 15) km/s in NGC 1566. This indicates that this velocity dispersion decreases with increasing galactocentric distance in a manner consistent with that predicted by the constant thickness disks of van der Kruit and Searle. The observations suggest that the old disk populations in spiral disks have mass-to-light ratios of (5-10) solar mass/solar luminosity, B, similar to those inferred from H I velocity dispersion and layer thickness and for the solar neighborhood. It is noted that the disks are then not massive enough to account for the amplitude of the rotation curves, and that more than half of the mass within the optical radius must therefore reside outside the disks in a dark halo.

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