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

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We examine the stellar velocity dispersions (sigma) of a sample of 48 galaxies, 35 of which are spirals, from the Palomar nearby galaxy survey. It is known that for ultra-luminous infrared galaxies (ULIRGs) and merger remnants thesigma derived from the near-infrared CO band-heads is smaller than that measured from optical lines, while no discrepancy between these measurements is found for early-type galaxies. No such studies are available for spiral galaxies - the subject of this paper. We used cross-dispersed spectroscopic data obtained with the Gemini Near-Infrared Spectrograph (GNIRS), with spectral coverage from 0.85 to 2.5um, to obtain sigma measurements from the 2.29 μm CO band-heads (sigmaCO), and the 0.85 um calcium triplet (sigmaCaT). For the spiral galaxies in the sample, we found that sigmaCO is smaller than sigmaCaT, with a mean fractional difference of 14.3%. The best fit to the data is given by sigmaopt = (46.0+/-18.1) + (0.85+/-0.12)sigmaCO. This "sigma discrepancy" may be related to the presence of warm dust, as suggested by a slight correlation between the discrepancy and the infrared luminosity. This is consistent with studies that have found no sigma-discrepancy in dust-poor early-type galaxies, and a much larger discrepancy in dusty merger remnants and ULIRGs. That sigmaCO is lower than sigmaopt may also indicate the presence of a dynamically cold stellar population component. This would agree with the spatial correspondence between low sigmaCO and young/intermediate-age stellar populations that has been observed in spatially-resolved spectroscopy of a handful of galaxies.