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We have obtained projected rotation velocities (vsini) of a sample of 19ultracool dwarfs with spectral types in the interval M6.5-T8 usinghigh-resolution, near-infrared spectra obtained with NIRSPEC and the Keck IItelescope. Among our targets there are two young brown dwarfs, two likely fieldstars, and fifteen likely brown dwarfs (30-72 Mjup) of the solar neighborhood.Our results indicate that the T-type dwarfs are fast rotators in markedcontrast to M-type stars. We have derived vsini velocities between <15 and 40km/s for them, and have found no clear evidence for T dwarfs rotating stronglyfaster than L dwarfs. However, there is a hint for an increasing lower envelopeon moving from mid-M to the L spectral types in the vsini-spectral type diagramthat was previously reported in the literature; our vsini results extend it toeven cooler types. Assuming that field brown dwarfs have a size of 0.08-0.1Rsol, we can place an upper limit of 12.5 h on the equatorial rotation periodof T-type brown dwarfs. In addition, we have compared our vsini measurements tospectroscopic rotational velocities of very young brown dwarfs of similar massavailable in the literature. The comparison, although model-dependent, suggeststhat brown dwarfs lose some angular momentum during their contraction; however,their spin down time seems to be significantly longer than that of solar-typeto early-M stars.Comment: Accepted for publication in ApJ (revised version

Spectroscopic Rotational Velocities of Brown Dwarfs