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The vertical velocity dispersion of the stars in the disks of two spiral galaxies
The velocity dispersion in the interarm regions in the disks of twoface-on spiral galaxies has been observed. At approximately oneluminosity scale length from the center, the Z-velocity dispersion ofthe 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 dispersiondecreases with increasing galactocentric distance in a manner consistentwith that predicted by the constant thickness disks of van der Kruit andSearle. The observations suggest that the old disk populations in spiraldisks have mass-to-light ratios of (5-10) solar mass/solar luminosity,B, similar to those inferred from H I velocity dispersion and layerthickness and for the solar neighborhood. It is noted that the disks arethen not massive enough to account for the amplitude of the rotationcurves, and that more than half of the mass within the optical radiusmust therefore reside outside the disks in a dark halo.
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