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We examine the stellar velocity dispersions (sigma) of a sample of 48galaxies, 35 of which are spirals, from the Palomar nearby galaxy survey. It isknown that for ultra-luminous infrared galaxies (ULIRGs) and merger remnantsthesigma derived from the near-infrared CO band-heads is smaller than thatmeasured from optical lines, while no discrepancy between these measurements isfound for early-type galaxies. No such studies are available for spiralgalaxies - the subject of this paper. We used cross-dispersed spectroscopicdata obtained with the Gemini Near-Infrared Spectrograph (GNIRS), with spectralcoverage from 0.85 to 2.5um, to obtain sigma measurements from the 2.29 $\mu$mCO band-heads (sigma_{CO}), and the 0.85 um calcium triplet (sigma_{CaT}). Forthe spiral galaxies in the sample, we found that sigma_{CO} is smaller thansigma_{CaT}, with a mean fractional difference of 14.3%. The best fit to thedata is given by sigma_{opt} = (46.0+/-18.1) + (0.85+/-0.12)sigma_{CO}. This"sigma discrepancy" may be related to the presence of warm dust, as suggestedby a slight correlation between the discrepancy and the infrared luminosity.This is consistent with studies that have found no sigma-discrepancy indust-poor early-type galaxies, and a much larger discrepancy in dusty mergerremnants and ULIRGs. That sigma_{CO}$ is lower than sigma_{opt} may alsoindicate the presence of a dynamically cold stellar population component. Thiswould agree with the spatial correspondence between low sigma_{CO} andyoung/intermediate-age stellar populations that has been observed inspatially-resolved spectroscopy of a handful of galaxies.

Differences between CO- and calcium triplet-derived velocity dispersions in spiral galaxies: evidence for central star formation?